Compressor capacity controls

Evaluate them carefully to determine their effect on energy cost.

By Henry Van Ormer

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Consultants reviewing compressed air systems find that the single biggest energy recovery opportunity, particularly in applications with multiple compressor units, lies with misused or poorly applied compressed air unloading controls. We have seen many instances of a plant that embarked on a significant compressed air conservation program on the demand side addressing issues such as:

  • Identifying and repairing leaks.
  • Reducing open blows.
  • Adjusting automatic condensate drains.
  • Managing potential inappropriate use.

When the dust settles, some only find that, although they are using less compressed air for production, the actual electric energy consumed did not decrease proportionately and sometimes did not reduce at all. In one recent case, after repairing the leaks, the system pressure rose and the electric power consumed actually increased.

A common reason for this outcome is that the unloading controls are not working correctly. They are simply not responding to the system demand.

What are capacity (unloading) controls?

The two most effective ways to run an air compressor are at full load and off. Continuously-operating unloading controls restrict the air delivered to the system while the unit is still running. This is always a compromise and, on the basis of specific power (BHP/CFM), is never as efficient as operating at full load.

There has been a great deal written on how capacity controls function. Now it is time to look at what they can and can't do and some of the common problems that significantly affect their ability to perform.

Basic objectives of unloading

When working effectively, unloading controls do several things. They:

  • Match supply of air to the demand at the appropriate time and minimize or eliminate system overpressure while maintaining the necessary minimum acceptable operating pressure.
  • Reduce the input power cost to the optimum point (proportional to the air flow demand).
  • Turn off unneeded air compressors and activate them when required.

Basic types of controls

Before reviewing the types of capacity controls, we should define the operating pressure band, also called the dead band or proportional band. This is the pressure range the control can span from fully loaded flow to fully unloaded (no flow). Compressors larger than 50 hp use a 10-psi band, although others can be and are used.

There is a high cost to higher system pressure (1/2% per psi) and increased flow through unregulated users (1% per psi). Most well planned systems try to hold as narrow a band as possible unless there is a specific requirement for a larger band. Regardless of air compressor type, capacity controls fall into several basic categories. Some will only be available on certain types of compressors.

First is automatic start/stop. On any compressor, this control refers to the automatic starting and stopping of the electric motor or driver. Usually, a pressure switch shuts off the motor at the upper pressure limit and restarts the motor at a lower system pressure. Although operating either at full load or off is the most efficient way to run an air compressor, most AC electric motors tolerate only a limited number of starts over a given time for reasons of heat build up. This limits the application of automatic start/stop, particularly for motors larger than 10 to 25 hp. A large system must run above minimum system pressure to hold minimum pressure and performance is dependent on adequate effective storage.

Unloading with continuous-run controls implies the driver or electric motor continues to run while the air compressor is unloaded in some manner. The objective is to match supply to demand, usually on the basis of system pressure. Continuous run controls either can be step or modulating type.

Step controls are also called on line/off line; cut in/cut out; load/no load; two-step (or three-step, five-step). The most common is two-step control, which keeps the compressor inlet either fully open or fully shut. Over the complete operational band, the unit is at full load from the preset minimum pressure point (load point) to the preset maximum pressure (no load point). This control is available on every type of air compressor as either a primary unit or part of a dual-control system.

The basic performance is fully loaded or full flow at points throughout the operational pressure band up to the final preset maximum pressure when the air flow shuts off completely. The unit then stays at no flow and full idle until the system pressure falls to the preset minimum when the unit immediately goes to full flow capacity.

In this mode of control, the compressor runs at its two most efficient modes — full load and full idle — which represents the lowest possible input power cost. Full idle at lowest input power occurs almost immediately, except in the case of lubricated or lubricant-cooled rotary screw and centrifugal compressors.

With lubricant-cooled rotary screws, full idle and lowest input power does not occur until the oil sump pressure is bled down. This can often represent a time delay from 20 seconds to as much as two minutes. Centrifugal compressors often have some time delay built into the controls before they go to full idle. Double acting reciprocating compressors can also be equipped with three- and five-step unloading.

It is usually easy to tell if the unit is loaded or unloaded. Comparing the duty cycle gives an accurate reflection of actual flow as a percent of rated capacity.

Correct piping and adequate storage is necessary to allow sufficient idle time over the operational pressure band to generate significant energy savings. This is particularly true with lubricant-cooled rotary screws, which must cover the bleed down time before any significant power cost savings can occur.

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