1. The Steam and Condensate Loop 1.1.1Steam - The Energy Fluid Module 1.1Block 1 IntroductionModule 1.1Steam - The Energy Fluid

2. The Steam and Condensate LoopSteam - The Energy Fluid Module 1.11.1.2Block 1 IntroductionFig. 1.1.1 An 18th century steam engine.Photography courtesy ofKew Bridge Steam Museum, LondonFig. 1.1.2 A modern packaged steam heatexchange system used for producing hot waterIt is useful to introduce the topic of steam by considering its many uses and benefits, beforeentering an overview of the steam plant or any technical explanations.Steam has come a long way from its traditional associations with locomotives and the IndustrialRevolution. Steam today is an integral and essential part of modern technology. Without it, ourfood, textile, chemical, medical, power, heating and transport industries could not exist or performas they do.Steam provides a means of transporting controllable amounts of energy from a central, automatedboiler house, where it can be efficiently and economically generated, to the point of use. Thereforeas steam moves around a plant it can equally be considered to be the transport and provisionof energy.For many reasons, steam is one of the most widely used commodities for conveying heat energy.Its use is popular throughout industry for a broad range of tasks from mechanical power productionto space heating and process applications.Steam - The Energy FluidSteam is efficient and economic to generateWater is plentiful and inexpensive. It is non-hazardous to health and environmentally sound. In itsgaseous form, it is a safe and efficient energy carrier. Steam can hold five or six times as muchpotential energy as an equivalent mass of water.When water is heated in a boiler, it begins to absorb energy. Depending on the pressure in theboiler, the water will evaporate at a certain temperature to form steam. The steam contains alarge quantity of stored energy which will eventually be transferred to the process or the spaceto be heated. 3. The Steam and Condensate Loop 1.1.3Steam - The Energy Fluid Module 1.1Block 1 IntroductionFig. 1.1.3Steam can easily and cost effectivelybe distributed to the point of useSteam is one of the most widely used media to convey heat over distances. Because steam flowsin response to the pressure drop along the line, expensive circulating pumps are not needed.Due to the high heat content of steam, only relatively small bore pipework is required to distributethe steam at high pressure. The pressure is then reduced at the point of use, if necessary. Thisarrangement makes installation easier and less expensive than for some other heat transfer fluids.Overall, the lower capital and running costs of steam generation, distribution and condensatereturn systems mean that many users choose to install new steam systems in preference to otherenergy media, such as gas fired, hot water, electric and thermal oil systems.It can be generated at high pressures to give high steam temperatures. The higher the pressure,the higher the temperature. More heat energy is contained within high temperature steam so itspotential to do work is greater.o Modern shell boilers are compact and efficient in their design, using multiple passes andefficient burner technology to transfer a very high proportion of the energy contained in thefuel to the water, with minimum emissions.o The boiler fuel may be chosen from a variety of options, including combustible waste, whichmakes the steam boiler an environmentally sound option amongst the choices available forproviding heat. Centralised boiler plant can take advantage of low interruptible gas tariffs,because any suitable standby fuel can be stored for use when the gas supply is interrupted.o Highly effective heat recovery systems can virtually eliminate blowdown costs, return valuablecondensate to the boiler house and add to the overall efficiency of the steam and condensateloop.The increasing popularity of Combined Heat and Power (CHP) systems demonstrates the highregard for steam systems in todays environment and energy-conscious industries. 4. The Steam and Condensate LoopSteam - The Energy Fluid Module 1.11.1.4Block 1 IntroductionFig. 1.1.4 Typical two port control valve with a pneumatic actuator and positionerEnergy is easily transferred to the processSteam provides excellent heat transfer. When the steam reaches the plant, the condensationprocess efficiently transfers the heat to the product being heated.Steam can surround or be injected into the product being heated. It can fill any space at auniform temperature and will supply heat by condensing at a constant temperature; this eliminatestemperature gradients which may be found along any heat transfer surface - a problem which isso often a feature of high temperature oils or hot water heating, and may result in quality problems,such as distortion of materials being dried.Because the heat transfer properties of steam are so high, the required heat transfer area isrelatively small. This enables the use of more compact plant, which is easier to install and takesup less space in the plant. A modern packaged unit for steam heated hot water, rated to1 200 kW and incorporating a steam plate heat exchanger and all the controls, requires only0.7 m floor space. In comparison, a packaged unit incorporating a shell and tube heatexchanger would typically cover an area of two to three times that size.The modern steam plant is easy to manageIncreasingly, industrial energy users are looking to maximise energy efficiency and minimiseproduction costs and overheads. The Kyoto Agreement for climate protection is a major externalinfluence driving the energy efficiency trend, and has led to various measures around the globe,such as the Climate Change Levy in the UK. Also, in todays competitive markets, the organisationwith the lowest costs can often achieve an important advantage over rivals. Production costs canmean the difference between survival and failure in the marketplace.Steam is easy to controlBecause of the direct relationship between the pressure and temperature of saturated steam, theamount of energy input to the process is easy to control, simply by controlling the saturated steampressure. Modern steam controls are designed to respond very rapidly to process changes.The item shown in Figure 1.1.4 is a typical two port control valve and pneumatic actuator assembly,designed for use on steam. Its accuracy is enhanced by the use of a pneumatic valve positioner.The use of two port valves, rather than the three port valves often necessary in liquid systems,simplifies control and installation, and may reduce equipment costs. 5. The Steam and Condensate Loop 1.1.5Steam - The Energy Fluid Module 1.1Block 1 IntroductionFig. 1.1.5 A modern boiler house packageWays of increasing energy efficiency include monitoring and charging energy consumption torelevant departments. This builds an awareness of costs and focuses management on meetingtargets. Variable overhead costs can also be minimised by ensuring planned, systematicmaintenance; this will maximise process efficiency, improve quality and cut downtime.Most steam controls are able to interface with modern networked instrumentation and controlsystems to allow centralised control, such as in the case of a SCADA system or a Building/EnergyManagement System. If the user wishes, the components of the steam system can also operateindependently (standalone).BoilerFig. 1.1.6 Just some of the productsmanufactured using steam as an essentialpart of the processWith proper maintenance a steam plant will last for many years, and the condition of manyaspects of the system is easy to monitor on an automatic basis. When compared with othersystems, the planned management and monitoring of steam traps is easy to achieve with a trapmonitoring system, where any leaks or blockages are automatically pinpointed and immediatelybrought to the attention of the engineer.This can be contrasted with the costly equipment required for gas leak monitoring, or the time-consuming manual monitoring associated with oil or water systems.In addition to this, when a steam system requiresmaintenance, the relevant part of the system is easy toisolate and can drain rapidly, meaning that repairs maybe carried out quickly.In numerous instances, it has been shown that it is farless expensive to bring a long established steam plantup to date with sophisticated control and monitoringsystems, than to replace it with an alternative methodof energy provision, such as a decentralised gas system.The case studies refered to in Module 1.2 provide reallife examples.Todays state-of-the-art technology is a far cry from thetraditional perception of steam as the stuff of steamengines and the Industrial Revolution. Indeed, steamis the preferred choice for industry today. Name anywell known consumer brand, and in nine cases out often, steam will have played an important part inproduction. 6. The Steam and Condensate LoopSteam - The Energy Fluid Module 1.11.1.6Block 1 IntroductionSteam is flexibleNot only is steam an excellent carrier of heat, it is alsosterile, and thus popular for process use in the food,pharmaceutical and health industries. It is also widelyused in hospitals for sterilisation purposes.The industries within which steam is used range fromhuge oil and petrochemical plants to small locallaundries. Further uses include the production ofpaper, textiles, brewing, food production, curingrubber, and heating and humidification of buildings.Many users find it convenient to use steam as the sameworking fluid for both space heating and for processapplications. For example, in the brewing industry,steam is used in a variety of ways during different stagesof the process, from direct injection to coil heating.Steam is also intrinsically safe - it cannot cause sparks and presents no fire risk. Many petrochemicalplants utilise steam fire-extinguishing systems. It is therefore ideal for use in hazardous areas orexplosive atmospheres.Other methods of distributing energyThe alternatives to steam include water and thermal fluids such as high temperature oil. Eachmethod has its advantages and disadvantages, and will be best suited to certain applications ortemperature bands.Compared to steam, water has a lower potential to carry heat, consequently large amounts ofwater must be pumped around the system to satisfy process or space heating requirements.However, water is popular for general space heating applications and for low temperature processes(up to 120C) where some temperature variation can be tolerated.Thermal fluids, such as mineral oils, may be used where high temperatures (up to 400C) arerequired, but where steam cannot be used. An example would include the heating of certainchemicals in batch processes. However thermal fluids are expensive, and need replacing everyfew years - they are not suited to large systems. They are also very searching and high qualityconnections and joints are essential to avoid leakage.Different media are compared in Table 1.1.1, which follows. The final choice of heating mediumdepends on achieving a balance between technical, practical and financial factors, which will bedifferent for each user.Broadly speaking, for commercial heating and ventilation, and industrial systems, steam remainsthe most practical and economic choice.Fig. 1.1.8 These brewing processes all use steamFig. 