Lesson test: Refrigeration and heat pumps

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Refrigeration and heat pumps

1.[2p]
Why is a coefficient of performance used instead of an efficiency for a refrigerator?

What you want is heat moved and what you pay for is work, and the ratio routinely exceeds one

Correct

Refrigerators are less efficient than heat engines

The answer is: What you want is heat moved and what you pay for is work, and the ratio routinely exceeds one

The Second Law forbids defining an efficiency below ambient temperature

The answer is: What you want is heat moved and what you pay for is work, and the ratio routinely exceeds one

Work and heat cannot be compared in the same ratio

The answer is: What you want is heat moved and what you pay for is work, and the ratio routinely exceeds one

2.[3p]
A vapour-compression cycle has $h_{1} = 238.40$ , $h_{2} = 273.11$ and $h_{3} = h_{4} = 95.47$ kJ/kg. What is its coefficient of performance as a refrigerator?

CorrectNot quite: 4.12

3.[3p]
The compressor of that cycle has an isentropic efficiency of $0.80$ . What is the coefficient of performance now?

CorrectNot quite: 3.29

4.[2p]
For the same machine, ${COP}_{H P} = {COP}_{R} + 1$ .

True

Correct

False

The answer is: True

5.[2p]
A cold room needs $12$ kW of cooling on a cycle with $q_{L} = 142.9$ kJ/kg. What refrigerant flow is needed, in kg/s?

CorrectNot quite: 0.084

6.[3p]
Why is the expansion device a valve rather than a turbine?

Select all that apply

A domestic machine would need a two-phase turbine producing under a watt

Correct

Liquid droplets would erode the turbine blades

The answer is: A domestic machine would need a two-phase turbine producing under a watt, The recoverable work is only about a tenth of the compressor work

The recoverable work is only about a tenth of the compressor work

Correct

A valve is isentropic and a turbine is not

The answer is: A domestic machine would need a two-phase turbine producing under a watt, The recoverable work is only about a tenth of the compressor work

7.[3p]
Match each deliberate departure from the ideal cycle to its reason.

Superheat at compressor inlet
Subcooling at condenser exit
Cascaded loops
Absorption system

no liquid reaches the compressor
pay in waste heat, not electricity
one stage cannot span a large lift
no flash vapour reaches the valve

Show the answer

Superheat at compressor inlet: no liquid reaches the compressor Subcooling at condenser exit: no flash vapour reaches the valve Cascaded loops: one stage cannot span a large lift Absorption system: pay in waste heat, not electricity

8.[2p]
A heat pump with ${COP}_{H P} = 4.29$ serves a house needing $12$ kW. What electrical power does it draw, in kW?

CorrectNot quite: 2.79

9.[2p]
Why is an air-source heat pump least effective when it is most needed?

The coefficient of performance falls as the temperature lift grows, and the lift is largest in a frost

Correct

Its compressor stalls at low ambient temperature

The answer is: The coefficient of performance falls as the temperature lift grows, and the lift is largest in a frost

The refrigerant freezes below zero degrees Celsius

The answer is: The coefficient of performance falls as the temperature lift grows, and the lift is largest in a frost

Electricity is more expensive in winter

The answer is: The coefficient of performance falls as the temperature lift grows, and the lift is largest in a frost

1.[2p] Why is a coefficient of performance used instead of an efficiency for a refrigerator?

2.[3p] A vapour-compression cycle has h1=238.40, h2=273.11 and h3=h4=95.47 kJ/kg. What is its coefficient of performance as a refrigerator?

3.[3p] The compressor of that cycle has an isentropic efficiency of 0.80. What is the coefficient of performance now?

4.[2p] For the same machine, COPHP=COPR+1.

5.[2p] A cold room needs 12 kW of cooling on a cycle with qL=142.9 kJ/kg. What refrigerant flow is needed, in kg/s?

6.[3p] Why is the expansion device a valve rather than a turbine?

7.[3p] Match each deliberate departure from the ideal cycle to its reason.

8.[2p] A heat pump with COPHP=4.29 serves a house needing 12 kW. What electrical power does it draw, in kW?

9.[2p] Why is an air-source heat pump least effective when it is most needed?

1.[2p]
Why is a coefficient of performance used instead of an efficiency for a refrigerator?

2.[3p]
A vapour-compression cycle has $h_{1} = 238.40$ , $h_{2} = 273.11$ and $h_{3} = h_{4} = 95.47$ kJ/kg. What is its coefficient of performance as a refrigerator?

3.[3p]
The compressor of that cycle has an isentropic efficiency of $0.80$ . What is the coefficient of performance now?

4.[2p]
For the same machine, ${COP}_{H P} = {COP}_{R} + 1$ .

5.[2p]
A cold room needs $12$ kW of cooling on a cycle with $q_{L} = 142.9$ kJ/kg. What refrigerant flow is needed, in kg/s?

6.[3p]
Why is the expansion device a valve rather than a turbine?

7.[3p]
Match each deliberate departure from the ideal cycle to its reason.

8.[2p]
A heat pump with ${COP}_{H P} = 4.29$ serves a house needing $12$ kW. What electrical power does it draw, in kW?

9.[2p]
Why is an air-source heat pump least effective when it is most needed?