Refrigerators and heat pumps are essentially the same device and differ only in their objectives. A refrigerator's objective is to remove heat from the cold medium, while a heat pump's objective is to supply heat to a warm medium. Both use the vapor compression refrigeration cycle which involves five basic components - a refrigerant, compressor, condenser coils, evaporator coils, and expansion device. The refrigerant absorbs heat inside the fridge/building and is compressed, releasing heat outdoors in the condenser coils, before repeating the cycle. Heat pumps transfer heat between outdoors and indoors using this cycle to provide both cooling and heating with one system.

1. Refrigeration & Heat Pumps
Refrigerators and heat pumps are essentially the same devices; they differ in
their objectives only.
The objective of a refrigerator is to remove heat (Qin) from the cold medium.
The objective of a heat pump is to supply heat (Qout) to a warm medium.

2. Vapour Compression Refrigerator System (VPRS)
Qout
Qin
How does a refrigerator work?
In the refrigeration cycle, there are five basic components: fluid
refrigerant; a compressor, which controls the flow of refrigerant; the
condenser coils (on the outside of the fridge); the evaporator coils (on
the inside of the fridge); and something called an expansion device.
Here's how they interact to cool your food.
1. The compressor constricts the refrigerant vapour, raising its
pressure, and pushes it into the coils on the outside of the
refrigerator. It gets hot.
2. When the hot gas in the coils meets the cooler air temperature of
the kitchen, it becomes a liquid. It cools and condenses.
3. The liquid at high pressure passes through the expansion valve, as
the pressure reduces the temperature reduces greatly.
4. The refrigerant at low temperature absorbs the heat inside the
fridge, cooling down the air.
5. Last, the refrigerant absorbing heat evaporates to a gas, then flows
back to the compressor, where the cycle starts all over.

3. How Does a Heat Pump Work
A heat pump is part of a home heating and cooling system and is installed outside your home. Like an air
conditioner such as central air, it can cool your home, but it’s also capable of providing heat. In cooler months, a
heat pump pulls heat from the cold outdoor air and transfers it indoors, and in warmer months, it pulls heat out of
indoor air to cool your home. They are powered by electricity and transfer heat using a refrigerant to provide
comfort all year round. Because they handle both cooling and heating, homeowners may not need to install
separate systems to heat their homes.

4. The Refrigerant
Several refrigerants may be used in refrigeration systems such as chlorofluorocarbons (CFCs),
ammonia, hydrocarbons (propane, ethane, ethylene, etc), carbon dioxide, air (in the air-conditioning
of aircraft), and even water (in applications above the freezing point).
R-11, R-12, R-22, R-134a and R-502 account for over 90% of the market.
R-11 is used in large capacity water chillers
serving AC systems in buildings.
The industrial and heavy commercial sectors use ammonia (it is toxic).
R-134a (replaced R-12, which damages the
ozone layer) is used in domestic refrigerators
and freezers, as well as automotive air
conditioners.
R-22 is used in window air conditions, heat
pumps, AC in commercial buildings, it offers
strong competition to ammonia.

5. R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems
such as those in supermarkets.
CFCs allow more ultraviolet radiation into the earth’s atmosphere by destroying the protective ozone
layer and thus contributing to the greenhouse effect that causes global warming. Fully halogenated CFCs
(such as R-11, R-12 and R-115) do the most damage to the ozone layer. Refrigerants that are friendly to
the ozone layer have been developed.
Two important parameters that need to be considered in the selection of a refrigerant are the
temperatures of the refrigerated space and the environment.
The saturation pressures of the refrigerant at the temperatures Tout and Tin also effect the Work into the
compressor.

8. Qout
Qin
Heat from cold
compartment
to atmosphere
from compressor
enthalpy
pressure
Temperature
entropy
Qout
Qin
Graph of Pressure against enthalpy Graph of Temperature against entropy
You need a change in temperature to allow heat transfer. Remember heat always flows from hot to cold.
The evaporator temperature is lower than the refrigerated space temperature.
The condenser temperature is higher than the ambient temperature.

9. Coefficient of Performance (COP)
The performance of refrigerators and heat pumps is expressed in terms of the coefficient of performance (COP), which is
defined as:
𝐶𝑂𝑃𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑜𝑟 = 𝐶𝑂𝑃𝑟 =
𝑑𝑒𝑠𝑖𝑟𝑒𝑑 𝑜𝑢𝑡𝑝𝑢𝑡
𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛𝑝𝑢𝑡
=
𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑒𝑓𝑓𝑒𝑐𝑡
𝑤𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡
=
𝑄𝑖𝑛
𝑊𝑛𝑒𝑡𝑡,𝑖𝑛
=
𝐶𝑂𝑃ℎ𝑒𝑎𝑡 𝑝𝑢𝑚𝑝 = 𝐶𝑂𝑃ℎ𝑝 =
𝑑𝑒𝑠𝑖𝑟𝑒𝑑 𝑜𝑢𝑡𝑝𝑢𝑡
𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛𝑝𝑢𝑡
=
ℎ𝑒𝑎𝑡𝑖𝑛𝑔 𝑒𝑓𝑓𝑒𝑐𝑡
𝑤𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡
=
𝑄𝑜𝑢𝑡
𝑊𝑛𝑒𝑡𝑡,𝑖𝑛
=
𝐶𝑂𝑃𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑜𝑟 𝑚𝑎𝑥 = 𝐶𝑂𝑃𝑟𝑚𝑎𝑥 =
𝑇𝑖𝑛
𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛
=
𝐶𝑂𝑃𝑟 𝑟𝑒𝑣 = 𝐶𝑂𝑃𝑟 𝑐𝑎𝑟𝑛𝑜𝑡 =
1
𝑇𝑜𝑢𝑡
𝑇𝑖𝑛
− 1
=
The Reference COP
The maximum COP of a refrigeration cycle operating between temperature limits of Tin and Tout.
Actual refrigeration cycles are not as efficient as ideal ones like the Carnot cycle because of the irreversibility's involved.
ℎ1 − ℎ4
ℎ2 − ℎ1
ℎ2 − ℎ3
ℎ2 − ℎ1
ℎ1 = ℎ𝑔 at P1, and ℎ3 = ℎ𝑓 at P3 for the ideal case.

