College of Engineering
Spring Session- 2016
THERMODYNAMICS II - ME 272
Dr. Saeed J. Almalowi, smalowi@taibahu.edu.sa
Problem 10.44 Statement
A steam power plant operates on an ideal regenerative Rankine cycle. Steam enters the turbine at
6MPa and 450C and is condensed in the condenser at 20 kPa. Steam is extracted from the turbine
at 0.4 MPa to heat the feed water in an open feed water heater. Water leaves the feed water
heater as a saturated liquid. Show the cycle on T-s diagram, and determine
(a) The net work output per kilogram of steam flowing through the boiler.
(b) The thermal efficiency of the cycle.
[Ans. (a) 1017, (b) 37.8 percent]
SOLUTION
m6 h6 + m2 h2 = m3 h3 ,
m5 = m2 + m6 , y =
College of Engineering-Taibah University
m6
→
m5
College of Engineering
Spring Session- 2016
THERMODYNAMICS II - ME 272
Dr. Saeed J. Almalowi, smalowi@taibahu.edu.sa
m6 h6 + h2 m5 − h2 m6 = m5 h3 →
y=
m6
m6
h6 −
h = (h3 − h2 )
m5
m5 2
m6 h3 − h2 604.8 − 251.7
=
=
= 0.1463
m5 h6 − h2 2665 − 251.7
q out = (1 − y) × (h7 − h1 ) = (1 − 0.1463) × (2213 − 251.3) = 1674.7
q in = (h5 − h4 ) = 3302 − 610.9 = 2691.1
kJ
kg
wnet = q in − q out = 2691.1 − 1674.7 = 1016.4
ηth = 1 −
kJ
kg
kJ
kg
q out
1674.7
=1−
= 0.37768 = 37.8%
q in
2691.1
Problem 11.20 Statement
Refrigeration- 134a enters the compressor of a refrigerator at 140 kPa and -10C at a rate of 0.3
m3/min and leaves at 1 MPa. The isentropic efficiency of the compressor is 78 percent. The
refrigerant enters the throttling valve at 0.95 MPa and 30C and leaves the evaporator as saturated
vapor at -18.5C. Show the cycle on a T-s diagram with respect to saturation lines, and determine
(a) the power input to the compressor, (b) the rate of heat removal from the refrigerated space, and
(c) the pressure drop and rate of heat gain in the line between the evaporator and the compressor.
[Ans. (a) 1.88 kW, (b) 4.99 kW, (c) 1.65 kPa,0.241 kw]
SOLUTION
College of Engineering-Taibah University
College of Engineering
Spring Session- 2016
THERMODYNAMICS II - ME 272
Dr. Saeed J. Almalowi, smalowi@taibahu.edu.sa
Problem 11.20 Statement
Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.2
MPa and 200 kPa with refrigerant-R134a as the working fluid. Heat rejection from the lower cycle
to the upper cycle takes place in and adiabatic counter flow heat exchanger where the pressure in
the upper and lower cycles are 0.4 and 0.5 MPa, respectively. In both cycles, the refrigerant is a
saturated liquid at the condenser exit and a saturated vapor at the compressor inlet, and the
isentropic efficiency is 80 percent. If the mass flow rate of the refrigerant through the lower cycle
is 0.15 kg/s, determine (a) the mass flow rate of the refrigerant through the upper cycle, the rate
heat removal from the refrigerated space, and (c) the COP of this refrigerator.
[Ans. (a) 0.212 kg/s, (b) 25.7 kW,(c) 2.68]
PLEASE SEE YOUR NOTES FOR SOLUTION!!!!!!
Fig.11.20 Cascade Refrigeration Cycle
College of Engineering-Taibah University