redmondjp
Well-known member
Thanks for posting this! Very useful info. I know that the interstage pressure relief valve is typically set at around 75psi on two-stage compressors, but I never knew what the actual operating pressure was.
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How can one measure compressor performance?
- Thread starter IT_Architect
- Start date
Thanks for posting this! Very useful info. I know that the interstage pressure relief valve is typically set at around 75psi on two-stage compressors, but I never knew what the actual operating pressure was.
This has been way overthought. A flow meter is going to be overkill. It should be possible to calculate this to between 2 and 3 significant digits of accuracy using a gauge and a stop watch. SCFM can be calculated using 3 measurements: Change in pressure, time required for that change in pressure, and tank volume (perform testing with tank valve closed, so that tank volume is a known constant).
Determine the point at which you want to measure. For example, say you want to know compressor output at 90 psi. We need to use a pressure range for a delta P. Compressor output diminishes nearly linearly as output pressure increases, so we will use 90 psi as an average of our start and end figures. We will time the pressure rise from 80 psi to 100 psi. Output at 80 is a little higher than output at 100, but using those points at beginning and ending effectively averages them for a calculated output at 90 psi. Lets say we have a 60 gallon tank, and it takes 33 seconds to rise from 80 psi to 100. I will show you the formula and explain it as we step through it.
((Delta P)/14.7)*((tank volume in gallons)/7.48)/(time in seconds/60)=SCFM
First we divide our Delta P by 14.7. That converts our pressure changes from psi to atm (atmospheres). If you push 1 cubic foot of air, into 1 cubic foot of space, the pressure within that space increases by 1 atmosphere. Our delta P/14.7 becomes 1.36
Our tank volume is divided by 7.48 to convert our volume measurement from gallons to cubic feet. Our tank volume becomes 8.02 cubic feet.
Our time in seconds is divided by 60 to convert it into time in minutes. Our time in minutes becomes 0.55 minutes.
(1.36 * 8.02)/(0.55) = 19.8 SCFM
To truly make this representative of SCFM, there would be some calculations necessary to adjust for variations in temperature, barometer, humidity, etc, but unless there's an actual reason to justify that kind of high precision, it would be unnecessary overkill (redundancy intended for extra emphasis).
Determine the point at which you want to measure. For example, say you want to know compressor output at 90 psi. We need to use a pressure range for a delta P. Compressor output diminishes nearly linearly as output pressure increases, so we will use 90 psi as an average of our start and end figures. We will time the pressure rise from 80 psi to 100 psi. Output at 80 is a little higher than output at 100, but using those points at beginning and ending effectively averages them for a calculated output at 90 psi. Lets say we have a 60 gallon tank, and it takes 33 seconds to rise from 80 psi to 100. I will show you the formula and explain it as we step through it.
((Delta P)/14.7)*((tank volume in gallons)/7.48)/(time in seconds/60)=SCFM
First we divide our Delta P by 14.7. That converts our pressure changes from psi to atm (atmospheres). If you push 1 cubic foot of air, into 1 cubic foot of space, the pressure within that space increases by 1 atmosphere. Our delta P/14.7 becomes 1.36
Our tank volume is divided by 7.48 to convert our volume measurement from gallons to cubic feet. Our tank volume becomes 8.02 cubic feet.
Our time in seconds is divided by 60 to convert it into time in minutes. Our time in minutes becomes 0.55 minutes.
(1.36 * 8.02)/(0.55) = 19.8 SCFM
To truly make this representative of SCFM, there would be some calculations necessary to adjust for variations in temperature, barometer, humidity, etc, but unless there's an actual reason to justify that kind of high precision, it would be unnecessary overkill (redundancy intended for extra emphasis).
rburke65
Well-known member
Sounds like a lot of BS to me........maybe try "The Physics Journal"....might get a better answer over there!
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justanengineer
Well-known member
This has been way overthought.
I disagree bc unless I've missed it nobody has hit on the most important concept of sizing any compressor or machine - duty cycle and demand. The #1 cause of low-end compressor failures I've seen is simply bc folks overwork them, commonly theyre rated ~50% with a max of 4-6 starts/hour which can dramatically derate them over an hour's use. If youre blasting or otherwise using a lot of air there's a significant difference between even 10 cfm pumped continuous EVERY minute for 60 minutes vs 15 cfm for only 20-30.