1.1.7 Clean steam pipeline equipmentused in pharmaceutical process plant 7. The Steam and Condensate Loop 1.1.7Steam - The Energy Fluid Module 1.1Block 1 IntroductionTable 1.1.1 Comparison of heating media with steamSteam Hot water High temperature oilsHigh heat content Moderate heat content Poor heat contentLatent heat approximately Specific heat Specific heat often2 100 kJ/kg 4.19 kJ/kgC 1.69-2.93 kJ/kgCInexpensive InexpensiveSome water treatment costs Only occasional dosingExpensiveGood heat transfer Relatively poorcoefficientsModerate coefficientscoefficientsHigh pressure required High pressure needed Low pressures onlyfor high temperatures for high temperatures to get high temperaturesNo circulating pumps required Circulating pumps required Circulating pumps requiredSmall pipes Large pipes Even larger pipesMore complex to control - More complex to control -Easy to control with three way valves or three way valves ortwo way valves differential pressure valves differential pressure valvesmay be required may be required.Temperature breakdown is Temperature breakdown Temperature breakdowneasy through a reducing valve more difficult more difficultSteam traps required No steam traps required No steam traps requiredCondensate to be handled No condensate handling No condensate handlingFlash steam available No flash steam No flash steamBoiler blowdown necessary No blowdown necessary No blowdown necessaryWater treatment requiredLess corrosion Negligible corrosionto prevent corrosionReasonable pipework Searching medium, Very searching medium,required welded or flanged joints usual welded or flanged joints usualNo fire risk No fire risk Fire riskSystem very flexible System less flexible System inflexible 8. The Steam and Condensate LoopSteam - The Energy Fluid Module 1.11.1.8Block 1 IntroductionSystem benefitsSmall bore pipework, compact sizeand less weightNo pumps, no balancingTwo port valves - cheaperMaintenance costs lower thanfor dispersed plantCapital cost is lower than fordispersed plantSCADA compatible productsAutomation; fully automated boiler housesfulfil requirements such as PM5 andPM60 in the UKLow noiseReduced plant size(as opposed to water)Longevity of equipmentBoilers enjoy flexible fuelchoice and tariffSystems are flexible andeasy to add toThe benefits of steam - a summary:Table 1.1.2 Steam benefitsInherent benefitsWater is readily availableWater is inexpensiveSteam is clean and pureSteam is inherently safeSteam has a high heat contentSteam is easy to control due to thepressure/temperature relationshipSteam gives up its heat at aconstant temperatureEnvironmental factorsFuel efficiency of boilersCondensate management and heat recoverySteam can be metered and managedLinks with CHP/waste heatSteam makes environmental andeconomic senseUsesSteam has many uses -chillers, pumps, fans, humidificationSterilisationSpace heatingRange of industries 9. The Steam and Condensate Loop 1.1.9Steam - The Energy Fluid Module 1.1Block 1 IntroductionQuestions1. How does the heat carrying capacity of steam compare with water ?a| It is about the same b| It is less than water c| More than water d| It depends on the temperature 2. Which of the following is true of steam ?a| It carries much more heat than water b| Its heat transfer coefficient is more than thermal oil and water c| Pumps are not required for distribution d| All of the above 3. The amount of energy carried by steam is adjusted bya| Controlling steam pressure b| Controlling steam flow c| Controlling condensation d| Controlling boiler feeedwater temperature 4. Approximately how much potential energy will steam hold compared to an equivalentmass of water?a| Approximately the same b| Half as much c| 5 to 6 times as much d| Twice as much 5. How does steam give up its heat ?a| By cooling b| By radiation c| By conduction d| By condensation 6. Which of the following statements is not true ?a| Steam is less searching than high temperature oil or water b| Steam pipes will be smaller than water or high temperature oil pipes c| Temperature breakdown of water and oil is easier than steam d| Steam plant is smaller than water plant. 1:c,2:d,3:a,4:c,5:d,6:c Answers 10. The Steam and Condensate LoopSteam - The Energy Fluid Module 1.11.1.10Block 1 Introduction 11. Steam and the Organisation Module 1.2Block 1 IntroductionThe Steam and Condensate Loop 1.2.1Module 1.2Steam and the Organisation 12. The Steam and Condensate LoopSteam and the Organisation Module 1.21.2.2Block 1 IntroductionSteam and the OrganisationThe benefits described are not of interest to all steam users. The benefits of steam, as a problemsolver, can be subdivided according to different viewpoints within a business. They are perceiveddifferently depending on whether you are a chief executive, a manager or at operating level.The questions these people ask about steam are markedly different.Chief executiveThe highest level executive is concerned with the best energy transfer solution to meet the strategicand financial objectives of the organisation.If a company installs a steam system or chooses to upgrade an existing system, a significant capitalinvestment is required, and the relationship with the system, and the system provider, will be longand involved.Chief executives and senior management want answers to the following questions:Q. What kind of capital investment does a steam system represent ?A steam system requires only small bore pipes to satisfy a high heat requirement. It does notrequire costly pumps or balancing, and only two port valves are required.This means the system is simpler and less expensive than,for example, a high temperature hot water system. Thehigh efficiency of steam plant means it is compact andmakes maximum use of space, something which is oftenat a premium within plant.Furthermore, upgrading an existing steam system withthe latest boilers and controls typically represents 50%of the cost of removing it and replacing it with adecentralised gas fired system.Q. How will the operating and maintenance costs ofa steam system affect overhead costs ?Centralised boiler plant is highly efficient and can use low interruptible tariff fuel rates. The boilercan even be fuelled by waste, or form part of a state-of-the-art Combined Heat and Power plant.Steam equipment typically enjoys a long life - figures of thirty years or more of low maintenancelife are quite usual.Modern steam plant, from the boiler house to the steam using plant and back again, can be fullyautomated. This dramatically cuts the cost of manning the plant.Sophisticated energy monitoring equipment will ensure that the plant remains energy efficientand has a low manning requirement.All these factors in combination mean that a steam system enjoys a low lifetime cost.Q. If a steam system is installed, how can the most use be made of it ?Steam has a range of uses. It can be used for space heating of large areas, for complex processesand for sterilisation purposes.Using a hospital as an example, steam is ideal because it can be generated centrally at highpressure, distributed over long distances and then reduced in pressure at the point of use. Thismeans that a single high pressure boiler can suit the needs of all applications around the hospital,for example, heating of wards, air humidification, cooking of food in large quantities and sterilisationof equipment.It is not as easy to cater for all these needs with a water system.Fig. 1.2.1 13. Steam and the Organisation Module 1.2Block 1 IntroductionThe Steam and Condensate Loop 1.2.3Q. What if needs change in the future ?Steam systems are flexible and easy to add to. They can grow with the company and be altered tomeet changing business objectives.Q. What does using steam say about the company ?The use of steam is environmentally responsible. Companies continue to choose steam because itis generated with high levels of fuel efficiency. Environmental controls are increasingly stringent,even to the extent that organisations have to consider the costs and methods of disposing of plantbefore it is installed. All these issues are considered during the design and manufacture of steamplant.Management levelA manager will consider steam as something that will provide a solution to a management problem,as something that will benefit and add value to the business. The managers responsibility is toimplement initiatives ordered by senior executives. A manager would ask How will steamenable successful implementation of this task ?Managers tend to be practical and focused on completing a task within a budget. They willchoose to use steam if they believe it will provide the greatest amount of practicality and expediency,at a reasonable cost.They are less concerned with the mechanics of the steam system itself. A useful perspectivewould be that the manager is the person who wants the finished product, without necessarilywanting to know how the machinery that produces it is put together.Managers need answers to the following questions:Q. Will steam be right for the process ?Steam serves many applications and uses. It has a high heat content and gives up its heat at aconstant temperature. It does not create a temperature gradient along the heat transfer surface,unlike water and thermal oils, which means that it may provide more consistent product quality.As steam is a pure fluid, it can be injected directly into the product or made to surround theproduct being heated. The energy given to the process is easy to control using two port valves,due to the direct relationship between temperature and pressure.Q. If a steam system is installed, how can the most use be made of it ?Steam has a wide variety of uses. It can be used for space heating over large areas, and for manycomplex manufacturing processes.On an operational level, condensate produced by a manufacturing process can be returned tothe boiler feedtank. This can significantly reduce the boiler fuel and water treatment costs, becausethe water is already treated and at a high temperature.Lower pressure steam can also be produced from the condensate in a flash vessel, and used inlow pressure applications such as space heating.Fig. 1.2.2 14. The Steam and Condensate LoopSteam and the Organisation Module 1.21.2.4Block 1 IntroductionQ. What does steam cost to produce ?Water is plentiful and inexpensive, and steam boilers are highly efficient because they extract alarge proportion of the energy contained within the fuel. As mentioned previously, central boilerplant can take advantage of low interruptible fuel tariffs, something which is not possible fordecentralised gas systems which use a constant supply of premium rate fuel.Flash steam and condensate can be recovered and returned to the boiler or used on low pressureapplications with minimal losses.Steam use is easy to monitor using steam flowmeters and SCADA compatible products.For real figures, see The cost of raising steam, later in this Module.In terms of capital and operating