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Vac pump substitute?

Qwkretrofit on Tue July 05, 2011 1:54 PM User is offline

Stupid question here...

I'm a A/C repair novice (to put it mildly). A friend's Dad (old school hot rodder) made a vac pump for me to use out of an old fridge compressor. Gave it to me in exchange for some wiring I did on a retrofit engine swap. I used the pump to vac down my system and it seemed to pull to 29.5 fairly quickly. What's the general consensus on continued use? Viable? Or should I buy a purpose built, new unit? As to expected use, I have a few cars I am doing Vintage Air installs over the next year or so, then just maintenance after that.....

JJM on Wed July 06, 2011 12:22 AM User is offline

We've heard of some folks using old refrigerator compressors as vacuum pumps, better than nothing, but best bet is a vacuum pump, preferably a two stage. The site sponsor a wide variety of pumps very competitively priced.

Just curious, if you have access to a micron gauge, next service you do hook it up and see what the old compressor pulls down. If less than 750, probably good to stay with what you've got!


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bromodragonfly on Wed July 06, 2011 12:39 AM User is offlineView users profile

Hermetic compressors aren't really designed to operate within a vacuum, but they can and will, if you force them to.

You will not get as good of a vacuum as with a 2 stage, oil sealed pump. I've used hermetics as recovery units, by attaching them to a condenser and pumping the liquid into a cylinder. They will pump well, but I find that they don't pull enough vacuum to satisfy my standards when it comes to getting the most moisture out of a system as possible.

You really do need a micron gauge to judge what kind of vacuum you're pulling. Gauges that show -29.5"hg could be reading anything around there. 10,000 microns or 500.

Also keep in mind, electricity will arc within a vacuum, you could get a shock or blow a fuse/breaker if the windings decide to arc to the metal hermetic case. Also, some hermetics rely on cold suction vapor for cooling the internals, which you will not get if you're pulling vacuum. So if you're gonna vacuum and walk away for a couple hours, you may just burn the thing out.

But, as JJM said, some vacuum is better than no vacuum If you're planning on doing prolonged AC work on a number of vehicles, I'd definitely invest in a vacuum pump. It is worth it to pay a few more bucks to get a dual stage pump. CFM isn't critical unless you want to evacuate a system as quickly as possible, higher is obviously more convenient.

And lastly, just remember to change your oil frequently, and use only vacuum pump oil, since it has much lower viscosity and vapor pressure than other lubricant oils. I change almost every time I use the pump, especially if it's been sitting for awhile between uses.

I looked at the sponsor site, and the Mastercool brand is quite decent. The same brand is sold at the local refrigeration wholesalers around here. For those not willing to shell out a thousand bucks for a Ritchie Yellow Jacket, it's a good alternative.

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