View Full Version : Which air compressor?

12-11-2011, 12:13 PM
Hi all,
I'm getting an air compressor to run a small workshop with several airlines, 2 cnc turret mills with power drawbars, and in the future a Dugard ECO 760 VMC.

The dugard requires 70 PSI and 200L/min air.

This is the compressor I'm looking at getting:

Its 20% off Clarke stuff for next 10 days as well.

My question is, would this compressor be up to the job?


13-11-2011, 12:34 PM
200l/min is roughly 7cfm.
That compressor will be rated at 30cfm free air delivery.
So with a bit basic physics (boyle's law) and given that you need 7cfm at 70psi, which is roughly 5 times atmospheric pressure, means that compressor could deliver about 6cfm at 70psi.

Does the dugard need that amount of air continually?
If it does, than that compressor is undersized, but I can't see what on a VMC would need that amount of air continually, unless it's using a mister.

13-11-2011, 12:43 PM
Thanks for your reply m_c, I'll contact dugard to see if that machine requires it continously but i have a feeling it does. I use a Haas VMC at work which alarms out if air pressure falls below 80 PSI.

I may need to spend a bit more on a compressor i think! I'll let you know how i get on. Thanks

John S
13-11-2011, 03:04 PM
How do you get 6 CFM from a 7.5 Hp compressor ?

13-11-2011, 04:15 PM
Boyles law.

The rated 30cfm is for FAD (Free Air Delivery), which is basically what it can provide while spinning freely while generating no relative pressure.
Now take Boyles law that states that provided the temperature remains constant, the volume and pressure are directly proportional, i.e. as you increase pressure, you proportionally reduce volume.

As atmospheric pressure is around 15psi (14.7psi to be exact, but it varies depending on height above sea level/weather), the compressor can pump out 30cfm at 15psi (this is called absolute pressure - a pure vacuum is 0psi absolute).
So for example if you then require the compressor to double the pressure to 30psi (or 15psi relative to atmoshperic pressure), the volume of air produced drops by half, provided the temperature remains the same.

As we need 75 psi (I've rounded this up for easy figures!), we need the compressor to produce 90psi absolute pressure, which is 6 times the FAD pressure, so FAD volume then drops by a factor of 6, which gives us 5 CFM at 75psi.
And now I've just realised my calculation earlier was wrong!

Those figures don't allow for temperature change, which will affect the actual figures, along with the compressor effiency, but they still give a rough idea of what the compressor can deliver.

13-11-2011, 04:24 PM
I'm actually wondering if the CFM figure Dugard state is for FAD delivery or peak volume, as I can't think why any machining centre would need that kind of volume/pressure continually?

13-11-2011, 07:17 PM
if your spending that sort of money then why dont you look at the hydro-vain compressors on ebay the deliver high CFM and low noise, should pic a gooden up for 1K friend of mind got one for 800 quid, was just 12 month old, that was a 70cfm jobbie

12-12-2011, 03:07 PM
Most rating in cfm are always expressed relative to atmospheric pressure, not output pressure, even if this is for tool air consumption.
But don't be fooled, most compressor rating are more than optimistic. They are given without pressure at the output, so in perfect condition.
When pressure is needed, the rating goes low (because of mechanical efficiency and temperature rise). For 90psi, don't expect more than 2cfm per minute per HP.