Performance Testing an Air Compressor

Using a load bank to determine compressor performance:

1. Shut off valve B to the plant. This will allow the full flow of the compressor to be directed to atmosphere through the load bank.
2. Begin by opening orifices on the meter until the compressor can just keep up at test pressure. Add the valves that are open and you have the flow.
3. Next, open or close more valves to test at other loads and make note of the resulting pressure. The pressure/flow points can then be plotted to form a performance curve for the compressor.

Leak Testing an Air System

The load bank can be used to acquire an accurate quantification of leaks in an air system.

1. A capacity test is performed on the compressor at system pressure
2. The system is pressurized without operating any equipment
3. A second flow test is done at the same system pressure
4. The difference in measured volumes is the leaks in the system

This approach can give very accurate leak quantities which can be easily converted into annual costs.

By isolating sections of the plant or system when testing leak quantity, the volume of leaks in any given building, process or system can be determined. This will help maintenance personnel to prioritize where they should focus their efforts.

Another benefit is to check for leaking block valves, leak test underground pipe, and check complex piping or process for total leak volume.

Consumption Test of Compressed Air System

Once the compressor(s) have been performance tested and system leak quantity determined, any individual end use can be tested or the entire system as an aggregate can be tested.

1. When system is at full pressure, test total flow to the system by loading the online compressor(s) to the desired system pressure and do not allow other compressors to come on. Document the flow through the meter.
2. Start the process or end use which you want to test.
3. Perform a second system flow test with the meter at the same pressure and document the flow.
4. Subtract the flow with the end use "on" from the flow with the end use "off". The difference in flow measurements is the consumption of the end use.

Performance Curves

Important conclusion drawn from the performance test:

• The compressor was rated at over 1,000cfm at 100psig but was only able to deliver 799scfm at 100psig.
• The modulating valve was prematurely closing the inlet on the compressors starting at 95 psig.

Imagine there are four of these compressors in the plant (1,000scfm each) plus a backup. If the plant air demand was 3800scfm and just two of the four had this problem (a 200cfm shortfall in air production), then the total capacity from the compressors would only be 3,600scfm and the backup compressor would be forced to run to keep up with demand. The facility may also begin to contemplate purchasing a new compressor because they are "running without backup" now.

At tests of over 230 plants, 80% of compressors tested which featured modulating inlet valves exhibited the graphed behavior above of not producing their rated air flow.

Testing Purge Volume on a Desiccant Dryer

Follow the standard procedures for testing air compressor performance. The first test should be performed either with the load meter installed prior to the dryer or piping system in bypass configuration so the dryer is not in the air stream. After the compressor output has been tested, start up the dryer purge and perform a second compressor capacity test at the same pressure as the first. The difference in the two measurements is the consumption of the dryer purge.

This is essentially the same as a leak or consumption test procedure except the "leak" is being used to dry the wet desiccant tower. The same principle can be applied to any piece of equipment or process which can be isolated.

General notes applicable to Desiccant dryers:

• Typical purge volumes will be from 15-17% of flow diverted from the "dry" side into the "wet" side to dry out the desiccant.
• Operating cycle is usually defined as the amount of time it takes for a regeneration cycle to dry the desiccant in each tower. For example, if it takes 4 hours to dry the desiccant in a tower, the operating cycle is 8 hours.
• Some manufacturers of Desiccant dryers with a heat reactivated (regenerating) system will direct up to 4% of system compressed air from the dry tower for some period of time to cool the hot desiccant.

For additional information or assistance, contact Kelly Beiermann, Nebraska Public Power District.

Note: We encourage requests from Nebraska customers. Businesses outside of Nebraska will be directed to their utility for further information.

Source: Airometrix Mfg., Inc. 1999-2003