why do you need to evacuate a auto a/c unit?

Answer 1

there are 2 basic reasons to have system evacuated prior to charging.
1: air does not compress and expand at the same rate as freon. the difference will cause high pressure on both the low side and high side. this increase in pressure will cause premature system wear as well as reduced cooling efficiency.

2:when the system gets air in it, the air contains some amount of moisture. when moisture mixes with freon it becomes corrosive which will cause premature system failure also.

also when the system is evacuated it will draw in an initial amount of freon easier.

Answer 2

I'll go with Sundew's answer. It is a federal regulation now that you have to have the system evacuated to prevent releasing the freon into the air. Also you need to have the air out for the reasons sundew listed. The way you get cooling, is by the expansion of the freon gas, with air in the system, it just messes everything up for pressures and it won't cool properly

Answer 3

The moisture is true but not for all systems. If the system uses ester oil and 134a moisture isn't a problem...with r-12 and/or mineral oil(as in a converted system) moisture causes chlorine and acid to be created and it slowly eats away at your a/c system.

As for what the air does it's already been explained.

Answer 4

for several reasons it can be to perform repairs or replacements on the a/c system or if the a/c lines or units are in the way of some major repairs on the engine compartment / dashboard, or to retrofit to the R-134 refrigerant.... etc.

Answer 5

i own a shop and the reason we do this is to find out if it will hold pressure,and also if it leaks down then we wont re-fill it until we find the leak in it,because charging the system back up ,with a leak would just be a waste of your money,and most shops don't want to have to do this job twice,they would rather do it once,and do it right,good luck with it hope this helps.