Control Gap: When air compressors fight

Synopsis: This article discusses some compressor sizing mistakes when it comes to using VSD air compressor control in a multi-compressor system.

Ron Marshall

So often when reviewing the data from systems that have VSD compressors installed we find that there is a “control gap” problem. During the initial plant tour the compressor operator may have proudly showed off his shiny new variable speed compressor, a piece of equipment designed to improve his compressed air system efficiency, all the while not knowing he had made a very common mistake with compressor sizing.

A quick glance of the collected data from a compressor system would tell an experienced compressor auditor what he needed to know, that the compressors are “fighting” for control, and that the system is not as efficient as it could be if certain rules had been followed before the new equipment was purchased. This is the value of monitoring to capture a baseline.

Figure 1: The system VSD compressor should always be the “trim” unit that takes up the partial load. (Source Compressed Air Challenge – Advanced seminar)

What is Control Gap?
Control gap occurs within a range of compressed air flows when the variable speed compressors have not been properly sized to match the fixed speed base compressors. Within any system there will be compressors that run in “base” duty, always running fully loaded, or turned off. These compressors are typically selected to be he most efficient full load units.

There are also compressors that are “trim” units, or the compressors that run in partial loading. These typically have characteristics selected to be the most efficient compressors when running in partial loading condition. VSD compressors are almost always the trim units in a system.

Variable speed compressors have a range of flow outputs within which the VSD drive adjusts the speed of the compression element under automatic control, this is the variable range, typically spanning 80% of the full load rating of the compressor, but not always. Some variable compressors have limited “turn down” range. A VSD compressor cannot turn down a full 100% of full flow capacity due to mechanical limitations relating to the compression efficiency of the screw element, and the cooling of the compressor motor. There is a minimum speed, below which some other method of compressor control takes over, like load/unload or start/stop operation. Compressor manufacturers warn compressor operators not to run the VSD compressors below minimum speed for extended periods of time because undesirable problems will occur, like high oil carryover and/or water contamination in the lubricant. This lack of 100% turn down becomes a problem when trying to coordinate multiple compressors.

Figure 2: Control gap occurs when the variable part of the VSD compressor is smaller than the base unit capacity. (Source Compressed Air Challenge – Advanced seminar)

Let’s look at an example system of three 160 kW (200 hp) compressors each capable of producing 26 m3/min (925 cfm). Two of the compressors are fixed speed and one is a VSD unit with a variable range of 17 m3/min (about 80%). The VSD compressor is set up to always run as a trim unit (Figure 1), and all will go well as long as the system flow is less than 26 m3/min.

But what happens if the system flow increases to 31 m3/min (1,000 cfm)? The VSD can only produce 26 m3/min, so the pressure will fall to the point a fixed speed compressor must start, bringing on 26 m3/min of capacity. This will push the pressure up to the VSD target setpoint, slowing the VSD down to its minimum speed. But this 9 m3/min plus the 26 m3/min of the fixed speed compressor is more than the 31 m3/min system demand, so the pressure will rise to the unload point of the fixed speed compressor, removing 26 m3/min of capacity.

But, once the base unloads the VSD compressor is again too small to carry the load alone, so the pressure will fall and load the fixed speed compressor back up. This condition will continue, rapidly cycling, as long as the system demand remains within the control gap (Figure 2). The fixed speed loading and unloading, at the same time the VSD is speeding up to full load and dropping back down to minimum speed is a sign of a control gap problem, any time the flow is within the 26 to 35 m3/min control gap the compressors will fight. This condition will repeat again when two base compressors plus the VSD are running within a band of 52 to 61 m3/min.

Sizing Rules
To avoid control gap, we must always make sure to follow these rules:

  1. The variable range of the VSD trim compressor must be at least equal to or larger than the base compressors with which it must work.
  2. The VSD compressor must always be the trim compressor, this requires special pressure settings coordination (Figure 3) that differ from the standard cascaded control.
    Thus, in the previous example, the variable range for the VSD compressor should have been at least 26 m3/min, requiring a compressor the next size up or about 35 m3/min rated flow capacity (200 kW). But care should be taken in checking the sizing and the variable range before a purchase, always carefully examine the rated compressor turndown capability, some 200 kW compressors might only have, for example, a 50% turn down range, making this a poor choice to match with 160 kW fixed speed units.
    It should be noted that there may be exceptions to these rules, if for example the plant loading is vary flat and unchanging or if there are compressors of multiple size in the system, and perhaps more than one VSD compressor. To be sure of proper operation always check with an expert before purchasing and for advice on pressure settings coordination.

Figure 3: To ensure a properly sized VSD compressor runs as the trim unit in all conditions its target pressure must be nested within the pressure control bands of all the other compressors. This differs from the typical standard cascaded pressure bands. (Source Compressed Air Challenge – Advanced seminar)

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