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Questions and Answers of Mechanical Engineering

An aircraft on the ground is to be cooled by a gas refrigeration cycle operating with air on an open cycle. Air enters the compressor at 30oC and 100 kPa and is compressed to 250 kPa. Air is cooled
Consider a regenerative gas refrigeration cycle using helium as the working fluid. Helium enters the compressor at 100 kPa and - 10oC and is compressed to 300 kPa. Helium is then cooled to 20oC by
A refrigerator operates on the ideal vapor-compression refrigeration cycle and uses refrigerant-134a as the working fluid. The condenser operates at 300 psia and the evaporator at 20oF. If an
An absorption refrigeration system is to remove heat from the refrigerated space at 2oC at a rate of 28 kW while operating in an environment at 25oC. Heat is to be supplied from a solar pond at 95oC.
Reconsider Prob. 11-120. Using EES (or other) software, investigate the effect of the source temperature on the minimum rate of heat supply. Let the source temperature vary from 50 to 250oC. Plot the
A gas refrigeration system using air as the working fluid has a pressure ratio of 5. Air enters the compressor at 0oC. The high-pressure air is cooled to 35oC by rejecting heat to the surroundings.
The refrigeration system of Fig. P11-123 is another variation of the basic vapor-compression refrigeration system which attempts to reduce the compression work. In this system, a heat exchanger is
Repeat Prob. 11-123 if the heat exchanger provides 9.51oC of subcooling.Prob. 11-123The refrigeration system of Fig. P11-123 is another variation of the basic vapor-compression refrigeration system
An ideal gas refrigeration system with three stages of compression with intercooling operates with air entering the first compressor at 50 kPa and - 30oC. Each compressor in this system has a
Using EES (or other) software, investigate the effect of the evaporator pressure on the COP of an ideal vapor-compression refrigeration cycle with R-134a as the working fluid. Assume the condenser
Using EES (or other) software, investigate the effect of the condenser pressure on the COP of an ideal vapor-compression refrigeration cycle with R-134a as the working fluid. Assume the evaporator
Derive a relation for the COP of the two-stage refrigeration system with a flash chamber as shown in Fig. 11-14 in terms of the enthalpies and the quality at state 6. Consider a unit mass in the
Consider a heat pump that operates on the reversed Carnot cycle with R-134a as the working fluid executed under the saturation dome between the pressure limits of 140 and 800 kPa. R-134a changes from
An ideal vapor-compression refrigeration cycle that uses refrigerant-134a as its working fluid maintains a condenser at 800 kPa and the evaporator at - 12oC. Determine this system's COP and the
A refrigerator removes heat from a refrigerated space at 0oC at a rate of 2.2kJ/s and rejects it to an environment at 20oC. The minimum required power input is (a) 89 W (b) 150 W (c) 161 W (d) 557
A refrigerator operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 120 and 800 kPa. If the rate of heat removal from the
A heat pump operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 0.32 and 1.2 MPa. If the mass flow rate of the refrigerant is
An ideal vapor compression refrigeration cycle with R-134a as the working fluid operates between the pressure limits of 120 kPa and 700 kPa. The mass fraction of the refrigerant that is in the liquid
Consider a heat pump that operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 0.32 and 1.2 MPa. The coefficient of performance
An ideal gas refrigeration cycle using air as the working fluid operates between the pressure limits of 80 and 280 kPa. Air is cooled to 35oC before entering the turbine. The lowest temperature of
Consider an ideal gas refrigeration cycle using helium as the working fluid. Helium enters the compressor at 100 kPa and 17oC and compressed to 400 kPa. Helium is then cooled to 20oC before it enters
An absorption air-conditioning system is to remove heat from the conditioned space at 20oC at a rate of 150kJ/s while operating in an environment at 35oC. Heat is to be supplied from a geothermal
Consider a refrigerator that operates on the vapor compression refrigeration cycle with R-134a as the working fluid. The refrigerant enters the compressor as saturated vapor at 160 kPa, and exits at
Consider a 300 kJ/min refrigeration system that operates on an ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. The refrigerant enters the compressor as
Reconsider Prob. 11-14. Using EES (or other) software, investigate the effect of evaporator pressure on the COP and the power input. Let the evaporator pressure vary from 100 to 400 kPa. Plot the COP
Repeat Prob. 11-14 assuming an isentropic efficiency of 85 percent for the compressor. Also, determine the rate of exergy destruction associated with the compression process in this case. Take T0 5
Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.20 MPa and 25oC at a rate of 0.07 kg/s, and it leaves at 1.2 MPa and 70oC. The refrigerant is cooled in the
A commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at - 30oC by rejecting its waste heat to cooling water that enters the condenser at 18oC at
Refrigerant-134a enters the compressor of a refrigerator at 100 kPa and - 20oC at a rate of 0.5 m3/min and leaves at 0.8 MPa. The isentropic efficiency of the compressor is 78 percent. The
Why do we study the reversed Carnot cycle even though it is not a realistic model for refrigeration cycles?
