Planning air system upgrades

Ten steps to successful system improvements.

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By By David M. McCulloch and Bill Scales, P.E.

PlantServices.com

Most compressed air systems are relatively modest at first, but grow and develop as production, inappropriate uses and leaks increase over time. Even assuming that air leaks and inappropriate uses have been investigated and reduced, adding production equipment raises demand for compressed air. Some equipment may need a different operating pressure, and the required air quality may change. These modifications represent potential problems for the plant engineer who wonders:

• What is the required volumetric flow rate and pressure of compressed air, and what size of air compressor should be added?
• What type of compressor and control system offers greatest reliability and lowest life-cycle operating costs?
• Where should the compressor be located?
• Is sufficient power, ventilation and cooling capacity available?
• What type of system capacity controls would be best?
• Is primary compressed air storage sufficient?
• Is distribution piping adequate?
• Is secondary compressed air storage sufficient?
• Is the current compressed air quality satisfactory?
• Does this project require professional help?

The current average and peak compressed air flow rates, in cubic feet per minute (cfm), should have been established before considering any proposed additions. The rated output of the existing compressor(s) also should have been established. The specifications for the proposed equipment that needs additional compressed air should state the required flow rate, pressure and air quality. This information provides the new total flow rate. Deducting the rated output of the existing compressor(s) gives the additional air flow required from a new compressor.

This calculation ignores potential leakage and increased inappropriate compressed air use. Also, it doesn’t account for differences in the frequency of operation of each piece of production machinery. Individual consumption peaks might not occur simultaneously. Nevertheless, you’ll need to determine the average and peak flow rates (Table 1).

Don’t add a fudge factor to the required air flow when calculating the rated capacity for the additional air compressor(s) because it could result in the new unit(s) operating at less than full capacity most of the time, robbing efficiency. If you decide to oversize the additional compressor by 20% or more, select the compressor that combines the best full-load and part-load economy to minimize the operating cost over the full range of compressed air requirements.

Another important consideration is standby capacity in case of compressor malfunction or needed repair. This may require at least one additional compressor. Conventional wisdom says that three 50%-capacity compressors are better than two 100% compressors, because this provides more flexibility without sacrificing system reliability, particularly during periods of reduced consumption. It’s also beneficial to operate the smallest total compressor horsepower, particularly for periods of reduced capacity requirements, such as a second- or third-shift operation. These factors, combined with reliable compressor service, are keys to maintaining energy and production efficiencies and profitable outcomes.

Each type of compressor has its advantages, disadvantages and preferred range of capacity and pressure. Table 2 provides a simple method for comparing different compressor types. Life-cycle cost analysis always is recommended, and should include specified maintenance.

Compressor cooling is a major consideration. If water-cooled, the important issues include availability and quality of cooling water, disposal or recirculation, possible treatment and overall cost. If compressors are air-cooled, adequate room ventilation is essential. Heat recovery also is a potential opportunity.

Compressor size and type determines the electrical power requirements. Additional ancillary equipment may require a different voltage and current. Consider, too, the availability of the required electrical supply and its support equipment. Ensure that proper circuit protection is provided for the added electrical load.

Many plants have a compressor room and, in some cases, it’s shared with other equipment. Several factors must be considered if a compressor is to be added. These include the need for a foundation, space for maintenance activities, space for drying and filtration equipment, room ventilation to handle the added heat release and the sound level.

It might make sense to install the new compressor in a different location, perhaps closer to the point of greatest demand or at the application requiring the highest pressure.

There are several types of capacity control for individual compressors, sequencing controls for multiple compressors and pressure and flow controls for compressed air systems. The correct selection of each determines system efficiency over the anticipated operating ranges.

Two rules for achieving optimum efficiency are (1) only the number of compressors needed to maintain the required system pressure should be in operation at any given time, and (2) all but one, a trim compressor, should be running at full capacity and pressure. The trim compressor should have an efficient capacity-control mode. If it’s a reciprocating compressor, this could be unloading in a series of capacity steps. For a rotary compressor, variable speed control or variable displacement is most efficient.