Need help finding the output of the air compressor I'm building. It has two cylinders, 3.25" bore X 3.5"stroke. That gives it an output of 57 cubic inches per rev. If it runs at 800 RPM then that works out to 45,600 cubic inches per minute. Divide by 144 gives 316 CFM at 0 PSI. How do you convert to output at different PSI's, like 125psi, or 150psi. Thanks for any help

 

Albertafarmer:

This can be solved using s simple equation called the Combined Gas Law
where Pressure 1 X Volume 1 = Pressure 2 X Volume 2. So take 361 cft. X 1 psi divide that by 125 psi and that's your answer. Or use the link below and have it calculate the answer. Just use the same temperature on both sides. The answer won't be absolutely perfect but very close.

http://www.1728.com/combined.htm

Rander

 

45,600 cubic inches = 26.4 cubic feet.

You need to divide by (12x12x12) or 1728, not 144.

Your calculations were a little off. You also need to figure the volumetric efficiency which for a normal compressor works out to be 85%. So your real cubic inches would be about 58 x 800 x 0.85 = 39,488 cubic inches divided by 1728 = 22.8 SCFM.

The gas laws posted above will give you the volume that air will occupy at a higher pressure but I don't know what you want that for. 1 SCFM is one Standard Cubic Foot of air per Minute at standard temperature and pressure which is room temp and atmospheric pressure. Now if you need CFM at say 90 PSIG then you have a 3.2 CFM compressor at 90 PSIG. Use that calculator and use 15 for P1 and 105 for P2 because we start out with atmospheric air at ~15 PSIA (0 PSIG) and we end up with 105 PSIA (90 PSIG) air.

HP to compress that air to 130PSIG is 22.8 x .204 = 4.6 HP then add 5% for mechanical losses = 4.88HP. Therefore it will require a REAL 5HP motor to compress to 130PSIG. Not some "special" rated "compressor duty" motor. If you have single phase electrical power like most of us that means you will need 20-23 FLA at 230V depending on motor efficiency. With 3-phase power you would need around 14 FLA at 230V. I suppose you could buy bogus rated 3-phase motors but all the ones I have looked at have been real HP.

 

Here's what a continuous-rated General Electric 5 Hp 230 volt motor looks like. It may possibly be slightly oversized for that pump. The pressure switch operates a 7.5 Hp rated contactor.
http://persweb.direct.ca/dangus/truc...ompressor1.jpg
http://persweb.direct.ca/dangus/truc...ompressor2.jpg

 

Probably a TEFC in a 184T or 215T frame etc, hard to tell without making some measurements.

Looks like you need to check into some corrosion control tho...

 

Thanks for the reply, all very informative, (and confusing). I'm using a 10HP Briggs and Stratton gas engine on it, so should have lots of power. Only thing I don't have yet is a pressure tank. Need one 80-100 gallon range. I have two tanks approx. 30 gal each, is there any advantage/disadvantage to using two smaller tanks vs one large tank?

 

Two smaller tanks give greater surface area for cooling the air and removing the moisture.

Derate the gas engine because it is a "impulse" motivator.

 

Albertfarmer,

An easy way to figure capacity in your air compressor is to consider any brand or configuration of reciprocating compressor will produce about 4.5 CFM per horsepower @ 125 PSIG. The higher in pressure you go the less effecient it is. This figure is applicable only to commercial machines. The Box store variety where they quote " Peak Horsepower " will not meet these specs as they are grossly overrated for marketing purposes.

A question a while back asked about two stage vs. single stage. If you plan on operating your system at or above 125 PSIG. the two stage is more efficient. Comparing the volumetric efficiency ( VE ) curves for the two types you will see they cross right about 120PSIG. where the single stage pump VE falls rapidly the two stage is still rising.

I would recommend you operate your shop at 130 PSIG. If you work with tires the lugnuts need higher pressure to be tightened properly. ( the tire stores all use 175 PSIG to tighten lugnuts with and you won't be able to remove them @ 100 PSIG.

