Refrigeration

Question 1

What is Refrigeration?

Answer 1

Refrigeration is a branch of science that deals with the removal of heat from a substance or space to a temperature lower than that of the surrounding environment by means of heat absorber

Question 2

What are the major applications of refrigeration?

Answer 2

1) Food processing
2) Chemical and process industries - Separation of gases- Dehumidification of air
3) Special Applications - Medicines- Ice skating rinks- Desalination of water
4) Comfort air-conditioning

Question 3

Describe the vapour compression system and its components?

Answer 3

1) A mixture of gas and liquid refrigerant is in the evaporator where it is a low temperature low pressure state.
2) The refrigerant is then fed to the compressor where it is compressed to a high pressure high temperature gas
3) From here it enters the condenser where it is cooled to a low pressure high temperature liquid
4) the refigerant is then epanded via the expansion valve to a cool temperature low pressure liquid gas mixture and then eneters the evaporator and the whole process takes place again.

Question 4

Describe how an absorption chiller operates and its components?

Answer 4

1) in the lecture example, ammonia will be in the evaporator and is fed to the absorber that is filled with water and ammonia.
2) the solution pump will the pump this to the generator as there is no compressor.
3) The Generator then feeds this to the condenser making sure that water doesn’t get we place a rectifier in between
4) The water rejected in the rectifier is then pumped back into the absorber
5) Meanwhile the amonnia is now taken to the condenser, low pressure high temp and then exapnsion and back to the evaporator

Question 5

What is the first law of thermodynamics?

Answer 5

The change to the internal energy of a system is equal to the heat added and work done to it

Question 6

What is the second law princple of thermodynamics?

Answer 6

It is impossible to convert heat completely into work.

Heat cannot be transferred from a colder body to a hotter body without net changes occuring in the system i.e. work has to be done to it. Entropy always icreases and the process is irrevisible.

Question 7

What is the 3rd law of thermodynamics?

Answer 7

It is impossible to reduce the temperature of a system to absolute zero in a finte number of steps.

Question 8

What is the eqaution for principle of conservation?

Answer 8

Q1 - Q2 = W

Question 9

What is the zeroth law of thermodyanamics?

Answer 9

The principle that if two bodies are in thermal equilbirium with a third body then the first two bodies are in thermal equilbrium with each other

Question 10

What is the thermal efficiency of heat engine defined by?

Answer 10

n (effi) = Work done/heat supplied

= W/Q1

= Q1-Q2/Q1

= 1-Q2/Q1

Question 11

How do you calculate the COP of refrigeration system?

Answer 11

COP ref = Q1/W

COP ref = Q2/W (for heat pump)

Question 12

What is the equation for COP heat Pump

Answer 12

COP hp = Q1 + W/W

= 1 + Q1/W

= 1 + COP ref

Question 13

What is Enthaply?

Answer 13

Enthalpy is designated the letter H and is the internal energy of the system plus the sum of the pressure and volume of the system.

H= U +pV

Question 14

What is Entropy?

Answer 14

Entropy is a thermodynaic property that is a reversible process by an amount equal to the heat absorbed or emitted divided by the thermodynamic temeperature.

deltaS = calculus dQrev/T

Question 15

What is the reversed carnot cycle?

Answer 15

The reversed carnot cycle is the process of a heat engine cycle but reversed to give cooling values.

1-2 is isentropic which is adiabatic compress essentially the compressor

2-3 isothermal which heat rejection ultimaltely the condenser

3-4 isentropic adiabatic expansion

4-1 which isothermal heat addition.

Q1 = T1 (S1- S4)

Q2 = T2 (S2 -S3)

W = Q2 - Q1 = T2 (S1 -S4) - T1 (S1 - S4)

= (T2 -T1) x (S1- S4)

Carnot COP ref = T1/ T2 -T1) in Kelvin

Question 16

Give the steady flow energy equation?

Answer 16

h_in + q = h_out + W

Question 17

Give the equation at points 1-2 for the isentropic compression process

Answer 17

-W_1-2 = h₂ - h₁

Question 18

What is the eqaution for the condensation process

Answer 18

Q_2-3 = h₃ - h_{<span>2</span>}

Question 19

What is the eqaution for 3-4 process with expansion valve?

Answer 19

h₃ = h₄

Question 20

What is the eqaution for refrigerant effect?

Answer 20

Q_4-1 = h₁ - h₄

Question 21

What is the eqaution for cooling capacity?

Answer 21

Qc = m(dot)r (h1 -h4) in KW

COP = Qc/W = (h1 - h4)/ (h2 - h1)

Question 22

What is the heating capacity equation?

Answer 22

Qh = m(dot) r (h2 - h3)

Question 23

What is the equation of the power consumption

Answer 23

Wc = m(dot) r (h2 -h1)

Question 24

What is the effect of suction vapour superheating?

Answer 24

1. Increase the refigerant effect but decreases the cooling capacity of the system
2. There is an increase in the heating effect but a decrease in the heating capacity of the system
3. Reduction in the compressor power consumption
4. Reduction in the heating and cooling COP of the system
5. The reduction in the compressor power is not enough to outweigh the effect of the reduced heat transfer rates in the heat exchangers

Question 25

What is the effect of liquid subcooling?

Answer 25

1. Reduces the flashing in the liquid during expansion
2. Increases both the refergeration and heating effect of the cycle
3. Liquid subcooling can be achieved in several ways

Question 26

what are the types of compressors used for refrigeration and heat pump applications?

Answer 26

Reciprocating

Centrifugal

Rotary

Question 27

What is meant by positive displacement compressors such as the rotary and reciprocating types?

Answer 27

Positive displacement compressors increase the pressure of the refrigerant vapour by reducing the volumen in the compression chamber by mechanical means.

Question 28

How is compression achieved in a centrifugal compressor?

Answer 28

Through a rotating impeller.

Question 29

What are the three compressor types and explain how each one works?

Answer 29

Depending on the driving and sealing of refrigeration agianst the atmosphere there three types are:

Hermetic

Semi-hermetic

Open Type

Question 30

Describe how a hermetic compressor works?

Answer 30

It is a completely sealed unit with the motor and other mechanical parts sealed within a steel casing.

Question 31

Describe what a semi-hermetic compressor is?

Answer 31

in a bolted casing rather than a steel casing and is an intergral unit with motor and compressor

Question 32

Describe a open type compressor?

Answer 32

the motor is completely external to the compressor. The open type compressor is driven by an external prime mover coupled to the crankshaft which protrudes from the crankcase through shaft seals