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Compressor RPM question

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Zira

02-02-2002 06:01:26




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Well I started working on converting this Speedaire compressor from electric motor to gas motor, and I need to get the proper sized pulley for the gas motor. I looked at the old posts here, it looks like the compressor should run at 800 - 1000 rpm. Looking in the Grainger catalog, it looks like 1000 or 1100 rpm so that makes sense. BUT, when I looked at the original electric motor on the compressor, it ran at 1725 rpm & drove a 2" pulley. The compressor has a 10" pulley/flywheel. My admittedly weak arithmatic says that the compressor must have only turned at 345 rpm (1725/5=345). What am I missing here?

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Chicken George

02-03-2002 05:08:47




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 Re: Compressor RPM question in reply to Zira, 02-02-2002 06:01:26  
Zira, I don't know how big a motor we are talking about,But I have a set up that I have been using for 31 years. It consists of a 10 hp.gas motor and it runs two compressers, one is a refrigerator compressor out of a big cooler and the other is a air compressor out of a truck (air brakes) I think, anyway that is what I could find back then. I have a 3"-4 groove pulley on the motor and it runs the origional pulleys on the compressors. I run the motor at a little above Idle may be 1800 or so and If I need more volume, for what ever,I speed the motor up. I personally don't like a motor screaming at 3600 rpm's.If you get a motor a little bigger than you need,you can run it at a slower speed more efficiently.I have worn out two sets of rings and one piston on that motor in a zillion hours of operation I personally don't think rpms are all that critical,
at least I have not had a problem.The motor is an old Tecumseh CAST IRON,#HH-100 with roller bearings on the crank. Luck with your project!!

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Neil

02-02-2002 18:44:12




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 Re: Compressor RPM question in reply to Zira, 02-02-2002 06:01:26  
Sounds like someone replaced the pully or the motro at some time in the past.
From what I understand if you get a splash lubed compressor running much under 500 or 400 RPM, you will not get enough oil splashing around.
When measuring pully sizes, you have to put a belt on and compare the diameter of the midline of the belt. Actually the diameter at the top of the belt is close enough for most things.
The pully on the pump probably has fan blades in it and is weighter to act as a flywheel, so don't replace it. If you can find the exact type of pump in the grainger catalog, then it will give you the recommended speed range for various HP engines, and sometimes even the correct pully to use. It may also tell you if it was designed for intermitant or continious opperation. If it is not rated for continuous opperation, it probably should not be used with a gas engine.

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george md

02-02-2002 11:32:29




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 Re: Compressor RPM question in reply to Zira, 02-02-2002 06:01:26  
zira,

does that compressor have an unloader built in,if not you will have to fabricate one. I have used an air operated ball valve , controlled be an air governor from truck air brake system. put tee in line from comp to tank and let let ball
valve dump air when set press is reached.it will
close when governor closes.

george



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Junkman

02-02-2002 08:47:17




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 Re: Compressor RPM question in reply to Zira, 02-02-2002 06:01:26  
Zira, I did some calculating for you. Supposing your engine runs at 3600 RPM and you want your pump to run at 1000 RPM. You need the engine pulley to turn 3.6 times for 1 revolution of the pump(engine RPM divided by pump RPM). Next you need to find out pulley circumferences(distance around the pulley). A 3" pulley is 9.42" around, a 3.5" pulley is 10.99" around, and a 4" pulley is 12.56" around. Next we need to find a pair of pulleys that come close to the bigger one being 3.6 times bigger than the small one(remember we figured this first). I figured a 12" pulley on the pump, With a 3" engine pulley you get 4 engine RPM's to 1 pump RPM yielding 900 RPM's on the pump. With a 4" pulley you get 3:1 giving you a pump speed of 1200 RPM. With a 3.5" pulley you get 3.42:1 which gives you 1053 RPM's on the pump. Forgive me if I sound like I am talking down to you. I try to explain in detailto everyone. Not knowing how much of this you know, it is easier to just explain it all at once. If you need any more help, feel free to E-mail me.
John

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G Taylor

02-02-2002 08:27:41




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 Re: Compressor RPM question in reply to Zira, 02-02-2002 06:01:26  
Your are correct on the calculations. The gas engine will have to output approx three times more power to run the compressor the new higher speed. Don't forget a belt guard & exhaust shield. No use getting sued if the visiting neighbour kid pokes thier fingers in somewhere.



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Don Young

02-02-2002 21:22:47




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 Re: Re: Compressor RPM question in reply to G Taylor , 02-02-2002 08:27:41  
I think the above post has a good point sometimes overlooked. The compressor was originally limited in output by the power available from the motor, as is practically always the case. If your motor or engine is smaller than the maximum rated driving horsepower for the compressor, it may not be possible to reach the desired PSI with the ratio set for maximum rated compressor speed.



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Rory

02-02-2002 06:29:28




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 Re: Compressor RPM question in reply to Zira, 02-02-2002 06:01:26  
It looks correct to me. It also looks like you will need 20" pully on the compressor if you plan on running your gas engine at 3600 rpm assuming you can get a 2" pulley for the engine.



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