Modern Engineering Thermodynamics

586 CHAPTER 14: Vapor and Gas Refrigeration Cycles
evaporator is to be buried in the ground below the frost line
and consequently will remain at 14.0°C year-round. The
condenser is to be inside a house and will operate at a
constant 20.0°C. The refrigerant enters the condenser as a
saturated vapor at 7.30 kg/min and exits as a saturated liquid.
Assume the isentropic efficiency of the compressor is 91.0%.
Determine
a. The coefficient of performance of this system.
b. The amount of heat (in kW) transferred into the house.
27.* A vapor-compression cycle heat pump has been developed that
uses R-22 as the working fluid. The evaporator is at 0.00°C and
the condenser is at 30.0°C. The refrigerant enters the condenser
as a saturated vapor at 0.0800 kg/s and exits as a saturated
liquid. The isentropic efficiency of the compressor is 83.0%.
Determine
a. The coefficient of performance of this system.
b. The power (in kW) required to drive the unit.
28.* A company wants to design a vapor-compression cycle heat
pump that uses ammonia as the working fluid. The evaporator
is at 0.00°C and the condenser is at 30.0°C. The refrigerant
enters the condenser as a saturated vapor at 0.120 kg/s and exits
as a saturated liquid. The compressor has an isentropic efficiency
of 85.0%. Determine
a. The coefficient of performance of this system.
b. The amount of heat (in kW) transferred from the cold to the
warm region.
29.* A special high-temperature vapor-compression cycle heat pump
for a spacecraft is to be designed that uses water as the
working fluid. The evaporator is at 100.°C and the condenser is
at 300.°C. The refrigerant enters the condenser as a saturated
vapor at 1.80 kg/min and exits as a saturated liquid. Assume
that the isentropic efficiency of the compressor is 91.0%.
Determine
a. The coefficient of performance of this system.
b. The amount of heat (in kW) transferred from the cold to the
warm region.
30. Use Eq. (14.8) to determine the proper chemical formula and
chemical name for the following refrigerants: (a) R-10,
(b) R-110, and (c) R-214.
31. Find the chemical formula and chemical name for the
following refrigerants using Eq. (14.8): (a) R-30, (b) R-40,
and (c) R-50.
32. Use Eq. (14.8) to find the chemical formula and chemical
name for the following refrigerants: (a) R-113, (b) R-114, and
(c) R-123.
33. Determine the chemical formula and R number of the following
refrigerants (see Eq. (14.8)): a) pentachloroethane,
b) trichloroethane, and c) octafluoropropane.
34. Use Eq. (14.8) to determine the chemical name and R number
of the following refrigerants: (a) CClF 3 , (b) CHF 3 , and
(c) CH 2 ClF.
35. Provide the chemical name and refrigerant R number
of the following materials: (a) NH 3 ,(b)CO 2 ,and
(c) H 2 O.
36.* A dual-cascade system using R-22 in both loops is used to
produce 30.0 tons of refrigeration in a large refrigeration
unit with an evaporator temperature of −40.0°C anda
condenser temperature of 25.0°C.Theintermediateheat
exchanger between the loops operates at −20.0°C, and the
isentropic efficiencies of the compressors in each loop are
both 83.0%. The following design specifications have been
defined for the loops:
Loop A
Station 1A Station 2sA Station 3A Station 4hA
T 1A = −20:0°C s 2sA = s 1A T 3A = 25:0°C
Compressor Compressor Condenser Expansion
Ainlet Aoutlet Aoutlet valve A outlet
x 1A = 1:00 p 2sA = 1500: kPa x 3A = 0:00 h 4hA = h 3A
Loop B
Station 1B Station 2sB Station 3B Station 4hB
Compressor Compressor Condenser Expansion
Binlet Boutlet Boutlet valve B outlet
x 1B = 1:00 p 2sB = 300: kPa x 3B = 0:00 h 4hB = h 3B
T 1B = −40:0°C s 2sB = s 1B T 3B = −20:0°C
For this design, determine
a. The mass flow rate of refrigerant in loops A and B.
b. The system’s coefficient of performance.
c. The pressure ratios across both of the compressors.
37.* A new ultralow-temperature, dual-cascade refrigeration system
using R-22 in both loops is used to produce 5.00 tons of
refrigeration with an evaporator temperature of −60.0°C and a
condenser temperature of 25.0°C. The intermediate heat
exchanger between the loops operates at −20.0°C and the
isentropic efficiencies of the compressors in each loop are both
75.0%. The following operating specifications have been
determined for the loops:
Loop A
Station 1A Station 2sA Station 3A Station 4hA
Compressor Compressor Condenser Expansion
A inlet A outlet A outlet valve A outlet
x 1A = 1:00 p 2sA = 1500: kPa x 3A = 0:00 h 4hA = h 3A
T 1A = −20:0°C s 2sA = s 1A T 3A = 25:0°C
Loop B
Station 1B Station 2sB Station 3B Station 4hB
Compressor Compressor Condenser Expansion
B inlet B outlet B outlet valve B outlet
x 1B = 1:00 p 2sB = 300: kPa x 3B = 0:00 h 4hB = h 3B
T 1B = −60:0°C s 2sB = s 1B T 3B = −20:0°C
For this design, determine:
a. The mass flow rate of refrigerant in loops A and B.
b. The system’s coefficient of performance.
c. The pressure ratios across both of the compressors.
38.* A dual-cascade system using R-22 in both loops is used to
produce 300. tons of refrigeration in a ice-skating rink with
an evaporator temperature of −30.0°C and a condenser
temperature of 25.0°C. The intermediate heat exchanger
between the loops operates at 0.00°C and the isentropic
efficiencies of the compressors in each loop are both 85.0%.
The following design specifications have been defined for
the loops:
Loop A
Station 1A Station 2sA Station 3A Station 4hA
Compressor Compressor Condenser Expansion
Ainlet Aoutlet Aoutlet valve A outlet
x 1A = 1:00 p 2sA = 1500: kPa x 3A = 0:00 h 4hA = h 3A
T 1A = 0:00°C s 2sA = s 1A T 3A = 25:0°C