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Almost all compressor bulding projects end up as expensive, or more expensive than buying a new one in the first place. Inevitably, if you do manage to come in much lower than the price of a new unit, you are usually left with a dog's breakfast of marginal tank, marginal motor, usually running at the wrong speed and a nearly worn out pump that is almost always passing oil. But, if you really want to tackle this you will need 1) A real air compressor, since refrigerant compressors almost always are designed to pump oil with the refrigerant. Some don't even have rings or ring grooves in the pistons. 2) A motor of adequate horsepower. Preferably one with start and run capacitors (single phase for home use). You will have to figure out the requirements of the pump you have. Anything over 5hp will require you to slow the pump down to run it on household current with a 3hp motor or less. If you have to go slower than 500 RPM you will probably not have adequate lubrication for a splash-lube pump. IF you do not have the HP requirements of the pump at hand, you can measure the displacement of all 1st stage cylinders and calculate 1hp for every 10 cubic inches of 1st stage displacement. This will require you to pull the head of the pump and measure bore and stroke, so figure on making some new head gaskets at the very least. Don't be surprised to find scored cylinders or broken valve springs on a pump with a lot of hours on it. 3) An ADEQUATE tank. Water tanks are a ticking time bomb when used as air tanks. Don't do it. You can only judge the condition of a tank by unscrewing one of the large caps and looking inside with a light or a candle on a stick. Don't be surprised to find a lot of rust or oil. A guy I know had his tank erupt into flames on the inside because there was so much oil and debris in there. 4) A pressure switch. Used switches are rarely any good. Why else would they have been removed from service. Usually you have to buy one with an unloader valve on it, to blow down the pump after each cycle. 5) A safety relief valve, for when it all goes wrong. Preferably one with a large CFM rating to relieve pressure as quickly as possible. 6) New belts 7) New pulley and sheave for the motor, which are sized according to the formula above. You have to figure out the balance of volume/pressure you want to achieve. The compressor should be set to cut out at maximum amp draw on the motor. If this occurs at say, 90 psi and you want to go to a higher pressure than that, you will have to put a smaller pulley on the motor to reach the higher pressure while not exceeding the max amp draw of the motor. i.e. slowing the pump down 8) New air filter 9) Drain valve for the tank to release the condensate from the tank 10) A valve to shut off the air to your shop, preferably a ball valve that opens and closes in 90 degrees of travel with the valve handle. 11) Belt guard, which you can fabricate yourself. 12) Proper electrical hookup, which will require the services of an electrician. I have seen some pretty scary electrical installations, including a guy who hooked up a 3 phase motor using two hot leads from a 220V breaker and 1 from a 110V breaker. For the life of me I don't know how some people get away with doing things like that. 13) A check valve to isolate the pump from tank pressure after cut out and to seal the air inside the tank 14) A discharge tube to bring the air from the pump into the tank. This will probably be made up of some 3/4 copper tubing and some compression fittings. I don't want to be too pessimistic. By all means go for it if you think you can do it cheaply and safely. My own friend just found out the hard way that rolling your own compressor is not always a good idea. He has a marginal motor, a rebuilt aluminum pump, a new tank, to replace the $50 one that ended up being full of pin holes (after he had it powder coated!), a pre fab belt guard and still has to put in a couple of hours labour to put it all together. After it is all said and done, I think he saved $50 over the price of a new unit with all new components and a warranty.
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