What is in this article?:
Although air compressor operating specifications may look the same on paper, their fundamental designs and controls can make major differences in how they perform.
Importance of capacity controls
Many compressed-air conservation program on the demand side target such issues as:
- identifying and repairing air leaks,
- eliminating open blowing,
- fixing malfunctioning condensate drains, and
- managing all potential inappropriate uses.
When these programs are completed successfully, often it is found that the facility consumes less compressed air for production, but electrical energy consumption does not go down proportionally. The reason: without appropriate capacity controls operating correctly on compressors, it is impossible to effectively translate lower air use into lower electrical energy input.
When working effectively, compressor-unloading controls should:
- match air supply to demand when needed,
- eliminate or minimize system overpressure,
- maintain the necessary minimum acceptable operating system pressure,
- reduce the input power cost to the optimum point proportional to the air flow demand, and
- turn off unneeded air compressors and bring them back on when required.
Regardless of the type of air compressor, the operating principles of capacity controls can be grouped into several basic categories. (Note that some will only perform on certain types of compressors.) Here are descriptions of these categories with some of the pros and cons of each.
Automatic start-stop control - This control simply starts and stops the electric motor or driver automatically. It can operate any type of compressor. A pressure switch usually accomplishes this function, shutting off the motor at the upper pressure limit, restarting it at the minimum system pressure.
Pro: the air compressor runs at its two most efficient modes, fully loaded and off.
Con: most AC electric motors can survive only a finite number of starts over a given time frame, primarily due to heat build up. This limits the application of automatic start-stop controls - particularly for motors larger than 10 to 25 hp.
Con: the compressor must run above minimum system pressure to hold that pressure.
Con: the system must have adequate air-storage capacity to perform satisfactorily.
Continuous-run controls (step type) - With these controls, the driver or electric motor runs continuously while the air compressor is unloaded in some manner to match supply to demand. System pressure usually commands the unloading arrangement. Continuous-run controls can be categorized as step or modulating type.
The most common is the two-step control which holds the compressor inlet either fully open or fully shut. Over the complete operational band, the compressor runs fully loaded (or at full flow) from the preset minimum pressure (or load point) to the preset maximum pressure (or no-load point). At the latter, the control shuts off air flow completely. The unit then runs at no flow and full idle until system pressure falls back to the load point. The control then goes immediately to full-flow capacity. A pressure switch typically actuates the two-step control, which can be either the primary control or part of a dual-control system on virtually every type of air compressor. (Some reciprocating compressors can be fitted with 3- and 5-step controls.)
Pro: the compressor runs at its two most efficient modes - full load and full idle - which results in the lowest possible input power cost. Full idle at lowest input power is accomplished almost immediately, except in the case of lubricated or lubricant-cooled rotary-screw compressors.
Con: both correct piping and adequate air storage are necessary to allow enough idle time over the operational pressure band to generate any significant energy savings.
Con: when two-step controls are misapplied, not only is there little or no power cost savings, but short cycling (i.e.: 20 sec. on/ 20 sec. off) can damage the equipment and shorten the life of normal wearing parts.
Con: too much backpressure in the interconnecting system can cause short cycling or ineffective unloading.
Con: at 85% to 95% loads, step controls consume some extra power because they have to compress at full capacity to a higher pressure just to hold a lower design system pressures.
Continuous-run controls (modulating) - These controls match supply to demand very accurately all along the operating band pressure range. Most incorporate some type of regulator, which in effect converts the operating pressure control band into a proportional band. If system pressure fluctuates as little as 1 psi, the modulating control immediately decreases or increases flow proportionally, depending on the signal. (This control generally is installed only on lubricant-cooled rotary-screw and centrifugal compressors.)
Pro: the minimum set system pressure draws the most power. As system demand falls, pressure rises, flow cuts back, and power usage also falls. This results in a savings at higher demand (and is the opposite of 2-step unloading where the power draw actually increases as system demand falls).
Pro: more efficient at high loads.
Pro: holds a relatively steady pressure when demand is stable, and responds responds quickly to any change.
Pro: does not depend on storage capacity to operate effectively.
Con: is generally more inefficient at lower loads.
Con: too much backpressure in the interconnecting piping can force multiple units into running on part load, when one or more could be shut off.
Controls for rotary screws
Industry's most commonly used air compressor in sizes above 30 hp today is the lubricant-cooled rotary-screw compressor. A significant number (80% to 85%) of these compressors use some form of modulating control as the primary unloading control or as the upper-range portion of a dual control. Two types of these controls for oil-injected rotary-screw compressors are throttled inlet and variable displacement.
In a throttled inlet control, the compressor's inlet valve is opened or closed to match supply to demand as sensed by a pressure regulator. The inlet valve modulates continuously and responds immediately in to any change in the sensed system pressure. In effect, flow capacity is controlled by restricting air intake. The control holds a constant system pressure with minimal valve movement at any given steady system demand.
Pro: smooth, non-cycling control of system pressure is easier on the power train and most other components.
Pro: is relatively efficient at loads from 60% to 100%.
Pro: will not short cycle, regardless of storage capacity and or piping.
Pro: simple to operate and maintain.
Pro: usually results in lower lubricant carryover in lubricated units.
Con: relatively inefficient at loads below 60%.
Con: backpressure must be overcome in order to reach full capacity.
Con: instant response may make the machine back down and unload, even when flow is needed for the base load.
Con: sensitivity and rapid reaction make correct piping and backpressure control necessary for optimum operation. (Note: this is true for all types of unloading controls.)