Nithish-Influence of Component Efficiencies on Cycle Performance

8/13/2019 Nithish-Influence of Component Efficiencies on Cycle Performance

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STEAM PIPING

The steam piping system is an essential part of steam power

plant. These are the following requirements.1) The piping adopted must ensure maximum reliability.

2) It should be possible to carry out inspection andmaintenancewithout complete shutdown of the plant.

3) Piping should be of necessary size to carry the requiredflow of fluids.

4) The piping should withstand high temperatures andpressures.

5) The number of fittings and bends required to make theconnections should be as minimumas possible.

6) The steam traps should be provided to drain the line and

prevent accumulation of water during the steam flow.


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STEAM PIPE FITTINGS Steam pipe fittings are used to assemble piping system and

make connections. Fittings can be in the form of screwed or welded fittings.

These are generally used in sizes up to 8cm in diameter.

For large pipe sizes flanges are used.

The various fittings which are in common use are:

1. Elbows

2. Bends

3. Tees4. Crosses

5. Plugs

6. Reducers.


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Elbows are used to change the directions of two pipes.

Tees are used for joining two pipe running's in same

direction and to provide outlet for a branch pipe. Reducers are used to join the pipes of different sizes.

Plugs and caps are used to close the ends of fittings and

pipes.

The present day power plants use welded connections forsteam lines to make them light.

Welding method makes pipe insulation easier.


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PIPING SUPPORTS

Steam pipes are usually supported by hangers, brackets,

rollers etc. The piping system should be anchored to limitthe maximum expansion and contraction strains.

The supported pipes should be free to move on their

supports in any direction except at the point of anchorage.

The supports should be arranged in such a way that any

one pipe can be withdrawn without disturbing the piping

system.

Rigid hangers are used to fasten overhead beams orembedded in concrete.

Rollers can be incorporated if axial movement is desired.

Spring type hangers can be used when the dimensional

changes are caused by temperature variations


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DESIGN OF STEAM PIPING

The design of pipe size should include the internal

pressure of steam, thermal expansion restraints and deadweight of pipes, fitting and insulation.

The pipe wall thickness to meet the requirement can be

calculated using

Tm=((d*p)/(2f+Bp))+C

Where Tm is minimum thickness of the pipe.(cm)

p is the steam pressure.

d is the outside diameter

f is allowable stress

B & C are the constants


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The steam flow rate through the pipe is given by

ms=A*Vs*

To determine the correct size of pipe with least cost, the

usual practice is to assume a velocity based on previous

experience and use the above equation for determining the

diameter if the mass of steam with its condition to be

carried is known.The higher the velocity of steam used, the smaller the

required size of pipe, but friction loss increases as a square

of the velocity.

Every care should be taken in the pipe layout to reduce thelength of the piping to minimum.


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The materials commonly used for steam piping insulation

are asbestos, magnesia, cork, hair felt, wool felt, rock wool

and diatomaceous earth.

A very common and effective insulation for temperature upto 4000C is the molded 85% magnesia.

A layer of glass silk before giving the layer of magnesia is

generally used for pipe insulation when the temperature is

above 500oC.

Glass silk has an advantage of cleanliness, non-inflammable

and can withstand vibration and rough handling without

losing its form or thermal efficiency.The heat loss through insulation is given by

Q=2**k[(T1-T2)/ln(r2/r1)] watts


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WASTE HEAT MANAGEMENT

Waste heat is the heat which is not used and exhausted out

as a waste product.

Recovery of exhaust heat can reduce fuel consumption and

increase thermal efficiency.

When the temperature of the exhaust gases exceeds 300oC,

steam generation becomes the most economical method. All the available waste heat appears as the low

temperature heat.

The heat discharged is either in the form of sensible heat orin the form of latent heat.

The waste heat is classified into high grade and low grade.


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High grade waste heat can be recovered easily through

properly designed heat equipments. The most economical

method of utilization is by waste heat boiler because steam

generated by cooling the gas can be used in the process as

well in turbines.

These flue gases justify as a viable source of energy for

industries as cement, iron, paper, glass and ceramicindustries as potential sources for heat recovery.

Low grade waste heat is usually in the form of flue gases

and drain waters. These are found in food processing and

chemical industries.


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THERMODYNAMIC CYCLES FOR WASTE HEAT

RECOVERY To recover the waste heat optimally, it is necessary to

bottom the heat energy to the maximum extent and at the

same time at its greatest efficiency.

Rankine cycles are attractive because they have relatively

high efficiency at low temperatures compared to otherdynamic energy conversion systems.

% of waste heat

That is recovered

Waste heat temp

Practicalrange

Carnot limit


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The higher the waste stream temperature and lower the

cooling medium temperature, the greater is the amount of

energy recovered.

