Compressed air fail: Verification spots trouble

An aerospace plant purchased a new high efficiency variablespeed drive compressor to replace an older VFD unit that had cometo the end of its lifespan. The new compressor was more efficientthan the old unit and was expected to save significant energy;calculated specific power was projected at 22 kW per 100 cfmincluding dryer purge.

Confident in good efficiency, the plant maintenance manager invited a compressed air auditor in to do efficiency measurements so the project could qualify for a utility incentive. Measuring devices were placed on the system to measure pressure, flow and power. Within hours, the auditor delivered the bad news — the actual specific power of the system was 79 kW/100 cfm, a sky-high number!

Investigation revealed a few issues. The first was one of two existing desiccant air dryers was left in fixed cycle mode. During the measurement period, the average compressed air flow to the plant was only 19 cfm, yet the air dryer purge was consuming 40 cfm of purge flow. This purge was the largest air flow in the plant — and was unnecessary!

The second problem was a leaky condensate drain; this flow was consuming an estimated 10 cfm of wasted flow.

A third problem is significant. The compressor installed is a 55 kW unit rated at 390 cfm and with a minimum speed of 55 cfm, but the average plant air flow is much less than 55 cfm (with very high peaks). This means this VSD unit will be running in start/stop mode for the majority of its life. Operation in this range is not desirable, and in this case results in inefficiency, because the compressor chosen uses recirculation to generate heat to boil off any water in the condensate. This recirculation uses extra power, reducing system efficiency.

This experience illustrates the value of verifying compressed air installations after a project takes place, not to mention the problems with oversizing VSD compressors. The customer is investigating ways to improve the situation to get expected power consumption.

Figure 1. The expected specific power of this system was 22 kW per 100 cfm, but the actual was sky high at almost 80 kW per 100 cfm.