216 POWER PLANT ENGINEERING
3.Steam at pressure of 30.23 bar and 400°C temperature is supplied to a steam turbine and is
exhausted at a pressure of 0.06865 bar. A single bleed is taken between the H.P. cylinder and
L.P. cylinder of the turbine at 2.45 bar for regenerative feed heating. The isentropic efficiency
for both the cylinders of the turbine is 85%. The temperature of the bleed condensate coming
out of the heat exchanger is 10°C lower than the temperature of the bled steam. Determine:
(a) Amount of bled steam per kg of steam supplied to the steam turbine.
(b) The thermal efficiency of the plant.
Consider no losses and pump work as negligible. Let the condensate coming out from the
heat exchanger and condenser be led to the hot well. [Ans. 0.158 kg/kg of steam, 32%]
4.In a condenser test, the following observations were made:
Vacuum = 69 cm of Hg
Barometer = 75 cm of Hg
Mean temperature of condenser = 35°C
Hot well temperature = 28°C
Amount of cooling water = – 50,000 kg/hr
Inlet temperature = 17°C
Outlet temperature = 30°C
Amount of condensate per hour = 1250 kg
Find
(a) the amount of air present per m^3 of condenser volume.
(b) the state of steam entering the condenser.
(c) the vacuum efficiency.
R for air = 287 J/kgK. [Ans. 0.0267 kg, 0.883, 97.48%]
5.In a power plant, steam at a pressure of 27.46 bar abs. and temperature 370°C is supplied by
the main boilers. After expansion in the H.P. turbine to 5.$84 bar abs., the steam is removed
and reheated to 370°C. Upon completing expansion in L.P. turbine, the steam is exhausted at
a pressure of 0.0392 bar abs. Find the efficiency of the cycle with and without reheating.
[Ans. 37.19%, 36.38%]
6.A fuel contains the following percentage of combustibles by mass C: 84%, H 2 : 4.1 %. If the
air used from burning of coal in a boiler is 16.2 kg per kg of fuel, find the total heat carried
away by dry flue gases if they escape at 300°C. The specific heats of CO 2 , OZ, NZ are 0.892,
0.917 and 1.047 respectively. Find the minimum amount of air required from the complete
combustion of 1 kg of this fuel and the excess O 2 supplied. [Ans. 4806.68 kJ, 11.16, 1.158]
7.A sample of coal has the percentage analysis by mass, C-85%, H-5%, incombustible-10%. In
a combustion chamber, the coal is burnt with a quantity of air 50% in excess of that theoreti-
cally required for complete combustion. Obtain the volumetric composition of dry flue gases.
When final temperature of flue gases is 307°C and boiler house temperature is 27°C, estimate
the maximum quantity of heat available for steam raising per kg of coal. Take CP for dry flue
gases as 1.005 and assume that the total heat of vapour in flue gases as 2683.87 kJ/kg. The
C.V. of coal is 32784.2 kJ/kg.