Using EES (or other) software, investigate the effects of varying the compressor isentropic efficiency over the range 60 to 100 percent and the compressor inlet volume flow rate from 0.1 to 1.0 m3/
A refrigerator uses refrigerant-134a as the working fluid and operates on the ideal vapor-compression refrigeration cycle except for the compression process. The refrigerant enters the evaporator at
The manufacturer of an air conditioner claims a seasonal energy efficiency ratio (SEER) of 16 (Btu/h)/W for one of its units. This unit operates on the normal vapor compression refrigeration cycle
An actual refrigerator operates on the vapor-compression refrigeration cycle with refrigerant-22 as the working fluid. The refrigerant evaporates at - 15oC and condenses at 40oC. The isentropic
How is the second-law efficiency of a refrigerator operating on the vapor-compression refrigeration cycle defined? Provide two alternative definitions and explain each term.
How is the second-law efficiency of a heat pump operating on the vapor-compression refrigeration cycle defined? Provide two alternative definitions and show that one can be derived from the other.
Consider isentropic compressor of a vapor-compression refrigeration cycle. What are the isentropic efficiency and second-law efficiency of this compressor? Justify your answers. Is the second-law
A space is kept at - 15oC by a vapor-compression refrigeration system in an ambient at - 5oC. The space gains heat steadily at a rate of 3500 kJ/h and the rate of heat rejection in the condenser is
Bananas are to be cooled from 28oC to 12oC at a rate of 1140 kg/h by a refrigerator that operates on a vapor-compression refrigeration cycle. The power input to the refrigerator is 8.6 kW.
A vapor-compression refrigeration system absorbs heat from a space at 0oC at a rate of 24,000 Btu/h and rejects heat to water in the condenser. The water experiences a temperature rise of 128C in the
A steady-flow Carnot refrigeration cycle uses refrigerant-134a as the working fluid. The refrigerant changes from saturated vapor to saturated liquid at 60oC in the condenser as it rejects heat. The
A refrigerator operating on the vapor-compression refrigeration cycle using refrigerant-134a as the refrigerant is considered. The temperature of the cooled space and the ambient air are at 10oF and
A room is kept at - 5oC by a vapor-compression refrigeration cycle with R-134a as the refrigerant. Heat is rejected to cooling water that enters the condenser at 20oC at a rate of 0.13 kg/s and
A refrigerator operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. The refrigerant evaporates at - 10oC and condenses at 57.9oC. The refrigerant
A refrigeration system operates on the ideal vapor-compression refrigeration cycle with ammonia as the refrigerant. The evaporator and condenser pressures are 200 kPa and 2000 kPa, respectively. The
Using EES (or other) software, repeat Prob. 11-33 if ammonia, R-134a, and R-22 is used as the refrigerant. Also, for the case of ammonia, investigate the effects of evaporator and condenser pressures
When selecting a refrigerant for a certain application, what qualities would you look for in the refrigerant?
Consider a refrigeration system using refrigerant-134a as the working fluid. If this refrigerator is to operate in an environment at 30oC, what is the minimum pressure to which the refrigerant should
A refrigerant-134a refrigerator is to maintain the refrigerated space at - 10oC. Would you recommend an evaporator pressure of 0.12 or 0.14 MPa for this system? Why?