The bottoming cycles can operate practically in the range

shown in the graph.

In a diesel engine, 38% of the fuel energy is converted into

work and remaining is waste heat.

Based on the efficiency limitations of the cycle, only

rankine, stirling and absorption cycles can be considered for

waste heat temperature of 100-400oC.


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HEAT RECOVERY METHODS

1) Sensible heat recovery: The gas-air heaters which are

commonly used are of counter-flow type.

By selecting various values of Tho , we can calculate the %

increase in recovery.

To double the heat recovery the area should be increased

by three times.

The recovery can be increased by increasing the value of

U.

The optimum heat extraction should be done byincreasing A as well as U such a way that extraction of

heat costs are minimum.


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2) Latent heat recovery:this is the most common method of

waste heat recovery when used for power generation.

Waste heat boilers can operate in any size and pressure

range of 2 to 50bar, the advantages arei. This form of recovery is attractive when latent heat is high

ii. A reasonably low temperature difference can be

maintained even when recovery is high.

iii. They require smaller pipe work as they operate in liquidphase which has high density.

iv. It gives higher values of U.

v. The system pressure can be kept low.


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OTHER USES OF WASTE HEAT

1. Agriculture:spray irrigation and soil heating can be used

to prevent the frost production in certain regions.

favorable temperatures govern the germination of the

seeds and increase the yield.

Heated water is used for soil sterilization for cotton seeds.

2. Green houses: it has been suggested that green houses

might be constructed adjacent to nuclear plants in cold

countries to use the waste heat and replace cooling

towers.3. Animal shelters: the growth rate of some animals is

strongly influenced on the environmental temperature.

This is particularly more effective for small animals like

poultry and swine.


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4. Aqua cultural uses: The fish species are cultivated in

controlled environments.

5. Waste heat utilization from waste water treatment:

municipal and industrial waste water effluents are used ascooling water in power plant. These are being looked upon

not as pollutants but as a secondary water source if

treated properly using waste heat.

6. Power generation7. Steam generation which can used for different industrial

purposes

8. Can be used as supplementary fuel in boilers.

9. Gas production.

WASTE HEAT BOILERS


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WASTE HEAT BOILERS


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INFLUENCE OF COMPONENT EFFICIENCIES ON

CYCLE PERFORMANCE

Components of a cycle :

1) Boiler

2) Nozzles

3) Turbine4) Condenser

5) Feed pump


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BOILERS

Evaporative capacity of a boiler is expressed in terms of kg

of steam evaporated/hr.

Equivalent evaporation may be defined as the amount of

water evaporated from water at 100oC to dry and

saturated steam at 100o

C.As per the standard conditions 1kg of water at 100oC

necessitates 2257kJ to get converted to steam at 100oC.

Boiler efficiency is the ratio of heat actually utilized in

generation of steam to the heat supplied by the fuel in the

same period.

Boiler efficiency = ma(h-hf)/C


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where,

ma= mass of water evaporated into steam per kg of

fuel at working pressure

h= enthalpy of steam/kg @ generating conditionshf= enthalpy of water @ given feed temperature and

C = calorific value of the fuel in kJ/kg.

The factors on which the boiler efficiency depends:

1. Boiler design

2. Built in losses

3. Properties and characteristics of fuel burnt4. Actual firing rate

5. Conditions of fuel absorbing surfaces

6. Humidity and temperature of the combustion air.

HEAT LOSSES IN A BOILER


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HEAT LOSSES IN A BOILER :

1. Heat lost due to flue gases.

The flue gases contain dry products of combustion as

well as steam generated due to combustion. The heat lost to flue gases can be reduced by passing

the flue gases through the economizer and preheater.

2. Heat lost due to incomplete combustion. The combustion is said to be incomplete if the carbon

burns to CO instead of CO2.

1kg of carbon releases 10120 kJ of heat if it burns to CO

and can release 33800 kJ/kg if it burns to CO2. This can be reduced by supplying excess quantity of air

and giving turbulent motion to the air before it enters

the furnace.

3 H t l t d t b t f l


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3. Heat lost due to unburnt fuel.

If mf1is the mass of unburnt fuel per kg of fuel used, then

heat lost is Q= mf1*C.

If solid fuels are used this loss cannot be completelyavoided.

4. Convection and radiation heat losses.

These losses can be reduced by providing insulation on

boiler surface.


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NOZZLES

A steam nozzle may be defined as a passage of varying

cross-section, through which heat energy of steam is

converted to kinetic energy.

In impulse turbine nozzles are fixed and in reaction turbine

are free to move.

NOZZLE EFFICIENCY:

When the steam flows through a nozzle the final velocity of

steam for a given pressure drop is reduced due to following

reasons1. Friction b/n nozzle surface and steam.

