Amount of Time for Refrigerant to Complete One Circuit

69-er on Tue December 07, 2010 3:39 PM

I have an unusual question I haven't seen addressed before. Has there been any studies to determine how long it takes a typical AC system to circulate the refrigerant for one circuit? Since one of the obvious variables in determining this would be the RPM of the compressor, I would guess about 2,000 RPM would be a good choice as a starting point.

The reason I am asking is that I am researching the idea of using a Sanden compressor to be used as an onboard air compressor. It's been done thousands of times over the years by off roaders. Yorks have been used successfully because the oil sump keeps them alive.

The biggest issue is how to lubricate a Sanden compressor. Typical solutions range from packing the bottom end with grease, pouring in oil on a regular basis, to using air tool oil injectors.

I think the air injector is the way to go. I figure if I can get the data on the time it takes for one lap around the system, I can calculate the amount of oil needed to keep a compressor alive based on the amount of oil that is present in a typical system. I can then see if an oil injector is capable of keeping up with the lubrication demands.

Any ideas or published data out there?

ice-n-tropics on Thu December 09, 2010 8:25 PM

Generally the mass flow rate varies from1 pound/min to 7 lb/min. 3lb /min for a SD508 (SD5H14) would be about average for 2000 rpm at medium/warm ambients.
A SD508 requires 4 to 6% OCR(oil circulation ratio = weight of oil divided by (weight of oil + Refrigerant). That's the circulated oil, not the crankcase and misc. oil).
The SD508 will survive longer at 100 psi max if you back off the adjusting screw about 45 degrees (and restake). The adjusting screw (behind the fixed gear) is factory set for discharge pressures up to 450 psi.
Large oil input rates would be required, but a Temp Rite oil separator could partially clean the discharge air and recycle the oil (after cooling the oil).
hotrodac

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

Edited: Thu December 09, 2010 at 8:35 PM by ice-n-tropics

bohica2xo on Thu December 09, 2010 9:01 PM

Tex:

Just an FYI, that motorhome I did years ago with the oil separator is still working fine. They called me with a question a year or so ago - road debris took out a line & the tech had no idea where to start on a recharge after repair. He said the lines were dry inside at the front of the coach. The SD709 is still rumbling along, but needed a clutch bearing at some point in it's life. I guess the oil flow is good enough to keep it happy. The 1 quart reserve of BVA 100 in the separator drained out clean, and was probably still just fine. It does cool well with no oil in the evaporator...

For 69'er, I would recommend (as you just did) an oil separator on the discharge line, and using the tank pressure to return the oil to the suction side. If you plan on running atmosphere instead of refrigerant gas, I would suggest you pick a lubricant with good corrosion protection properties. Anything suitable for use in an oil flooded rotary screw compressor should be a good choice.

You will need to run a fair bit of oil into that pump. An air line lubricator will not cut it at all. I was shooting for about 10% mass on the oil in that system.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Fri December 10, 2010 6:23 AM

Here we go again in trying to make a vacuum pump out of an old refrigerator compressor or using a MVAC compressor to compress air. Key parameters of an air compressor is the pressure you need and the air volume you require in cfm, and must have its own lubrication system. A York would be a good choice, has its own sump with even a sight gauge to check your oil reserve. Or even buy an air compressor and mounted it.

Even my Caddy has an electric air compressor that outputs 150 psi, not very great on volume, but could augment that with a 5 gallon tank for short bursts to do crazy things.

ice-n-tropics on Fri December 10, 2010 5:01 PM

A A/C separator also improves the condenser primary side heat exchange (removes insulated boundary layer and may improve Neslett and Prindell no's.)
The Sanden valves are carbon steel with no corrosion resistance.
The York is definitely first prize because the most of the oil remains in the crankcase. I still have a set of York's Berillium Copper valves to prevent valve rusting when converting a York to air. When I was testing in the York wind tunnel, the chief engineer of York pulled them out of his desk and said "These are hard to come buy. Here, you may want these some day".
I like to return the cooled separated oil directly into the compressor crankcase, which will promote compressor survival under border line lubrication. The oil plug can be drilled out and a oil return fitting added.
hotrodac

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

bohica2xo on Sat December 11, 2010 9:28 PM

Well, if you really want an underhood AIR compressor:

70 CFM underhood Oil flooded Screw Compressor

We used several of these in foam systems installed on firefighting vehicles. They are great compressors. Only problem we ever had was a third party diesel shop that thought it "ran too fast" and installed a much smaller crankshaft pulley. As supplied, they will run a jackhammer all day long from under the hood of a pickup.

But of course this is no 20 dollar solution from a junkyard.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Mon December 13, 2010 6:03 AM

150 cfm @ up to 175 psi is impressive, but can only wonder what 69er's application is? Using a jack hammer or just filling tires?

69-er on Tue May 01, 2012 5:39 PM

Been a while since I last saw this thread. My project has been put aside for a year or two, (or three!)

I'll be using the proposed compressor to operate the suspension air bag system. The electric pump I have will do the trick, it's just the noise I would like to eliminate.

To simplify the system, I might just go with the proven York compressor. However, instead of trying to fit it to the engine accessory drive system, I might experiment with using the driveshaft to power it like a rearend oil pump on a racecar. It will only be pumping while going down the road but as little as I will be using the airbags, between the York and the electric pump, (as a backup) I think it will work fine.

Larry

NickD on Wed May 02, 2012 6:52 AM

If just for level control, those 12V inflater air compressors work just fine, got one that can output as high as 150 psi, or you can pull one out of an old Cadillac. Can add a pressure switch for automatic operation.

Have fairly large air bags on my motorhome, that fill so fast don't dare use my air compressor with a pressure gauge chuck on it, finger isn't fast enough to avoid an overfill. Normally set those at 60 psi, so set my regulator at that value with a straight chuck, they reach that pressure in a fraction of a second, can hear the air flow, when it stops, quickly pull off the air chuck.

Another way to get more reserve is to buy one of these five gallon portable air tanks, the compressor and tank can be mounted anywhere you can get 12V to and the output air line.

But if you are considering making one of these jumping jack cars, have no idea why, that is when you need a lot of air and quick. There are other boards that get into stuff like this.

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