costs, it was seen when answering the concerns of the chiefexecutive that steam plant can represent value for money in both areas.Q. Is there enough installation space ?The high rates of heat transfer enjoyed by steam means that the plant is smaller and more compactthan water or thermal oil plant. A typical modern steam to hot water heat exchanger packagerated to 1 200 kW occupies only 0.7 m floor space. Compare this to a hot water calorifier whichmay take up a large part of a plant room.Q. Not wishing to think too much about this part of the process, can a total solution beprovided ?Steam plant can be provided in the form of compact ready-to-install packages which are installed,commissioned and ready to operate within a very short period of time. They offer many years oftrouble-free operation and have a low lifetime cost.Technical personnel /operatorsAt the operating level, the day-to-day efficiency and working life of individuals can be directlyaffected by the steam plant and the way in which it operates. These individuals want to knowthat the plant is going to work, how well it will work, and the effect this will have on their timeand resources.Technical personal/operators need answers to the following questions:Q. Will it break down ?A well designed and maintained steam plant should have no cause to break down. The mechanicsof the system are simple to understand and designed to minimise maintenance. It is not unusualfor items of steam plant to enjoy 30 or 40 years of trouble-free life.Q. When maintenance is required, how easy is it ?Modern steam plant is designed to facilitate rapid easy maintenance with minimum downtime.The modern design of components is a benefit in this respect. For example, swivel connectorsteam traps can be replaced by undoing two bolts and slotting a new trap unit into place. Modernforged steam and condensate manifolds incorporate piston valves which can be maintainedin-line with a simple handheld tool.Sophisticated monitoring systems target the components that really need maintenance, ratherthan allowing preventative maintenance to be carried out unnecessarily on working items ofplant. Control valve internals can simply be lifted out and changed in-line, and actuators can bereversed in the field. Mechanical pumps can be serviced, simply by removing a cover, which hasall the internals attached to it. Universal pipeline connectors allow steam traps to be replaced inminutes. 15. Steam and the Organisation Module 1.2Block 1 IntroductionThe Steam and Condensate Loop 1.2.5An important point to note is that when maintenance of the system is required, a steam system iseasy to isolate and will drain rapidly, meaning that repairs can be quickly actioned. Any minorleaks that do occur are non-toxic. This is not always the case with liquid systems, which areslower and more costly to drain, and may include toxic or difficult to handle thermal fluids.Q. Will it look after itself ?A steam system requires maintenance just like any other important part of the plant, but thanksto todays modern steam plant design, manning and maintenance requirements and the lifetimecosts of the system are low. For example, modern boiler houses are fully automated. Feedwatertreatment and heating burner control, boiler water level, blowdown and alarm systems are allcarried out by automatic systems. The boiler can be left unmanned and only requires testing inaccordance with local regulations.Similarly, the steam plant can be managed centrally using automatic controls, flowmetering andmonitoring systems. These can be integrated with a SCADA system.Manning requirements are thus minimised.Industries and processes which use steam:Table 1.2.1 Steam usersHeavy users Medium users Light usersFood and drinks Heating and ventilating ElectronicsPharmaceuticals Cooking HorticultureOil refining Curing Air conditioningChemicals Chilling HumidifyingPlastics FermentingPulp and paper TreatingSugar refining CleaningTextiles MeltingMetal processing BakingRubber and tyres DryingShipbuildingPower generation 16. The Steam and Condensate LoopSteam and the Organisation Module 1.21.2.6Block 1 IntroductionInteresting uses for steam:o Shrink-wrapping meat.o Depressing the caps on food jars.o Exploding corn to make cornflakes.o Dyeing tennis balls.o Repairing underground pipes (steam is used to expand and seal a foam which has been pumpedinto the pipe. This forms a new lining for the pipe and seals any cracks).o Keeping chocolate soft, so it can be pumped and moulded.o Making drinks bottles look attractive but safe, for example tamper-proof, by heat shrinking afilm wrapper.o Drying glue (heating both glue and materials to dry on a roll).o Making condoms.o Making bubble wrap.o Peeling potatoes by the tonne (high pressure steam is injected into a vessel full of potatoes.Then it is quickly depressurised, drawing the skins off).o Heating swimming pools.o Making instant coffee, milk or cocoa powder.o Moulding tyres.o Ironing clothes.o Making carpets.o Corrugating cardboard.o Ensuring a high quality paint finish on cars.o Washing milk bottles.o Washing beer kegs.o Drying paper.o Ensuring medicines and medical equipment are sterile.o Cooking potato chips.o Sterilising wheelchairs.o Cooking pieces of food, for example seafood, evenly in a basket using injected steam forheat, moisture and turbulence at the same time.o Cooking large vats of food by direct injection or jacket heating.and hundreds more. 17. Steam and the Organisation Module 1.2Block 1 IntroductionThe Steam and Condensate Loop 1.2.7The cost of raising steamIn todays industry, the cost of supplying energy is of enormous interest. Table 1.2.2 showsprovisional industrial fuel prices for the United Kingdom, obtained from a recent Digest of UKEnergy Statistics, which were available in 2001.Table 1.2.2 UK fuel prices - 2001 (provisional)Fuel Size of consumer 2001Small 55.49Coal ( per tonne) Medium 46.04Large 33.85Small 142.73Heavy fuel oil ( per tonne) Medium 136.15Large 119.54Small 230.48Gas oil ( per tonne) Medium 224.61Large 204.30Small 4.89Electricity (pence per kWh) Medium 3.61Large 2.76Small 1.10Gas (pence per kWh) Medium 0.98Large 0.78The cost of raising steam based on the above costsAll figures exclude the Climate Change Levy (which came into force in April 2001) although theoil prices do include hydrocarbon oil duty.The cost of raising steam is based on the cost of raising one tonne (1 000 kg) of steam using thefuel types listed and average fuel cost figures.Table 1.2.3 UK steam costs - 2001 (provisional)FuelAverage unitUnit of supplyCost of raisingcost () 1 000 kg of steam ()Heavy (3 500 s) 0.074 0 Per litre 9.12OilMedium oil (950 s) 0.091 8 Per litre 11.31Light oil (210 s) 0.100 0 Per litre 12.32Gas oil (35 s) 0.105 4 Per litre 12.99Natural gasFirm 0.006 3 Per kWh 6.99Interruptible 0.005 0 Per kWh 5.55Coal 35.160 0 Per Tonne 3.72Electricity 0.036 7 Per kWh 25.26 18. The Steam and Condensate LoopSteam and the Organisation Module 1.21.2.8Block 1 IntroductionFig. 1.2.3Boiler efficiencyA modern steam boiler will generally operate at an efficiency ofbetween 80 and 85%. Some distribution losses will be incurredin the pipework between the boiler and the process plantequipment, but for a system insulated to current standards, thisloss should not exceed 5% of the total heat content of the steam.Heat can be recovered from blowdown, flash steam can be usedfor low pressure applications, and condensate is returned to theboiler feedtank. If an economiser is fitted in the boiler flue, theoverall efficiency of a centralised steam plant will be around 87%.This is lower than the 100% efficiency realised with an electricheating system at the point of use, but the typical running costsfor the two systems should be compared. It is clear that thecheapest option is the centralised boiler plant, which can use alower, interruptible gas tariff rather than the full tariff gas orelectricity, essential for a point of use heating system. The overallefficiency of electricity generation at a power station isapproximately 30 to 35%, and this is reflected in the unit charges.Components within the steam plant are also highly efficient. For example, steam traps only allowcondensate to drain from the plant, retaining valuable steam for the process. Flash steam fromthe condensate can be utilised for lower pressure processes with the assistance of a flash vessel.The following pages introduce some real life examples of situations in which a steam userhad, initially, been poorly advised and/or had access to only poor quality or incompleteinformation relating to steam plant. In both cases, they almost made decisions which wouldhave been costly and certainly not in the best interests of their organisation.Some identification details have been altered.Case study: UK West Country hospital considers replacing their steam systemIn one real life situation in the mid 1990s, a hospital in the West of England considered replacingtheir aged steam system with a high temperature hot water system, using additional gas firedboilers to handle some loads. Although new steam systems are extremely modern and efficientin their design, older, neglected systems are sometimes encountered and this user needed totake a decision either to update or replace the system.The financial allocation to the project was 2.57 million over three years, covering professionalfees plus VAT.It was shown, in consultation with the hospital, that only 1.2 million spent over ten yearswould provide renewal of the steam boilers, pipework and a large number of calorifiers. It wasalso clear that renewal of the steam system would require a much reduced professional input.In fact, moving to high temperature hot water (HTHW) would cost over 1.2 million morethan renewing the steam system.The reasons the hospital initially gave for replacing the steam system were:o With a HTHW system, it was thought that maintenance and operating costs would be lower.o The existing steam plant, boilers and pipework needed replacing anyway.Maintenance costs for the steam system were said to include insurance of calorifiers, steam trapmaintenance, reducing valves and water treatment plant, also replacement of condensate pipework.Operating costs were said to include water treatment, make-up water, manning of the boilerhouse, and heat losses from calorifiers, blowdown and traps.The approximate annual operating costs the hospital was using for HTHW versus steam, aregiven in the Table 1.2.4. 