Capacity of an air compressor 1

[skuntz]

If a compressor is rated at a certain flow and pressure (i.e. 25 SCFM at 100 psig), does this mean that I can get a higher flow at a lower pressure. If so how can I determine the pressure available at the higher flow and how can I control the compressor output to achieve this operating condition?

 

[handleman]

No. SCFM is a flow rate corrected for temperature and pressure.

-handleman, CSWP (The new, easy test)

 

[Compositepro]

There are many designs of compressors. How they behave will depend on the specifics of their design. So consult the manufacturer.

Standard reciprocating air compressors will put out less flow at higher pressures because of dead or unswept volume in the cylinders. So, if the dead volume of a cylinder is 50%, then the output will drop to zero at a compression ratio of two. This is why you will often see flow rate given at 40 psi and 90 psi output.

 

[crjones]

handleman is close to correct. The pressure is barometric pressure. SCFM is at standard conditions of 68defF and 14.5 psia. ACFM is at site conditions at the inlet to the system. Where ICFM is at the inlet flange of the compressor with no inlet losses.

 

[zdas04]

To try to clarify, let's assume a reciprocating compressor because people find it easier to visualize that.

The piston displaces a certain fixed volume. That volume is filled with suction gas and then compressed to discharge pressure. It does this some fixed number of times per minute. So, if you keep the suction pressure approximately constant and keep the piston speed constant, then the amount of suction gas compressed is a constant and is not a function of discharge pressure.

On the other hand, energy consumed is a function of the ratio of suction pressure to discharge pressure. If you lower the discharge pressure then you will use less energy for the same CFM.

David

 

[MintJulep]

No, for a reciprocating compressor the throughput of a compress IS a function of the discharge head because of re-expansion of the air in the clearance volume during the suction stroke.

This is why a proper designation of capacity must include the discharge pressure.

 

[zdas04]

OK MintJulep, I went into Ariel Performance and defined a set of conditions at a discharge pressure and ran it. Then I dropped discharge pressure and ran it again and the calculated flow went up. Your explaination of re-expansion of the air in the clearance volume makes sense (i.e., at higher pressure there are more undischarged gas molecules to take up the volume than there is at than at lower pressures).

I guess that is what I get for developing a "logical" answer without doing the math.

David

 

[crjones]

The discharge pressure is critical to the sizing. But not for capacity. A shot at the adiabatic horsepower is the capacity x the discharge pressure. Divide by a presumed efficiency and you have motor sizing.

 

[zdas04]

crjones,
That is just not right for a machine with clearance (i.e., any real machine). Mintjulep's description is correct--you just don't completly empty the cylinder on the upstroke (on a recip) and the amount of gas that is left in the clearance changes with changing discharge pressure. That changes the amount of gas that can enter the cylinder on the downstroke (or when the suction valves open)

Run it in a program like Ariel or Frick's Coolware and you'll see that if you only change the discharge pressure from one run to the next, the flow rate will change.

David

 

[JRLAKE]

It sounds like you may be buying an air compressor. If so:

Yes. Compressors are rated at a Flow and a Pressure. Yes they will deliver higher flows with lower discharge pressures. How much more depends on the design and or type of compressor.

I prefer to preface flow units with FAD (Free air delivery) rather than SCFM, CFM, ICFM, etc. One cubic foot of free air at atmospheric conditions is pulled into the compressor, compressed to 100 PSI and delivered out the machine, to the pipe, in one minute would define a machine rated for 1 CFM FAD @ 100 PSI.

Compressor manufacturers, or their marketing departments, use the ICFM, CFM, SCFM terminologies to confuse buyers. Period. There is no standard for what ICFM, SCFM, CFM means across the various vendors. Some say ICFM is the inlet to compression chamber, neglecting inlet filter. Some say it includes the inlet filter. Some say CFM is from inlet filter to aftercooler. Some say SCFM is inlet filter to separator and some say this is ICFM. Blah blah blah blah blah. It is all smoke and mirrors.

Make the vendor show you FAD at a defined pressure, with defined atmospheric conditions (Temp, Relative Humidity) and make sure it includes ALL the components you intend to buy (inlet filter, oil separator, separator element, aftercooler, afterfilter, etc, etc, etc). This is the only way to compare two machines apples to apples. Some manufacturers are famous for showing horsepower at a point (defined by FAD and pressure) and comparing it to a competitor. However they will neglect to tell the customer that their data doesn't include pressure/flow drops through various components standard on the package.

 

[crjones]

zdas04, the equation I gave is the equation for work and whether I have designed rotary screw compressors, recips or scroll compressors and it seems to apply.

 

[zdas04]

The question wasn't about motor/engine sizing or hp requirements, it was about the air-end capacity being a function of (among other things) discharge pressure. The thread started out saying a resounding "No, capacity is not a function of discharge pressure". Then MintJulep pointed out that machines do not have zero clearance and the re-expansion of the gas in the clearance volume reduces the amount of gas that can flow into the cylinder on the intake stroke. Several manufacturer's programs support MJ's hypotheses that a machine at a constant speed with a constant suction pressure will have a lower flow capacity at a higher discharge pressure than at a lower discharge pressure.

David

 

[skuntz]

Thanks to all that have responded to this thread so far. The reason for my question is that we have a reciprocating compressor where the rating is stated as 26.1 SCFM @ 100 psig. We wanted to now if it could deliver 50 SCFM @ 2.5 psig. I called two sources - the manufacturer and the distributor - and got two answers! I finally just but a flowmeter at the output and ran the compressor with the output full open. I could not get more than about 30 SCFM out and the pressure right at the outlet (of the tank) was 5 psig. I believe I was limited buy the outlet pipe diameter. It was in fact the manufacturer that predicted that I would not be able to get 50 SCFM even at 100% duty cycle.

For whatever reason a complete understanding of compressor theory and operation often eludes me, but I have learned quite a bit on this forum.

Thank You.

 

[hydtools]

The maximum you could ever get out of a compressor is its swept volume. That is at atmospheric inlet and atmospheric outlet pressure. At other outlet pressures the delivered volume is less. At higher outlet pressures some residual air remains in the compressor, it is not discharged. So less air can be taken in. So if the swept volume is less than the 50scfm it will never deliver that output. The manufacturer probably knew that and thats why he told you that you could not get more than 30scfm at any outlet pressure.

Ted

 

[MarioOliveira]

Hi,
Consider a one-liter bottle (like the cylinder, can only 1 liter of displacement, if you want more volume Fad. Is to increase the size of the cylinder (not viable) or increase the speed of the compressor, if technically have structure to it.

Mário
Lisbon
Portugal