Modern Engineering Thermodynamics

Problems 587
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 = −30:0°C s 2sB = s 1B T 3B = 0:00°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.
39.* A small, two-stage, vapor-compression refrigeration unit using
R-134a produces 15.0 tons of refrigeration with an evaporator
pressure of 100. kPa and a condenser pressure of 1200. kPa. The
intermediate flash chamber operates at 400. kPa and the
isentropic efficiencies of the compressors in each loop are both
89.0%. The following operating specifications have been
developed for the stages (loops):
Stage (Loop) A
Station 1A Station 2sA Station 3A Station 4hA
Compressor Compressor Condenser Expansion
A inlet A outlet A outlet valve A outlet
p 1A = 400: kPa p 2sA = 1200: kPa x 3A = 0:00 h 4hA = h 3A
s 2sA = s 1A p 3A = 1200: kPa
Stage (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 = 400: kPa x 3B = 0:00 h 4hB = h 3B
p 1B = 100: kPa s 2sB = s 1B p 3B = 400: kPa
For this design, determine
a. The mass flow rate of the refrigerant.
b. The system’s coefficient of performance.
c. The total power required by the compressor.
40.* A large, two-stage, vapor-compression packing house refrigeration
unit using R-134a produces 55.0 tons of refrigeration with an
evaporator pressure of 80.0 kPa and a condenser pressure of 1.00
MPa. The intermediate flash chamber operates at 400. kPa and
the isentropic efficiencies of the compressors in each loop are
both 92.0%. The following operating specifications have been
determined for the stages (loops):
Stage (Loop) A
Station 1A Station 2sA Station 3A Station 4hA
Compressor Compressor Condenser Expansion
A inlet Aoutlet Aoutlet valve A outlet
p 1A = 400: kPa p 2sA = 1:00 MPa x 3A = 0:00 h 4hA = h 3A
s 2sA = s 1A p 3A = 1:00 MPa
Stage (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 = 400: kPa x 3B = 0:00 h 4hB = h 3B
p 1B = 80:0kPa s 2sB = s 1B p 3B = 400: kPa
For this design, determine
a. The mass flow rate of the refrigerant.
b. The system’s coefficient of performance.
c. The total power required by the compressor.
41.* A very large, two-stage, vapor-compression refrigeration unit
using R-134a produces 3000 tons of refrigeration with an
evaporator pressure of 100 kPa and a condenser pressure of
2000 kPa. The intermediate flash chamber operates at 800 kPa,
and the isentropic efficiencies of the compressors in each loop
are both 91%. The following operating specifications have been
determined for the stages (loops):
Stage (Loop) A
Station 1A Station 2sA Station 3A Station 4hA
Compressor Compressor Condenser Expansion
A inlet A outlet A outlet valve A outlet
p 1A = 800: kPa p 2sA = 2000: kPa x 3A = 0:00 h 4hA = h 3A
s 2sA = s 1A p 3A = 2000: kPa
Stage (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 = 800: kPa x 3B = 0:00 h 4hB = h 3B
p 1B = 100: kPa s 2sB = s 1B p 3B = 800: kPa
For this design, determine
a. The mass flow rate of the refrigerant.
b. The system’s coefficient of performance.
c. The total power required by the compressor.
42. Laboratory measurements on an absorption refrigeration
system produced the following results: the evaporator absorbs
15.0 × 10 3 Btu/h while 14.0 × 10 3 Btu/h of heat is added to
the generator. The carrier liquid pump requires 0.500 hp to
operate during the test. Determine the coefficient of
performance of this unit.
43.* A new absorption refrigeration system is designed to operate in
a hazardous environment, where the temperature is 40.0°C. If
the generator temperature is 100.0°C and the evaporator
temperature is 20.0°C, determine the Carnot absorption
refrigeration coefficient of performance of this unit.
44. If the Carnot absorption refrigeration coefficient of performance
of a new refrigeration unit is 4.20 and the environmental and
generator temperatures are 70.0°F and 300.°F, respectively,
determine the cooling (evaporator) temperature.
45.* A new household refrigerator-freezer combination unit is
designed with a dual-evaporator system. The freezer
compartment is at −20.0°C and the refrigerator compartment is
at 4.00°C. The outlet of the condenser is at 28.0°C. The cooling
capacities of both the refrigeration and the freezer compartments
are to be 400. kJ/h each. The system uses refrigerant R-134a with
a compressor isentropic efficiency of 87.0%. Determine (a) the
coefficient of performance for this design and (b) the mass flow
rate of refrigerant required.
46.* A new commercial refrigerator-freezer combination unit is being
designed with a dual-evaporator system. The freezer
compartment is to be at −24.0°C and the refrigerator
compartment is to be at 8.00°C. The outlet of the condenser is
at 26.0°C. The cooling capacities of both the refrigerating and
the freezer compartments are to be 800. kJ/h each. The system
uses refrigerant R-134a with a compressor isentropic efficiency
of 89.0%. Determine
a. The coefficient of performance for this design.
b. The mass flow rate of refrigerant required.
c. The quality at the outlet of the refrigeration evaporator.