19. Steam and the Organisation Module 1.2Block 1 IntroductionThe Steam and Condensate Loop 1.2.9Table 1.2.4 Operating costsUtility Steam () HTHW ()Fuel245 000 180 0000 37 500Attendance 57 000 0Maintenance 77 000 40 000Water treatment 8 000 0Water 400 100Electricity 9 000 12 000Spares 10 000 5 000Total 406400 274600Additional claims in favour of individual gas fired boilers were given as:o No primary mains losses.o Smaller replacement boilers.o No stand-by fuel requirement.The costings set out above made the HTHW system look like the more favourable option interms of operating costs.The new HTHW system would cost 1 953 000 plus 274 600 per annum in operating andmaintenance costs. This, in effect, meant decommissioning a plant and replacing it at a cost inexcess of 2 million, to save just over 130 000 a year.The following factors needed to be taken into account:o The 130 000 saving using HTHW is derived from 406 400 - 274 600. The steam fuel costcan be reduced to the same level as for HTHW by using condensate return and flash steamrecovery. This would reduce the total by 65 000 to 341 400.o The largest savings claimed were due to the elimination of manned boilers. However, modernboiler houses are fully automated and there is no manning requirement.o The 37 000 reduction in maintenance costs looked very optimistic considering that the HTHWsolution included the introduction of 16 new gas fired boilers, 4 new steam generators and9 new humidifiers. This would have brought a significant maintenance requirement.o The steam generators and humidifiers had unaccounted for fuel requirements and watertreatment costs. The fuel would have been supplied at a premium rate to satisfy the claim thatstand-by fuel was not needed. In contrast, centralised steam boilers can utilise low costalternatives at interruptible tariff.o The savings from lower mains heat losses (eliminated from mains-free gas fired boilers) wereminimal against the total costs involved, and actually offset by the need for fuel at premiumtariff.o The proposal to change appeared entirely motivated by weariness with the supposed lowefficiency calorifiers however on closer inspection it can be demonstrated that steam towater calorifiers are 84% efficient, and the remaining 16% of heat contained in the condensatecan almost all be returned to the boiler house. Gas fired hot water boilers struggle to reach the84% efficiency level even at full-load. Unused heat is just sent up the stack. Hot water calorifiersare also much larger and more complicated, and the existing plant rooms were unlikely tohave much spare room.o A fact given in favour of replacing the steam system was the high cost of condensate pipereplacement. This statement tells us that corrosion was taking place, of which the commonestcause is dissolved gases, which can be removed physically or by chemical treatment. Removingthe system because of this is like replacing a car because the ashtrays are full !o A disadvantage given for steam systems was the need for insurance inspection of steam/water 20. The Steam and Condensate LoopSteam and the Organisation Module 1.21.2.10Block 1 Introductioncalorifiers. However, HTHW calorifiers also require inspection !o A further disadvantage given was the need to maintain steam pressure reducing valves. Butwater systems contain three port valves with a significant maintenance requirement.o The cost of make-up water and water treatment for steam systems was criticised. However,when a steam system requires maintenance, the relevant part can be easily isolated and quicklydrained with few losses (this minimises downtime). In contrast, a water system requires wholesections to be cooled and then drained off. It must then be refilled and purged of air aftermaintenance. HTHW systems also require chemical treatment, just like steam systems.Presented with these explanations, the hospital realised that much of the evidence they had beenbasing their decision on was biased and incomplete. The hospital engineering team reassessedthe case, and decided to retain their steam plant and bring it up to date with modern controls andequipment, saving a considerable amount of money.Trace heatingTrace heating is a vital element in the reliable operation of pipelines and storage/process vessels,across a broad range of industries.A steam tracer is a small steam pipe which runs along the outer surface of a (usually) larger processpipe. Heat conductive paste is often used between the tracer and the process pipe. The two pipesare then insulated together. The heat provided from the tracer (by conduction) prevents the contentsof the larger process pipe from freezing (anti-frost protection for water lines) or maintains thetemperature of the process fluid so that it remains easy to pump.Tracing is commonly found in the oil and petrochemical industries, but also in the food andpharmaceutical sectors, for oils, fats and glucose. Many of these fluids can only be pumped attemperatures well above ambient. In chemical processing, a range of products from acetic acidthrough to asphalt, sulphur and zinc compounds may only be moved through pipes if maintainedat a suitable temperature.For the extensive pipe runs found in much of process industry, steam tracing remains the mostpopular choice. For very short runs or where no steam supply is available, electrical tracing isoften chosen, although hot water is also used for low temperature requirements. The relativebenefits of steam and electric tracing are summarised in Table 1.2.5.Table 1.2.5 The relative merits of steam and electric trace heatingSteam Electrictrace heating trace heatingRobustness - ability to resist adverse weather and physical abuse Good PoorFlexibility - ability to meet demands of different products Excellent PoorSafety - suitability for use in hazardous areas Excellent Cannot be used in all zonesEnergy costs per GJ 0 to 2.14 8.64System life Long LimitedReliability High HighEase by which the system can be extended Easy DifficultTemperature control - accuracy of maintaining temperature Very good/high ExcellentSuitability for large plant Excellent ModerateSuitability for small plant Moderate GoodEase of tracer installation Moderate Requires specialist skillsCost of maintenance Low ModerateSpecialised maintenance staff requirement No YesAvailability as turnkey project Yes YesCase study: UK oil refinery uses steam tracing for 4 km pipeline 21. Steam and the Organisation Module 1.2Block 1 IntroductionThe Steam and Condensate Loop 1.2.11In 1998, a steam trace heating system was installed at one of the UKs largest oil refineries.BackgroundThe oil company in question is involved in the export of a type of wax product. The wax hasmany uses, such as insulation in electric cabling, as a resin in corrugated paper and as a coatingused to protect fresh fruit.The wax has similar properties to candle wax. To enable it to be transported any distance in theform of a liquid, it needs to be maintained at a certain temperature. The refinery therefore requireda pipeline with critical tracing.The project required the installation of a 200 mm diameter product pipeline, which would runfrom a tank farm to a marine terminal out at sea a pipeline of some 4 km in length.The project began in April 1997, installation was completed in August 1998, and the first successfulexport of wax took place a month later.Although the refinery management team was originally committed to an electric trace solution,they were persuaded to look at comparative design proposals and costings for both electric andsteam trace options.The wax applicationThe key parameter for this critical tracing application was to provide tight temperature control ofthe product at 80C, but to have the ability to raise the temperature to 90C for start-up orre-flow conditions. Other critical factors included the fact that the product would solidify attemperatures below 60C, and spoil if subjected to temperatures above 120C.Steam was available on site at 9 bar g and 180C, which immediately presented problems ofexcessive surface temperatures if conventional schedule 80 carbon steel trace pipework were tobe used. This had been proposed by the contractor as a traditional steam trace solution for the oilcompany.The total tracer tube length required was 11.5 km, meaning that the installation of carbon steelpipework would be very labour intensive, expensive and impractical. With all the joints involvedit was not an attractive option.However, todays steam tracing systems are highly advanced technologically. Spirax Sarco andtheir partner on the project, a specialist tracing firm, were able to propose two parallel runs ofinsulated copper tracer tube, which effectively put a layer of insulation between the product pipeand the steam tracer. This enabled the use of steam supply at 9 bar g, without the potential forhot spots which could exceed the critical 120C product limitation.The installation benefit was that as the annealed ductile steam tracer tubing used was available incontinuous drum lengths, the proposed 50 m runs would have a limited number of joints, reducingthe potential for future leaks from connectors.This provided a reliable, low maintenance solution.After comprehensive energy audit calculations, and the production of schematic installationdrawings for costing purposes, together with some careful engineering, the proposal was to usethe existing 9 bar g distribution system with 15 mm carbon steel pipework to feed the tracingsystem, together with strainers and temperature controls. Carbon steel condensate pipework wasused together with lightweight tracing traps which minimised the need for substantial fabricatedsupports.The typical tracer runs would be 50 m of twin isolated copper tracer tubing, installed at the 4 and8 oclock positions around the product pipe, held to the product pipeline with stainless steelstrap banding at 300 mm intervals.The material and installation costs for steam trace heating were about 30% less than the electric 22. The Steam and Condensate LoopSteam and the Organisation Module 1.21.2.12Block 1 Introductiontracing option. In addition, ongoing running costs for the steam system would be a fraction ofthose for the electrical option.Before the oil company management would commit themselves to a steam tracing system, theynot only required an extended product warranty and a plant performance guarantee, but alsoinsisted that a test rig should be built to prove the suitability of the self-acting controlled tracer forsuch an arduous application.Spirax Sarco were able to assure them of the suitability of the design by referral to an existinginstallation elsewhere on their plant, where ten self-acting controllers were already installed andsuccessfully working on the trace heating of pump transfer lines.The oil company was then convinced of the benefits of steam tracing the wax product line andwent on to install a steam tracing system.Further in-depth surveys of the 4 km pipeline route were undertaken to enable full installationdrawings to be produced. The company was also provided with on-site training for personnel oncorrect practices and installation procedures.After installation the heat load design was confirmed and the product was maintained at theFig. 1.2.4LaggingWaxSteamrequired 80C.The oil company executives were impressed with the success of the project and chose to installsteam tracing for another 300 m long wax product line in preference to electric tracing, eventhough they were initially convinced that electric tracing was the only solution for criticalapplications. 23. Steam and the Organisation Module 1.2Block 1 IntroductionThe Steam and Condensate Loop 1.2.13Questions1. How does the cost of upgrading a steam system compare with installing a decentralisedgas fired system ?a| It costs the same to upgrade the steam system. b| It costs twice as much to upgrade the steam system. c| It costs 75% as much to upgrade the steam system. d| It costs half as much to upgrade the steam system. 