building a compressor


Old 04-20-05, 11:42 AM
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building a compressor

I bought an old compressor at a yard sale. The tank had a big rust hole in it and both copper tubes were pinched. I bought an old compressor tank at the salvage yard for $5 and sandbalsted and painted it. I've got new copper lines, pressure switch, belt, and etc. I tested the motor and it was fine. Now-I don't know anything about air pumps and need to know how to disassemble and service and test this one. Is there a manual for beginners? Also can changing pulley size increase CFM or is a different pump necessary? I wish I could give you manufacturer's name and model # but all I know is the donor compressor for the pump was from montgomery ward. How can I determine CFM once I get it running. And lastly, I would like to build a gas one after I complete this one; how is a compressor converted?

Thanks for your help!!!
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Old 04-20-05, 02:18 PM
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I made one this time last year and its been working great. I used a 7hp gasoline engine for a pump, I gutted the valves out of it and machined a custom cyldiner head for it to pump air. It was a fun project.

You sound like your on the right track. Basicly you said everything except you need a check valve going into the tank so the pump isnt under pressure while starting. And also youll need to make sure your pressure switch has an "unloader" valve (look for a 1/4" compression fitting on the switch), and of course this needs to be plumbed in between the pump and tank. Most compressor check valves have a connection for the unloader. Changing pulley sizes will change CFM but it may overload the motor or damage the pump. I probably try running it at 1000 rpm if you have no idea where to start.

I'm not sure on determining CFM. I'd like to know this myself. I did time mine once. It takes just over 30 seconds to go from 90-125psi and its a 26g tank. Anybody wanna take a stab at what CFM that is?
Old 04-21-05, 11:23 AM
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As far as a way to test cfm capacity, shops use a calibrated venturi.
I am working on another way for myself but don't have it completely figured out yet.
It is possible though to have a pretty good idea as to whether it's close by using the pump mfr's tables to get the pump's capacity and then use a tool of known consumption to see if it keeps up.

If you describe the pump, number of cylinders and any markings on it, along with the motor hp, and amperage along with the motor pulley and compressor flywheel sizes.
If you have a place to host a picture of the unit and link to it we could take a look.

As far as dismantling the pump you are better to just clean it up on the outside and just try it before you rip it apart.
Changing the pulley size to increase the rpm is the way to increase the pump's capacity, but you have to make sure the motor can handle the increased load.
Also, there is a maximum rpm that a compressor can be run at.
It would overload the pump and there is a speed at which the pump becomes inneficient.
The exact details on what your pump is capable of will be found in the mfr's tables.

In order to use a gas engine on a compressor you would eliminate the pressure switch and replace it with governor controller.
This would do two things.
When the pressure built up in the tank a valve would open, bleeding air from the discharge line of the compressor.
More importantly is a connection on the controller that goes to an air operated solenoid which when the controller is bypassing also removes pressure from a tiny air cylinder that controls the engine's throttle to slow it down.
You need this throttle control because the compressor would produce more air than the bleed on the control valve can exhaust.
These parts are sometimes tricky to find.

I built mine with a two cylinder Cambell Hausfield pump and a 5 hp Briggs and Stratten engine. I figure it's putting out around 10 cfm based on the rpm it's turning.
It is pretty handy. I'm using it this week with 150 feet of hose to blow out a/c coils I can't use water on.
Old 04-21-05, 07:01 PM
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thanks guys

lotsa good info--do you (all) feel that the manufacturer isn't as important as the number of cylinders and size of pulley and etc--because the size of pump from one manufacturer will do approximately the same as from another manu?

Also what type of shop can I find a venturi? A machine shop--and industrial machinery repair shop?

Can I use muratic acid to clean it before painting? Where is the air intake? Perhaps plugging those will allow me to clean it without damaging the innards.

There isn't a data plate of any sort on the pumps, so i don't know the manu. I don't have access to a digi cam (yet) so I can't post a pic.

How can I determine if I am pairing the pump with a motor that's too powerful?--Any rule of thumb?

Once again thanks for your help!!!
Old 04-23-05, 08:25 AM
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i have used a old Mopar a/c compressor driven by a 3.5 hp gas engine, it was OK for little thing's not enough CFM
Old 04-23-05, 08:51 AM
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1. It's the compressor displacement and rpm that determines the capacity of a pump, not who makes it.
If you have a 2 cylinder pump you will have twice the displacement as one with the same sized single cylinder.
I won't talk about two stage.

2. The equipment for determining capacity would cost way more than any air compressor you would buy for a home shop.

3. Muriatic acid will not penetrate the grease that will be in the pores of the cast and it would also leave a residue that will need to be neutralized, plus muriatic will attack the metal as well as any rust.
Use varsol to remove the oil and a wire brush to get any paint left behind.

4. Don't know where the air intake is because we don't know which head you have.

5. There is sometimes a casting number on the body of the pump.

6. Like I said before you need to figure out which head you have and then use the mfr's tables to determine motor size.

You are asking about motor sizes. Did you not get a motor on this unit?
Old 04-23-05, 03:49 PM
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yeah, I got a motor--I was just wondering if a higher HP motor would increase CFM--or if a pump is shipped with a motor that utilizes all its capacity.

If I remove the top plate of the pump will I be able to determine the 1 cylinder/2 cylinder thing?

So much like an engine displacement determines power pump displacement determines power!?

What is varsol?
Yes, I remember numbers are cast into the pump body--I'll post it when I get out there to read it. But I remember they seemed to be a 4 or 5 digit number and didn't seem like a model or serial #.

Thanks Greg and all the rest
Old 04-23-05, 03:59 PM
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Most compressor mfr's turn the pump at close to its maximum rpm and use the proper motor to do this.
Again, you would need the info on the pump to know.

If the shape of the cylinder is mostly square it will be a single cylinder, if rectangular, twice as long as wide it will be a two cylinder.

Displacement determines a comp's capacity, the same as an internal combustion engine.

Sorry about the Varsol term.
It is a trade name for solvent.

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