Compressor controls limit performance and/or displacement volumetrically, normally based on pressure. The controller will attempt to keep the signal pressure between the set points or at the set point. The set points are normally adjusted by the manufacturer's representative, and many operators and plant engineers are unaware of how these controls are set.

Most compressors use two set points. These represent the deadband of the unit controls. The deadband is the differential pressure between the highest pressure at which the compressor pumps the least mass of air and the lowest pressure at which it pumps the most mass of air. These two setpoints also normally represent the unload and load pressure. Adjusting the set points has a significant effect on how the compressor operates. The differential determines how the unit will respond to manage the pressure at the signal location.

A typical set of control points on a pressure switch is shown in Figure 2. A 10-psig differential exists between the control set points. If the controls on this compressor are set to modulate, this implies that each 1-psi increment represents 10% of the capacity within the set points. IThe volume is at pressure. As the displaced capacity of the machine increases, the pressure will also drop. This means that the work energy is increased less than linearly.

Different types of compressors will respond in different ways. Rotary-screw compressors will displace between 40% and 100% of capacity. Thus, in the dead band modulating, 60% of the capacity can be displaced. If the unit were modulating at 50% of the dead band (as indicated in Figure 2), the compressor would be putting out 50% of the top 60%, or 700 cfm at 113 psig. Just before reaching the unload point, the unit will throttle back to 40%, or 400 cfm at 118 psig.

If the machine was in the load-unload operation, Figure 3, it would displace its capacity at variable pressures between the load and unload pressure. If the compressor is a positive-displacement design, the volume will remain constant from the load to the unload pressure at a rising pressure — thereby increasing density at rated mass or work energy available. In the case of recriprocating compressors, some are multi-step. In a two-step unit, the compressor will function at 0% and 100%, such as deadband A in Figure 3. Deadband B in Figure 3 is a three-step control which would displace 0%, 50%, and 100%. This would require either two pressure switches or a control regulator to perform the same function as two pressure switches. Most larger recriprocating compressors are capable of at least three-step operation. Some manufacturers offer five-step compressors, which displace 0%, 25%, 50%, 75%, and 100%.

If a unit would achieve its optimum mass at a given pressure, setting the unit controls below this point would achieve less than optimum performance. Setting the controls above this point would overload the drive motor. Careful planning of set points can determine how many compressors are necessary to achieve a particular result.

Some compressors, such as centrifugal designs, use a single setpoint controller. Most of these types of controls either attempt to maintain the signal pressure at the set point with modulating inlet and blow-off controls, or they operate load-no load within an adjustable proportional band related to the set point. The objective should be to keep the unit on a natural curve and out of the limiting controls for this type of base load compressor. As the signal pressure approaches the set points, the controls engage and move the operation off the curve and into the limited control range on the throttle line. In load-no load, the signal pressure rides the natural curve of the compressor, Figure 4.

When the demand exceeds the capacity of a compressor, the pressure drops below the lowest set point below the deadband. This condition is called the draw down. This is an important condition to understand, because all compressors that are base loaded on their own controls or with a sequencer must be in draw-down before the next compressor becomes loaded. Figure 5 demonstrates a cascade of three compressors with two units in draw down.

If all of the compressors in Figure 5 were in modulation and the demand reduced, the signal pressure would rise and all compressors would throttle and remain part loaded. If the deadband spreads for each machine had a different Dp, the signal pressure that resulted would be a function of the way the set points were adjusted.