2. Which of the following uses for steam could be found in a hospital ?a| Space heating. b| Sterilisation. c| Cooking. d| All of the above. 3. Which of the following statements is true ?a| Steam creates a temperature gradient along the heat transfer surface,ensuring consistent product quality.b| Steam gives up its heat at a constant temperature without a gradient along theheat transfer surface, ensuring consistent product quality.c| High temperature oils offer a constant temperature along theheat transfer surface, which leads to poor product quality.d| High temperature oils can be directly injected into the product to be heated. 4. A hot water calorifier can occupy much of a plant room. How much floor space does amodern steam to hot water packaged unit need if it is rated at 1200 kW ?a| 0.7 m b| 7.0 m c| 1.2 m d| 12 m 5. Why is steam inexpensive to produce ?a| Steam boilers can use a variety of fuels. b| Steam boilers can utilise the heat from returned condensate. c| Steam boilers can be automated. d| All of the above. 6. Which of the following statements best describes steam tracing ?a| Steam is injected into the process pipe to keep the contents moving. b| An electric jacket is used to heat the process piping. c| A steam tracer is a small steam pipe which runs along the outside of a process pipe. d| A tracer is a small water filled pipe which runs along the outside of a process pipe. 1:c,2:d,3:b,4:a,5:d,6:c Answers 24. The Steam and Condensate LoopSteam and the Organisation Module 1.21.2.14Block 1 Introduction 25. The Steam and Condensate Loop Module 1.3Block 1 IntroductionThe Steam and Condensate Loop 1.3.1Module 1.3The Steam and Condensate Loop 26. The Steam and Condensate LoopThe Steam and Condensate Loop Module 1.31.3.2Block 1 IntroductionThe Steam and Condensate LoopThis Module of The Steam and Condensate Loop is intended to give a brief, non-technical overviewof the steam plant. It offers an overall explanation of how the different parts of the steam plantrelate to each other - and represents useful reading for anyone who is unfamiliar with the topic,prior to progressing to the next Block, or, indeed, before undertaking any form of detailed studyof steam theory or steam plant equipment.The boiler houseThe boilerThe boiler is the heart of the steam system. The typical modern packaged boiler is powered by aburner which sends heat into the boiler tubes.The hot gases from the burner pass backwards and forwards up to 3 times through a series oftubes to gain the maximum transfer of heat through the tube surfaces to the surrounding boilerwater. Once the water reaches saturation temperature (the temperature at which it will boil at thatpressure) bubbles of steam are produced, which rise to the water surface and burst. The steam isreleased into the space above, ready to enter the steam system. The stop or crown valve isolatesthe boiler and its steam pressure from the process or plant.Fig. 1.3.1 Typical heat path through a smoke tube shell boilerIf steam is pressurised, it will occupy less space. Steam boilers are usually operated under pressure,so that more steam can be produced by a smaller boiler and transferred to the point of use usingsmall bore pipework. When required, the steam pressure is reduced at the point of use.As long as the amount of steam being produced in the boiler is as great as that leaving the boiler,the boiler will remain pressurised. The burner will operate to maintain the correct pressure. Thisalso maintains the correct steam temperature, because the pressure and temperature of saturatedsteam are directly related.The boiler has a number of fittings and controls to ensure that it operates safely, economically,efficiently and at a consistent pressure.FeedwaterThe quality of water which is supplied into the boiler is important. It must be at the correcttemperature, usually around 80C, to avoid thermal shock to the boiler, and to keep it operatingefficiently. It must also be of the correct quality to avoid damage to the boiler.Steam at 150C3rd Pass (tubes)2nd Pass (tubes)1st Pass (furnace tube(s))400C600C200C350C 27. The Steam and Condensate Loop Module 1.3Block 1 IntroductionThe Steam and Condensate Loop 1.3.3Ordinary untreated potable water is not entirely suitable for boilers and can quickly cause themto foam and scale up. The boiler would become less efficient and the steam would become dirtyand wet. The life of the boiler would also be reduced.The water must therefore be treated with chemicals to reduce the impurities it contains.Both feedwater treatment and heating take place in the feedtank, which is usually situated highabove the boiler. The feedpump will add water to the boiler when required. Heating the water inthe feedtank also reduces the amount of dissolved oxygen in it. This is important, as oxygenatedwater is corrosive.BlowdownChemical dosing of the boiler feedwater will lead to the presence of suspended solids in theboiler. These will inevitably collect in the bottom of the boiler in the form of sludge, and areremoved by a process known as bottom blowdown. This can be done manually - the boilerattendant will use a key to open a blowdown valve for a set period of time, usually twice a day.Other impurities remain in the boiler water after treatment in the form of dissolved solids. Theirconcentration will increase as the boiler produces steam and consequently the boiler needs to beregularly purged of some of its contents to reduce the concentration. This is called control of totaldissolved solids (TDS control). This process can be carried out by an automatic system which useseither a probe inside the boiler, or a small sensor chamber containing a sample of boiler water, tomeasure the TDS level in the boiler. Once the TDS level reaches a set point, a controller signalsthe blowdown valve to open for a set period of time. The lost water is replaced by feedwater witha lower TDS concentration, consequently the overall boiler TDS is reduced.Level controlIf the water level inside the boiler were not carefully controlled, the consequences could becatastrophic. If the water level drops too low and the boiler tubes are exposed, the boiler tubescould overheat and fail, causing an explosion. If the water level becomes too high, water couldenter the steam system and upset the process.For this reason, automatic level controls are used. To comply with legislation, level control systemsalso incorporate alarm functions which will operate to shut down the boiler and alert attention ifthere is a problem with the water level. A common method of level control is to use probes whichsense the level of water in the boiler. At a certain level, a controller will send a signal to thefeedpump which will operate to restore the water level, switching off when a predetermined levelis reached. The probe will incorporate levels at which the pump is switched on and off, and atwhich low or high level alarms are activated. Alternative systems use floats.Fig. 1.3.2 A sophisticated feedtank system wherethe water is being heated by steam injection 28. The Steam and Condensate LoopThe Steam and Condensate Loop Module 1.31.3.4Block 1 IntroductionIt is a legal requirement in most countries to have two independent low level alarm systems.The flow of steam to the plantWhen steam condenses, its volume is dramatically reduced, which results in a localised reductionin pressure. This pressure drop through the system creates the flow of steam through the pipes.The steam generated in the boiler must be conveyed through the pipework to the point where itsheat energy is required. Initially there will be one or more main pipes or steam mains which carrysteam from the boiler in the general direction of the steam using plant. Smaller branch pipes canthen distribute the steam to the individual pieces of equipment.Steam at high pressure occupies a lower volume than at atmospheric pressure. The higher thepressure, the smaller the bore of pipework required for distribution of a given mass of steam.Steam qualityIt is important to ensure that the steam leavingthe boiler is delivered to the process in the rightcondition. To achieve this the pipework whichcarries the steam around the plant normallyincorporates strainers, separators and steamtraps.A strainer is a form of sieve in the pipeline.It contains a mesh through which the steammust pass. Any passing debris will be retainedby the mesh. A strainer should regularly becleaned to avoid blockage. Debris should beremoved from the steam flow because it can bevery damaging to plant, and may alsocontaminate the final product.High alarmControllersBoiler shellSecond low alarmFirst low alarmProtectiontubesPump onPump offFig. 1.3.3 Typical boiler level control/alarm configurationFig. 1.3.4 Cut section of a strainer 29. The Steam and Condensate Loop Module 1.3Block 1 IntroductionThe Steam and Condensate Loop 1.3.5o Condensate does not transmit heat effectively. A film of condensate inside plant will reducethe efficiency with which heat is transferred.o When air dissolves into condensate, it becomes corrosive.o Accumulated condensate can cause noisy and damaging waterhammer.o Inadequate drainage leads to leaking joints.A device known as a steam trap is used to release condensate from the pipework whilst preventingthe steam from escaping from the system. It can do this in several ways:o A float trap uses the difference in density between steam and condensate to operate a valve. Ascondensate enters the trap, a float is raised and the float lever mechanism opens the main valveto allow condensate to drain. When the condensate flow reduces the float falls and closes themain valve, thus preventing the escape of steam.o Thermodynamic traps contain a disc which opens to condensate and closes to steam.o In bimetallic thermostatic traps, a bimetallic element uses the difference in temperature betweensteam and condensate to operate the main valve.o In balanced pressure thermostatic traps, a small liquid filled capsule which is sensitive to heatoperates the valve.Once the steam has been employed in the process, the resulting condensate needs to be drainedfrom the plant and returned to the boiler house. This process will be considered later in this Module.Pressure reductionAs mentioned before, steam is usually generated at high pressure, and the pressure may have tobe reduced at the point of use, either because of the pressure limitations of the plant, or thetemperature limitations of the process.This is achieved using a pressure reducing valve.Fig. 1.3.5 Cut section of a separatorshowing operationAir to atmospherevia an air ventThe steam should be as dry as possible to ensureit is carrying heat effectively. A separator is a bodyin the pipeline which contains a series of platesor baffles which interrupt the path of the steam.The steam hits the plates, and any drops ofmoisture in the steam collect on them, beforedraining from the bottom of the separator.Steam passes from the boiler into the steammains. Initially the pipework is cold and heatis transferred to it from the steam. The airsurrounding