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A few air conditioner compressor questions.

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Arcstarter

Mon Dec 28 2009, 10:15PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

As a few of you know, i recently got an A/C compressor. I got it running, after quite awhile was spent trying to find a pinout, and coming up with the right start capacitor...

I am wondering if mine is getting hotter than what would be considered 'normal'. After about 5 minutes the pump is quite hot, and not too much longer before it is too hot to comfortably touch for any period of time. You can feel heating within the first minute, give or take.

I doubt a fan would make even the slightest impact on the heating, and there is really no other way to cool it. Unless i made a jacket around it, which pumped liquid nitrogen

Also, i am wondering about how long it should take to evacuate stuff. Say i have a 1 liter mason jar. How long should i expect it to take to evacuate that to near the full capability of the pump?

After ever few minutes of operation i add a bit of low viscosity oil into the intake. It is around the same viscosity as the pump oil found in the compressor upon arrival.

And a last question, which is not too important, is what type of pump is most often found in A/C compressors? Apparently it can be a piston, scroll, or even rotary vane pump.

Thanks for any answers!

Zum Beispiel

Mon Dec 28 2009, 10:40PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

1. A/C compressors rely on 'suction gas cooling', that is cold gas being sucked into the compressor from the evaporator. Short runs are ok, but long runs can cook it (I say can because I've seen systems that lost their refrigerant charge and had run for days. I once got a nasty burn from a big scroll comp from just briefly touching it. The paint had turned soft and the casing was probably way over 100 deg C. The compressor survived, smaller ones might not be so lucky). The motor coils may or may not have internal thermistor protection and if the windings don't burn out, it'll just seize from thermal expansion and poor lubrication.

2. Depends completely on the type of the compressor. Give me the model number and I'll see if I can find a datasheet for it. Post a picture if you can.

3. Don't expect to pull too tight a vacuum and also don't be surprised if the compressor dies. Moisture in the oil means a swift death for the little bugger.

rp181

Mon Dec 28 2009, 10:41PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

You do not need to add oil every couple of min, every week depending on use is fine.

Those do get hot, and a fan actually does help. I have seen peopleuse two 120mm fans.

Excessive heat may be caused by lack of lubrication, try putting a lot of oil, and run it. cycle that a couple of times and see what happens.

Arcstarter

Mon Dec 28 2009, 11:11PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Thanks for the quick replies!

Zb, that sounds about right. Thanks for the reassurance that my pump will be fine for at least short runs...

The model number is 39R131F. Made from Rechi Precision. A few pics can be found here:

That is not mine, but these are the same (from the same eBay seller, too) For the record, that thing that the OP calls a 'capacitor' is a thermal cut-off, and i do use it, along with the associated spring and the cap and gasket (even though it will never be outside

)

The oil i am using is made for air compressor powered tools. This is the red colored stuff. It may be far from ideal. If it'd be any better, i can use two different kinds of power steering oil, or hydraulic jack oil. I am sticking with this oil, unless for some reason one of the others are better.

I do not want any incredible vacuum. Nothing like making vacuum tubes, just gas discharge tubes and stuff like that. Sort of like plasma globes i suppose.

I will provide any more information i can if needed. Thanks alot!

Zum Beispiel

Tue Dec 29 2009, 01:06AM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Yup, that's a rotary compressor (not a rotary vane, though. See:

Didn't find any sheets in my library, but a quick look at the manufacturers site says its a 620W (electrical) compressor. Compared to a one you'd find in a fridge it's pretty damn big. 1L should go down pretty quickly.

Should be fine for things like plasma globes. See:

It seems this compressor is for R22 (a great refrigerant, too bad it's being phased out...), which is good as these use regular mineral oils and are generally more forgiving (newer ones use esther oils which are horrible for moisture absorption, among other things). I once worked on a R22-based industrial freezer that had had the compressor's oil charge replaced with 5W-40. Apparently the last person to service it thought that what's good for my car is good enough for this. It worked until it got cold outside...

Also a few more points:
1. Fan cooling won't do much good, I'm afraid. The internals will still overheat and the motor windings cook, or it'll just stall (permanently).
2. Do not over-fill the oil. At best it'll just spit it all over the place from the exhaust, at worst it'll just stall and break.

Hope this helps. Bed time for me.

rp181

Tue Dec 29 2009, 01:20AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

well I was wrong :)
what motor run capacitor did you use? I was considering getting one of those.

Steve Conner

Tue Dec 29 2009, 10:48AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I once made my own ghetto compressed air supply, using a fridge compressor and a tank from a truck air brake. I went to a truck scrapyard and got the smallest tank I could find.

The compressor did tend to spit oil from its discharge pipe along with the air, so I installed an oil trap. When the trap got full (this usually took about a week) I just poured the contents back into the intake. If you keep adding fresh oil, it'll get overfull.

The other problem I found was that the compressor couldn't start under back pressure. Once started, it could get well over 100psi, but then once the pressure switch stopped it, it couldn't start again. I guess this isn't a problem in fridges and AC systems, because the pressure goes when the compressor stops.

However, I only needed a low pressure supply, about 15-20psi if I remember right, so it worked fine. The compressor probably got cooled better than it would in a vacuum system, since it was sucking in air at room temperature.

Experimentonomen

Tue Dec 29 2009, 06:00PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

This is why real air compressors have a electronic pressure relief valve on the compressor discharge line before the check valve that stay open for a few seconds to allow the motor to get up to operating speed before the load of compressing air into the tank is applied. Atleast bigger compressors have this to limit the startup current.

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