Your understanding of compressor controls and how to use them can be an important factor in how well your air system functions and the amount of energy it consumes. Unfortunately, many systems consist of a series of individual unit controls, with no provision for meeting the challenge of getting two or more compressors to operate together efficiently. Once compressors are purchased and installed, attention usually is limited to keeping the machines running — but not necessarily efficiently. Unless the output falls below the minimum acceptable pressure or the equipment fails for one reason or another, the compressor's controls are ignored.

Air compressor basics

Compressors don't make pressure; they pump volume. If a compressor is rated at a given pressure, that means it is constructed to safely contain that pressure. The controls limit the pressure relative to the configuration into which the compressor is pumping. If more volume is pumped into the downstream system than is removed by the users, the pressure rises. If more air volume is removed by users than is pumped into the system, the pressure falls. The job of the controls is to limit the pumped volume partially — or entirely — relative to the pressure set points adjusted on the controls. At full load, the compressor pumps 100% of its volumetric capacity and the controls perform no function.

Whether looking at single or multiple compressors, it is important that each compressor that is running should be optimized — this means pumping the optimum volume at the optimum pressure, or the maximum pounds of gas per minute at whatever pressure is accomplished with the least amount of energy. In addition, controls on compressors protect the equipment. They are usually integrally tied to safety alerts, alarms, or shutdowns. It is to your advantage to know what safety features are in the control system and how they function.

The vast majority of older compressors have few and relatively simple safety features — sometimes only encompassing high pressure and temperature shutdowns. Many newer compressors use microprocessors for controls and safety functions. Their control consoles include many more safety features and complex functions.

Control signals and signal location

Regardless of the type of control on the compressor, there must be a pressure signal to operate the control. The location from which the signal originates is critical to how the unit will function, how much energy it will consume, whether it will function with other compressors, and whether the desired performance will be achieved.

The idea behind control signals is to take a pressure from a location inside or downstream from the compressor and transmit it either pneumatically or electronically to the compressor controller. This signal interfaces with a pressure switch or transducer. In most compressors, the intent is to move air from the compressor air end downstream, to keep the pressure at the signal location at or between an adjustable set point (or set points) preloaded in the compressor controller.  Many different potential locations for pressure signals  are designated by compressor manufacturers, Figure 1.

Because the location will change the amount of energy the compressor consumes and the performance, the following should be noted:
• Compressor manufacturers have rated their performance based on packaged volume, pressure, and energy upstream of the aftercooler. Signal location is normally not discussed in the product literature. In some cases, performance is stated as "packaged," but this does not indicate what is in the package or where the signal is located.
• Some manufacturers have used signal location A for many years, but recently have begun to use location B.
• Other manufacturers state performance downstream of the aftercooler, interpolating the Dp across the aftercooler in the performance, even though the signal originates upstream of the aftercooler, and
• Many manufacturers use the same flow components in a package frame size to accommodate many capacity sizes - up to twice the size at the top of the frame as at the bottom of the frame. The effect of the components' Dp and the signal location will have a dramatic influence on performance. As the volume displaced in a given frame size increases, the Dp increases as a square function of flow, and the performance decreases.

One of the problems with installing the signal location upstream is that the quality of the air there is poor. The air is saturated with moisture and can be laden with lubricant. Both the water and lubricant will condense in the signal line, fouling the controls or transducer. Another prevalent problem is the effect of accumulating liquids in the control line — this can result in too high a pressure signal. Taking the signal from downstream of the aftercooler, at the separator, will reduce this problem by about 80%, but will still only delay the inevitable. Some manufacturers install signal line blow-outs, so that the lines can be kept clean on a daily basis. However, most have no means of correcting this problem other than disassembling the control lines or using a different signal from a dry, clean source downstream of the cleanup equipment.