10. Analysis
Each component is treated separately as an open system with steady flow.
𝑞𝑖𝑛 − 𝑞𝑜𝑢𝑡 + 𝑤𝑖𝑛 − 𝑤𝑜𝑢𝑡 = ℎ𝑜𝑢𝑡 − ℎ𝑖𝑛
Steady Flow Energy Balance
Component Process First Law of
Thermodynamics
Compressor (1-2) Constant entropy Win = m(h2 - h1)
Condenser (2-3) Constant pressure Qout=m(h2-h3
Throttle Valve (3-4) Δs > 0
Wnett= 0
Qnet = 0
Evaporator (4-1) Constant pressure

11. Example
Refrigerant – 134a is the working fluid in an ideal compression refrigeration cycle. The refrigerant leaves
the evaporator at -20oC and has a condenser pressure of 0.9MPa. The mass flow rate is 3kg/min. Find
COPr and COPr, max for the same Tmax and Tmin and the tons of refrigeration.
Qout
Qin
Heat from cold
compartment
to atmosphere
from compressor
enthalpy
pressure
Graph of Temperature against entropy Graph of Pressure against enthalpy
Temperature
entropy

12. Compressor Inlet (stage 1)
𝑇1 = −20𝑜𝐶 𝑓𝑟𝑜𝑚 𝑡𝑎𝑏𝑙𝑒𝑠 ℎ1 = ℎ𝑔 = 235.31𝑘𝐽/𝑘𝑔 𝑠1 = 𝑠𝑔 = 0.9332𝑘𝐽/𝑘𝑔
Saturated pure vapour, x = 1
Compressor Exit (stage 2)
𝑃2 = 0.9𝑀𝑃𝑎 = 0.9 × 106𝑃𝑎
𝑓𝑟𝑜𝑚 𝑡𝑎𝑏𝑙𝑒𝑠 𝑠2 𝑜𝑐𝑐𝑢𝑟𝑠 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 40𝑜𝐶 𝑎𝑛𝑑 50𝑜𝐶
𝑠1 = 𝑠2 = 0.9332𝑘𝐽/𝑘𝑔 from T-S diagram
0.9566 − 0.9217 = 0.0349
By interpolation
0.9332 − 0.9217 = 0.0115
𝑅𝑎𝑡𝑖𝑜 =
0.0115
0.03495
= 0.3353
Using Ratio 282 − 271.25 × 0.3353 = 3.6045
ℎ2 = 271.25 + 3.6045 = 274.85𝑘𝐽/𝑘𝑔
Using Ratio 50 − 40 × 0.3353 = 3.353
𝑇2 = 40 + 3.353 = 43.353𝑜𝐶
= 253𝐾
= 316.353𝐾

13. Condenser Exit (stage 3)
𝑃2 = 𝑃3 = 0.9𝑀𝑃𝑎 = 0.9 × 106𝑃𝑎 (𝑝 − ℎ 𝑑𝑖𝑎𝑔𝑟𝑎𝑚)
Saturated pure liquid, x = 0
𝑓𝑟𝑜𝑚 𝑡𝑎𝑏𝑙𝑒𝑠 ℎ3 = ℎ𝑓 = 99.56𝑘𝐽/𝑘𝑔 𝑠3 = 𝑠𝑓 = 0.3656𝑘𝐽/𝑘𝑔
Throttle Exit (stage 4)
𝑇1 = 𝑇4 = −20𝑜𝐶
ℎ3 = ℎ4 = 99.56𝑘𝐽/𝑘𝑔
𝐶𝑂𝑃𝑟 =
𝑄𝑖𝑛
𝑊𝑛𝑒𝑡𝑡,𝑖𝑛
=
ℎ1 − ℎ4
ℎ2 − ℎ1
=
235.31 − 99.56
274.85 − 235.31
=
235.31 − 99.56
274.85 − 235.31
= 3.433
𝑇𝑖𝑛
𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛
=
𝐶𝑂𝑃𝑟 𝑚𝑎𝑥 = 𝐶𝑂𝑃𝑟 𝑐𝑎𝑟𝑛𝑜𝑡 = 253
316.353 − 253
= 3.98
This is the best possible
ratio of the amount of
heat removed to the
work done.

14. One ton of refrigeration is the heat transfer rate required to melt 1 ton of ice at 0oC in 1 day (24 hrs)
3.5kW or 211KJ/min
The tons of refrigeration, often called the cooling load or refrigeration effect, are:
𝑄𝑖𝑛 = 𝑚 ℎ1 − ℎ4 = 3 235.31 − 99.56 = 407.25𝑘𝐽/𝑚𝑖𝑛
𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝐿𝑜𝑎𝑑 = 407.25𝑘𝐽/𝑚𝑖𝑛 ×
1 𝑇𝑜𝑛
211𝑘𝐽/𝑚𝑖𝑛
= 1.93 𝑇𝑜𝑛

15. Heat Pump Systems
A heat pump can be used to heat a house in winter but
also to cool the house in summer.