Support
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 .
EVACUATING A SYSTEM CONTINUED
By Grant Laidlaw
In the last Support article published in the RACA Journal , Jabo asked : Mr Grant , you say that we should use a vac pump , but many do not use this , can you explain ? This article continues as part 2 of answering this question .
Jabu , it is extremely important that a refrigeration system is completely free of moisture and non- condensables ( such as air , nitrogen , etc ). 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 . In the last issue I discussed the why . Let us move forward and look at how .
We know that evacuation is the process used to remove noncondensables and moisture from a refrigeration system . This is accomplished by the use of vacuum pumps specially designed for this purpose . A new system or a system that has been opened for repair must be completely evacuated before ( re ) charging . Prior to evacuation the system must be pressure tested to ensure that the system is leak proof .
The two methods of removing moisture and non-condensables from the system are :
• The deep vacuum method , and
• The triple evacuation method
In the “ deep vacuum method ”, reducing the pressure in the system boils off the moisture in the system which the vacuum pump then removes . In order to make sure that all the moisture is boiled off , the pressure must be reduced to well below the pressure corresponding to the saturation temperature of the water at the plant temperature ( plant room ambient temperature ).
The non-condensables ( air ) in the system are removed by reducing the pressure to about 500 microns . After the system has been evacuated to 500 microns it is allowed to stand for at least two hours - depending on the size of the system . Any moisture present in the system will boil off and increase the system pressure ( decrease the vacuum ) which you can observe on your vacuum gauge .
The system pressure will always increase to a certain extent because of pressure equalization in the system . If , however , the
pressure in the system does not rise further than 500 microns , the system may be assumed to be dry and leak proof .
As the boiling point of water at 500 microns is very low , this means that if any water is still present in the system it will boil off as long as the temperature of the plant is above this level ( which in South Africa it usually is ).
The deep vacuum method is used on systems containing only a minimum of contaminants ( for instance when a system has only been open to the atmosphere for a short period ); whereas the triple evacuation method is used on systems containing a greater amount of contaminants .
With the triple evacuation method there are three stages . Typically we only use this method for problematic systems known to have moisture . This method is effective for moisture removal as dry nitrogen contains no moisture and therefore will absorb more moisture before becoming saturated . In addition , at each stage fresh dry nitrogen is introduced .
Stage 1 The plant is evacuated to an absolute pressure of 5 000μ ; this vacuum is broken with dry nitrogen and the pressure in the plant is increased with dry nitrogen to approximately 50 kPa . This pressure is maintained for 15 minutes to disperse the nitrogen throughout the system and for the nitrogen to absorb the water vapour . The nitrogen is very dry and therefore will absorb the moisture very quickly . After the 15 minutes the nitrogen is released together with the moisture it has absorbed .
Stage 2 This stage is basically the same as stage 1 . Before connecting the vacuum pump to the system , the nitrogen must be released . The vacuum pump may only be connected once the pressure inside the plant is 0 kPa ( g ).
The plant is again evacuated to 2 000 microns and then pressurized again to 50 kPa with dry nitrogen . The nitrogen is left in the plant for 15 minutes and then released .
www . hvacronline . co . za RACA Journal I May 2022 53