Increasing Air Output (CFM) of my compressor?

by Alex
(London, UK)

Hi Bill


I have an old compressor I use in my garage. it has a V Twin pump and appoximately 150 litre tank.

It is currently running a 1.5hp 1500 rpm, 240volt motor. however, it does not pump enough air to run my Eastwood soda blaster or my blast cabinet. It starts out fine but after a minute or two it pressure drops.

I guess my question is will changing the motor to a 3hp unit, 3000rpm improve the air flow? I presume it won't double it as the pump probably has a maximum displacement, but I would like a few extra CFMs if possible without having to buy a complete new compressor.

Thanks in advance for any advice you can give.
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Alex, typically a 1.5 HP air compressor will give you about 6 CFM of compressed air at about 90 PSI.

If your blast cabinet uses more air than that, then you will always be starving it for air as your compressor is too small.

By changing the motor to a 3 HP from a 1.5 HP, all you will have is a motor that works less to reciprocate the pump. It will not change the pump speed, and that will not result in any change in air capacity.

If you want more air, you need to cycle the pump faster, and that means a change in sheave / belt.

A larger sheave at the motor side will result in a faster pump action.

However, at what point will you destroy your pump? It was designed to run at a specific cycle per minute and by increasing the speed, you may (and likely will) exceed the design capabilities of your compressor pump and destroy it. Or, you may not! :-) That's a chance you will take if you wish.

You may find that with a larger pump sheave the 1.5 HP hasn't got enough power to work properly, and then you'll need to move up in HP.

Find out what the cabinet needs in terms of CFM if you intend in using it a lot. Then, get a compressor that has that capacity, and then some, so that you won't run out of air.

Cheers,

Bill

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Compress air consumption versus needed compressor hp

by Jarid
(Dale, IN)

We want to use compressed air to blow off cnc shavings during a trimming process.

To perform this operation we open a ¾” air line for 11 seconds at 100psi.

How much of our compressor hp is required to supply us with this (blow off) feature?

Generally speaking…., it would be nice to know the hp required to supply 100 psi through a ¾” line per second.
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Jarid, that info is on this site. Check under pressure drop for more details.

As a blow off - and I think you wrote you are using a 3/4" line ? - that is too large in my opinion.

You need an air knife or a venturi device to maximize the compressed air force. Exair sells a bunch of these.

Most blow-offs I've installed use a 1/4" line with a valve to meter further, and if more force is needed, a manifold to allow broader air coverage at higher volume.

Bill

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SCFM vs. Bar Vs. PSI

Die grinder (Photo: www.ls1.com)

Die grinder (Photo: www.ls1.com)

I have purchased a Hypertherm 30 Plasma cutter and was told by one person it requires 6.7 scfm @90 psi, and the owners manual says min 80 psi or 5.5 bar and max 135 psi or 9.3 bar. What I can't understand is the whole "bar psi, and cfm" Jargon. What compressor would you reccomend to fullfill all three of these , and can it handle a 7" grinder as well ? Thank you very much. I have bookmarked your site it is awesome!!!!
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Glad that you enjoy the website. It`s been over 7 years in the building.

All the info you need to understand the lingo of Bar, PSI and CFM is noted on the pages with those names. Look under CFM on the site map page for links to all those pages.

When the specs say that an air tool needs 6.7 SCFM, then be a bit careful. I don`t know if they truly mean pre-compressed free air, and that is waht Standard Cubic Feet per Minute (SCFM) of air means, or if they really mean you need 6.7 CFM of air at 90 PSI, which is air that has already been compressed.

As to your die grinder requirements, you need also to look at the specs for the flow and pressure needed for that tool.

As a rule of thumb you can expect to get about 4 CFM of compressed air at 90 PSI for each HP of air compressor motor.

If you plan on running your tools continuously, and at the same time, you need to add up all the CFM required, and divide that by 4 to get an approximate horsepower.

But first, read the page on sizing your air compressor for more info.

Cheers,

Bill

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