Compressed Air Systems Equipment Reliability & Maintenance Center

Heed These Signs of Inefficiency

EP Editorial Staff | February 16, 2018

Some compressed air systems are more efficient than others in serving end-use applications in plants.

By Ron Marshall

While compressed air is one of the most inefficient ways to power mechanical systems, it’s also one of the most widely used sources of such power in industrial operations. For various reasons, some systems are simply better than others at converting atmospheric air into compressed air, and efficiently delivering it to end users. Is yours one of them? The following seven signs may point to inefficiency problems:

Low-loaded vs. run-time ratio

If your system runs in load/unload mode, i.e., wherein any compressor alternates between loaded and unloaded, record its loaded and run-time hours. If the loaded hours are less than 70% of total run-time hours, it’s likely a compressor is running inefficiently. Are any of yours running in modulation mode? Modulation mode is the least-efficient way to run compressors in part load. If you don’t know the mode, ask your service provider.

Storage receiver size

How large is your storage receiver in relation to your trim compressor? The trim compressor is the one that loads and unloads to take partial load when the plant air demand only requires a fraction of a compressor. Sometimes, there could be more than one trim. Take the number of gallons of storage capacity and divide it by the CFM (cu-ft./min.) rating of the trim compressor. (Usually, CFM is about 4 times the horsepower rating). Is the ratio less than 5?  For sites with lubricated screw compressors, the effective storage should be larger than 5-gal. per trim CFM. Best practice is about 10-gal. per CFM. For variable-speed compressors, the storage volume would be 10 times the flow at minimum speed.

Excessive cycle frequency

When monitoring your load/unload operation, measure the number of seconds the compressor is in the loaded condition and the time it’s in the unloaded state. An efficiently running system would have total cycle time (loaded plus unloaded) above two minutes at around 50% loading (when load and unload are equal).

High system pressure

If system pressure is running above 100 psi, the system may be inefficient. It takes 1% more energy to produce compressed air for every two psi of higher pressure.

High levels of leaks

Visit your plant during non-production hours. Does the compressed air system sound like a pit of angry vipers? Obvious audible leakage is indicative of inefficiency.

Lack of system monitoring

How much is the energy input to your compressed air system compared to the flow produced? If you can’t answer this question, your system is likely inefficient.

High drainage levels

Do you have condensate drains that have been cracked open manually or timer units that are wasting expensive compressed air? These situations typically reflect efficiency problems. EP

Ron Marshall has spent almost 25 years working with compressed air systems, first as an industrial-systems officer with Manitoba Hydro (hydro.mb.ca) and, since his retirement, as owner of Marshall Compressed Air Consulting, based in Winnipeg, Manitoba. For more information, email ronm@mts.net, or visit compressedairaudit.com.

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