Lecture 12 : Refrigeration

Question 1

What is the main purpose of a refrigerator?

Answer 1

to create cooler temperatures as compared to surrounding environment (for preservation of food products etc)

Question 2

Whats is a refrigerant?

Answer 2

It is a cooling agent that absorbs heat (from heat evolved from respiring fruits etc) and leaves cool air behind when passed through a compressor and evaporator

Question 3

The refrigerant has a higher boiling point than water (100°C). True or False, why?

Answer 3

False, the refrigerant needs to have a very low boiling point so that it can turn into gas very easiy when it absorbs heat
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• else high amounts of pressure needed to vapourise the refrigerant into gas (cost) –> need to vapourise refrigerant before it is recooled back into liquid to get re-used
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• need to heat into vapour for efficient transfer of heat from vapour to cooling medium in condenser ++ compressor is designed for gases only, not liquids

Question 4

What are the 3 most important aspects when selecting a refrigerant?

Answer 4

1) Latent heat of vapourisation (amount of heat required to cause change from liquid to gas without change in pressure or temperature)
- Should be high, so that refrigerant can absorb a lot of heat before changing state, and so that less volume of refrigerant needed to flow

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2) Freezing temperature of refrigerant
- Should be below evaporation temperature to minimise equipment damage (see eg in notes)

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3) refrigerant should have high critical temperature (temperature where saturated liquid + saturated vapour is present at the same time)

Question 5

Although it is desirable for latent heat of vapourisation of refrigerants to be high, it should not be too high. Why?

Answer 5

With excessively high latent heat of vapourisation, this means a high amount of pressure is needed to condense the refrigerant back → high expenditure on heavy construction of condenser and piping.

Question 7

What is meant by critical temperature and why should refrigerants have high critical temperature?

Answer 7

Critical temperature is the temperature where saturated vapour and saturated liquid is present.

Refrigerants should have high critical temperature, higher than the highest expected temperature in surroundings of refrigerator,** to ensure that refrigrant vapours can be liquiefied back into liquid **

Question 8

Refrigeration system

What are the 4 main parts of a refrigeration system?

Answer 8

1. Evaporator
2. Compressor
3. Condenser
4. Expansion valve

Question 9

Refrigeration system

What happens in the evaporator? What is the state of the refrigerant when leaving evaporator/entering condenser?

Answer 9

Heat from the surroundings (carried by water in tubes) is transferred to liquid refrigerant, causing it to vapourise

• Saturated vapour

Question 10

Refrigeration system

What is the main purpose of compressor? What state does refrigerant leave the compressor as?

Answer 10

The purpose of the compressor is to compress the saturated vapours at high pressure and temperature to make it into superheated vapor.
- This ensures that vapor is very hot to have efficient heat exchange in the condenser.

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Refrigerant leaves as superheated vapours.

Question 11

Refrigeration system

Name the 3 kinds of compressors used and the most commonly used type of compressor.

Answer 11

1. Centrifugal
2. Rotary
3. Reciprocating (most commonly used)

Question 12

Refrigeration system

Refer to notes for the complete cycle of a reciprocating compressor (first diagram)

What is point 1 called, and what is the volume of the cylinder?

Answer 12

Point A : initial point

• cylinder is at 100% volume with vapors/gas

Question 13

Refrigeration system

Refer to notes for the complete cycle of a reciprocating compressor.

Compression occurs from which point to which point?
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How does pressure and volume of cylinder change?
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Do any valves open?

Answer 13

Compression : point 1 to 2

• Pressure increases, vol decreases from 100% to 15%
• both suction and discharge valves remain closed

Question 14

Refrigeration system

Refer to notes for the complete cycle of a reciprocating compressor.

Discharge of gas occurs from which point to which point?
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How does volume of the cylinder change?
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Do any valves open?

Answer 14

Point 2 to 3.
- cylinder decreases from 15% volume to 5% volume.
- Discharge valve opens

Question 15

Refrigeration system

Refer to notes for the complete cycle of a reciprocating compressor.

What occurs from point 3 to 4?
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How does volume of the cylinder change?
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Do any valves open?

Answer 15

Expansion
- volume increases
- Suction valve open

Question 16

Refrigeration system

Refer to notes for the complete cycle of a reciprocating compressor.

