Refrigeration

Question 1

1. With reference to a refrigeration system:
   a) Describe how air is removed from a system. (8)
   b) Describe how and where refrigerant gas is added to a system. (8)

Answer 1

a) Pump the system down until the condenser is full.
Using an approved service vessel and pump, the refrigerant may be extracted. Re-charge with fresh refrigerant.
Fill out the log book for the system recording type of gas and quantities.
b) Attach the filling line to the suction side of the compressor tightly.
Invert the refrigerant canister and start the compressor.
Observe the condenser sight-glass for the correct level.
Log how much and type of gas added.

Question 2

2. With reference to a refrigeration system, state the indications of EACH of the following faults: a) air in the system; (8)
   b) undercharge of refrigerant gas. (8)

Answer 2

a) Bubbles seen within condenser sight glass  
Compressor knocking  
Icing of pipe-work  
Fluctuating pressures  
b) Compressor Over-running  
In-effective cooling  
No liquid visible within condenser sight-glass  
Low suction pressure

Question 3

3. a) Sketch a simple refrigeration system showing the FOUR major components. (8) b) State the condition of the refrigerant between EACH component in the sketch of Q2(a). (8)

Answer 3

b) Evaporator - Compressor Superheated vapour.
Compressor - Condenser Superheated vapour.
Condenser - Thermostatic Expansion Valve Sub cooled liquid.
Thermostatic Expansion Valve - Evaporator Liquid & vapour mix.

Question 4

4. a) Describe the operation of a domestic refrigeration plant, referencing the refrigerant condition at the main components. (12)
   b) State how liquid is prevented from returning to the compressor. (4)

Answer 4

a) The gas in the system is compressed in the compressor, raising its temperature and pressure to a superheated vapour. The vapour is then led to the condenser where its cooled by seawater and changes to a sub cooled liquid, due to it remaining at high pressure at this stage. A thermostatic expansion valve then dramatically drops the pressure of the sub cooled liquid which causes the saturation temperature to fall. As it crossed the valve the saturation temperature falls below its actual temperature causing some liquid to flash off before reaching the evaporator where latent heat in the surrounding are boils off remaining liquid meaning a superheated gas leaves ready to re-enter the compressor.
John Willison IAMI Revision 39
b) To stop liquid returning to the compressor, a solenoid is connected to a thermostat inside the evaporator. As the room is cooled to the pre-determined temperature, the solenoid shuts off the expansion valve, stopping the flow of refrigerant in the system. The compressor will continue to run briefly but will be stopped once drop in pressure.