I was in no way suggesting one should not properly size a compressor choice. My comment was in regard to some people suggesting very complicated ways of measuring compressor output, when all one needs to do this is a stopwatch, calculator (both of which are found on anyone's phone these days), and the gauge on the tank.
In my field of work we take phased P/T (pressure vs time) data from directly in the cylinder(on site in field conditions). Plotting how long it takes to get from suction to discharge pressure and what percent of the stroke it is actually drawing in or discharging the gases. We then can determine lots of things including volume moved, ring leakage, valve issues, etc. I was a little unclear on the question asked, so this may not be useful but if you want more information send me a PM. As compressor analysis is to deep a subject to post all the details but I can send you as much information as you would want if interested.
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Boy did you open a can of worms. I have spent my life in the air compressor business. I am not an engineer but I can tell you that a reputable industrial manufacturer will have published volumetric efficiency curves and piston displacement. From this data you can calculate SCFM at any given pressure within the limits of the machine.
To test these claims they adhere to exact standards. These include barometric pressure, humidity altitude and temperature. To duplicate these standards in the field would be expensive to say the least. To test any other way would generate more questions than answers.
As far as wear affecting performance there is an easy test for a two stage compressor. Interstage pressure is directly related to capacity. It should be anywhere from 28 to 32 PSIG. If it is lower than that delivery is deminished. If it is higher than that delivery is not diminished much but power consumption goes up as well as heat.
The compressor repair guys need to know a lot of about a lot of things, lol. I hired one and trained him as an hvac tech, he is on his own now, but he was aweosoe and already knew so much from working on compressors, he already had the idea of a/c, he could charge a system braze in components, he knew basic plumbing principals, he understood the importance of properly sizing, it was great...
I have a speedaire air dryer that I need to fix, the compressor is dead (just a 1/5hp r134 tecumseh, Ill replace it with an embraco) and it needs a fan, one lead to the others demise obviously. I was going to just replace the entire unit, BUT they are insane, $900+ for a silly air dryer???? That is crazy... I will fix this one for $135-compressor, $19 fan motor (which I should have somewhere), and 10oz's of 134a... Doesn't look like much else can go wrong with them?
This has been done by a number of governing bodies and industry societies (ASME, ISO, API, CAGI). The current best practice is a simplified version of ISO 1217, found in appendix C.
CAGI has a synopsis here: http://www.airbestpractices.com/standards/iso-and-cagi/displacement-compressor-performance-standards
Note that the volume measurements clearly differentiate between ACFM, ICFM, and SCFM.
The small compressors in question here, are typically not of much interest in industry. Consequently adherence to performance test standards isn't commonly seen. The 25hp recip installed to actuate the inlet dampers on a 33,000 hp air blower didn't get tested to ASME PTC 9, but the blower certainly did.
CAGI has a synopsis here: http://www.airbestpractices.com/standards/iso-and-cagi/displacement-compressor-performance-standards
Note that the volume measurements clearly differentiate between ACFM, ICFM, and SCFM.
The small compressors in question here, are typically not of much interest in industry. Consequently adherence to performance test standards isn't commonly seen. The 25hp recip installed to actuate the inlet dampers on a 33,000 hp air blower didn't get tested to ASME PTC 9, but the blower certainly did.
sberry
Banned
I agree with the engineer and with the post above in some sense. There is no fine mathematical line that matters to the avg user on this board and without slide rule can tell for almost certain a 1/2 black pipe will carry all the air they need.
I read some of the design parmeters and the differences in measurement and many are rather insignificant and not that I ever knew it all that well don't need to know this to get rust off with a sander.
They may not be insignificant in design and some so fussy a little dust on a blower blade is factor but to a user if it makes 7.1 or 7.3 dosnt mean much especially if he had a demand of 15 at the tool and needs a unit with 12 minimum to produce for the tool at a given duty cycle.
I read some of the design parmeters and the differences in measurement and many are rather insignificant and not that I ever knew it all that well don't need to know this to get rust off with a sander.
They may not be insignificant in design and some so fussy a little dust on a blower blade is factor but to a user if it makes 7.1 or 7.3 dosnt mean much especially if he had a demand of 15 at the tool and needs a unit with 12 minimum to produce for the tool at a given duty cycle.
sberry
Banned
I agree and my point also, as an engineering exercise its great, as a practical tool for sizing a common comp not very much.So whats the point of all of this?
You buy a product from a reputable company, take good care of it, and spend your time using it NOT thinking about how to make sure it is usable....