Heat transferHeat is a form of energy It is transferred from a region of high temperature to a region of low temperature. Mechanisms of heat transfer:1. Conduction: It is the transfer of energy from one molecule to the other adjacent to it without displacement of particles. It is the main mechanism in case of solids (metal rods)2. Convection: Energy transferred from region to another by the motion of fluid (water).Heat transfer from lower layer to middle to upper.Its confined to liquid and gases (not solids)Convection may be:a. Natural: due to difference in density arising from temperature gradient.b. Forced: convection by mixing3. Radiation: All bodies with temperature above absolute zero radiate heat in the form of electromagnetic waves which travel in straight line at the speed of light. Radiation may be transmitted, reflected or absorbed by matterThe fraction absorbed is transformed into heat ex. Sun Sources of heat energy:1. Direct e.g. flame and electrical heater.2. Indirect it involves 2 steps:a. Transfer of heat from one system to heat transfer agent (HTA) (Brine, steam, water, oil and air)b. Transfer of heat from HTA to second system.

Steam (as a Heat Transfer Agent) Steam usually under pressure. Steam properties: Has very high heat content. Its used to give both:1. Sensible heat: amount of energy required to raise the temperature of substance by 1C2. Latent heat: amount of energy required to convert from state to another Its raw material (water) is cheap, clean, odorless and tasteless. Therefore its accidental contamination is not serious (as oil). Steam is generated in boilers (away from the factory) By fitting a valve that will not open to allow the escape of steam until a certain pressure is reached As pressure increased, the boiling point of water is increased and the steam temperature is increased. As P B.P of Water Steam temperature

The relation between pressure and temperature of steam is not a simple one and tables have been worked out of which the following data were abstracted:PressureSteam Temperature

(Psig)FC

0212100

5228109

10240116

15250121

20259126

25266130

30274134

Steam classification 1. Saturated steam (wet): Steam arises directly from boiling water. Easily condensed on any cooler surface due to presence of suspended water droplets2. Unsaturated steam (dry): Its pressure is less than that of wet steam at the same temperature.Dont condense on a slightly cooler surface 3. Superheated steam: Produced by heating a saturated steam away of contact with its liquid state. It behaves as a gas.

Steam dyers (water separator)They are used to separate water from steam and to convert wet steam to dry steamSteam a gas lighter than a liquid condensateASteam separator, sometimes referred to as amoisture separator, is a device for separating water droplets from steam.It is important to remove water droplets from steam because:1. In all engines, wet steam reduces thethermal efficiency2. In piston engines, water can accumulate in the cylinders and cause ahydraulic lockwhich will damage the engine3. Inthermal power stations, water droplets in high velocity steam coming from nozzles (or vanes) in asteam turbinecan impinge on and erode turbine internals such as turbine blades.When steam rapidly changes its direction, the water in the steam is left behind.There are two types of steam dryers as shownSimple typeScrew type

Consists of a vessel with vertical plate (baffle) reaching almost the bottom.Wet steam enters at the top, pass down one side and up on the other side leaving the water at the bottom of the vesselThe wet steam passes into the upper part of a cylindrical vessel contains a central pipe, its outside wall is screw so the wet steam take spiral movement round the pipe and centrifugal force generated throws water on the inner wall of the outer vessel and water drips down into the bottom, while dry steam which changes its direction rapidly passes up and out by the pipe

Steam traps: Steam trap is used to prevent outlet of live steam and removal of water collected in steam dryers.They are divided into two types:1. MechanicalDepends on the physical difference between vapor and liquid

a. Float type trap:b. Bucket steam trap

1) As the condensate level rises, 2) The float rises3) Outlet is opened by the float. 4) Till the condensate is discharged, 5) The float falls and outlet is closed.The outlet is closed when the bucket floats on the condensate. As more condensate drains, some over flows into the bucket so it sinks and the outlet is opened and condensate is blown out by the pressure of the steam and bucket floats up to close the outlet once more.

