Industrial Utility Efficiency    

Displacement Compressor Performance Standards

Acceptance Test Codes

If you have ever looked at the small print of a compressor brochure or a CAGI Data Sheet or a compressor technical information page, you have probably seen some reference to one of the above standards. At one time or another, US compressor manufacturers have used these standards to test and report compressor performance. These are referred to as “Acceptance Test” codes.

Before beginning a discussion of acceptance test codes, it should be noted that ASME PTC9 is no longer maintained as an active standard. You can still purchase a copy from ASME for reference purposes, but manufacturers are unlikely to test and rate to an inactive standard. The CAGI/Pneurop standard has also been dropped from active status, but since it is based on Annex C of ISO 1217 you may still see references to this standard in literature or technical data that has not been updated to reference ISO 1217, Annex C.

Acceptance test codes were originally written to be a test method that could be used to confirm that a compressor built to a customer’s specific requirements for flow, pressure and power actually met those requirements. If the requirements were met, the customer would “accept” the compressor and the manufacturer could be paid. That is fine for custom-built compressors, but most compressors sold to industry are not custom-built. Most compressors are of a standard design and built in batches or in continuous production quantities and are fully piped and wired as a complete, self-contained compressor. For these types of compressors, ISO developed Annex C for ISO 1217. Annex C is a “Simplified acceptance test for electrically driven packaged displacement air compressors”.
The purpose of the simplified test code is to provide a consistent method of measuring the volume flow (at the terminal point of the compressor package and at a standardized set of inlet conditions) and total package input power requirements of a displacement compressor so that interested parties can make a direct comparison on an apples to apples basis.

Standardized Reference Terms & Conditions

It is important to understand what the term “volume flow” means. Volume flow has been referred to as actual cubic feet per minute (ACFM), cubic feet per minute (CFM), free air delivered (FAD) and various metric equivalents. Manufacturers of compressors in the United States have agreed to standardize their terminology and use only ACFM when stating volume flow. Volume flow is the volume of air at the standardized reference condition delivered to the terminal point of the compressor package. For example, a 1000 ACFM, 125 PSIG rated compressor will “inhale” a little more than 1000 cubic feet of ambient air, compress that air to the rated discharge pressure, perhaps use some of that compressed air for control purposes and deliver the equivalent of 1000 cubic feet of inlet air to the terminal point of the compressor package. Remember that this is a volume measurement, so think of it as a bucket that moves 1000 cubic feet of something every minute.

If you had a pneumatic device that required 1000 CFM (at standardized reference conditions) at 90 PSIG, that 1000 ACFM compressor should do the trick, right? Well, maybe. This is where the standardized reference conditions come into play. The standardized reference conditions for ISO 1217 are found in Annex E and describe the inlet conditions for the acceptance test. Those conditions are:

 

Inlet Air Pressure 14.5 PSIA (absolute)
Inlet Air Temperature 68 °F
Relative Water Vapor Pressure 0
Cooling Water Temperature (if water-cooled) 68 °F

 

Since sea level air pressure is 14.7 PSIA, the reference conditions are set at an elevation slightly above sea level. A relative water vapor pressure of zero means that water vapor is not taken into account when determining the volume rating of the compressor. The reason not to consider water vapor in calculating volume flow is because water vapor varies greatly from location to location and the amount of water vapor that is condensed varies from application to application. By eliminating it from the rating criteria, the customer can use the volume rating as a starting point to calculate his supply volume based on his specific ambient conditions. The customer will also need to use the reference pressure and temperature in that calculation. At these conditions, one cubic foot of air weighs about 0.075 pounds. A compressor rated at 1000 ACFM operating at these conditions would put about 75 pounds of air into the system every minute.

If the facility was in a location where the ambient pressure was 14.5 PSIA, the temperature was a constant 68_F and the relative humidity was 7%, the 1000 ACFM compressor would work fine for the 1000 CFM demand. Under those conditions, with a 90 PSIG operating pressure, the pressure dew point would be 49.4_ F. Water vapor drawn into the compressor would never see a temperature lower than that and no water would condense. There would likely be no need to dry the air since the pressure dew point is almost 20_F lower than the ambient temperature.

