Why Compressor Size Matters

Photo courtesy Marshall Compressed Air Consulting

Unlike the oversizing of air compressors, going ‘bigger’ really can be better for associated storage receivers and piping.

By Ron Marshall

In compressed air systems, as in other areas, the size of equipment does matter. For air compressors, it’s crucial to ensure sufficient output capacity to supply peak flows without causing pressure problems, as well as provide extra capacity to cover unexpected flow caused by leakage, intermittent loads, and future equipment additions.

Grossly oversizing compressors, however, can increase energy costs. For example, with load/unload-controlled compressors, doubling the equipment size from a 50-hp to a 100-hp unit could increase operating costs by about 70%, even though the same amount of air is being produced. Consequently, in terms of air compressors, bigger is not usually better.

WHEN TO GO BIG

Bigger can, in fact, be better for other components within a compressed air system. Specifically, if the storage receiver and system piping are oversized, energy efficiency will improve. In short, if the size of your storage receiver doesn’t make visitors to your compressor room do a double take, then the tank is probably too small.

Large storage tanks provide the greatest benefit for lubricated screw compressors running in load/unload mode. These units normally cycle from loaded to unloaded at a specific frequency—and each of those cycles wastes energy. Thus, increasing the size of the storage receiver reduces the number of cycles and improves efficiency. For example, installing a storage receiver sized at 4,000 gal. for a 100-hp compressor would save about 25% in operating costs, compared with one sized at 400 gal. This is a savings of about $19,000/yr. at 10 cents/kWh for a compressor running full time at 50% average loading.

Installing piping that’s too small causes higher-than-normal compressor discharge pressures, which increase the power consumed per unit output. For every 2 psi of extra pressure that a compressor must produce, power consumption increases by 1%. In addition, for unregulated compressed air demands, each psi of extra pressure increases the flow by about 1%, further loading the equipment. 

Right-sizing your compressor room and distribution piping so there’s less than 2% pressure loss across the entire system (excluding air dryers and filters) ensures that the equipment will run efficiently at the lowest acceptable pressure. Of course, you should always carefully monitor the pressure to ensure personnel aren’t unexpectedly increasing compressor settings for some reason.

MEASURING UP

How does your plant’s equipment measure up? To answer this question, sites should have their compressed air systems monitored by professional service providers. They can check for compressor size and pressure drop and make recommendations for improvements. 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. To learn more, email ronm@mts.net, or visit compressedairaudit.com.