Refrigeration Flashcards
What is Refrigeration?
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
What are the major applications of refrigeration?
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
Describe the vapour compression system and its components?
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.
Describe how an absorption chiller operates and its components?
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
What is the first law of thermodynamics?
The change to the internal energy of a system is equal to the heat added and work done to it
What is the second law princple of thermodynamics?
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.
What is the 3rd law of thermodynamics?
It is impossible to reduce the temperature of a system to absolute zero in a finte number of steps.
What is the eqaution for principle of conservation?
Q1 - Q2 = W
What is the zeroth law of thermodyanamics?
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
What is the thermal efficiency of heat engine defined by?
n (effi) = Work done/heat supplied
= W/Q1
= Q1-Q2/Q1
= 1-Q2/Q1
How do you calculate the COP of refrigeration system?
COP ref = Q1/W
COP ref = Q2/W (for heat pump)
What is the equation for COP heat Pump
COP hp = Q1 + W/W
= 1 + Q1/W
= 1 + COP ref
What is Enthaply?
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
What is Entropy?
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
What is the reversed carnot cycle?
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
Give the steady flow energy equation?
hin + q = hout + W
Give the equation at points 1-2 for the isentropic compression process
-W1-2 = h2 - h1
What is the eqaution for the condensation process
Q2-3 = h3 - h<span>2</span>
What is the eqaution for 3-4 process with expansion valve?
h3 = h4
What is the eqaution for refrigerant effect?
Q4-1 = h1 - h4
What is the eqaution for cooling capacity?
Qc = m(dot)r (h1 -h4) in KW
COP = Qc/W = (h1 - h4)/ (h2 - h1)
What is the heating capacity equation?
Qh = m(dot) r (h2 - h3)
What is the equation of the power consumption
Wc = m(dot) r (h2 -h1)
What is the effect of suction vapour superheating?
- Increase the refigerant effect but decreases the cooling capacity of the system
- There is an increase in the heating effect but a decrease in the heating capacity of the system
- Reduction in the compressor power consumption
- Reduction in the heating and cooling COP of the system
- The reduction in the compressor power is not enough to outweigh the effect of the reduced heat transfer rates in the heat exchangers
What is the effect of liquid subcooling?
- Reduces the flashing in the liquid during expansion
- Increases both the refergeration and heating effect of the cycle
- Liquid subcooling can be achieved in several ways
what are the types of compressors used for refrigeration and heat pump applications?
Reciprocating
Centrifugal
Rotary
What is meant by positive displacement compressors such as the rotary and reciprocating types?
Positive displacement compressors increase the pressure of the refrigerant vapour by reducing the volumen in the compression chamber by mechanical means.
How is compression achieved in a centrifugal compressor?
Through a rotating impeller.
What are the three compressor types and explain how each one works?
Depending on the driving and sealing of refrigeration agianst the atmosphere there three types are:
Hermetic
Semi-hermetic
Open Type
Describe how a hermetic compressor works?
It is a completely sealed unit with the motor and other mechanical parts sealed within a steel casing.
Describe what a semi-hermetic compressor is?
in a bolted casing rather than a steel casing and is an intergral unit with motor and compressor
Describe a open type compressor?
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
