CONDENSOR In Thermal Power Plant
GUIDED BY:Proff., Mayur MakvanaCONDENSOR In Thermal Power PlantNAME ENROLLMENT NO.Patel Mihir130450119107Patel Mubarak130450119110Patel Parth130450119116Patel Pathik130450119117Pathan Uvesh130450119125
INDEXRankine CycleIntroduction to CondenserSources of Air in CondenserEffect of Air Leakage in CondenserDalton's Partial Pressure LawVacuum efficiencyCondenser efficiency
RANKINE CYCLEThe Rankine cycle used in modern power plant has many more components, but the four components are common to all power plants.In this cycle, water is heated in the steam generator to produce high temperature and pressure steam.This steam is then expanded in a turbine to produce electricity from a generator that is connected to the turbine.The steam from turbine is then condensed back into water in the condenser.The pump then returns the water to the steam generator.
INTRODUCTION TO CONDENSERIn systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant.Condensers are typically heat exchangers which have various designs and come in many sizes ranging from rather small (hand-held) to very large industrial scale units used in plant processes.
A surface condenser is an example of such a heat-exchange system. It is a shell and tube heat exchanger installed at the outlet of every steam turbine in thermal power stations. Commonly, the cooling water flows through the tube side and the steam enters the shell side where the condensation occurs on the outside of the heat transfer tubes. The condensate drips down and collects at the bottom, often in a built-in pan called a hotwell.The shell side often operates at vacuum or partial vacuum.
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SOURCES OF AIR IN CONDENSERFollowing are the chief sources of air found in condensers : Air leaks in condenser from atmosphere at the joints of the parts which are internally under a pressure less than that of atmosphere- The amount of air leaking in, mainly depends upon the accurate workmanship and can, with care in the design and making of the-vacuum joints, be reduced to a very small quantity. Air also comes in with the steam from the boiler into which it enters dissolved in feed water. The amount of air coming in depends upon the treatment the feed water receives before it enters the boiler. The air entering through this source is relatively small.In case of jet condensers, some air comes in with the injection water (cooling water) in which it is dissolved.
In the surface condensers of well designed and properly maintained steam turbine plants, the amount of air entering condensers is about 5 kg per 10,000 kg of steam. With reciprocating steam engines, the air entering is about 15 kg per 10,000 kg of steam.In case of jet condensers the amount of air dissolved in injection water is about 0-5 kg per 10,000 kg of water.
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How to check the air leakage in condenser?In order to check the following procedure is adopted:1.Keep the plant running until the temperature and pressure condition are steady in the condenser.2.The steam condenser be isolated by shutting off steam and simultaneously closing the condensate and air extraction pumps.In case there is a leakage the readings of vacuum gauge and thermometer will record a fall.
Following method is used to check the source of air leakage:1. Put the effect of steam condenser under the air pressure and note its effect on soap water at the points where infiltration is likely occur.2.Put the pepprament oil on the suspected joints (when the condenser is operating) and make a check on the pepprament odour in the discharge of air ejector.3.Large leakage in steam condenser under the vacuum can be detected by passing candle flame over possible openings.
Effect of air leakage:The effect of air leakage in a condenser are given below:(1)It increase the back pressure on the turbine with the effect of steam at with the effect that there is less enthalpy drop and low thermal efficiency of the plant.(2)The pressure of air in the condenser lowers the partial pressure of steam will condense at a lower temperature and that will require greater amount of cooling water.(3) It reduces the rate of condensation of steam, because air having poor thermal conductivity reduces the overall heat transfer from the steam air mixture.(4)The pressure of air in the condenser increase the corrosive action.