the pipes is also cooler than thesteam, so the pipework will begin to lose heat tothe air. Insulation fitted around the pipe willreduce this heat loss considerably.When steam from the distribution system entersthe steam using equipment the steam will againgive up energy by: a) warming up the equipmentand b) continuing to transfer heat to the process.As steam loses heat, it turns back into water.Inevitably the steam begins to do this as soon asit leaves the boiler. The water which forms isknown as condensate, which tends to run to thebottom of the pipe and is carried along with thesteam flow. This must be removed from thelowest points in the distribution pipework forseveral reasons:Steam outSteam inCondensate to drainvia a float trap 30. The Steam and Condensate LoopThe Steam and Condensate Loop Module 1.31.3.6Block 1 IntroductionSteam at the point of useA large variety of steam using plant exists. A few examples are described below:o Jacketed pan - Large steel or copper pans used in the food and other industries to boilsubstances - anything from prawns to jam. These large pans are surrounded by a jacket filledwith steam, which acts to heat up the contents.o Autoclave - A steam-filled chamber used for sterilisation purposes, for example medicalequipment, or to carry out chemical reactions at high temperatures and pressures, for examplethe curing of rubber.o Heater battery - For space heating, steam is supplied to the coils in a heater battery. The air tobe heated passes over the coils.o Process tank heating - A steam filled coil in a tank of liquid used to heat the contents to thedesired temperature.o Vulcaniser - A large receptacle filled with steam and used to cure rubber.o Corrugator - A series of steam heated rollers used in the corrugation process in the productionof cardboard.o Heat exchanger - For heating liquids for domestic/industrial use.Control of the processAny steam using plant will require some method to control the flow of steam. A constant flow ofsteam at the same pressure and temperature is often not what is required a gradually increasingflow will be needed at start-up to gently warm the plant, and once the process reaches thedesired temperature, the flow must be reduced.Control valves are used to control the flow of steam. The actuator, see Figure 1.3.6, is the devicethat applies the force to open or close the valve. A sensor monitors conditions in the process, andtransmits information to the controller. The controller compares the process condition with theset value and sends a corrective signal to the actuator, which adjusts the valve setting.Fig. 1.3.6 A pneumatically operated two port control valveValve stemValve plugActuatorValveSpringsDiaphragmMovement 31. The Steam and Condensate Loop Module 1.3Block 1 IntroductionThe Steam and Condensate Loop 1.3.7A variety of control types exist:o Pneumatically actuated valves - Compressed air is applied to a diaphragm in the actuator toopen or close the valve.o Electrically actuated valves - An electric motor actuates the valve.o Self-acting - There is no controller as such - the sensor has a liquid fill which expands andcontracts in response to a change in process temperature. This action applies force to open orclose the valve.Condensate removal from plantOften, the condensate which forms will drain easily out of the plant through a steam trap. Thecondensate enters the condensate drainage system. If it is contaminated, it will probably be sentto drain. If not, the valuable heat energy it contains can be retained by returning it to the boilerfeedtank. This also saves on water and water treatment costs.Sometimes a vacuum may form inside the steam using plant. This hinders condensate drainage,but proper drainage from the steam space maintains the effectiveness of the plant. The condensatemay then have to be pumped out.Mechanical (steam powered) pumps are used for this purpose. These, or electric powered pumps,are used to lift the condensate back to the boiler feedtank.A mechanical pump, see Figure 1.3.7, is shown draining an item of plant. As can be seen, thesteam and condensate system represents a continuous loop.Once the condensate reaches the feedtank, it becomes available to the boiler for recycling.Control valveSteamCondensate collecting receiverHeated mediumCondensate returns to the feedtankPlantCondensateCondensate SteamMechanical pumpFig. 1.3.7 Condensate recovery and returnAirEnergy monitoringIn todays energy conscious environment, it is common for customers to monitor the energyconsumption of their plant.Steam flowmeters are used to monitor the consumption of steam, and used to allocate costs toindividual departments or items of plant. 32. The Steam and Condensate LoopThe Steam and Condensate Loop Module 1.31.3.8Block 1 IntroductionQuestions1. What is the purpose of the multi-flue passes in a boiler ?a| To reduce the amount of flue gases exhausted b| To help produce drier steam c| To provide more even generation of steam bubbles d| To give a greater heat transfer area to the water 2. What is the purpose of the boiler feedtank ?a| To store chemically treated water for the boiler b| To provide a reservoir of hot water for the boiler c| To collect condensate returning from the plant d| All of the above 3. The boiler feedtank is heated to approximately what temperature ?a| 80C b| 20C c| Steam temperature d| It isnt heated, all heating takes place in the boiler 4. What is the purpose of boiler bottom blowdown ?a| To remove total dissolved solids in the boiler water b| To remove separated out oxygen c| To dilute the boiler water to reduce TDS d| To remove solids which collect in the bottom of the boiler 5. What is used to remove suspended water particles in a steam main ?a| A separator and steam trap b| A strainer and steam trap c| A strainer d| A reducing valve 6. Which of the following is the purpose of a boiler automatic level control ?a| To provide TDS control b| To maintain a specified level of water c| To comply with legislation d| To take corrective action if the boiler alarms sound 1:d,2:d,3:a,4:d,5:a,6:b Answers 33. The Steam and Condensate Loop 2.1.1Block 2 Steam Engineering Principles and Heat Transfer Engineering Units Module 2.1Module 2.1Engineering Units 34. The Steam and Condensate LoopEngineering Units Module 2.12.1.2Block 2 Steam Engineering Principles and Heat TransferEngineering UnitsThroughout the engineering industries, many different definitions and units have been proposedand used for mechanical and thermal properties.The problems this caused led to the development of an agreed international system of units (orSI units: Systme International dUnits). In the SI system there are seven well-defined unitsfrom which the units of other properties can be derived, and these will be used throughout thispublication.The SI units include length (in metres), mass (in kilograms), time (in seconds) and temperature(in Kelvin). The first three will hopefully need no further explanation, while the latter will bediscussed in more detail later.The other SI units are electric current (in amperes), amount of substance (in moles) and luminousintensity (in candela). These may be familiar to readers with a background in electronics, chemistryand physics respectively, but have little relevance to steam engineering nor the contents ofThe Steam and Condensate Loop.Table 2.1.1 shows the derived units that are relevant to this subject, all of which should befamiliar to those with any general engineering background. These quantities have all been assignedspecial names after famous pioneers in the development of science and engineering.Table 2.1.1 Named quantities in derived SI unitsQuantity Name Symbol SI units Derived unitForce newton N m kg/s J/mEnergy joule J m kg/s N mPressure or stress pascal Pa kg/m s N/mPower watt W m kg/s J/sThere are many other quantities that have been derived from SI units, which will also be ofsignificance to anyone involved in steam engineering. These are provided in Table 2.1.2.Table 2.1.2 Other quantities in derived SI unitsQuantity SI units Derived unitsMass density kg/m kg/mSpecific volume (vg) m/kg m/kgSpecific enthalpy (h) m/s J/kgSpecific heat capacity (cp) m/s K J/kg KSpecific entropy m/s K J/kg KHeat flowrate m kg/s J/s or WDynamic viscosity kg/m s N s/mTemperatureThe temperature scale is used as an indicator of thermal equilibrium, in the sense that any twosystems in contact with each other with the same value are in thermal equilibrium.The Celsius (C) scaleThis is the scale most commonly used by the engineer, as it has a convenient (but arbitrary) zerotemperature, corresponding to the temperature at which water will freeze.The absolute or K (kelvin) scaleThis scale has the same increments as the Celsius scale, but has a zero corresponding to theminimum possible temperature when all molecular and atomic motion has ceased. Thistemperature is often referred to as absolute zero (0 K) and is equivalent to -273.15C. 35. The Steam and Condensate Loop 2.1.3Block 2 Steam Engineering Principles and Heat Transfer Engineering Units Module 2.1Fig. 2.1.2 Comparison of absolute and gauge pressuresFig. 2.1.1 Comparison of absolute and gauge temperaturesAbsolute temperaturedegrees kelvin (K)Temperature relative to thefreezing point of waterdegrees Celcius (C)373 K 100C273 K 0C0 K -273CAtmospheric pressure(approximately 1 bar a = 0 bar g)Perfect vacuum(0 bar a)GaugepressureAbsolutepressureVacuumDifferential pressurebar a bar g + 1The SI unit of temperature is the kelvin, which is defined as 1 273.15 of the thermodynamictemperature of pure water at its triple point (0.01C). An explanation of triple point is given inModule 2.2.Most thermodynamic equations require the temperature to be expressed in kelvin. However,temperature difference, as used in many heat transfer calculations, may be expressed in either Cor K. Since both scales have the same increments, a temperature difference of 1C has the samevalue as a temperature difference of 1 K.PressureThe SI unit of pressure is the pascal (Pa), defined as 1 newton of force per square metre (1 N/m).As Pa is such a small unit the kPa (1 kilonewton/m) or MPa (1 Meganewton/m) tend to be moreappropriate to steam engineering.However, probably the most commonly used metric unit for pressure measurement in steamengineering is the bar. This is equal to 105 N/m, and approximates to 1 atmosphere. This unit isused throughout this publication.Other units often used include lb/in (psi), kg/cm, atm, in H2O and mm Hg. Conversion factorsare readily available from many sources.The two scales of temperature are interchangeable, as shown in Figure 2.1.1 and expressed inEquation 2.1.1.Absolute pressure (bar a)This is the pressure measured from the datum of a perfect vacuum i.e. a perfect vacuum has apressure of 0 bar a.Equation 2.1.17 . 36. 