Modern Engineering Thermodynamics

Problems 585
13.* A refrigeration unit is to be designed for a meat market using
R-22 to maintain meat at 0.00°C while operating in an
environment at 30.0°C. The refrigerant enters the condenser as a
saturated vapor and exits as a saturated liquid. Determine the
coefficient of performance (COP) for this refrigerator, using
a. A reversed Carnot cycle operating between these temperature
limits.
b. An isentropic, vapor-compression refrigeration cycle with an
aergonic, adiabatic throttling valve installed between the
high-pressure condenser and the low-pressure evaporator.
14. A vapor-compression refrigeration unit uses R-22 and has an
isentropic efficiency of 88.0% when working between the
temperature limits of −20.0°F and80.0°F. The refrigerant enters
the condenser as a saturated vapor and exits as a saturated liquid.
Determine the coefficient of performance (COP) of this unit.
15.* A vapor-compression cycle refrigeration system using R-22 has an
evaporator temperature of −20.0°C and a condenser temperature
of 20.0°C. The refrigerant exits the compressor as a saturated
vapor and exits the condenser as a saturated liquid. Determine
a. The isentropic coefficient of performance of this system.
b. The coefficient of performance of a Carnot refrigerator
operating between the same temperature limits.
c. The “tons” of isentropic refrigeration per unit mass flow rate
of refrigerant.
16.* Repeat Problem 15 using R-134a instead of R-22 as the
refrigerant.
17.* A vapor-compression cycle refrigerator uses R-22 as the working
fluid. The evaporator temperature is −20.0°C, the condenser
temperature is 40.0°C, and the flow rate of R-22 is 0.100 kg/s. If
the refrigerant exits the compressor as a saturated vapor and
exits the condenser as a saturated liquid, determine
a. The isentropic coefficient of performance.
b. The equivalent reversed Carnot cycle coefficient of
performance.
c. The amount of refrigeration (in tons) that this system can
provide.
18.* Repeat Problem 17 using R-134a instead of R-22 as the
refrigerant.
19.* A vapor-compression cycle refrigerator using R-22 as the working
fluid has an evaporator temperature of −30.0°C and a condenser
temperature of 50.0°C. The refrigerant enters the compressor at
7.50 kg/min as a saturated vapor and exits the condenser as a
saturated liquid. The isentropic efficiency of the compressor is
82.5%. Determine
a. The coefficient of performance for the system.
b. The coefficient of performance for a reversed Carnot cycle
with the same temperature limits.
c. The refrigeration (in tons) that this system can provide.
20.* Repeat Problem 19 using R-134a instead of R-22 as the
refrigerant.
21. The states in a vapor-compression cycle air conditioner using
R-22 as the working fluid are as follows:
Station 1 Station 2s
Compressor Compressor
inlet outlet
T 1 = 20:0°F T 2s = 80:0°F
s 1 = s 2s x 2s = 1:00
Station 3 Station 4h
Throttle
Throttle
valve inlet valve outlet
T 3 = T 2s = 80:0°F T 4h = 20:0°F
x 3 = 0:00 h 4h = h 3
Determine the actual coefficient of performance of this cycle if
the compressor isentropic efficiency is 79.0%.
22. A large vapor-compression cycle refrigeration manufacturer tests
its units by directing the condenser heat to the evaporator and
adding additional cooling as necessary to fully cool the unit.
In the test unit shown Figure 14.37, the refrigerant is R-22 and
the refrigerant mass flow rate is 1500. lbm/min.
3
4
FIGURE 14.37
Problem 22.
Expansion
valve
Additional cooling
Q H
Condenser
Evaporator
Q L
Station 1 Station 2s
Compressor inlet Compressor isentropic outlet
T 1 = 20:0°F p 2s = p 3 = 98:7 ≈ 100: psia
p 1 = 30:0 psia s 2s = s 1
Station 3
Station 4h
Condenser outlet Expansion valve outlet
p 3 = 98:87 ≈ 100: psia
x 3 = 0:00
Assuming the isentropic efficiency of the compressor is 100.%,
determine:
a. The compressor power input.
b. The cooling capacity in tons of refrigeration.
c. The unit’s COP.
23. A vapor compression cycle heat pump using R-22 is used to
provide 20.0 × 10 3 Btu/h of heat to a house. The evaporator
temperature is 14.0°F and the condenser temperature is 70.0°F.
The refrigerant exits the compressor as a saturated vapor and
exits the condenser as a saturated liquid. The isentropic
efficiency of the compressor is 80.0%. Determine
a. The mass flow rate of the refrigerant.
b. The power input to the compressor.
c. The coefficient of performance of this system.
24. Repeat Problem 23 using R-134a instead of R-22 as the
refrigerant.
25. A vapor-compression cycle heat pump using low-pressure water
as the working fluid is proposed to heat a house. The evaporator
is to be buried in the ground below the frost line and
consequently will remain at 50.0°F year-round. The condenser is
to be inside the house and will operate at a constant 80.0°F. The
water enters the condenser as a saturated vapor at 12.6 lbm/min
and exits as a saturated liquid. Assume the isentropic efficiency
of the compressor is 100.%. Determine
a. The coefficient of performance of this system.
b. The amount of heat (in Btu/h) transferred into the house.
26.* A vapor-compression cycle heat pump using R-134a as the
working fluid is to be designed for heating a house. The
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Compressor
1
W C