Compressed air: How to manage multiple compressors

Trying to wrangle multiple compressors at your plant? Take control for better, more-efficient operation.

By Ron Marshall

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Trying to coordinate control of compressed air systems can be a frustrating experience at best, especially if there are multiple compressors located in different compressor rooms. If your compressed air system is controlled manually and you set up a control strategy using local compressor pressure settings, odds are that your system is consuming a lot more energy than it needs. Addressing this issue can not only reduce your power bill but also give you more-reliable and stable air pressure.

How do compressors control pressure?

We'll focus here on the control of rotary screw compressors, the most common kind found in industrial plants. The strategies discussed here may or may not apply to systems with centrifugal compressors. 

Pressure is the most important parameter in a compressed air system. If the level is not maintained above the minimum required pressure, then compressed-air-powered machines will start to malfunction, affecting production. In a perfect world, you could set your compressors exactly at the minimum required pressure and they would control themselves, starting and stopping, loading and unloading, to maintain perfect pressure output at optimum energy consumption.  But, the world of compressed air is far from perfect, and the biggest problem is pressure differential.

Consider Figure 1, which is a simplified diagram of a compressed air system. The compressors in this system control the pressure at location P1. If left to control the pressure with a typical cascaded pressure band arrangement (pressure set points overlapped by 5 psi), these compressors would provide a pressure of, say, 100 psi when both are fully loaded and perhaps 115 psi during the lightest loads. But it's important to understand that it is not the pressure at point P1 that's important. The pressure at P6, right at the critical end user, is the location that should be controlled accurately. Real-world conditions conspire against this requirement, however.

A real-world system has pressure differential across system components. At full load, there may be 4 psi across the filter, 6 psi across the dryer, 5 psi drop in the system piping and 15 psi drop across the filter, regulator, lubricator (FRL), hoses and connectors. This leaves the critical user with only 70 psi of air pressure during the worst-case scenario, system full load, when the compressor discharge pressure is being controlled at 100 psi. During light loading conditions, because the pressure differential drops with lower flow, the pressure at the critical user may float as high as 115 psi, depending on the local regulator setting.

Can the compressor control help stabilize the pressure at the critical end user? No, because the compressors are controlling the pressure at point P1. Therefore, as system loading varies higher or lower through the course of the day, the pressure at the critical user points P4 to P6 will vary widely. This wide variation can cause inconsistent production output if the end use is pressure-sensitive, or it can shut down the operation completely. To solve these problems, compressed air operators will artificially increase compressor discharge pressure to compensate, say in this case by 10 psi, so that the pressure is well above the minimum requirement at full loading conditions.

Energy penalty for higher pressure

If this adjustment is made, then the compressors would regulate the pressure at a discharge to between 110 and 125 psi through the changing loading conditions. This higher pressure, though, comes at a price. As a general rule of thumb, at around 100 psi, the power consumed by an air compressor increases by about 1 percent for every 2 psi in discharge pressure increase. Also, for every 1 psi increase in pressure, the flow consumed by unregulated compressed air users goes up by slightly less than 1 percent. This higher flow further increases the compressor power consumption by a level that depends on system characteristics.

The higher pressure may also cause other efficiency issues. A common maximum pressure rating for air compressors is 125 psi. Perhaps as the filters in the system age and get clogged with dirt, system operators must increase the pressure at the compressor discharge to compensate. But if this is done, the compressor will exceed its rating, which would overload the motor and activate over-pressure protection. This higher pressure may also require the compressor to operate in modes such as modulating control that are inefficient at partial loads.

Multiple compressor rooms

If more compressors come into the picture – perhaps as with multiple compressor locations in a system set up like Figure 2 – then the problems multiply. Each compressor room would have different loading conditions and different pressure differentials. The plant distribution header might have a different pressure on one end than the other. But to control the system efficiently, the compressors in one room must be properly coordinated with the other rooms. To properly coordinate the air compressors with local compressor set-point adjustment, a very wide pressure band would be required. In systems with widely varying loads, this coordination is pretty much impossible unless the compressors somehow share the load with the units running at partial loads.

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