2. Thermostatic:Depends on the condensate can lose its sensible heat and have low temperature than steam.

a. Simple thermostatic trapb. Balanced pressure expansion steam trapBellows trap

slight variation in the steam pressure, and temperature, may cause the trap either open or closed (disadvantage)It is used for steam pans, steam heated stills. It consists of a closed capsule (made of cupper and corrugated like a below) and held at the top, so it will expand and contract, at the bottom of the capsule is a valve seat. The capsule contains a liquid having a boiling point a few degrees below the boiling point of water. Thus when surrounded by stem, the liquid in the capsule boils, causing the element to expand and close the outlet. Condensate cools the capsule, the liquid then ceases to boil, and the bellows contract, opening the outlet and discharging the condensate. When all the condensate is removed, steam causes the trap to close again

Thermal insulation:Used to prevent heat loss in an economic state. Is the reduction ofheat transfer(the transfer of thermal energybetween objects of differing temperature) between objects in thermal contact.Thermal insulation provides a region of insulation in whichthermal conductionis reduced than absorbed by the lower-temperature body.The insulating capability of a material is measured withthermal conductivity (k). Thermal conductivity is measured inwatts-per-meter perkelvin(Wm1K1), represented ask.Low thermal conductivity is equivalent to high insulating capability (R-value). Inthermal engineering, other important properties of insulating materials are productdensity ()andspecific heat capacity (c).Factors influencing performanceInsulation performance is influenced by many factors the most prominent of which include:1. Thermal conductivity("k" or "" value)2. Surfaceemissivity("" value)3. Insulation thickness4. Density5. Specific heat capacity6. Thermal bridging

A good lagging materials used for hot surface should:1. Have low thermal conductivity2. Should suppress convection currents.Examples of lagging materials are: a) Crock very good insulator but charred at moderate temperature (used in refrigerating)b) Asbestos used for steam pipes and hot plastic materials.c) 85% Magnesia with Asbestos used for steam pipes and hot plastic materials.Economic thickness of lagging as thickness increased the heat loss decreased and increased the cost. Therefore there is an optimum thickness. Small pipes need small thickness.

Heat-Exchange equipment:1. Tubular heat exchanger:Heat exchangers are so important and so widely used in the process industries that their design has been highly developed.a) Single-Pass Tubular Heater (1-1 exchanger) It consists from bound of parallel tubes which is surrounded by a cylinder shell and provided with two distribution chambers, condensed vapor is drained using steam trap (drips) and non condensed gas and vapor are removed from vent.b) Fixed-Head Heat exchanger (1-4 exchanger)The tubes have at each end plate or sheets fixed to shell. Internal fluid to be heated makes several passes in the exchanger (4 passes). Baffles increase the velocity of flow outside the tubes, this giving better heat transfer. Disadvantage tube bundle can not be removed for cleaning and tubes has no change for expansion due to baffles.c) Floating-Head Heat exchanger (1-2) and (2-4) exchangerOne tube plate is fixed from one end and the second is bolted to a floating head cover, so that the tube bundle can move relative to the shell. Advantages tube bundle can be removed for cleaning and can be expanded.Factors to be considered in the design of heat exchangers:Any heat exchanger consists from shell side and tube side:Shell side: heat transfer can be increased by fitting alternating baffles to multiply the path of heating liquid, so it flow first in one direction and then in the opposite direction and also help their support.Tube side: when heat is flowing under steady state condition the amount of heat transfer (Q) is given by Fouriers equation:

K= thermal conductivity coefficient of materialX= Thickness of tubes T= Temperature difference A= Area Q= rate of heat transfer Tubes must be of high thermal conductivity to absorb more heat e.g. copper, long thin and high number of tubesInternal tube composed from copper and the external tube composed from cast iron.

2. Special forms of heat exchangers A number of special types of equipment have been developed to give improved heat transfer under conditions where the standard forms of tubular-units are unsatisfactory.Finned tube exchanger: for viscous liquids (have large surface area)Plate heat exchanger: series of parallel plates are held firmly together and supported in frame. It produces large surface area for viscous liquids, easy to clean. The plates are spaced about 5 mm apart by rubber gasket around the edge of plates. The hot fluid passes between alternate pairs of plates exchanging heat with the cold fluid in the adjacent plates.Spiral Heat Exchanger:Two liquids are flowing countercurrent. Frytherm is suitable for heating and cooling products, which are highly visous or containing liquid matters as well as products whose viscosity changes during the heat exchange processes or which exhibit tendency to crystallize. The scrapers cause the product to be conveyed from the inner surfaces of each chamber to the outer surfaces. This results in rapid change process without any significant amount of heat lost to the environment. The telescopic scraped surface heat exchange elements enables the unit dimensions to be reduced to a minimum. Advantages: Large surface area in small place, low cost, drastic reduction in energy consumption, velocity very high and overall heat transfer coefficient is high.