Compressor Controlers

Pros and Cons of Individual Compressor Controls

Previously we described the 6 different types of Basic Air Compressor Controls but what kind of control would you want for your compressor. Her are few pros and cons of the controls to help you assess the needs of your compressor.

Start/Stops

Pros

  • Simple control using only a pressure switch
  • Motor and compressor operate only when needed which saves energy
  • Good for small compressors that are 25 HP or less (depending on application)

Cons

  • Frequent starting wears down motor and compressor
  • Pressure setting to stop must be higher than required system pressure to build storage and may increase energy use
  • Loses of pressure control in the range of 35 psi
  • Limited to small compressors

Load/Unload

Pros

  • Motor compressor runs continuously which reduces wear  and tear associated with too many frequent motor starts
  • Tighter range of pressure (approx 10 psi)
  • Provides adequate storage and offers energy-efficient control of rotary screw, reciprocating and some centrifugal compressors

Cons

  • If applied incorrectly short cycles cause  premature wear and tear. There is minimal or no power savings on lubricant-injected rotary screw compressors
  • There needs to be proper blow down time and the storage capacity required for lubricant-injected rotary compressors to achieve energy savings and prevent lubricant foaming
  • Requires over-pressurizing to maintain minimum system pressure

Modulating

Pros

  • The motor and compressor run continuously reducing wear
  • Tighter range of pressure control (10 PSI)
  • Steady progressive capacity control that matches demand

Cons

  • Pressure ratios increase as inlet pressure is throttled
  • Inefficient at lower loads(lubricant-injected rotary compressors limited to 40-60% capacity; centrifugal compressors limited by potential surge and may require discharge blow off)

Dual/Auto Dual

Pros

  • Combines features of modulating, load/unload. and start/stop
  • Shuts down compressors when unloaded for pre-set duration which in turn saves energy
  • Better selects operation mode for small reciprocating compressors

Cons

  • Makes the control complex
  • The Over-run timer must be set to limit premature starting and stopping

Variable Displacement

Pros

  • Energy-efficient control scheme that gets down to 50% of capacity
  • Matches displacement to demand without reducing inlet pressure or increasing ratios of compression

Cons

  • Makes the control complex
  • High initial cost
  • Only available for 50 HP+ compressors

Variable Speed

Pros

  • Energy-efficient and precise control
  • Various rotating speeds and giving more displacement and power. These are directly proportional to speed rotation

Cons

  • Makes the control complex
  • High initial cost
  • Reduced full load efficiency
  • Efficiency of rotary screw compressor ends drop at lower or higher speeds

The 6 Types of Basic Individual Compressor Controls

Compressor controls are designed to match compressor delivery with compressed air demand, by maintaining the compressor discharge pressure within a highly specified range.  This discharge pressure should be set as low as possible to minimize the energy usage.

Compressor systems are typically composed of multiple compressors delivering air to a common plant air header. The combined capacity of these machines is generally sized to meet the maximum plant air demand. System controls are almost always needed to orchestrate a reduction in the output of the individual compressors during the times of having lower demand. Compressed air systems are usually designed to operate within a fixed pressure range and to deliver a volume of air that varies with system demand. System pressure is monitored and the control system decreases compressor output when the pressure reaches a predetermined level. Compressor output is then increased again when the pressure drops to a lower predetermined level.

There are 6 basic types of individual compressor controls that a person has to take into account when looking into purchasing and using air compression:

Start/Stop

  • Turns the motor which drives the compressor on or off in response to a pressure signal (seen on reciprocating and rotary compressors)

Load/Unload

  • Allows the motor to run constantly but unloads the compressor when a predetermined pressure is reached. The compressor reloads at a predetermined lower discharge pressure. This is also sometimes referred to as constant speed or constant run control (seen on reciprocating, rotary, and centrifugal compressors).

Modulating

  • Restricts passage of air to the compressor to progressively reduce compressor output to a specified minimum, when the compressor is then unloaded. This is also referred to as throttling or capacity control (seen on rotary and centrifugal compressors).

Dual/Auto Dual

  • This controller is commonly seen in small reciprocating compressors, allows the selection of either Start/Stop or Load/Unload. When used in a lubricant-injected rotary compressor it provides modulation or load/unload control to a preset reduced capacity. When unloading the addition of an over-run timer will stop the compressor after running unloaded for a preset time.

Variable Displacement

  • This controller allows progressive reduction of the compressor displacement without reducing the channel opening (seen on reciprocating and rotary compressors).

Variable Speed

  • This controller adjusts the compressor capacity by varying the speed of the electric motor driving the compressor in response to system signals.

Learn more about the these six basic compressor controllers and more about other FS Curtis products