7HPSHUDWXUH & 37. = 38. The Steam and Condensate LoopEngineering Units Module 2.12.1.4Block 2 Steam Engineering Principles and Heat TransferEquation 2.1.3Equation 2.1.2HQVLW RI VXEVWDQFH6SHFLILF JUDYLWHQVLW RI ZDWHU=VZP 9 YJGauge pressure (bar g)This is the pressure measured from the datum of the atmospheric pressure. Although in realitythe atmospheric pressure will depend upon the climate and the height above sea level, a generallyaccepted value of 1.013 25 bar a (1 atm) is often used. This is the average pressure exerted bythe air of the earths atmosphere at sea level.Gauge pressure = Absolute pressure - Atmospheric pressurePressures above atmospheric will always yield a positive gauge pressure. Conversely a vacuum ornegative pressure is the pressure below that of the atmosphere. A pressure of -1 bar g correspondsclosely to a perfect vacuum.Differential pressureThis is simply the difference between two pressures. When specifying a differential pressure, it isnot necessary to use the suffixes g or a to denote either gauge pressure or absolute pressurerespectively, as the pressure datum point becomes irrelevant.Therefore, the difference between two pressures will have the same value whether these pressuresare measured in gauge pressure or absolute pressure, as long as the two pressures are measuredfrom the same datum.Density and specific volumeThe density (r) of a substance can be defined as its mass (m) per unit volume (V). The specificvolume (vg) is the volume per unit mass and is therefore the inverse of density. In fact, the termspecific is generally used to denote a property of a unit mass of a substance (see Equation 2.1.2).Where:r = Density (kg/m)m = Mass (kg)V = Volume (m)vg = Specific volume (m/kg)The SI units of density (r) are kg/m, conversely, the units of specific volume (vg) are m/kg.Another term used as a measure of density is specific gravity. It is a ratio of the density of asubstance (rs) and the density of pure water (rw) at standard temperature and pressure (STP). Thisreference condition is usually defined as being at atmospheric pressure and 0C. Sometimes it issaid to be at 20C or 25C and is referred to as normal temperature and pressure (NTP).The density of water at these conditions is approximately 1 000 kg/m. Therefore substanceswith a density greater than this value will have a specific gravity greater than 1, whereas substanceswith a density less than this will have a specific gravity of less than 1.Since specific gravity is a ratio of two densities, it is a dimensionless variable and has no units.Therefore in this case the term specific does not indicate it is a property of a unit mass of asubstance. Specific gravity is also sometimes known as the relative density of a substance.Heat, work and energyEnergy is sometimes described as the ability to do work. The transfer of energy by means ofmechanical motion is called work. The SI unit for work and energy is the joule, defined as 1 N m.The amount of mechanical work carried out can be determined by an equation derived fromNewtonian mechanics:Work = Force x Displacement 39. The Steam and Condensate Loop 2.1.5Block 2 Steam Engineering Principles and Heat Transfer Engineering Units Module 2.1It can also be described as the product of the applied pressure and the displaced volume:Work = Applied pressure x Displaced volumeExample 2.1.1An applied pressure of 1 Pa (or 1 N/m) displaces a volume of 1 m. How much work has beendone?Work done = 1 N/m x 1 m = 1 N m (or 1 J)The benefits of using SI units, as in the above example, is that the units in the equation actuallycancel out to give the units of the product.The experimental observations of J. P. Joule established that there is an equivalence betweenmechanical energy (or work) and heat. He found that the same amount of energy was required toproduce the same temperature rise in a specific mass of water, regardless of whether the energywas supplied as heat or work.The total energy of a system is composed of the internal, potential and kinetic energy. Thetemperature of a substance is directly related to its internal energy (ug). The internal energy isassociated with the motion, interaction and bonding of the molecules within a substance. Theexternal energy of a substance is associated with its velocity and location, and is the sum of itspotential and kinetic energy.The transfer of energy as a result of the difference in temperature alone is referred to as heat flow.The watt, which is the SI unit of power, can be defined as 1 J/s of heat flow.Other units used to quantify heat energy are the British Thermal Unit (Btu: the amount of heat toraise 1 lb of water by 1F) and the calorie (the amount of heat to raise 1 kg of water by 1C).Conversion factors are readily available from numerous sources.Specific enthalpyThis is the term given to the total energy, due to both pressure and temperature, of a fluid (suchas water or steam) at any given time and condition. More specifically it is the sum of the internalenergy and the work done by an applied pressure (as in Example 2.1.1).The basic unit of measurement is the joule (J). Since one joule represents a very small amount ofenergy, it is usual to use kilojoules (kJ = 1 000 joules).The specific enthalpy is a measure of the total energy of a unit mass, and its units are usually kJ/kg.Specific heat capacityThe enthalpy of a fluid is a function of its temperature and pressure. The temperature dependenceof the enthalpy can be found by measuring the rise in temperature caused by the flow of heat atconstant pressure. The constant-pressure heat capacity cp, is a measure of the change in enthalpyat a particular temperature.Similarly, the internal energy is a function of temperature and specific volume. The constant-volume heat capacity cv, is a measure of the change in internal energy at a particular temperatureand constant volume.Because the specific volumes of solids and liquids are generally smaller, then unless the pressureis extremely high, the work done by an applied pressure can be neglected. Therefore, if theenthalpy can be represented by the internal energy component alone, the constant-volume andconstant-pressure heat capacities can be said to be equal.Therefore, for solids and liquids: cp cvAnother simplification for solids and liquids assumes that they are incompressible, so that theirvolume is only a function of temperature. This implies that for incompressible fluids the enthalpyand the heat capacity are also only functions of temperature. 40. The Steam and Condensate LoopEngineering Units Module 2.12.1.6Block 2 Steam Engineering Principles and Heat TransferEquation 2.1.44 P F 7SThe specific heat capacity represents the amount of energy required to raise 1 kg by 1C, and canbe thought of as the ability of a substance to absorb heat. Therefore the SI units of specific heatcapacity are kJ/kg K (kJ/kg C). Water has a large specific heat capacity (4.19 kJ/kg C) comparedwith many fluids, which is why both water and steam are considered to be good carriers of heat.The amount of heat energy required to raise the temperature of a substance can be determinedfrom Equation 2.1.4.Where:Q = Quantity of energy (kJ)m = Mass of the substance (kg)cp = Specific heat capacity of the substance (kJ/kg C )DT = Temperature rise of the substance (C)This equation shows that for a given mass of substance, the temperature rise is linearly related tothe amount of heat provided, assuming that the specific heat capacity is constant over thattemperature range.Example 2.1.2Consider a quantity of water with a volume of 2 litres, raised from a temperature of 20C to 70C.At atmospheric pressure, the density of water is approximately 1 000 kg/m. As there are1 000 litres in 1 m, then the density can be expressed as 1 kg per litre (1 kg/l). Therefore themass of the water is 2 kg.The specific heat capacity for water can be taken as 4.19 kJ/kg C over low ranges of temperature.Therefore: Q = 2 kg x 4.19 kJ/kg C x (70 - 20)C = 419 kJIf the water was then cooled to its original temperature of 20C, it would also release this amountof energy in the cooling application.Entropy (S)Entropy is a measure of the degree of disorder within a system. The greater the degree of disorder,the higher the entropy. The SI units of entropy are kJ/kg K (kJ/kg C).In a solid, the molecules of a substance arrange themselves in an orderly structure. As the substancechanges from a solid to a liquid, or from a liquid to a gas, the arrangement of the moleculesbecomes more disordered as they begin to move more freely. For any given substance the entropyin the gas phase is greater than that of the liquid phase, and the entropy in the liquid phase ismore than in the solid phase.One characteristic of all natural or spontaneous processes is that they proceed towards a state ofequilibrium. This can be seen in the second law of thermodynamics, which states that heatcannot pass from a colder to a warmer body.A change in the entropy of a system is caused by a change in its heat content, where the change ofentropy is equal to the heat change divided by the average absolute temperature, Equation 2.1.5.Equation 2.1.5&KDQJH LQ HQWKDOS + 41. &KDQJH LQ HQWURS 6 42. $YHUDJH DEVROXWH WHPSHUDWXUH 7 43. When unit mass calculations are made, the symbols for entropy and enthalpy are written in lowercase, Equation 2.1.6.Equation 2.1.6&KDQJH LQ VSHFLILF HQWKDOS K 44. &KDQJH LQ VSHFLILF HQWURS V 45. $YHUDJH DEVROXWH WHPSHUDWXUH 7 46. The Steam and Condensate Loop 2.1.7Block 2 Steam Engineering Principles and Heat Transfer Engineering Units Module 2.1To look at this in further detail, consider the following examples:Example 2.1.3A process raises 1 kg of water from 0 to 100C (273 to 373 K) under atmospheric conditions.Specific enthalpy at 0C (hf) = 0 kJ/kg (from steam tables)Specific enthalpy of water at 100C (hf) = 419 kJ/kg (from steam tables)Calculate the change in specific entropySince this is a change in specific entropy of water, the symbol s in Equation 2.1.6 takes thesuffix f to become sf.Example 2.1.4A process changes 1 kg of water at 100C (373 K) to saturated steam at 100C (373 K) underatmospheric conditions.Calculate the change in specific entropy of evaporationSince this is the entropy involved in the change of state, the symbol s in Equation 2.1.6 takes thesuffix fg to become sfg.Specific enthalpy of evaporation of steam at 100C (373 K) (hfg) = 2 258 kJ/kg (from steam tables)Specific enthalpy of evaporation of water at 100C (373 K) (hfg) = 0 kJ/ks (from steam tables)( )IV N- NJ .III&KDQJH LQ VSHFLILF HQWKDOS K 47. &DOFXODWH &KDQJH LQ VSHFLILF HQWURS V 48. $YHUDJH DEVROXWH WHPSHUDWXUH 7 49. 7KHUHIRUH V V( )=IJ N- NJ .VIJIJIJ&KDQJH LQ VSHFLILF HQWKDOS K 50. &DOFXODWH&KDQJH LQ VSHFLILF HQWURS V 51. $YHUDJH DEVROXWH WHPSHUDWXUH 7 52. 7KHUHIRUH V VThe total change in specific entropy from water at 0C to saturated steam at 100C is thesum of the change in specific entropy for the water, plus the change of specific entropy for thesteam, and takes the suffix g to become the total change in specific entropy sg.J I IJJJV N- NJ .7KHUHIRUH&KDQJH LQ VSHFLILF HQWURS V 53. V VV IURP ([DPSOH 54. IURP DERYH 55. D D DDD 56. The Steam and Condensate LoopEngineering Units Module 2.12.1.8Block 2 Steam Engineering Principles and Heat TransferAs the entropy of saturated water is measured from a datum of 0.01C, the entropy of waterat 0C can, for practical purposes, be taken as zero. The total change in specific entropy in thisexample is based on an initial water temperature of 0C, and therefore the final result happensto be very much the same as the specific entropy of steam that would be observed in steamtables at the final condition of steam at atmospheric pressure and 150C.J6SHFLILF WRWDO HQWKDOS RIVWHDP DW DWPRVSKHULF SUHVVXUHDQG DW & . 