Receiver size is not important other than to lessen the start / stop cycle for the compressor. A large receiver stores more volumn, true, but it also requires your compressor to run longer to restore the used air.

Hope this helps you guys. I really enjoy this forum.

 

The compressor I'm using is a DeVilbiss 432, run by a Briggs and Stratton 10HP engine. I want it to run a sandblaster that needs 6 to 25 CFM at 85 to 125 psi. The compressor is a 4 cylinder, two stage, V configuration. You all have been very helpful, Thanks

 

Did you get a different compressor from the one you started with or were those two cylinders the primary side of the 4 cylinder compressor?

Do you need 6-25 CFM at 85-125PSI or SCFM???

Confused... Do you understand the difference between PSIA (Absolute) and PSIG (Gauge)? Most people just use PSI which is commonly the same as PSIG but that can get confusing when you start using formulas and get to calculating things. PSIG is gage pressure referenced to earth's atmospheric pressure of about 15 PSI (14.7 PSI). When a PSI/PSIG gauge is sitting on the workbench it will read "0". When an absolute PSIA gauge is sitting on the workbench it will read 14.7 PSIA which will vary with altitude, weather, etc.

I hope this clears the confusion.

For a two stage compressor:
HP to compress that air to 130PSIG is 22.8 x .185 = 4.2 HP then add 5% for mechanical losses = 4.43HP.
HP to compress that air to 175PSIG is 22.8 x .21 = 4.788 HP then add 5% for mechanical losses = 5.03HP, which would require an electric motor with a higher Service Factor (SF), probably a 1.15 or 1.25.

Your 10HP brigs should be OK but you may not be able to set the pressure cutoff at 170PSI, you may want to back off to 150PSI. An internal combustion engine has to be derated about 50% for the equivalent electric motor HP (I forget the exact figures). Otherwise the engine might get to "smoking" a little.

Make sure you put a 1/2" or 3/4" ball valve vented to atmosphere into the compressor outlet line so you can open it when trying to start the engine. Wait for the engine to warm up before you close it. Does your compressor have "head unloaders" so you can keep the engine running? Unloaders are special intake valves that close when pressure is reached to shut off the compressor. They essentially make the compressor pump vacuum. Controlling an internal combustion engine setup is the fun part.

 

I dont know what a Dev 432 is but 4 cyl 2 stage are usually larger thn a 10 hp is going to push. https://www.ford-trucks.com/user_gal...=29639&width=0 This one is a 20 hp engine pushing a 10 hp pump, 4 cyl 2 stage making about 40 cfm running at rated speed. 2 100# lp tanks mounted under the truck in series. Whoops,,, I looked up at an earlier thread where Eric calculated it for you,, it will probably work but running it at 800 may be marginal for power, you may need to gear it down a touch,,, maybe not.

 

He may have to cut the pressure cutout down a little. It kind of depends on what he is going to be doing with it. If he is looking for home shop, -OK, but for commercial work, -probably not.

 

I would tend to believe it may take 12 hp to run it at full bore and full pressure with a little margin for wear factor. Mine was a 10 hp pump and the 20 engine runs it great. I see a 16 hp water cooled eng for sale, new, very reasonalble and I think if I geared it down a little to 700 or so it would run it ok. But,, hate to buy something like that and not have it work. It doesnt even seem to put a load on the 20 which is what it was spec'd out for.

 

That is why I told him in post #10 above:
=================
Your 10HP brigs should be OK but you may not be able to set the pressure cutoff at 170PSI, you may want to back off to 150PSI. An internal combustion engine has to be derated about 50% for the equivalent electric motor HP (I forget the exact figures). Otherwise the engine might get to "smoking" a little.

 

http://www.toolprice.com/product/1394X
You gentlemen are getting way to technical. Just want to know if i can run this sand blaster with a 10HP motor, because it's all I have right now.