A refrigerator that operates on the ideal vapor-compression cycle with refrigerant-134a is to maintain the refrigerated space at - 10oC while rejecting heat to the environment at 25oC. Select
A heat pump that operates on the ideal vapor-compression cycle with refrigerant-134a is used to heat a house and maintain it at 26oC by using underground water at 14oC as the heat source. Select
Refrigerant-134a enters the condenser of a steady-flow Carnot refrigerator as a saturated vapor at 90 psia, and it leaves with a quality of 0.05. The heat absorption from the refrigerated space takes
What is a water-source heat pump? How does the COP of a water-source heat pump system compare to that of an air-source system?
A heat pump that operates on the ideal vapor-compression cycle with refrigerant-134a is used to heat water from 15 to 45oC at a rate of 0.12 kg/s. The condenser and evaporator pressures are 1.4 and
A heat pump with refrigerant-134a as the working fluid is used to keep a space at 25oC by absorbing heat from geothermal water that enters the evaporator at 50oC at a rate of 0.065 kg/s and leaves at
Refrigerant-134a enters the condenser of a residential heat pump at 800 kPa and 50oC at a rate of 0.022 kg/s and leaves at 750 kPa subcooled by 3oC. The refrigerant enters the compressor at 200 kPa
A heat pump that operates on the ideal vapor-compression cycle with refrigerant-134a is used to heat a house and maintain it at 75oF by using underground water at 50oF as the heat source. The house
A heat pump using refrigerant-134a heats a house by using underground water at 8°C as the heat source. The house is losing heat at a rate of 60,000kJ/h. The refrigerant enters the compressor at 280
Reconsider Prob. 11-46. Using EES (or other) software, investigate the effect of varying the compressor isentropic efficiency over the range 60 to 100 percent. Plot the power input to the compressor
A certain application requires maintaining the refrigerated space at - 32oC. Would you recommend a simple refrigeration cycle with refrigerant-134a or a two-stage cascade refrigeration cycle with a
A two-stage compression refrigeration system operates with refrigerant-134a between the pressure limits of 1.4 and 0.10 MPa. The refrigerant leaves the condenser as a saturated liquid and is
Repeat Prob. 11-53 for a flash chamber pressure of 0.6 MPa. Prob. 11-53 A two-stage compression refrigeration system operates with refrigerant-134a between the pressure limits of 1.4 and 0.10 MPa.
Reconsider Prob. 11-53. Using EES (or other) software, investigate the effect of the various refrigerants for compressor efficiencies of 80, 90, and 100 percent. Compare the performance of the
Consider a two-stage cascade refrigeration system operating between the pressure limits of 0.8 and 0.14 MPa. Each stage operates on the ideal vapor-compression refrigeration cycle with
Repeat Prob. 11-56 for a heat exchanger pressure of 0.55 MPa. Prob. 11-56 Consider a two-stage cascade refrigeration system operating between the pressure limits of 0.8 and 0.14 MPa. Each stage
Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.4 MPa and 160 kPa with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the
Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.2 MPa and 200 kPa with refrigerant-134a as the working fluid. The refrigerant leaves the condenser as a
It is proposed to use water instead of refrigerant-134a as the working fluid in air-conditioning applications where the minimum temperature never falls below the freezing point. Would you support
A two-evaporator compression refrigeration system as shown in Fig. P11-60 uses refrigerant-134a as the working fluid. The system operates evaporator 1 at 08C, evaporator 2 at 226.48C, and the
A two-evaporator compression refrigeration system like that in Fig. P11-60 uses refrigerant-134a as the working fluid. The system operates evaporator 1 at 30 psia, evaporator 2 at 10 psia, and the
Repeat Prob. 11-61E if the 30 psia evaporator is to be replaced with a 60 psia evaporator to serve a 15,000 Btu/h cooling load.Prob. 11-61EA two-evaporator compression refrigeration system like that
Devise a refrigeration cycle that works on the reversed Stirling cycle. Also, determine the COP for this cycle.
How is the ideal-gas refrigeration cycle modified for aircraft cooling?
