Compressor Control Methods

As noted in the descriptions of the various compressor types, there are several control methods available for air compressors, and control methods may greatly affect the overall operating efficiency of the compressor.

Inlet Valve Modulation / Inlet Throttling

Used in: Oil Flooded Rotary Screw, Oil Flooded Rotary Vane Compressors

Inlet valve modulation (often just referred to as modulation) throttles off the air inlet to the compressor as pressure rises above the setpoint pressure. This causes the compressor to draw in less air, matching compressor capacity with air usage for relatively steady pressure control. However, this also causes the compressor to draw a vacuum at the inlet, so that it is trying to make high pressure from a lower starting pressure. This causes part load performance to be very poor (a machine that modulates to 0 capacity still uses about 70% of its full load power). The modulating compressor can typically regulate discharge pressure with no or minimal air receiver storage

Some modulation controlled machines may be adjusted to fully unload or “blow down” if capacity reduces to a certain level, such as 40%.  This saves some energy, but requires the use of air storage receivers to meet demand when in the fully unloaded state.

inlet-modulation-graph

Load / Unload (Dual) Control

Used In:  Virtually any type of compressor

Load/unload (sometimes called load / no-load or dual control) requires storage receiver volume, and operates the compressor at full capacity until the unload pressure (cut-out) setpoint is reached.  At the unload pressure, the compressor switches to unload, producing no compressed air, and venting its internal pressure (blowing down).  During this unload period, demand of the plant must be met by the stored air in the receiver(s) and piping.  Once a lower load (cut-in) pressure is reached, the compressor returns to full capacity, and the cycle repeats.

During the unload period, the compressor reaches a low unloaded power consumption (approximately 25% of full load for lubricated screw and vane compressors, and 10 to 20% for reciprocating, oil-free screw, and centrifugal compressors).  On oil flooded screw and vane compressors, however, it may take 30 to 60 seconds of unload time for the compressor to reach this lowest unloaded power consumption.  For this reason, the efficiency of these compressors is greatly affected by the amount of receiver volume present in the system, as well. More receiver volume allows longer unloaded periods with lower power consumption. In addition, more receiver volume causes the compressor to cycle less, reducing wear and tear on the machine.

dual-control-graph

Load/Unload control also includes variants, such as “Auto Dual” which shuts the compressor off totally if it remains in the unloaded state for a preset time delay.  Other variants may monitor motor starts and/or motor temperature to maximize off time of the motor, while limiting the motor starts to a reasonable frequency.

Variable Displacement Control

Used In: Oil Flooded Screw Compressors, Double Acting Reciprocating Compressors

Variable displacement controls change compressor capacity by opening ports in the pump that limit the amount of the cylinder or airend that is used for compression.  For double acting reciprocating compressors and some rotary screw designs, this is done in discrete steps (0, 25%, 50%, 75% and 100% for reciprocating compressors, 50%, 62.5%, 75%, 87.5% and 100% for rotary screw compressors). Other rotary screw designs are continuously adjustable from 50 to 100% capacity.  Efficiencies are typically good within the variable displacement range, but screw compressors sometimes sacrifice some full load performance to use this method, and screw compressors operating below 50% must resort to another control method, such as inlet modulation or load/unload.

variable-displacement-graph

Variable Speed Control

Used In: Oil Flooded Screw Compressors, Oil Flooded Vane Compressors, Water Flooded Screw Compressors

Variable speed drive (variable frequency drive) compressor controls use a frequency drive to control the frequency of the electrical signal to the motor.  This, in turn, varies the speed of the motor and airend, controlling capacity.  This provides a nearly proportional flow to power ratio at part load.  At full load, however, drive losses make the variable speed compressor slightly less efficient than a fixed speed compressor.

Variable speed compressors can turn down to approximately 25% load, at which the compressor must load/unload or start and stop.

variable-speed-graph

Variable speed drives are sometimes considered for retrofit applications to existing compressors, but this provides some unique complications.  Typical variable speed drive applications such as pumps and fans are variable torque applications, where torque drops significantly with speed.  Compressor applications are constant torque, where torque remains relatively constant (assuming constant air pressure) as speed varies.

Additionally, fixed speed compressor airends typically have been designed to operate over a narrow speed range, without regard for significant speed change.  In this case, efficiency of the pump often drops off substantially as speed varies from the design speed.

airend-speed-graph

Centrifugal Compressor Controls

Centrifugal compressors typically throttle their inlet controls to about 70 to 80 % of capacity. Below this range, there is insufficient airflow through the machine to overcome the pressure at the discharge. This causes the air to reverse flow through the compressor, a condition known as “surge”.

centrifugal-chart

Surge is not a desirable condition, and may damage the compressor or cause nuisance shutdowns, therefore, if demand is reduced below the turndown range at a given pressure, the compressor must control in a different manner. The traditional option is to maintain the minimum flow by venting the excess air through a discharge valve, called a blowoff valve (BOV) or modulating blowoff valve (MBOV). Manufacturers refer to this control method as “modulation” or “constant pressure” control, because it is able to meet air demand without significant storage volume or pressure fluctuation.

Alternatively, the compressor can unload, fully opening the discharge valve, and shutting the inlet. In this case, the discharge valve is referred to as a bypass valve (BPV) and may be a modulating valve that is operating only in the fully opened or fully closed position or may be a two position valve.  Unloading controls may be referred to by several different names, such as dual control, auto dual control, etc..  While unloading reduces power consumption to 10 to 20% of full load power consumption, it does require significant storage volume and allows pressure fluctuation to occur.

centrifugal-graph

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