Pros and Cons of Individual Compressor Controls

Previously we discussed The 6 Types of Basic Individual Compressor Controls to learn about the differences, but how do you determine which controls you would want for your compressor? To help with this decision here are few pros and cons of the basic air compressor controls:

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
• The pressure setting to stop must be higher than required system pressure to build storage. This may increase energy use.
• Loss 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