Max PSI in a compressor

by Andy Halliburton
(Baton Rouge, LA)

I have a 4 hp 30gal compressor with a max psi of 120. I want to increase the max.

Am I correct that replacing the pressure switch with a max 150 will accomplish this.

The CFM at 90psi is aroung 14cfm. Are there any watchouts here. I also might take my old 20gal tank without the pump and plumb it in to my 30 gal tank to get 50 total.

My compressor is in the attic of my garage for space reasons. Hauling and setting up a 60 gal vert compressor seems a bit overwhelming.

Will adding the 2nd 20gal tank increase my working CFM or is that a pump issue? I do not know if my current pressure switch is adjustable which is why I thought about changing it. Thanks





Hi Andy:

A bunch of good questions.

A simple answer to your first question is...yes, changing the setting on the existing pressure switch to a higher cut out pressure (which may void your warranty) or installing a new pressure switch with a higher cut out pressure, will allow the pressure in your compressor receiver to be higher...PROVIDED, the compressor is electro/mechanically capable.

By this I mean that the air tank is rated for the higher pressure, and that your air compressor has the ability to perform the work necessary to compress the air to that higher pressure.

Compressing air is simply a matter of converting electrical energy and mechanical leverage into stored energy in the form of compressed air in the air tank.

In order for a compressor to generate higher compressed air pressure, it must have the "oomph" in the motor to force the mechanics to compress air to higher pressures than the rating, and the mechanical components in the compressor must be able to handle the increased load.

Is yours capable of this? No way I can tell.

Yes, adding additional air reservoirs means that your compressor will run longer before the air pressure in the whole air system reaches cut out pressure, and the result is that you air tool will run longer before the air in the system is depleted to the point where the compressor has to kick in again to replenish the air.

You can get lots of information on this topic here. Read the page and follow the links.

And there's more info here.

Cheers,

Bill




Comments for
Max PSI in a compressor

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Mar 14, 2010
 No...5hp compressors are not the same
by: Anonymous

1st,if it is 110/115 it is not true 5 horsepower.

the max 115 current can put out electrically is 1.8 HP..

They rate compressors falsely using start up HP which only happens for a fraction of a second. A true 5 HP has to be at minimum 22 amps and 230 volts.

There are many misleading theories on calculating TRUE RUNNING HP..It takes a lot of typing to explain this but if anyone wants to calculate their true HP and needs info on them, I build them, own a group on this and have a lot of important info.

Feel free to e mail me at bareitallsandblasting@yahoo.com...I will invite you to my group and show you charts on how to dictate true horsepower and true cfm...115 compressors will only produce about 5 to 7 cfm @ 40 psi and at 90 psi maybe 2 cfm...feel free to e mail me concerning compressor calculations
Dec 14, 2008
 The truth
by: Andy

I want around 15 cfm at 90 psi in a compressor. Some 5hp models say they can do this and some do not. How can you read b/t the lines of the claims and tell for sure? Looking at 5 hp 30 gal belt driven oiled compressor. Should't all 5 hp be close?





Andy... all 5HP units can be close, but not the same. It depends on the engineering by the company that makes that unit.

For example, some 5HP might have a 9" sheave driving the mating sheave on the compressor head. Others might have a 7" sheave/pulley.

What that means is that even though the electric motor may be rotating at the same speed on both units, the compressor heads will be running at different speeds based on the different pulley sizes, and as a result, they generate different flow characteristics.

The engineers that designed each unit specified different components based on their need to build a unit that sells at the level they need to make a profit, and provides the life expectancy that fits within that price/profit portfolio.

By the way, the tank size has nothing to do with the flow characteristic of the compressor...just that with a larger tank you get more compressed air available for immediate use when the tank is full.

So no, all 5HP units don't produce the same.

The rule of thumb is that most compressors over 10HP generate about 4 CFM of compressed air at 90 PSI for each HP of motor size. That rule gets a bit wonky when you're under 10HP...but it should be pretty close.

I'm surprised then that you're seeing 5HP units that don't claim to produce 15 CFM @ 90 PSI. I would think that most would.

Bill

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