# How to figure out CFM at less than max



## Jonathan (Dec 28, 2019)

Hi all. Need some help. Looking to buy a new compressor. Ingersoll Rand 5hp 80 gallon. Literature says CFM is 14 at max pressure which is 175 psi. I only need 100 psi for the machine it will be hooked to. Is there a way to calculate what the cfm will be at 100psi? 

Thanks in advance. 
Jonathan


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## Jonathan (Dec 28, 2019)

I think I figured this out for myself if it helps anyone else I'll post it here. 
Here's a formula I found online:

P=K*(1/V)
90psi=K*(1/5cfm)
Solving for K, we get a value of K= 450
V=K*(1/P) so V=450*(1/35psi)= 12.86 cfm

In my case I have [email protected] but I want more cfm at 100psi:

175=K(1/14)
K=2450.9
V=2450.9(1/100)
V=24.5cfm @100psi. Which is all I need so this compressor will work for me. 
Thanks


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## Osviur (Sep 7, 2015)

Hi Jonnathan,

I think that this formula (Boyle's law) can't be applied to this case, K is a constant equal to the pressure exerted by a fixed mass of a gas in a fixed container volume. Any change in either factor implies the inverse change in the other factor, giving the same constant.

The volume aspired by the high-pressure cylinder (175 PSI normally= two stage), is fixed, if the RPM remains fixed, then the volume compressed will be the same (or so), I think that this equipment is running at the design RPM's, so there must be a very small margin to increase them, changing the pulleys.

A better solution (I think) would be to use a pressure regulator set at 100 PSI using the 175 PSI as the primary air supply. In this way you get the best motor and pump efficiencies.

Anyway, it is just an opinion coming not from a specialist. Let's wait for the specialist!


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## Jonathan (Dec 28, 2019)

Osviur
Thanks, I'm certainly no expert. But I did pull this formula off a air compressor specific forum where it supposedly came from an engineer in the field. 
I agree I don't think I can change the pressure produced by the compressor as you say the cylinders have a fixed volume and the motor a fixed speed hence no way to change pressure other that with the regulator. Here's where I get lost....If you have a tank of air at 175 and you release it through the regulator at 100, my believe is that you will have a larger volume of air. Stands to reason that a given volume of air at 175 has to expand to a larger volume in order to reduce pressure to 100? Hence more CFM than rated at the higher pressure.


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## Osviur (Sep 7, 2015)

Jonathan,

That is correct, here you can apply: P1 * V1 = P2 * V2 but these pressures are not manometric (as indicated by the instrument) they are absolute pressures (add 14.7 psi to P1 to obtain absolute), then, do the operation. In this operation P1 is 175 + 14.7 PSI, V1 is the tank volume, P2 is 114.7 PSI. I'm not sure if this formula can be applied to the volume displaced (14 CFM @ 175 PSI), the dynamic behavior may be different. If not, it would be: 
14 CF * (175 + 14.7) =V2 * (100 + 14.7)
V2 = 23.15 CF per minute 

But better, let's wait for the experts.


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## stevon (Mar 5, 2019)

Jonathan,

I think you are talking about the T-30 model which shows 15.8 CFM @ 90 PSI. Keep in mind, few compressors are designed to run at 100% duty cycle. The compressor has a fixed CFM at the specified RPM and duty cycle. If not changing the compressor size (bore and or stroke) the only other option to increase CFM is speed but there are limits going that route. I see that on most compressors they show only a slight increase of CFM at lower pressures like 1 to 2 more CFM. There are higher CFM target designs, like a single stage four cylinder focused on max CFM at lower air pressures. 18 CFM @ 100 psi is about the most you can get out of a 5 HP electric motor air compressor. To get more CFM at a lower pressure you need to increase pump displacement and lower top pressure requirement. I'm not sure I understand your question. I would need more information about your requirements. How much CFM is actually required and for how long ? If you need 20 CFM for more than an hour, I would recommend a 7.5 HP @25 or more CFM compressor that also has a piloted unloader valve (runs idle while not filling compressor)eg EMAX that keeps cooling pump and motor while not under load. This can be added separately to any compressor, you could run a setup like that and run all day long at a 20 CFM load.




Emax review with constant run feature:





5HP continuous run feature
https://www.powerequipmentdirect.com/EMAX-EP05V080I1/p3798.html

5-HP 80-Gallon 4-Cylinder
https://www.aircompressorsdirect.com/Campbell-Hausfeld-TQ3104-Air-Compressor/p7630.html. 

Compressor Pump 4 Cylinder
https://completeequipment.ca/produc...-cylinder-single-stage-emglo-mgu-dewalt-jenny

19CFM @100psi
https://www.powerequipmentdirect.com/Puma-PK7060V/p11114.html

30 CFM @100psi 
https://www.aircompressorsdirect.com/Eagle-C7180V1-MS-Air-Compressor/p14815.html

Stephen



Jonathan said:


> Osviur
> Thanks, I'm certainly no expert. But I did pull this formula off a air compressor specific forum where it supposedly came from an engineer in the field.
> I agree I don't think I can change the pressure produced by the compressor as you say the cylinders have a fixed volume and the motor a fixed speed hence no way to change pressure other that with the regulator. Here's where I get lost....If you have a tank of air at 175 and you release it through the regulator at 100, my believe is that you will have a larger volume of air. Stands to reason that a given volume of air at 175 has to expand to a larger volume in order to reduce pressure to 100? Hence more CFM than rated at the higher pressure.


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