2. Internal friction of steam itself.

3. Shock losses.

M t f th f i ti l l b t th th t


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Most of these frictional losses occur between the throat

and exit of the nozzle.

The friction tends to decrease the velocity of steam andincrease the final dryness fraction or super heat steam.

Nozzle efficiencyis defined as the ratio of the actual

enthalpy drop to the isentropic enthalpy drop between thesame pressures.

nozzle= (h-ha)/(h-hs)


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TURBINE

Steam turbines are classified into

1. Impulse turbine2. Reaction turbine

In theformertype steam expands in the nozzles and its

pressure does not alter as it moves over the blades while in

the lattertype the steam expands continuously as it passes

over the blades and thus there is a gradual fall in the pressure

during expansion.


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Impulse turbine

Work done on the blade

It is only velocity of whirl which performs work on the blade.Force= mass of steam * acceleration

= ms(Cw1-Cw2)

Work done on blades/sec=force*distance travelled= ms(Cw1-Cw2)*Cbl

Blade or diagram efficiency

= (work done on the blades)

(Energy supplied to blade)

St ffi i


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Stage efficiency

stage=(work done on the blade/kg of steam)

(Total energy supplied/kg of steam)

stage= blade* nozzle

Overall or turbine efficiency

turbine=(heat converted into useful work)

(total adiabatic heat drop)

It is the overall efficiency that is meant when the efficiency of

a turbine is spoken .

EROSION OF TURBINE BLADES


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EROSION OF TURBINE BLADES

The most effected portion is the back of the inlet edge of

the blade, where either grooves are formed or even some

portion breaks away.

Methods to prevent erosion:

i. By raising the temperature of steam at inlet, so that at

exit of the turbine the wetness does not exceed 10%.

ii. By adopting reheat cycle.

iii. Drainage belts are provided on the turbine on outer

periphery.iv. The leading edge is provided with a shield of hard

material.


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LOSSES IN STEAM TURBINES1) Residual velocity losses:

The steam leaves the turbine with some absolute velocity.

The energy loss due to absolute exit velocity is 10 to 12%.

This loss can be reduced by using multistage.

2) Loss due to friction:

friction losses occurs in nozzles, turbine blades andbetween steam and rotating discs.

The loss due to friction is about 10%.

3) Leakage losses:

Between turbine shaft and bearings.

Leakage of steam through the glands.

Leakage at blade tips.

The total leakage loss is about 1-2%.

4) Losses due to mechanical friction:


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4) Losses due to mechanical friction:

The loss due to friction between shaft and bearing comes

under this category. Some loss occurs in regulating the

valves.This losses can be reduced using a efficient lubrication

system.

5) Radiation losses:

The heat is lost from turbine to surroundings as itstemperature is higher than atmospheric temperature.

This losses is usually negligible as turbines are highly

insulated.

6) Loss due to moisture:The steam contains water particles passing through the

lower stages of turbines as it becomes wet.

The velocity of water < velocity of steam and hence part of

K.E. of the steam is lost.


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CONDENSER

The desirable features of a good condenser are:

i. Minimum quantity of circulating water.ii. Minimum cooling surface area per kW capacity.

iii. Minimum auxiliary power.

iv. Maximum steam condensed per m2of surface area.

Condenser

vacuum

pressure

Cooling water temp

AIR LEAKAGE IN CONDENSER


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AIR LEAKAGE IN CONDENSERSources:

1. The air leaks through the joints, packings and glands into

the condenser where the pressure is below theatmospheric.

2. The feed water contains air in dissolved condition. Thedissolved air gets liberated when the steam is formed and

it is carried with the steam into the condenser.Effects:

1. It increases the pressure in condenser or back pressure ofthe prime mover and reduces the work done/kg of steam.

2. The pressure of air lowers the partial pressure of steamand its corresponding temperature.

3. The heat transfer rates are reduced due to the presence of

air as it offers high resistance.

R l


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Removal:

1. The air from the condenser is removed with the help of air

pumps.

2. The primary function of air pump is to maintain the

vacuum in the condenser which corresponds to the

exhaust steam temperature by removing the air.

3. Another function of the pump is to remove thecondensate coming out from the bottom of the

condenser.

4. An air pump which removes both air and condensate is

called wet air pump while the pump which removes onlymoist air is called dry air pump.

VACUUM EFFICIENCY


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VACUUM EFFICIENCY It is defined as the ratio of the actual vacuum to the

maximum obtainable vacuum.

The higher vacuum is obtained when there is only steam

and no air is present in the condenser.

CONDENSER EFFICIENCY

condenser efficiencyis defined as the ratio of the rise in

temperature of cooling water to the difference between the

temperature corresponding to the vacuum in the condenser

and inlet temperature of cooling water.


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Nithish-Influence of Component Efficiencies on Cycle Performance

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