Don't err with Air Compressors

By by David M. McCulloh, Mac Consulting Services

ChemicalProcessing.com

Keywords: Air Compressors, David M. McCulloch and Mac Consulting Services

Understand what's involved in getting a suitable supply of compressed air

Chemical plants vary widely in size and complexity — and so do the types of compressors used in them, and the compressors’ capacity control systems. The units often differ significantly from those at most industrial manufacturing plants using compressed air, where the most common compressor now is the oil-injected rotary screw.

While chemical plants generally pose less variation in flow requirements from shift to shift than other manufacturing facilities, there can be differences in pressure requirements for the various processes, plus instrument air. Air quality requirements, including dryness, also may be stricter for some processes, requiring different drying and filtration methods. Each of these aspects may require segregation of some systems for more efficient operation. In addition, the cost of loss of production, due to an interruption of air supply, must be considered. We’ll look at these considerations.

Demand profile

It’s essential to know how, where and in what quantities compressed air is being used. List each process or application, together with the minimum, average and peak rates of flow and any cycle times on and off. Staggering the timing of peak demands can reduce the total maximum demand. One common problem is that distribution piping is sized for the average demand, with pressure drop based on that. When peak demand is significantly higher, the pressure drop also will be significantly higher, resulting in a lower pressure at the point of use or requiring a higher discharge pressure from the compressor(s). So, size distribution piping for the peak flow rate with an air velocity in the pipe not to exceed 30 ft/sec.

Also, establish the required operating pressure for each process or application. Significant differences in required operating pressure may identify potential segregation of processes or applications for overall system efficiency. Operating the total compressed air supply at the highest required pressure results in increased power consumption (an approximately 1% increase for every 2 psi). Having a separate air supply at the required pressure for a specific process or application can improve overall system efficiency. In each case, ensure that sufficient backup air supply is available.

While adequate primary compressed air storage (near the compressors) is essential for efficient capacity control of the units, adequate secondary storage (near each point of use) can prevent wide swings of pressure at each point of use.

Compressed air leakage in the distribution system also contributes to the cost of operation. Leak detection equipment is readily available and an ongoing leak detection and repair program should be standard practice. In addition, idle production machines or processes should be fitted with shut-off valves that close upon shut down.

Adequate pressure sensing points must be available from compressor discharge, before and after dryers and filters and throughout the distribution system to each point of use. This can allow monitoring the dynamic variations in the entire system.

Compressor types

Three kinds of compressors generally find use at chemical plants:

• Centrifugal. Larger plants often use centrifugal compressors, which are capable of substantial flows of air, although they’re now available in as low as 100 hp. These are dynamic compressors, where a continuous flow of air has its velocity increased by impellers rotating at high speeds. The velocity energy, or kinetic energy, is translated into pressure energy in diffuser or volute chambers. The number of impeller stages depends upon the desired final pressure and efficiency. Cooling occurs between each stage. An important advantage of centrifugal compressors is that they are inherently oil-free, with no lubricant coming into contact with the air passing through the compression process. They generally are water-cooled.

Figure 1. Reduced capacities in centrifugal compressors can lead to a reversal of flow.

Centrifugal compressors have a characteristic pressure curve, where pressure increases as capacity decreases. This is an important consideration in capacity control, which is further complicated because pressurehead- making ability varies inversely with absolute inlet temperature and resultant mass. This means that, when designed for a given inlet capacity in actual cubic feet per minute (acfm), the mass flow increases inversely with absolute inlet temperature and the pressure generated also increases because of the denser inlet air. So, for a given discharge pressure, the compressor will have “grown.” See Figure 1, which presumes the same inlet temperature to each stage. Cooling water temperatures can further affect the curve. Figure 1 has a control pressure of 125 psig. It also shows a “surge” line, to the left of which, at reduced capacities, a flow reversal can occur and should be avoided.

Generally, plants want to maintain a relatively constant pressure to the system; controls will reduce capacity if the pressure tends to rise above the set point due to decreasing demand. With centrifugal compressors, a common means of capacity reduction is by progressively closing an inlet valve or guide vanes. This causes a pressure drop at the compressor inlet, reducing the mass flow in proportion to the absolute inlet pressure. Because inlet guide vanes impart rotation in a similar direction to the impeller, this provides energy savings compared with a simple butterfly valve. However, as stated, flow reduction must be limited to avoid surge and generally flow shouldn’t go below 80% or 70% of capacity.