57. K 58. N- NJ IURP VWHDP WDEOHV 59. 6SHFLILF WRWDO HQWKDOS RIVWHDP DW DWPRVSKHULF SUHVVXUHDQG DW & . 60. K 61. &KDQJH LQ VSHFLILF HQWKDOS K 62. N- NJ$YHUDJH DEVROXWH WHPSHUDWXUH .N- NJ IURP VWHDP WDEOHV 63. $YHUDJH DEVROXWH WHPSHUDWXUHDExample 2.1.5A process superheats 1 kg of saturated steam at atmospheric pressure to 150C (423 K). Determinethe change in entropy.Equation 2.1.6&KDQJH LQ VSHFLILF HQWKDOS K 64. &KDQJH LQ VSHFLILF HQWURS V 65. $YHUDJH DEVROXWH WHPSHUDWXUH 7 66. J&KDQJH LQVSHFLILF HQWURS V 67. N- NJ .&KDQJH LQ VSHFLILF HQWURS V 68. 7RWDO FKDQJH LQ VSHFLILF HQWURS V 69. V DGGLWLRQDO HQWURS GXH WR VXSHUKHDWLQJ V 70. 7KH FKDQJH LQ WRWDO VSHFLILF HQW7KH WRWDO FKDQJH LQVSHFLILF HQWURS N- NJ .URS N- NJ . IURP ([DPSOH 71. N- NJ . 72. The Steam and Condensate Loop 2.1.9Block 2 Steam Engineering Principles and Heat Transfer Engineering Units Module 2.1Questions1. Given water has a specific heat capacity of 4.19 kJ/kg C, what quantity of heat is requiredto raise the temperature of 2 500 l of water from 10C to 80C?a| 733 250 kJ b| 175 000 kJ c| 175 kJ d| 41 766 kJ 2. A pressure of 10 bar absolute is specified. What is the equivalent pressure in gaugeunits?a| 8 bar g b| 11 bar g c| 9 bar g d| 12 bar g 3. A valve has an upstream pressure of 8 bar absolute and a downstream pressureof 5 bar g. What is the pressure differential across the valve?a| 3 bar b| 4 bar c| 7 bar d| 2 bar 4. What quantity of heat is given up when 1 000 l of water is cooled from 50C to 20C?a| 125 700 kJ b| 30 000 KJ c| 30 000 kJ/kg d| 125 700 kJ/kg 5. 500 l of fuel oil is to be heated from 25C to 65C. The oil has a relative density of 0.86and a specific heat capacity of 1.88 kJ/kgC. How much heat will be required?a| 17 200 kJ b| 37 600 kJ c| 32 336 kJ d| 72 068 kJ 6. A thermometer reads 160C. What is the equivalent temperature in K?a| 433 K b| 192 K c| 113 K d| 260 K 1:a,2:c,3:d,4:a,5:c,6:a Answers 73. The Steam and Condensate LoopEngineering Units Module 2.12.1.10Block 2 Steam Engineering Principles and Heat Transfer 74. The Steam and Condensate Loop2.2.1Block 2 Steam Engineering Principles and Heat Transfer What is Steam? Module 2.2Module 2.2What is Steam? 75. The Steam and Condensate Loop2.2.2Block 2 Steam Engineering Principles and Heat Transfer What is Steam? Module 2.2What is Steam?A better understanding of the properties of steam may be achieved by understanding the generalmolecular and atomic structure of matter, and applying this knowledge to ice, water and steam.A molecule is the smallest amount of any element or compound substance still possessing all thechemical properties of that substance which can exist. Molecules themselves are made up of evensmaller particles called atoms, which define the basic elements such as hydrogen and oxygen.The specific combinations of these atomic elements provide compound substances. One suchcompound is represented by the chemical formula H2O, having molecules made up of two atomsof hydrogen and one atom of oxygen.The reason water is so plentiful on the earth is because hydrogen and oxygen are amongst themost abundant elements in the universe. Carbon is another element of significant abundance,and is a key component in all organic matter.Most mineral substances can exist in the three physical states (solid, liquid and vapour) which arereferred to as phases. In the case of H2O, the terms ice, water and steam are used to denote thethree phases respectively.The molecular structure of ice, water, and steam is still not fully understood, but it is convenient toconsider the molecules as bonded together by electrical charges (referred to as the hydrogenbond). The degree of excitation of the molecules determines the physical state (or phase) ofthe substance.Triple pointAll the three phases of a particular substance can only coexist in equilibrium at a certain temperatureand pressure, and this is known as its triple point.The triple point of H2O, where the three phases of ice, water and steam are in equilibrium, occursat a temperature of 273.16 K and an absolute pressure of 0.006 112 bar. This pressure is veryclose to a perfect vacuum. If the pressure is reduced further at this temperature, the ice, instead ofmelting, sublimates directly into steam.IceIn ice, the molecules are locked together in an orderly lattice type structure and can only vibrate.In the solid phase, the movement of molecules in the lattice is a vibration about a mean bondedposition where the molecules are less than one molecular diameter apart.The continued addition of heat causes the vibration to increase to such an extent that somemolecules will eventually break away from their neighbours, and the solid starts to melt to a liquidstate (always at the same temperature of 0C whatever the pressure).Heat that breaks the lattice bonds to produce the phase change while not increasing the temperatureof the ice, is referred to as enthalpy of melting or heat of fusion. This phase change phenomenonis reversible when freezing occurs with the same amount of heat being released back to thesurroundings.For most substances, the density decreases as it changes from the solid to the liquid phase.However, H2O is an exception to this rule as its density increases upon melting, which is why icefloats on water. 76. The Steam and Condensate Loop2.2.3Block 2 Steam Engineering Principles and Heat Transfer What is Steam? Module 2.2Pressure bar gTemperatureC00 1 2 3 4 5 6 7 8 9 10 11 12 13 1450100200300400Fig. 2.2.1 Steam saturation curveSteam saturation curveWaterIn the liquid phase, the molecules are free to move, but are still less than one molecular diameterapart due to mutual attraction, and collisions occur frequently. More heat increases molecularagitation and collision, raising the temperature of the liquid up to its boiling temperature.Enthalpy of water, liquid enthalpy or sensible heat (hf) of waterThis is the heat energy required to raise the temperature of water from a datum point of 0C toits current temperature.At this reference state of 0C, the enthalpy of water has been arbitrarily set to zero. The enthalpyof all other states can then be identified, relative to this easily accessible reference state.Sensible heat was the term once used, because the heat added to the water produced a change intemperature. However, the accepted terms these days are liquid enthalpy or enthalpy of water.At atmospheric pressure (0 bar g), water boils at 100C, and 419 kJ of energy are required toheat 1 kg of water from 0C to its boiling temperature of 100C. It is from these figures that thevalue for the specific heat capacity of water (Cp) of 4.19 kJ/kg C is derived for most calculationsbetween 0C and 100C.SteamAs the temperature increases and the water approaches its boiling condition, some moleculesattain enough kinetic energy to reach velocities that allow them to momentarily escape from theliquid into the space above the surface, before falling back into the liquid.Further heating causes greater excitation and the number of molecules with enough energy toleave the liquid increases. As the water is heated to its boiling point, bubbles of steam form withinit and rise to break through the surface.Considering the molecular structure of liquids and vapours, it is logical that the density of steam ismuch less than that of water, because the steam molecules are further apart from one another.The space immediately above the water surface thus becomes filled with less dense steam molecules.When the number of molecules leaving the liquid surface is more than those re-entering,the water freely evaporates. At this point it has reached boiling point or its saturation temperature,as it is saturated with heat energy.If the pressure remains constant, adding more heat does not cause the temperature to rise anyfurther but causes the water to form saturated steam. The temperature of the boiling water andsaturated steam within the same system is the same, but the heat energy per unit mass is muchgreater in the steam.At atmospheric pressure the saturation temperature is 100C. However, if the pressure is increased,this will allow the addition of more heat and an increase in temperature without a change of phase.Therefore, increasing the pressure effectively increases both the enthalpy of water, and the saturationtemperature. The relationship between the saturation temperature and the pressure is known asthe steam saturation curve (see Figure 2.2.1). 77. The Steam and Condensate Loop2.2.4Block 2 Steam Engineering Principles and Heat Transfer What is Steam? Module 2.2Equation 2.2.1K K K=J I IJWater and steam can coexist at any pressure on this curve, both being at the saturation temperature.Steam at a condition above the saturation curve is known as superheated steam:o Temperature above saturation temperature is called the degree of superheat of the steam.o Water at a condition below the curve is called sub-saturated water.If the steam is able to flow from the boiler at the same rate that it is produced, the addition offurther heat simply increases the rate of production. If the steam is restrained from leaving theboiler, and the heat input rate is maintained, the energy flowing into the boiler will be greater thanthe energy flowing out. This excess energy raises the pressure, in turn allowing the saturationtemperature to rise, as the temperature of saturated steam correlates to its pressure.Enthalpy of evaporation or latent heat (hfg)This is the amount of heat required to change the state of water at its boiling temperature, intosteam. It involves no change in the temperature of the steam/water mixture, and all the energy isused to change the state from liquid (water) to vapour (saturated steam).The old term latent heat is based on the fact that although heat was added, there was no changein temperature. However, the accepted term is now enthalpy of evaporation.Like the phase change from ice to water, the process of evaporation is also reversible. The sameamount of heat that produced the steam is released back to its surroundings during condensation,when steam meets any surface at a lower temperature.This may be considered as the useful portion of heat in the steam for heating purposes, as it is thatportion of the total heat in the steam that is extracted when the steam condenses back to water.Enthalpy of saturated steam, or total heat of saturated steamThis is the total energy in saturated steam, and is simply the sum of the enthalpy of water andthe enthalpy of evaporation.Where:hg = Total enthalpy of saturated steam (Total heat) (kJ/kg)hf = Liquid enthalpy (Sensible heat) (kJ/kg)hfg = Enthalpy of evaporation (Latent heat) (kJ/kg)The enthalpy (and other properties) of saturated steam can easily be referenced using the tabulatedresults of previous experiments, known as steam tables.The saturated steam tablesThe steam tables list the properties of steam at varying pressures. They are the results of actualtests carried out on steam.