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Compressor calculations

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Gene Davis

02-15-2001 20:13:55

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HELP!!!! I need some of you air compressor savvy people out there to help me to maximize my compressor! It is an old, but very good condition, Curtis brand 2cylinder, 2stage, 3"x1-5/8" x 3"unit has 3/4" inlet, with an oil bath air cleaner, 1/2" outlet, with an intercooler between the high and low pressure cylinders. Mounted on 60 gallon tank(much newer than the compressor pump!) has input check valve into tank and is controlled by pressure switch. Had an unloader once upon a time but doesn't seem to need one now, always starts right up with the 1-1/2 hp 240v single phase motor. The motor is a 1-1/2 h.p. 1750 rpm capacitor start, farm duty w/ 3.5" pulley, the compressor has a 13.5" pulley at the bottom of the groove, (4"o.d. and 14" o.d. at the top of the 1/2" belt groove) and or as the proper name says sheaves, (pulleys to us red necks here in Ga.). O.K.--heres where I need the gurus in compressor savvy to help me! I don't think that this unit is doing all it is capable of. My calculations come out to about 450 +/-

rpms please correct my figures where necessary. I saw somewhere a formula for calculating the h.p. needed for the displacment,etc. what is the c.f.m. now? And what is the safe and most efficent output for this unit?What h.p. at preferably 1750 rpms? Not wanting to sand blast or anything like that just need to see if I can safely produce a little more volume, pressure is usually 90-100 p.s.i. to run air wrench or paint with. Would like to think in terms of 3-5 h.p.230v. capacitor start 1ph. I would greatly appreciate some wisdom on the subject.***Thanks!*** LGD

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.Gene Davis

02-17-2001 20:19:34

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Re: Compressor calculations in reply to Gene Davis, 02-15-2001 20:13:55

Thanks for all the help people! Displacement comes to 22.2 cid output at present speed which calculates to about 6.o6 cfm @ 492 rpm w/ 1-1/2 hp motor. Looks like to get most efficent output i will need a 3 h.p. motor w/ 6-7 drive sheave should give approx. 10-11 cfm @850-900 rpms, also will need an amp meter. Thanks again for the advice!*****lgd

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T_Bone

02-16-2001 19:49:26

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Re: Compressor calculations in reply to Gene Davis, 02-15-2001 20:13:55

Hi Gene, The book your wanting for the correct calculations is Modern Refrigeration by Goodheart Wilcox available at WW Grainger.

Sorry but I just have the time for 2hrs of math right now.

T_Bone

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ironhead

02-16-2001 18:47:25

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Re: Compressor calculations in reply to Gene Davis, 02-15-2001 20:13:55

I recently emailed curtis with basicly the same questions. I gave them the serial# from an old pump and they emailed back the rpm and hp requirments and rated cfm. I dont have the address now but you can just type in curtis compessor on a search and it will show up.

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S.AMMER

02-16-2001 16:36:19

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Re: Compressor calculations in reply to Gene Davis, 02-15-2001 20:13:55

About 6 or 8 years ago Air Supply Co. of Rockford MI. was a Curtiss-Toledo air compressor dealer, they may be able to help you if you can describe the unit you have. IF not maybe the address for the manufacturer. We have a Curtiss 10 hp 2 stage at work, good old unit. Hope this helps, sorry I don't have the phone number.

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Gene Davis

02-16-2001 06:16:48

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Re: Compressor calculations in reply to Gene Davis, 02-15-2001 20:13:55

This is good advice, now help with this! From the archives,"RE:NEED OLD KELLOGG COMPRESSOR HELP" dated 11-9-00 posted by ROGER PROSPER; "displacement of a cylinder is found by: (Pi/4)x diameter squared x stroke." I am confused as to where the {Pi/4} part of the equation comes from and how to calculate the displacment of a 2cyl, 2 stage 3x1-5/8-x3 pump where do I get a value for the Pi figure of this formula? What is Pi/4 is it kin to the Pi-r square used in circle calculations? HELP!!! Thanks!***** LGD

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Al English

02-16-2001 09:11:01

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Re: Re: Compressor calculations in reply to Gene Davis, 02-16-2001 06:16:48

Gene, I was unable to bring up the post you refered to. Pi is 3.14(more or less). Pi/4 is the same as: 3.14 divided by 4, which is also the same as Pi x r squared or 3.14 x (radius of bore x radius of bore). Unless I misunderstood what you are trying to do, I already took the time explain why your approach is not going to give you the results you desire. And the same applies to the information Bus Driver was nice enough to provide on motor sizing. While it would be possible to solve this all on paper, much more information would be needed than any of us is going to have, and it would involve a LOT more math than you're working with now...Al English

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Al English

02-16-2001 03:23:26

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Re: Compressor calculations in reply to Gene Davis, 02-15-2001 20:13:55

Hi Gene, Yes, you can make a calculation as to the theoretical pump speed and motor horsepower needed to produce a given output, but that doesn't really mean much in the real world. Every pump has a different level of efficiency, every pump has a diferent max RPM limit(some also have a minimum), and a pumps efficience decreases(also different factor from pump to pump) as it is spun faster and/or pressure is increased. There is a book containing horsepower, RPM, output, pressure, etc. data on everything from current pumps to ones that haven't been made in years. A shop that is seriously into selling and servicing compressors will have this book, or know someone that does. Unless you have the original documantation that came with your pump you need to find someone with one of these books to the the real answer to your question. Good luck...Al English

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Bus Driver

02-16-2001 05:44:33

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Re: Re: Compressor calculations in reply to Al English, 02-16-2001 03:23:26

Agree completely with Al English. Another way is to guess at pulley/ motor size. Install it. IMMEDIATELY check the amperage draw on the motor with a clamp-around ammeter. If amperage exceeds the motor nameplate, install a smaller pulley on the motor and try again. If amperage is more than 10% below the motor nameplate, try a larger pulley in the motor. Compressor manufacturers also specify a maximum RPM for their units to turn. No matter how you determine to drive your compressor, check the motor amperage as soon as the unit is started. Electric motors are most efficient operated at rated HP. Continuous overloading of an electric motor will lead to burnout.

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Bus Driver

02-17-2001 05:19:12

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Re: Re: Re: Compressor calculations in reply to Bus Driver, 02-16-2001 05:44:33

"Machinery's Handbook", which many of you already own, has a lengthy section on compressing air. It is not specific to a brand of equipment, but offers much information, plus the formulas for doing calculations.

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