Compressor size confusion

[pete1234]

Hi guys I hope someone might be able to help.

When searching compressors, all ratings are quoted at maximum PSI/CFM.

A tool I am hoping to use has a rating of 30CFM but only a PSI of 50. Could anyone help with choosing a compressor of the right size.

thanks in advance

 

[brightspark]

are you single phase or have you got 3 phase power supply. if only single your limited to a twin pump and motor setup like the clarke well over a grand for 28cfm .whats the tool

 

[pete1234]

are you single phase or have you got 3 phase power supply. if only single your limited to a twin pump and motor setup like the clarke well over a grand for 28cfm .whats the tool

i only have single phase so was potentially considering a petrol/diesel powered unit. The tool is a gun for spraying render

 

[carbon]

As the tool has a rated input of 50psi you can set the pressure to 50psi on the regulator that could be potentially fitted or fit a better quality one to one end of the receiver and use that for that particular application. If it requires 30cfm you need to get a compressor with more volume than that. The pumps volume is rated when it’s not compressing air, when the output from the pump vents to atmosphere. My 30cfm compressor achieves about 24cfm when spraying.

 

[Chippie]

The cfm figures are a bit misleading….free air delivery at pressure is the usual figure to be concerned about..

The compressor has a displacement figure, ( obviously based on its bore and stroke) but bear in mind this is at atmospheric pressure….

Typical pumps will displace something like 14cfm when spun at around 1500 rpm,taking several minutes to reach 100-120psi….As you draw from the reservoir the pressure will drop, depending on the dead band of the pressure switch, the comp may restart at around 90 psi…taking a few mins to top up the reservoir again..obviously as you are drawing off at 50 psi, ( assuming you have the regulator set correctly and it can provide the volume throughput ) these timings will be different….
The above may give you some indication what to look for…,but if you go the motor route, 3 phase is more likely to be the option for large pumps, then there is duty cycle/stop-start numbers to consider…engine driven units are slightly different, using an unloader to maintain pressure while the engine continues to run….

 

[frank horton]

Pete
I had one of those when I rebuilt my house in France....
I ran two biggish 2 cylinder comp "single phase", they worked very hard and burnt one out....
they basically ran for 8hrs per day....
in the end bought a road comp....no regulator, just stood further away to get the render on....
did two coats to match the rest of the house...
u'll struggle with normal comps.....
worked for me......

to add, I used to use my shop comp to sandblast chassis and tractor pieces....even as it's large it wont keep up....
3 phase, direct drive Atas Copco.... no idea on CFM....

 

[whm_fab]

nice bead breaker

 

[Jack Ryan]

I find compressor advertising very confusing. No one seems to use the same standard.

I'm not a compressor expert but here are my thoughts. I would value others' thoughts.

If the supplier states free air delivery (FAD) as it is officially defined, at least there is a starting point. FAD is the unloaded (zero PSIG) air delivery at the air intake temperature. Tools, on the other hand, have an air delivery requirement at a specified non-zero working pressure.

I don't think I have seen a compressor specification of max CFM & max pressure. If there is a max pressure specified, it is sometimes the maximum safe operating pressure and sometimes the pressure switch cut out pressure.

Your render spray tool requires 30CFM @ 50 PSIG. Using the ubiquitous ideal gas law, that is equivalent to a FAD of:

(50/15)*30 = 100 CFM

With a FAD of 100 CFM you could work continuously and your compressor would run continuously. This is very approximate as, for one thing, I have ignored the temperature rise of the air as it is compressed.

You don't need the full FAD to run your sprayer because the compressor has a tank containing air compressed to around 120~150 PSIG. You can spray until the tank pressure approaches 50 PSIG. At that point, the compressor can no longer keep up with the sprayer requirements so you will have to stop and wait for the tank to refill.

If you can run the compressor before you buy, it might be worth noting the low and high switching pressures and the time it takes to top up the tank. If that time is excessive (to you), the motor may be weak and/or the compressor efficiency may be low. These values can also be used to approximate the loaded air delivery.

Jack

 

[knighty]

for 30cfm and short term use you'll be best off hiring a diesel road tow compressor

phone around for prices - prices for 1 day will be extortionate, but don't be put off prices drop fast the longer you have it

 

[henry Kadzielski]

My two cents worth. I have a Sullair 115 (115liters/sec @7bar). These road tow compressors have the rating that the op is interested in. These also can be set to output the pressure you desire from their max operational pressure. From memory FAD to actual CFM is about .8. So if you require 30cfm @50psi, rule of thumb equated to 15cfm @100psi, which equates to about 20cfm FAD in a perfect world. Generally I go 50% greater that what theory says, so a 30cfm FAD compressor will get you going. This is not available in single phase. I have looked up the biggest single phase compressors here and you would need two in parallel to get what you need and they would be flogging themselves to death to achieve it. You would also need to change the air hose connection size to accommodate the actual air flow, not to get pressure drop at the gun. Just my two cents worth.