POWER PLANT ENGINEERING
Gas Turbine It is a mechanical device converts one form of energy into useful form of energy. Type of engine:-1} I C engine:- In the case of IC engine the product of combination itself is the working fluid and produces output Compression and expansion takes place at Same location 2}E C engine :- In the case the case of EC engine the product of combustion are transferred their heat to the another working fluid and which it used for providing some useful output. Advantage of gas Turbine over IC engine Simple mechanism ,High speeds can developed due to the rotary engine ,Easy balancing, Compact that is weight to power ratio is less Disadvantage :-1}The comp. Which is used in gas Turbine handling the gaseous phase of working fluid which is not negligible in comparison to the Turbine work, therefore the net workout put reduce and it's efficiency work, therefore the net workout put reduce and it's efficiency is less as that IC engine.2) high speed reduction gear are required as the value of centrifugal force are high. 3) High heat resistant material are required for the gas Turbine because these are subjected to higher temperature continuously for a long period of time. (Creep failures changes)
TYPES of gas Turbine Open cycle gas Turbine: - In the case of open cycle gas Turbine in every cycle a fresh charge is introduced. 1-2 is a Isentropic compression, 2-3 is a constant pressure head addition, 3-4 is a Isentropic expansion. Closed cycle and open cycle :-Closed cycle gas Turbine :- In the case of closed cycle gas Turbine same working fluid is circulated again and again Advantage of closed cycle gas Turbine over open cycle 1)Wide variety of fuel can be used because products of combustion doesn't enter the Turbine Better thermal efficiency can be obtain between the same maximum and minimum temperature limits r = 1.67 2}Better working fluid like argon or helium can be used having the value of v = 1.67 3}It can operate below atmospheric pressure Disadvantage 1}Coolant is used to cool down the Turbine exhaust before entering the compressor whereas in the case of open cycle atmosphere is acting as a sink 2}Absolute leak proving for the cycle is very difficult to achieve System is complicated , costly and bulky and it is not generally preferred for air craft application.
Effectiveness of Regeneration heat or thermal ratio or degree of Regeneration It is defined as the ratio of actual possible rise of temperature to the maximum possible rise of temperature in Regeneration generally the value of effectiveness is vary 80% to 70% Note :- In case of ideal regenerative cycle the value of effectiveness is 100%E = 100% Ta = T4 Tb = T2 Efficiency of ideal regenerative cycle :-Conclusion :- beyond a certain process ratio the use of Regeneration becomes ineffective because in that condition the components output temperature exceeds the Turbine exhaust temperature Reheating in gas Turbine :-Reheating means expansion of working fluid takes place more than one stage and with the help of Reheating the net work output.
Conditions for maximum workout with perfect reheating The scope of the regeneration process with the use of reheating because the Turbine exhaust temperature increase ( T6 > T4 ) therefore reheating generally coupled with regeneration Inter-cooling in gas Turbine :-It is improving the net work reducing the compressor work Conclusion :- The scope regeneration increase with the use of inter-cooler therefore inter-cooling is generally coupled with regeneration Condition for minimum work Input with perfect inter-cooling Rankine cycle ( steam power cycle ) :-Reason for using water as a working fluid It is harmless surface, It can be easily treated, Chemically stable, Cheaper an at economic, Easily available.
Efficiency of single Brayton cycle:-Assumption 1} Working fluid is air and its chemical composition doesn't change 2} the value of Cp Cv and r does not change with temperatures 3} Negating K. E and potential energy changes 4} Neglecting the pressure losses which passing through the c c and the duct connecting various equipment. Efficiency of a Brayton cycle is a function of pressure ratio and a diabetic index. Effect of pressure ratio on ‘n’ and net work output. There is a maximum limit of 1700k is imposed due to the metallurgical conduction of the Turbine blade with increase in pressure ratio 'n' will increase but the network decrease For the same power output mass flow rate decrease Optimum pressure ratio for maximum work output Isentropic efficiency of compression and Turbine :-1}Isentropic efficiency of compression is defined as the ideal enthalpy rise to that actual enthalpy rise 2}When as in Turbine Isentropic efficiency is the ratio of actual enthalpy drop to that of ideal enthalpy drop. Regeneration: - In the gas Turbine Regeneration is a process in which Turbine exhaust are utilized to increase the temperature of compress air before entering the combustion chamber.
Specific stream consumption:-It is the amount of stream consumed to produce 1 Kw power output Heat rate: - It is a rate of Heat required to produce 1 Kw power output . Carnot vapour cycle :-Drawback 1}Design and the control of partial condenses is difficult 2}Difficult to design a compression handling a mixture of liquid and vapour 3}The state of the working fluid at the entry of Turbine is saturated vapor at the end of Turbine is wet region which increases the change erosion of Turbine blades 4}The work of compression is high which reduce the work output of carnot cycle 5}As the wet region ( liquid of content ) increase blor the Turbine is rotated at high speed if any liquid particle impinge on it , it will Crack the blade. Rankine cycle :- Process :- 1-2 = isentropic expansion, 2-3= constant pressure heat rejection, 3-4 = pump work ,4-5-1 = constant pressure heat addition. 1-2 = Turbine - Wr= h1-h2, 2-3= condenses = Qr = h2-h3, 3-4 = pump = Wp = h4- h3, Or Wp = h4-h3 = Vf p = Vf3 ( PH - Pl ) , = V f3 ( pb - Pc ) , 1-5-1 = boiler = Qb = h1 - h4.
Reason for the lower efficiency of Rankin cycle with respect to control:-The net workout put is having higher value in case of Rankin cycle but is does not mean that it's efficiency is higher than that of carnot because mean temperature of heat addition for carnot is having higher value 1-2-3`-5-1 = carnot cycle 1-2-3-4-5-1 = Rankin cycle Methods of improving the performance of Rankin cycle :-The efficiency of the cycle cannot increase by increasing the T max A or by decreasing a Teman R ( Rejection ) Case -1 During condenser pressure :-Case -2 Increase in boiler pressure :-Case 3 superheating :-It is process of increasing the temperature at constant pressure above saturated vapor Case -4 Reading :- The primary aim if the reheating is to improve the quality ( dryness fraction ) of the steam at the Turbine exit . So that there will be less erosion of the Turbine blades. Reheating mass the expansion of working fluid takes place in more than one stage and with the use of reheating the net work output increase therefore specific stream consumption decreases which makes the plant compact. Case 4 = Reservation: - In a ideal regeneration cycle efficiency is increase by exchanging the heat internally between the expanding steam in the Turbine and the feed water before entering the boiler. Ideal regeneration cycle is not practically possible because the heat exchange cannot be erosion of Turbine blades (last stage of blade) practically the high temperature stream is extraction from the intermediate stage of the Turbine which is utilized to increase the temperature of feed water.
TYPES of regeneration (feed water heater ) Open feed water heater Open feed water heater is a mixing chamber when the high temperature stream extracted from the Turbine is utilized to increase the temperature of feed water. The pressure before mixing is made equal in order to minimize the loss of energy therefore more number of pumps is required. Ex. Deaerator: - The main purpose of Desecrator is to remove the air mixture with the feed water. If air will not remove if will increase the Rth because Kair is very low (0.026) Closed feed water heater :-In this type of heater the high temperature stream rejects it's heat energy to the feed water without mixing which are at different pressure . Effect of regeneration: - 1 efficiency increase (heat rate decrease) 2) for the same power output stream flow rate increase Turbine it requires large boiler size 3) for same mass flow rate the Turbine output decrease 4) For the decrease mass flow rate there is a decrease in the size of condenses 5) It is the convenient methods of removing the air from the condenser.