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Compressed Air Systems: Controlling Artificial Demand

Key Points
  • Small, undetected leaks can cause a compressed air system to operate at an artificially high pressure. 
  • Artificial demand can be eliminated by lowering demand-side pressure using intermediate control.
  • Select the right type of controller to match compressor output to demand.

Compressed air hosesLeaks in a compressed air system waste as much as 30 percent of the compressor’s output, costing thousands of dollars each year. Fixing these leaks is the first step in reducing this waste. However, because system pressure increases after the leaks are repaired, undetected leaks can become larger.

This can lead to a condition called artificial demand, in which the compressor system operates at an artificially high pressure in order to satisfy production needs at all times, including peak demand periods. The production machinery and equipment consume more air than is necessary. Artificial demand can be eliminated by lowering the demand-side pressure to the minimum level using intermediate control.

Stuck in the middle 

Intermediate control, which uses a pressure/flow controller like a demand expander valve, operates downstream of the cleanup and drying equipment and upstream of the main piping distribution system. Deviations in the demand-side pressure in relation to the set balance point pressure are monitored. The instantaneous deviation at the specific point in time is a good measure of the actual demand.

A signal is generated in accordance with the deviation measured, which directs the flow control in the header module to adjust the flow to correct the deviation. By introducing a controlled differential pressure across an upstream receiver and the associated piping, the compressors are isolated from event loads. This saves up to 50 percent of input horsepower and allows the resulting excess capacity to be stored as reserve energy. Peaks are dealt with using this stored air, allowing compressors to remain offline.

Other benefits of intermediate control include:

  • Stabilizes the system pressure supplied to the production floor
  • Delivers only the amount of air necessary to perform the task
  • Improves product quality and increases production efficiency
  • Reduces compressor wear and maintenance

Once all leaks are repaired, you may have to adjust the controls to reduce run time so that it matches the decrease in demand, especially if you're using load/unload blowdown control. In this case, reduce the pressure drop blow-off set point from 85 percent to 75 percent of rated pressure. You no longer have to maintain constant pressure. If the compressor has a variable frequency drive (VFD) controller, it will automatically be adjusted to match demand.

Control technology

For single compressors, the following types of primary controls are available:

  • Load/unload allows the motor to run continuously but unloads the compressor when the discharge pressure is sufficient.
  • Start/stop is when the compressor motor is turned on or off, depending on the discharge pressure.
  • Modulating controls vary the output to meet flow requirements using throttling, which closes down the inlet valve.
  • Part-load (VFD) controls adjust output pressure without requiring the compressor to start/stop or load/unload. 

For a system with multiple compressors, the situation is more complicated. Automated controls, either sequencing or network, match system demand with compressors operated at or near their maximum efficiency levels.

Keep in mind that some controls aren't suitable for all types of compressors. A variable displacement control only works for positive displacement compressors, such as screw and reciprocating types. Load/unload, modulating or VFD controls will work for any type of compressor.

We can help

National Grid has tools and energy solutions to grow your business and foster strong client relationships. For more information, contact us at 844-280-4325 or retrofit@nationalgrid.com

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