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EVACUATING A SYSTEM
By Grant Laidlaw
GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy ( ACRA ) in Edenvale . He holds a Bachelor of Business Administration and an associate degree in educational administration . He has a National Technical Diploma and completed an apprenticeship with Transnet . He has dual-trades status : refrigeration and electrical . He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018 . Currently he is the SAIRAC national treasurer .
This essential step avoids various operating issues and potential system damage .
Jabo asks : Mr Grant , you say that we should use a vac pump , but many do not use this , can you explain ?
Hi Jabo , it is extremely important that a refrigeration system is completely free of moisture and noncondensables . Recently I have been involved with the training of refrigeration technicians who are currently working in the field with more than one year ’ s experience . It is honestly quite concerning as to how many do not draw a vacuum when working on a refrigeration system .
It has been quite a while since I looked at evacuation , so having said that let us look into your question .
Let me begin by saying : A new system or a system that has been opened for repair must be completely evacuated before charging or re-charging .
The reasons for evacuation are to remove moisture and noncondensables ( such as air , nitrogen , etc ) from the system .
• Moisture in the system will combine with the refrigerant and form acids when the plant is in operation , and
• Non-condensables will cause an increase in the head pressure . The acids attack the internal parts of the system and will , in time , destroy the compressor . Hermetic or semi-hermetic type compressors will fail earlier because the acids will attack the windings of the electric motor . This will result in
a burnout , which creates products of combustion such as moisture , acid , soot , varnish and hard carbon .
Air or any other non-condensable gas in the refrigeration system will accumulate in the condenser . This reduces the condenser area available for condensing the refrigerant and hence will cause an increase in head pressure . This can cause overheating of the compressor motor , which can also result in a burnout . It will also increase the amperage drawn by the compressor and increase running costs .
Air and moisture can be left behind in the system during installation or service , or enter the system during operation through a leak in a part of the system , which is under a negative pressure .
Nitrogen , used during brazing or for leak testing must be removed before the system is charged with refrigerant . In order to ensure that the refrigeration system is leak-proof it must be leak-tested prior to evacuation and charging .
Now that we understand the issues let us move on to what level should a refrigeration system be evacuated , and the reasons for this .
The main purpose of evacuating a system is to remove the moisture and the non-condensable ( air ).
To remove all the air from a system is virtually impossible . If a system is evacuated to 6,65 kPa absolute pressure , or 96,6 kPa below standard atmospheric pressure at sea level , it is estimated that only 94 % of the air is removed from the system .
As with refrigerants , the boiling point ( saturation temperature ) of water will decrease with decreasing absolute pressure . At a pressure of 9203 microns ( abs ) or 1227 Pascal ( abs ), water will boil at 10 degrees Celsius . If the plant temperature is 20 degrees Celsius and the pressure inside the plant is 9203 microns , the water in the system will start boiling because heat flows from the plant into the water .
The bigger the temperature difference , the faster the water will boil . Thus , the lower the pressure in the system , the lower the boiling point of the water and the quicker the plant will be dehydrated .
In order to make sure that all the moisture inside the system is evaporated and removed , and that we remove as much air as possible it has been found that the system must be evacuated to an absolute pressure of 5000 microns ; this is equivalent to 667 Pascal ( abs ) or 6,67 mbar . At this pressure water will boil at ± 2 ° C . For South African conditions , where the ambient temperature is normally well in excess of 2 ° C this provides enough heat to evaporate any water that may be inside the system .
Jabo , it is very important to note that these pressures are so low they cannot be measured with the normal manifold pressure
www . hvacronline . co . za RACA Journal I April 2022 35