Use Correction Factors

If the facility happened to be in Denver, some calculations would have to be made to determine whether the compressor would actually operate the device. The ambient pressure in Denver is about 12.2 PSIA. Assuming a 95_F summer day with a 30% relative humidity, the calculations go as follows:

Temperature Correction –

(460+Standard Temperature in _F) / (460+Ambient Temperature in _F)

Or

(460+68) / (460+95) = 528/555 = 0.95

Altitude Correction –

Absolute Ambient Air Pressure / Standard Ambient Air Pressure<.

Or

12.2 / 14.5 = 0.84

Multiplying these two correction factors together give you the total change in density of the ambient air between the site conditions and the standard conditions. The air on that summer day in Denver is only 79.8% as dense as standard reference conditions. Additionally, at 95_F and 30% RH, the compressor is pulling in about 0.96 pounds of water vapor every minute. At Denver’s elevation that means that about 22 cubic feet of the inlet air stream to the compressor was water vapor. Since water is bad for most pneumatic devices, the compressed air will likely be run through an aftercooler and a dryer to get the pressure dew point down to about 40_F. This will cause some of the water vapor to condense into a liquid, removing its portion of the inlet air stream from what is available at the discharge point of the dryer. It actually reduces the available inlet air stream by about 16-17 CFM. So, take 16 CFM from the original 1000 ACFM capacity and then multiply by the correction factors.

1000 – 16 = 984 x 0.95 x 0.84 = 785 Standard Cubic Feet per Minute (SCFM)

The compressor is still “inhaling” 1000 cubic feet of volume, however, the air in Denver is less dense (fewer molecules) than the air at the standardized rating condition. The bucket size did not change but what went into the bucket did. 1000 cubic feet of volume yields the equivalent of 785 cubic feet of air at the standardized reference condition. At standard conditions, a full bucket held 75 pounds of air and in Denver the bucket is full with just 58.9 pounds of air.

   Win-Win  
  Changes in ambient temperature are why some marginal applications work fine in the winter and struggle in the summer.    

Sizing the Air Compressor

To find out what size compressor to use to deliver 1000 cubic feet of air at the reference condition, start by adding 17 to the 1000. Use 17 (or 18) instead of the 16 used above because the compressor is going to pull in more ambient air and thus more water vapor. Now divide 1017 by the correction factors.

1017 / 0.95 / 0.84 = 1274 ACFM

This calculation results in the volume required at the stated summer conditions to deliver the same mass as 1000 cubic feet of air at standard conditions. In the winter, colder air is more dense and the correction factor may go above 1.0. Changes in ambient temperature are why some marginal applications work fine in the winter and struggle in the summer.

A frequently asked question concerns why compressor manufacturers rate in ACFM and dryer manufacturers rate in SCFM. The short answer is that a dryer’s capacity is based on mass flow through the dryer and not volume flow. A refrigerated dryer, for example, is only capable of cooling a certain number of molecules of air down to its specified dew point. The dryer has to deal with a given number of pounds of air without regard to the volume that it took to make that number of pounds. Theoretically, an 800 SCFM dryer would work with a 1000 ACFM compressor at the Denver summer conditions stated earlier. A dryer’s capacity is also affected by the temperature of air at the inlet of the dryer and the pressure at the inlet, so “theoretically” assumes that the pressure is 100 PSIG and the temperature is 100_F. Dryer manufacturers have their own set of correction factors if the temperature or pressure varies from 100.

Conclusion

In conclusion, the ISO 1217 rating for a compressor serves as a volume rating, indicating how much ambient air the compressor will deliver to the terminal point of the package at the rated pressure. From there, calculations can easily be made to adjust the volume rating to the actual site conditions. Also worth noting is that ISO 1217 is subject to periodic review and rewrite. A new version is due out soon. This new version will include methods for testing and reporting variable speed compressors at less than full load levels. This method has already been adopted by members of the Compressed Air and Gas Institute and available on members CAGI Data Sheets. It shows power and flow at minimum speed, maximum speed and at least three equal points between those two speeds. This was done to allow users to look at performance over the flow range they actually need and not at just the full load point. To assure users that they are comparing true performance and not MCFM (marketing cubic feet per minute) the Compressed Air and Gas Institute sponsors a third-party testing and verification program for rotary screw compressors and refrigerated dryers. The testing lab performs random testing on participating manufacturers’ compressors to verify that the claimed performance meets the ISO 1217 standard and the appropriate dryer testing standard. For information about this program, visit www.cagi.org.

For more information please contact Wayne Perry at email: wayne.perry@kaeser.com or visit www.kaeser.com