What occurs from point 4 to 1?
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How does volume of the cylinder change?
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Do any valves open?

Answer 16

Suction
- Suction valve remains open, gas enters
- Piston retracts to location A (initial point)

Question 17

Refrigeration system

What occurs in the condenser?

Answer 17

To condense the refrigerant vapours back into liquid, by transferring heat from refrigerant to another medium (air/water)

Question 18

Refrigeration system

What are the 3 different types of condensers?

Answer 18

1. Shell and coil
2. Shell and tube
3. Double pipe

Question 19

Refriegeration system

After refrigerant condenses into a liquid, where does it enter?

Answer 19

expansion valve

Question 20

Refriegeration system

After condensed refrigerant passes through the expansion valve, it comes out of the valve as a mixture of liquid and gas.
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Explain how this happens and what is this phenomenon called?

Answer 20

As the condensed liquid refrigerant enters expansion valve, volume increases and pressure decreases. Thus, it allows liquid particles to be further apart, causing some of it to vapourise

• Flashing

Question 22

In evaporator and condenser, state change occurs (liquid → gas and gas → liquid respectively). Does pressure and temperature change in evaporator and condenser?

Answer 22

No, state change occurs by adding / removing heat without inducing a change in temperature or pressure. This heat is known as latent heat, which is the “hidden energy” to cause state change without change in pressure and temp

Question 23

During compression in the compressor, what happens to pressure and temperature, as well as enthalpy (heat content) of the refrigerant?

Answer 23

• Pressure and temperature increases
• Enthalpy increases as saturated vapours turn into superheated vapours which contain more heat.

Question 24

During expansion in the expander, what happens to pressure and enthalpy (heat content) of the refrigerant?

Answer 24

• Pressure decreases
• Enthalpy remains constant (only volume increases,** temp of expander is the same as ambient temp** → no additional heat added/removed from refrigerant)
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  check bolded info if its correct

Question 25

What is cooling load?

Answer 25

• Rate of heat energy removal from a given space in order to lower the temperature to a desired level
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• A mechanical refrigeration system that can remove heat from a
  space at a rate of 3.5168 kW/ton is considered as one ton of
  refrigeration (1 TR) capacity
  ✓ i.e., 1 TR = 3.5168 kJ/s = 3.5168 kW

Question 26

What is the equation for work done (power, q_w) in compressor?

Answer 26

q_w = ṁ (H3-H2)
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where
ṁ = refrigerant mass flow rate (kg/s),
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H3 = enthalphy of refrigerant at the end of compression stroke
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H2 = enthalphy of refrigerant at the start of compression stroke

Question 27

What is the equation for work done (power, q_c) in condenser?

Answer 27

q_c = ṁ (H3-H1)
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where
ṁ = refrigerant mass flow rate (kg/s),
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H3 = enthalphy of refrigerant at the end of compression stroke (when it first enter the condenser)
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H1 = enthalphy of refrigerant at the exit of condenser

Question 28

What is the refrigeration effect? What is the equation?

Answer 28

Difference in heat content when it enters and leaves the evaporator.
- Refrigeration effect = H2-H1
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Where:
H2 = enthalpy of refrigerant at exit of evaporator (and start of compression stroke)
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H1 = enthalpy of refrigerant at inlet of evaporator

(thus refrigeration effect = amount of heat lost by stuff in refrigerator = amount of heat gained by refrigerant)

Question 29

What is the equation for the rate of heat accepted by refrigerant (q_e) as it undergoes evaporation?

Answer 29

q_e = ṁ (H2-H1)
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where
ṁ = refrigerant mass flow rate (kg/s),
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H2 = enthalpy of refrigerant at exit of evaporator (and start of compression stroke)
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H1 = enthalpy of refrigerant at inlet of evaporator

Question 30

What is coefficient of performance and the equation?

Answer 30

Ratio between heat absorbed by refrigerant as it flows through evaporator / heat equivalent of energy supplied to compressor
- indicates how efficient the cooling of the refrigeration system is

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C.O.P = (H2-H1) / (H3-H2)
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H3 = enthalphy of refrigerant at the end of compression stroke (when it first enter the condenser)
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H2 = enthalpy of refrigerant at exit of evaporator (and start of compression stroke)
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H1 = enthalphy of refrigerant at the exit of condenser

refer to notes for more detailed explanation