Section 4 - Refrigeration and AC Plant Flashcards
List certain properties that refrigerant should include?
- Non Toxic
- Non Flammable
- Non Explosive
- Non Corrosive
- Leaks are easy to detect
- Operate under low pressure
- Chemically stable
- Don’t react with oil in the system
- Low boiling temp
- Low condensation temperature point, to allow sea water to be coolant
- High enthalpy of evaporation to produce good cooling effect per KG of vapour pumped.
- Low vapour volume per KG. therefore reducing compressor size
- Pressure difference between evaporating pressure and condensing pressure should be as little as possible to increase pumping efficiency.
- Relatively low cost
- Available worldwide
What is the terminology used for refrigerant?
Refrigerant 22
R22
What is the Montreal Protocol?
It is an international agreement that was founded after the effects of CFC and HCFC showed the damage to the ozone layer and restrictions were put in place.
What are the 3 values calculated for refrigerant environment rating?
GWP - The Global Warming Potential
TEWI - The Total Equivalent Warming Impact
ODP - The Ozone Depletion Potential
What are the 3 types of blended refrigerant?
- Azeotrope
- Near Azeotrope
- Zeotrope/ Non Azeotrope
Describe the 4 main components of a refrigeration system?
The Compressor
The compressor compresses the refrigerant as a gas, adding pressure to give a force to move it around the system. The compressor is commonly a reciprocating type, driven by an electric motor.
The Condenser
The condenser changes the refrigerant gas from a vapour into a liquid by transferring the enthalpy of condensation from the refrigerant to the cooling water. Most commonly found to be a shell and tube heat exchanger.
The Expansion Valve
The expansion valve lowers the liquid refrigerant temperature and pressure by controlling the flow of refrigerant.
The Evaporator
A series of heat exchanger grids inside the refrigeration chamber. The evaporator transfers the enthalpy of evaporation from the contents of the chamber to the refrigerant. In doing this the refrigerant changes state from a liquid to a vapour (boiling) then to a superheated gas. After leaving the evaporator the refrigerant returns to the compressor.
What are the 3 phases/ states of refrigerant?
- As a Liquid
- As a Vapour (or saturated gas)
- As a Gas ( or superheated gas)
Explain a Pressure- Enthalpy Diagram with an aid of a sketch?
To the left of the left-hand curve the refrigerant will exist as a liquid. Under the curve the refrigerant will exist as a vapour. To the right of the right-hand curve the refrigerant will exist as a gas.
- From 1 to 2 Evaporator process
- From 2 to 3 Compressor process
- From 3 to 4 Condenser process
- From 4 to 1 Expansion valve process
(2>3) represents the amount of superheat.
(4>1) represents the amount of sub-cooling.
Explain Superheat?
Superheat is then any temperature of a gas above the boiling point of that liquid.
For example, Water boiled at 100 degrees creates steam. if the steam is heated to 101 degrees then it has been superheated by 1 degree.
Explain Saturation?
The point where a change of state in a substance is taking place.
Explain Subcooling?
Subcooling is any temperature of a liquid or solid below its saturation temperature.
Example, Instead of boiling our 100 degree water by adding heat, we shall remove heat from the 100 degree water. As heat is removed from the liquid water it‘s temperature will drop below its boiling (saturation) temperature. Water at 99 degrees has been subcooled by one degree C.
Explain the difference between a Sensible Heat and Latent Heat?
Sensible heat is heat that can be measured by a thermometer. Anytime heat is added or removed from a substance and a temperature change occurs.
Latenheat is when a gas changed to liquid at a constant temperature as a thermometer cannot detect it.
What is the Bubble Point?
This is the point at which the first vapour is produced from the liquid refrigerant on changing phase.
What is the Dew Point?
The point at which the first liquid is produced in changing phase from a vapour to a liquid.
List the main protection devices on a Refrigeration plant?
- Low Pressure (LP) Control Switch
- High Pressure (HP) Cut Off
- Low oil pressure cut off
- Compressor lifting cylinder head
- Unloading gear
- Cylinder head relief valve/ bursting disk
- Oil separator
- Drier unit
Explain how the LP Control Switch works on a refrigeration plant?
It’s located on the suction side of the compressor. When the flow of refrigerant stops then the compressor will stop. Once flow returns then it will start the compressor. It uses the refrigerant gas pressure to operate.
Explain the HP cut out switch and the possible causes of over pressure on a refrigeration plant?
The HP switch is fitted on the discharge side of the compressor. This will stop the compressor in the event of over pressure and can only be rest manually.
Possible causes of over pressure are;
- Loss of condenser cooling water
- Air in the system
- Refrigerant overcharge
- Shut valve
What are the 5 good properties of a refrigerant oil?
- Low wax content
- Low pour point
- Low viscosity
- Good thermal stability
- Good chemical stability
How is it possible to get air in the refrigerant system and what effect does it have?
- From its initially charge
- Leaks in the system working at or below atmospheric pressure.
- When charging the system
It will cause the compressor to overheat and then the temperature/ pressure inside the condenser will be too high.
Explain how we can insure there is no air in the condenser when recharging the refrigeration plant?
The method is known as purging. By shutting the condenser valves and filling. Then after waiting the air will rise due to being lighter. We can then open a fitting on the top and monitor with a refrigerant gas detector. Pressure should never be more than the HP cut off switch.
Why is water in a refrigeration plant bad?
Water or moister will get to the expansion valve and then freeze. Usually this is an affect of air being in the system
What are the two basic methods for recovering refrigerant?
- Vacuum pump extraction
- Pumping out using systems compressor
What is the difference between Absolute, Relative and Specific Humidity?
Absolute Humidity is the actual mass of water vapour present in a mixture of air and water expressed in KG.
Relative Humidity is the amount of moisture in a sample of air compared to that of saturated air at the same temp.
Specific Humidity is ratio of water vapour present in moist air to that of dry air. Expressed in KG/ KG Dry Air.
What is the comfort zone in regards to a Psychrometric Chart?
Wet bulb temps of 18 - 23 degrees
Dry bulb temps of 20 - 26 degrees
Relative Humidity between 30 - 70%
