What can you do to initiate a plan for monitoring your compressed air? Install test points for pressure gauges at the compressor discharge, before the pre-filter and dryer, after the after-filter and throughout the demand side of the piping. This is especially true for loops where a series of equipment is supplied (Figure 7). Install flow meters at branches in the system. After you have installed flow meters, try to solve the material balance. Most of the time, you will find you missed something.
Figure 7. Install a loop to balance the load on the system.
Another common problem is gage accuracy. Although it may be useful to install gauges to monitor pressures at the demand points, don’t use these values in calculations! Unless instruments are periodically calibrated, and this goes for flow meters as well, they will drift. Because even calibrated gauges can differ in accuracy, measure differential pressures throughout the network with a single test gage. For our purposes, we rely on high quality digital gages. We use the same gauge, which has been calibrated prior to being employed. This is the only way to know what is going on.
Sometime during the discussion about a measurement plan someone will ask about measuring the dew point in the system. Technically, this is known as the pressure dew point (PDP). The damage that moisture causes to some equipment will be itemized as justification for a humidity meter. Generally, measuring the PDP is unnecessary. If you have a moisture problem eliminate it! Some industries, such as pharmaceuticals, food, cosmetics and fine chemicals (refer to ISO 8573.1) may have air quality standards that require permanent monitoring of humidity. These instruments are delicate and need annual calibration.
A well-managed system
How do you know when your air compressor system is performing at maximum efficiency?
Figure 8. In a poorly managed system, power usage does not respond to change in demand.
Figure 8 shows a stable well-managed system. A conservation program is in-place, air quality meets standards, and pressure drops have been eliminated on both the demand-side and supply-side. The trouble is that the cost savings are impressive but the power usage is still a little higher than expected — what’s wrong? Look at Figure 8. The chart shows that power doesn’t respond to demand, i.e., flow: reduced demand should mean less power — right? What is needed is an air management system to assure that all compressors run at full-load efficiency. This system must have software capable of analyzing operating condition, compressor load condition, and adjusting to pressure change — either rise or decay. The net result will be all units on at full demand (load), one on at partial load, and all other off, 80% to 85% of the time. Figure 9 shows the results of installing a management system.
Figure 9. In an optimized compressed air system, power responds to demand.
Another component of a well-managed system is keeping it in tune. This requires an annual check-up, and a monitoring system. Monitoring systems are available to do this on a continuous basis.
A final component is training. Operating and maintaining a compressed air system requires the efforts and talents of many people. To be effective, these people must understand how your system works and how interdependency of its components affects cost. It has been our experience that this type of training pays off quickly.
Energy savings go directly to the bottom line. Saving money in compressed air will not only reduce costs it will increase plant production.