DALTONS PARTIAL PRESSURE LAW
What is PARTIAL PRESSURE ?Partial Pressure is define as It is the pressure which each constituent of a gas mixture would exert if it alone occupied the volume of the mixture at the same temperature
Statement of Daltons partial pressure lawIn Thermodynamic Daltons partial pressure law is stated as:
The mixture of non-reacting gases, the total pressure exerted is equal to the sum of partial pressure of individual gases
This empirical law was observed by John Dalton in 1801.This law is related for ideal gas laws.As shown in figure the gases a and b, originally occupying volume V at temperature T are mixed in the third vessel which is of the same volume and same temperature.Mathematically,
For Air:For Number of gases:
IN POWER PLANT ENGINEERINGCondensers contains air and steam mixture.
So, The statement is The pressure of the air and steam is equal to the sum of the pressures which constituent would exert, if it occupied the same space by self
It means,
Where,pc = pressure in condenser,pa = partial pressure of air,ps = partial pressure of steam.
Measurement of vacuum
The vacuum in a condenser is usually expressed in millimeters of mercury and it is the difference between the barometric pressure (or barometric height) and absolute pressure in condenser. In order to know the absolute pressure in the condenser, both the vacuum gauge and barometer must be read. The difference between the barometerand vacuum gauge readings will give the absolute pressure in the condenser.Barometric pressure is a variable quantity and varies from place to place. Hence, it is more convenient for the purpose of comparison to refer vacuum gauge readings to a standard barometer of 760 mm of mercury (or 1.01325bar).
Standard or corrected vacuum in mm of Hg.= 760 mm of mercury - absolute pressure in condenser in mm of Hg. = 760 mm of Hg [Barometer reading in mm of Hg- Vacuum gauge reading in mm of Hg.]
Since one standard atmosphere = 760 mm of Hg =1.01325 bar (101.325 kPa),1 bar = 760 / 1.01325 = 750 mm of HgSo, Pressure equivalent of 1 mm of Hg = 1.01325/760 = 0.001333 bar or = 0.1333kPa.
Vacuum Efficiency
In a steam condenser we have a mixture of steam and air, and the total pressure which exists in the condenser is the sum of the partial pressures exerted by the steam and air. With no air present in the condenser, the total absolute pressure in the condenser would be equal to partial pressure of steam corresponding to the temperature of condenser,and maximum vacuum would be obtained in the condenser. The ratio of the actual vacuum obtained at the steam inlet to the condenser, to this maximum vacuum (or Ideal vacuum) which could be obtained in a perfect condensing plant (with no air present) is called thevacuum efficiency.
Vacuum efficiency = Actual vacuum at the steam inlet to the condenser [Barometric pressure -Absolute pressure correspondending to the temperature of Condensation]
Or
[Barometric pressure] - Absolute pressure in the condenser [Barometric pressure] - Absolute pressure corresponding to temperature of condensation
If the absolute pressure of steam corresponding to the temperature of condensation were equal to the absolute pressure in the condenser, the vacuum efficiency would be 100%. In fact, there will always be some air present in the condenser due to leakage and dissolved air present in the steam entering the condenser. The value of vacuum efficiency, therefore, depends upon the quantity of air removed from the condenser bythe air pump.
example : Steam enters a condenser at 32.88C and with barometer standing at 760 mm of Hg, a vacuum of 685 mm of Hg was produced. Determine the vacuum efficiency.From steam (Pressure) tables, at 32.88C, partial pressure of steam= 0.05 bar = 0.05 x 750 = 37.5 mm of Hg.
Vacuum efficiency =Actual vacuum/[Barometric Pressure-Absolute pressure corresponding to temperature of Condensation]=685/ [760-(0.05 X 750)]= 685/722.5= 0.9481 or 94.81 %
or Vacuum efficiency =Actual vacuum/ vacuum corresponding to saturation temperature of condensate=Actual vacuum/ Ideal vacuum =685/760 37.5= 0.9481 or 94.81 %(same as before).
Condenser Efficiency:
Reference:1.Thermal Engineering by R K Rajput, Laxmi publication(ninth edition) 2.Power plant engineering by H G Katariya and J P Hadiya, Books of India (Fifth edition)
