RACA Journal May 2022 | Page 57

Support systems and larger for bigger systems ). The advantage of using copper piping is that it has a much lower resistance .

The correct procedure to evacuate a system is as follows :

• Check that all equipment is suitable and safe to use .

• Check oil with regards to level and clarity .

• Test your vacuum pump with a micron meter attached to the suction port of the vacuum pump . Take note of the vacuum attained . The pump should achieve a vacuum of 500 microns or better .

• Connect the vacuum pump to the system .

• Start the vacuum pump , slowly open the valve on the vacuum pump and on the manifold with two full turns .

• Operate the vacuum pump until at least 500 microns of vacuum is obtained . Close the manifold valves and stop the vacuum pump . ( NB : in this order ). Observe the vacuum gauge for a few minutes . A rise in pressure would indicate a possible leak or moisture remaining in the system . ( Leak test , repair leak if needed and evaluate again ).

• When a vacuum is obtained , close all valves , stop the vacuum pump and leave it to stand for at least two hours ( during this period watch the vacuum gauge if the pressure increases rapidly then there is a leak that must be found and repaired ).

• If the system does hold its vacuum , remove the yellow hose from the vacuum pump and connect it to the charging cylinder .

• If the pressure increases rapidly the system has a big leak ; if the system pressure increases slowly the system has a small leak .

Finally , it is very important to remove the vacuum pump and instrumentation from the system without loss of vacuum , I have seen technicians draw a vacuum only to allow air into the system when disconnecting the vacuum pump .

Remember : close your manifold valves before turning off the vacuum pump . In addition , all the valves on the system must be closed before any of the hoses are disconnected .

If the hoses are connected to the compressor service valves these valves must be back seated before the hoses are removed .

If the system is to be charged immediately the vacuum can be broken with the correct refrigerant . If not , all valves must be tightly shut and capped .

Jabu , thank you for the question , I hope that this helps you moving forward . This is a very important technical aspect in the refrigeration and air conditioning industry that is often ignored .

Jabu , now how do you evaluate the system vacuum ?

Just obtaining a vacuum of 500 microns does not mean that the system is dry and airtight . If there is still water present in the system this will evaporate and increase the pressure to the vapour pressure of the water at the plant temperature . If there is a small leak in the system , the pressure will rise and eventually come up to atmospheric pressure ( 0 kPa g ). As I have said before , the above reason is why the system must be left standing for at least two hours or longer if possible .

The following is the correct procedure to evaluate the system vacuum :

• After the system has been standing for a minimum of two hours observe the vacuum gauge .

• If the system pressure stabilises at or below a pressure of 500 microns the system may be assumed to be dry and leak proof .

• If the system pressure stabilises at the vapour pressure of water at the system temperature ( this will be between 500 microns atmospheric pressure ) the system still contains moisture but is leak proof .

• If the system pressure increases to above the vapour pressure of water at the system temperature and continues to lose vacuum to atmospheric pressure the system has a leak , which must be repaired before charging the system with refrigerant .

Thank you for all your questions . Send your problems ( and sometimes your creative solutions ) to acra @ netactive . co . za with ‘ Solutions Page ’ in the subject line . You may include pictures . RACA

A four-hose manifold with large evacuation hoses .