Capacity of an air compressor
Capacity of an air compressor
(OP)
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?





RE: Capacity of an air compressor
-handleman, CSWP (The new, easy test)
RE: Capacity of an air compressor
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.
RE: Capacity of an air compressor
RE: Capacity of an air compressor
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
RE: Capacity of an air compressor
This is why a proper designation of capacity must include the discharge pressure.
RE: Capacity of an air compressor
I guess that is what I get for developing a "logical" answer without doing the math.
David
RE: Capacity of an air compressor
RE: Capacity of an air compressor
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
RE: Capacity of an air compressor
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.
RE: Capacity of an air compressor
RE: Capacity of an air compressor
David
RE: Capacity of an air compressor
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.
RE: Capacity of an air compressor
Ted
RE: Capacity of an air compressor
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