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MEC303 > Lecture 10 - Refrigeration Cycles 2 > Flashcards

Lecture 10 - Refrigeration Cycles 2 Flashcards

1
Q

Give COPHP in terms of COPR

A

COPHP = COPR + 1

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2
Q

What is the desired output of a heat pump?

A

QH (heat out)

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3
Q

What can be used as heat source for a heat pump?

A

Technically anything - but the lower it is, the more difficult and expensive producing work becomes

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4
Q

What does a heat pump do?

A

Takes low grade heat, usually extracted from the ground at low temperature. Heat pump is used to raise it to a usable temperature

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5
Q

Give the factors that affect heat pump performance

A

Capacity and efficiency fall significantly at low source temp
Must be sized for the largest heat load required

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6
Q

Water vs air as heat source

A

Air systems:
-May require supplemental heating
-Can have frosting problems at high humidity - forms on evaporator coils, reducing heat transfer
Water (or soil)
-More constant temperatures
-If too much is extracted ground can freeze - only remove heat that can be replaced

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7
Q

Give the advantages of a heat pump as heating system

A
  • Same components a refrigerator
  • Design a reversible system to both heat and cool
  • Free air conditioning
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8
Q

How does combined heating/cooling work?

A

Accomplished by reversing valve

Switches condenser

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9
Q

How does combined heating/cooling work?

A

Accomplished by reversing valve
Switches condenser and evaporator around
More practical for high cooling low heating loads

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10
Q

How do heat pumps compare with resistance heaters and gas boilers?

A

1/3 of the energy, C02 of resistance heater

3/4 of the c02 of gas boiler

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11
Q

Why dont we use heat pumps for domestic heating?

A
  • Large area required
  • Lower efficiencies - when taking losses into account
  • Fluctuations in demand - not stable
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12
Q

If only electricity is available is a heat pump a good option

A

yes

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13
Q

Can we use a refrigerator as AC?

A

No - dumping heat into same place trying to take heat from - one heat source

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14
Q

Refrigeration requirement of heat pump

A

reach temp lower than required cooling temp

temp higher than the temp where you want to dump the heat

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15
Q

Can water be used as a working fluid?

A

Yes, but it would be very inefficient

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16
Q

Give options for working fluid of heat pump

A

Ammonia - limited to below 80C and very toxic

C02 non toxic natural substance low COP usually used in cascade

17
Q

What working fluid does industry like and why?

A

Ammonia
Low cost and high COPs
However it is toxic

18
Q

What was the issue with early refrigerants?

Give examples of early refrigerants

A

Highly toxic - sulphur dioxide, ethyl chloride and methyl chloride

19
Q

Give the history of Freons

A

Frigidaire corp - requested GM to develop new refrigerant in 1928
Come up with CFCs - most suitable Freon
Wide Scale use in aerosols, foam insulation and as solvents
Ozone crisis caused new look

20
Q

Give types of refrigerants (organic and inorganic)

A

Inorganic - ammonia, C02 sulphur dioxide

Organic - methylchloride, trichlorodifluoromethane, dichlorofluoromethane, fluoromethanes and others

21
Q

For heat flow at reasonable rate, what should the difference in temperatures be between refrigerant and fluid?

A

5-10 degrees

22
Q

Refrigerant critical temperature

A

greater than condenser temperature - approach carnot cycle and achieve high COP

23
Q

Refrigerant Freezing temperature

A

Low - liquid only in evaporator - no freezing

24
Q

Refrigerant saturation pressure

A

above atmospheric - avoid air leaks into system

25
Q

Refrigerant Evaporation enthalpy

A

High - Reduces mass flow rate

26
Q

Refrigerant Specific Volume

A

Low - Reduces compressor work and system size

27
Q

Refrigerant Stability

A

Good - Both pure substances and mixtures

28
Q

Refrigerant Thermal Conductivity

A

High - Good heat transfer rates

29
Q

Refrigerant Solubility

A

Low - avoid water contamination

avoid oil contamination

30
Q

Refrigerant Toxicity/irritancy

A

Low - avoid poisoning and convenient handling

31
Q

Refrigerant Non Flammable

A

Safety in charging, handling and safety if leaks

32
Q

Refrigerant Detectability

A

Good - for tracing leaks

33
Q

Refrigerant Ozone Depletion

A

None

34
Q

Refrigerant Cost

A

Low

35
Q

Why was freon so good?

A

Met all the requirements of a refrigerant apart from ozone layer
Replaced CFCs with HFCs

36
Q

What is the gas refrigeration systems?

A

Reverse brayton cycle

37
Q

What is the gas refrigeration cycle used for?

A

Aircraft cooling

38
Q

What is an absorption refrigeration cycle?

A

A cycle that uses solar energy converted as a compressor

MEC303 (14 decks)

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