Air enters the compressor of an ideal gas refrigeration cycle at 40oF and 10 psia and the turbine at 120oF and 30 psia. The mass flow rate of air through the cycle is 0.5lbm/s. Determine (a) The rate
An ideal gas refrigeration cycle using air as the working fluid is to maintain a refrigerated space at - 23oC while rejecting heat to the surrounding medium at 27oC. If the pressure ratio of the
In a refrigeration system, would you recommend condensing the refrigerant-134a at a pressure of 0.7 or 1.0 MPa if heat is to be rejected to a cooling medium at 15oC? Why?
Air enters the compressor of an ideal gas refrigeration cycle at 7oC and 35 kPa and the turbine at 37oC and 160 kPa. The mass flow rate of air through the cycle is 0.2 kg/s. Assuming variable
Repeat Prob. 11-70 for a compressor isentropic efficiency of 80 percent and a turbine isentropic efficiency of 85 percent. Prob. 11-70 Air enters the compressor of an ideal gas refrigeration cycle at
Using EES (or other) software, study the effects of compressor and turbine isentropic efficiencies as they are varied from 70 to 100 percent on the rate of refrigeration, the net power input, and the
A gas refrigeration cycle with a pressure ratio of 4 uses helium as the working fluid. The temperature of the helium is 26oC at the compressor inlet and 50oC at the turbine inlet. Assuming isentropic
A gas refrigeration system using air as the working fluid has a pressure ratio of 4. Air enters the compressor at 27oC. The high-pressure air is cooled to 27oC by rejecting heat to the surroundings.
Repeat Prob. 11-74 assuming isentropic efficiencies of 75 percent for the compressor and 80 percent for the turbine.
A gas refrigeration system using air as the working fluid has a pressure ratio of 5. Air enters the compressor at 0oC. The high-pressure air is cooled to 35oC by rejecting heat to the surroundings.
An ideal gas refrigeration system with two stages of compression with intercooling as shown in Fig. P11-77 operates with air entering the first compressor at 90 kPa and - 24oC. Each compression stage
How will the answers of Prob. 11-77 change when the isentropic efficiency of each compressor is 85 percent and the isentropic efficiency of the turbine is 95 percent?Prob. 11-77An ideal gas
What is absorption refrigeration? How does an absorption refrigeration system differ from a vapor-compression refrigeration system?
Does the area enclosed by the cycle on a T-s diagram represent the net work input for the reversed Carnot cycle? How about for the ideal vapor-compression refrigeration cycle?
How is the coefficient of performance of an absorption refrigeration system defined?
An absorption refrigeration system that receives heat from a source at 95oC and maintains the refrigerated space at 0oC is claimed to have a COP of 3.1. If the environmental temperature is 19oC, can
An absorption refrigeration system receives heat from a source at 120oC and maintains the refrigerated space at 0oC. If the temperature of the environment is 25oC, what is the maximum COP this
Heat is supplied to an absorption refrigeration system from a geothermal well at 110oC at a rate of 5 × 105 kJ/h. The environment is at 25oC, and the refrigerated space is maintained at - 18oC.
A reversible absorption refrigerator consists of a reversible heat engine and a reversible refrigerator. The system removes heat from a cooled space at - 15oC at a rate of 70 kW. The refrigerator
An ammonia-water absorption refrigeration cycle is used to keep a space at 25oF when the ambient temperature is 70oF. Pure ammonia enters the condenser at 300 psia and 140oF at a rate of 0.04lbm/s.
Describe the Seebeck and the Peltier effects.
How does a thermocouple work as a temperature measurement device?
A thermoelectric generator receives heat from a source at 340oF and rejects the waste heat to the environment at 90oF. What is the maximum thermal efficiency this thermoelectric generator can have?
A thermoelectric refrigerator removes heat from a refrigerated space at 25oC at a rate of 130 W and rejects it to an environment at 20oC. Determine the maximum coefficient of performance this
A thermoelectric cooler has a COP of 0.15 and removes heat from a refrigerated space at a rate of 180 W. Determine the required power input to the thermoelectric cooler, in W.
A thermoelectric cooler has a COP of 0.18 and the power input to the cooler is 1.8 hp. Determine the rate of heat removed from the refrigerated space, in Btu/min.
What is the difference between partial differentials and ordinary differentials?

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