Gas Compressor Capacity Control

Gas compressors used in various industries, from oil and gas tank vapor control to biogas methane capture, require capacity control to handle the variability in gas production.  This article will provide a brief overview on methods of controlling positive displacement compressor capacity.

What is a Positive Displacement Compressor?

• A positive displacement compressor is a type of compressor where gas pressure is increased by physically reducing the volume of the gas.
• Examples of positive displacement compressors include:
  • Reciprocating compressors
  • Screw compressors
  • Diaphragm compressors
  • Sliding vane compressors
• By contrast, dynamic compressors operate differently:
  • They impart velocity to the gas, which is then converted to pressure through a diffuser.
  • Examples of dynamic compressors include:
    • Centrifugal compressors (axial and radial)
    • Ejectors

Why is Compressor Capacity Control Required?

Compressor capacity control is essential because gas streams often experience changing flow rates. These variations can occur due to production differences or consumption changes.

Examples of Flow Rate Variations in Gas Production

• Ambient temperature changes in oil tanks, such as heating during the day, can alter gas production.
• Feedstock differences entering a digester can result in varying gas generation rates.

Examples of Flow Rate Variations in Gas Consumption

• Gas compressors supplying fuel to gas engines generating electric power must adjust flow according to engine demand.
• During off-peak hours, electric power demand decreases, reducing the required fuel rate from the engine and, consequently, the flow rate needed from the gas compressor.

Key Point: Capacity control ensures that the compressor efficiently matches gas supply with varying production and consumption needs, maintaining safety, efficiency, and reliability.

Methods of Gas Compressor Control

Different options exist for controlling gas compressor capacity.  The items below will discuss common methods for controlling positive displacement gas compressor capacity.  Specific compressor technologies have additional methods of capacity control, but they are outside the scope of this discussion.

Compressor Start/Stop Capacity Control

Compressor start/stop control is one of the oldest methods of controlling gas compressor flow rate.  This method is commonly used in situations where gas can be stored in some type of accumulation system and/or gas pressure can be allowed to increase to a certain value before the gas compressor starts drawing the pressure down to a predetermined value.  Once the specified gas low-pressure value is reached the gas compressor is shut off.

This method can be used in systems that can tolerate varying pressures upstream of the compressor.  

Limitation

If upstream pressure must be controlled within a narrow operating band, it’s usually better to vary compressor capacity using other methods (suction throttling, speed control, or bypass control).

Compressor Operating Speed to Control Compressor Capacity

Direct control of compressor operating speed is one of the most frequently used methods of controlling gas compressor capacity. This method can be applied to both electric drive and gas engine drive compressors.

Electric Drive Compressors

• Use a Variable Frequency Drive (VFD) or Variable Speed Drive (VSD) to directly control operating speed.
• A VFD works by adjusting the frequency of power supplied to the motor from the base electrical signal (60 Hz in North America).
• Provides precise and efficient control of compressor capacity.

Gas Engine Drive Compressors

• Operating speed is adjusted through throttle control, typically managed by the engine management system.
• Allows the compressor to match capacity with process requirements.

Important Considerations

• Most compressors have minimum and maximum operating speeds, which may differ from the driver’s limits.
• Speed limits should be set in the drive controller (gas engine or electric) to prevent exceeding compressor design limits.
• Speed control is typically the most efficient method of capacity adjustment.

Limitation

If the minimum process flow requirement is less than the flow produced at the compressor’s minimum operating speed, additional capacity control methods (such as suction throttling or bypass control) may need to be used in combination with speed control.

Click to see how operating speed changes capacity with our visual chart.

Gas Recycle to Control Compressor Capacity/Flow Rate

Once a gas compressor has reached its minimum operating speed the second most common method for controlling compressor capacity is the use of a gas recycle loop.  This method takes gas from the discharge side of the compressor and allows some (or all) of the gas to return to the compressor inlet.  This method can allow for a net flow rate from the compressor package to reach 0 ACFM.  For this method to work successfully the gas should be taken from downstream of the aftercooler and returned to the suction separator. 

•  If the gas is not cooled, the compressor could recycle hot gas continually increasing the operating temperature of the compressor until the compressor shuts down due to reaching a maximum operating temperature.  
• If the gas is cooled, there is potential for liquids to form in the process stream.  These liquids must be removed prior to the compressor inlet to prevent damage to the compressor.  This is why the gas must be returned to the inlet separator.

This method is less efficient than direct compressor speed control, as work is being put into the gas that is not leaving the gas compression skid.  However, this method may be a more cost-effective solution for small, low power gas compression systems versus the cost of adding a variable frequency drive (VFD).

Suction Throttling to Control Compressor Capacity

Another method to control gas compressor capacity is to artificially restrict the gas inlet stream to reduce the amount of mass flow the gas compressor is able to achieve.  This method typically uses an active control system to control the position of a throttling valve.  This method is an acceptable means of compressor capacity control, but is typically the least efficient of the options available.  

Benefit: The benefit suction throttling offers is that it is simpler to implement than a gas recycle system.  

Disadvantage: The disadvantage is that the compressor capacity flow rate cannot be reduced to 0 ACFM, as damage would likely occur to the compressor.

Key Takeaways

• Compressor capacity control adjusts output to match demand, saving energy and reducing wear. 
• Common methods include start/stop cycling, speed control, gas recycle, and suction throttling. 
• Speed control is usually the most efficient, but the best method depends on your system and flow requirements.

Learn More With Ro-Flo Compressors 

All of the above methods are effective methods to control the capacity of your Ro-Flo sliding vane compressor.  Each method of compressor capacity control has advantages and disadvantages that should be considered during system design.  The goal is to cost-effectively implement a system with sufficient flexibility to handle process variation and provide operational reliability.  

Please contact Ro-Flo Compressor’s application engineering team to discuss the most effective means of capacity control for your compressor application.