A/C compressor for on-board compressed air

KyAllroad wrote: Wouldn't a purpose built onboard air compressor be easiest? My 2004 Expedition had a unit in the back somewhere, I never messed with it but it seems easy button would be to find one of those in a junkyard.

electric air compressors are so-so for stock air bags that don't have to fill quickly. Even the best of the best electrics can only muster about half the cfm of even a small belt-driven pump, and they require massive amperage. They also tend to make a very large amount of noise.

If I were just doing small air-assist bags, I might consider electric for a brief drunken moment, but this will be HD bags, operate things like impact wrenches, need to inflate 32" tires... way beyond the realm of a small electric pump.

Trying to air up a 32" tire with a stock Expy pump might take over an hour. Those pumps are also not designed for long duty cycles and would likely not last more than the first time filling a tire... or possibly just fry the alternator in the process.

The setup I'm planning should fill a tire in about 4 minutes after three or four cycles of a pump that is designed to operate nearly constantly for thousands of hours. Its really a massive difference. Its like trying to carve Mount Rushmore with a pocket knife. 4" into carving you realize you've worn the blade down to a nub and you now not only still have a flat tire, but deflated air bags, a melted compressor, and a smoked alternator as well.

For my money, I will never use another electric compressor for OBA. Ever.

java230 wrote: I did this on my old toyota. However I dont think you need to worry about high pressure so much, mine put out a ton of volume. It was a stock 85 pickup.

I agree with the volume/pressure thing. My main reason for going higher pressure is 2-fold: first, I have a tank that will hold 800+ psi, and an A/C compressor is capable of making 300 psi. The only reason I can see for not going higher pressure is compressor reliability. When it comes to compressed air, more is better. In this situation I have the opportunity to double the mass of air without needing more space.

So my end product will have to be engineered to make the most of things. If my research determines that compressor fatigue spikes significantly after 280 psi it might be smart to back it down to 250. If I find out that SCFM doubles below 150 psi, then it might be in my best interest to suck it up and get a larger tank and drop the psi. Does that make sense?

My existing knowledge base in my head from years of Automotive HVAC just knows that most compressors see 275 psi for hours at a time in the summer and last 20 years or more, so I can imagine that higher pressure is a logical choice in this case

If you are running that much pressure make sure you put a regulator on the tank outlet. Your hoses and air tools aren't going to like that much pressure.

I know you said no electric pumps, but the duty cycle and CFM of Land Rover electric pumps must be reasonably high. Those things lift the rear of an overweight Brit at every stoplight for years on end without dying.

You can probably lay hands on a working sanden at a junk yard for not much cash. Even if you burnt through one every year or two you'd get 10-20 years worth of junkyard sandens for the cost of the ready modded unit from Australia.

You strike me as a practical enough kind of guy that you'd find your way thru grease packing a sanden without much drama, if you fry it in a few months then at least you have a jumping off point and can look at revising the system from there.

I think your 275 psi is not worth pursuing, for two reasons. First, I think compressor efficiency will be compromised building the extra 100 psi. Second, I bet you will run into pretty bad overheating problems. With freon, its receiving a refrigerated gas under pressure. Air is coming in warm, at atmospheric pressure. That must make a pretty big difference to cooling.

In reply to Streetwiseguy:

A wise observation. Maybe some R&D in the form of using a manual switch and a laser thermometer to figure out a good max PSI before I set final pressure.

The other thing to think about is that A/C systems displace heat. It will take some maths, but if you think about it, the system removes heat from one spot and puts it somewhere else. It doesn't "generate cold." The net heat seen by the compressor in my application may not be much different from a closed freon system. There is an evaporator that absorbs heat and a condenser that sheds it. Meanwhile the compressor makes heat. In my case I'm basically removing the condenser and evaporator, so it depends on how much more heat a typical condenser sheds compared to how much heat a typical evaporator absorbs.

I'm basically making a household shop compressor, but using a compressor unit that is capable of a wider range of RPMs. So theoretically I shouldn't generate much more heat than a Campbell Hausfeld.

But your observation is wise and has convinced me to test it to be sure.

gearheadmb wrote: If you are running that much pressure make sure you put a regulator on the tank outlet. Your hoses and air tools aren't going to like that much pressure.

Already in the worx :) Probably a fixed regulator in the 110-125 psi range depending on air bag requirements.

How do you plan on draining the tank?

The talk of heat is true, but it will condense on the cold tank walls quickly.

Maybe your over thinking this... If you want a high pressure small volume tank, why not build a diy power tank or just use a cow tank. A cheap small backup compressor for when it goes empty. Just a thought. I used a cow tank for years, it was great, my only complaint is tires went flat faster in cold weather with straight co2

java230 wrote: How do you plan on draining the tank? The talk of heat is true, but it will condense on the cold tank walls quickly.

typical pressure relief valve like the one pictured below in an appropriate pressure. Installed at the lowest point in the tank. Give it a yank, let the flow hold it open for a while and it self-resets. I did this for the drain on my shop tank because it was up on a shelf. Yank a string, walk away.

Except in the case of my shop compressor, you first got a shower of rusty water. If you lingered too long you would get shards of ice shot at your face. Totally safe.

The only shortcoming I see to this plan is that there is a modest probability of debris causing the blowoff valve to continually leak. If that is the case, I can do an adequately rated ball valve or draincock and put the blowoff valve somewhere else.

java230 wrote: Maybe your over thinking this... If you want a high pressure small volume tank, why not build a diy power tank or just use a cow tank. A cheap small backup compressor for when it goes empty. Just a thought. I used a cow tank for years, it was great, my only complaint is tires went flat faster in cold weather with straight co2

You know me... I always overthink things. :)

I do have a tank and compressor. Useless. I want bulletproof and overkill. Not a fan of CO2 for obvious reasons, not the least of which is its cost.

Just to add to the party, modern A/C compressors are designed in such a way that they trap the oil before it goes out to the condenser, because modern condensers have tubes so small inside that oil fog can clog them or otherwise severely reduce the efficiency.

Some of the newer units have passages a tenth this size. (And think about this when you see an OE condensor is $500 and a RockAuto chinese knockoff is $80... think there's a difference? It's also why you have to throw away the condenser if you've had a compressor failure, they can't be flushed out. They do at least do a good job of filtering the debris)

So there may be no real problem with using a modern compressor instead of one of those huge old Yorks, the oil should mostly stay in the compressor. As a bonus, they tend to have safety valves way over 400psi.