Pumping down of the refrigeration plant or evacuation means removal of refrigerant, atmospheric air and moisture from the system. After the pressure testing of the refrigeration plant, the refrigeration plant must be evacuated in order to eliminate unwanted gases and moisture. Similarly, when refrigeration plant is to be shut down for dismantling, pumping down of the refrigerant to be carried out. Evacuation must be carried out on all types of refrigeration plants, regardless of the type of refrigerant with which the plant is to be charged.
Pumping Down of the Refrigeration Plant with Compressor
Before dismantling any parts of the refrigeration plant for inspection or repair, pump down must be carried out.
- Open suction and discharge stop valves on compressor.
- Close liquid stop valve after condenser or receiver so that liquid refrigerant can be collected in the tank. Any solenoid valves in the liquid line should be opened by force, adjusting the thermostat to its lowest position so that the liquid line can be bled of refrigerant. Adjust any constant-pressure valves to bring evaporator pressure down to atmospheric pressure.
- Start up the compressor. Adjust regulating system to lower suction pressure.
- Keep a close eye on the suction pressure gauge! When the suction pressure is equal to atmospheric pressure, stop the compressor and quickly shut off the discharge stop valve. Shut off any stop valve in the oil return line. If the receiver has an extra stop valve in the feed line, this can be closed; practically the entire refrigerant charge will then remain shut off in the receiver. Note: The receiver must not be overfilled! There should be a minimum gas volume of 5%.
- A slight over pressure should normally remain in the piping system – this safeguards the system from penetration of air and moisture.
- Before dismantling parts, the operator should put on a gas mask.
Please be aware of the fact that HCFC and HFC refrigerants mix only minimally with water, and it is therefore necessary to effect evacuation of such systems with particular care.
The boiling point of a fluid is defined as the temperature at which the steam pressure equals atmospheric pressure. For water, the boiling point is 100°C. Lowering of the pressure also lowers the boiling point of the water.
The table below shows the boiling point of water at very low pressures:
Pumping Down of the Refrigeration Plant with Vacuum Pump
For pumping down of the refrigeration plant or evacuation, use a vacuum pump, which bleeds the plant of air and steam. The vacuum pump must be able to lower the pressure to approx. 0.1 mm Hg (mercury column) and must be fitted with a gas ballast valve. This valve should be used wherever possible to prevent aqueous vapours from condensing in the vacuum pump.
For a satisfactorily performed evacuation, the final pressure must be lower than 5 mm Hg. Attention is drawn to the fact that there may be a risk of any water left in the refrigeration plant freezing if the ambient temperatures are lower than 10°C. In such instances, it will be necessary to supply heat to the component surroundings, as ice evaporates with difficulty.
It is recommended to carry out evacuation as follows:
- Evacuate to a pressure lower than 5 mm Hg.
- Blow dry air or nitrogen into the system to a pressure corresponding to atmospheric pressure. Never use OXYGEN cylinders.
- Repeat evacuation to reduce pressure to less than 5 mm Hg.
- Shut the vacuum pump off from the refrigeration plant and check that the pressure does not rise for the next couple of hours. If the system still contains water, this will evaporate and cause the pressure to rise, thereby indicating unsatisfactory evacuation and necessitating a repetition of the procedure.
REFERENCE: Instruction Manual for SABROE Reciprocating Compressors SBO 21, 22, 41, 42, 43