Control (Wet) Storage – a Misunderstood Concept
When evaluating a compressed air system and the proper application of storage, one of the most misunderstood concepts is control storage. It is often referred to as wet, primary, or in some cases demand storage. However, the term control storage is more reflective of its main function – to maximise the effective operation of the compressor control. For the purpose of this article, I have limited the definition of control storage to any storage created between the air compressor discharge, and before any cleanup equipment i.e. filters and air dryers. While one can argue that control storage shares some commonality of purpose with the more commonly applied demand (dry) storage (storage created after cleanup equipment and before demand regulation), it differs in the location of the storage and its functionality. It is not that unusual in compressed air system design to integrate both control and demand storage taking into account the primary function of both.
While control storage was originally an integral component of a reciprocating air compressor installation, this article will only briefly explore that application. The content of this article will focus instead on rotary screw installations since the rotary screw air compressor has essentially replaced the reciprocating as the compressor of choice in industrial applications. It should also be noted that in compressed air systems where reciprocating and rotary screw air compressors are operating together, control storage should always be utilised.
The reciprocating air compressor, up until the 1960’s, was the air compressor of choice and utilised extensively in general industrial plants. Control storage was always an integral part of a reciprocating air compressor installation; whether it was a tank mount configuration for 25 hp or smaller units, or a stand-alone vertical or horizontal configuration for larger hp.
Three main reasons determined why control storage was always used in conjunction with reciprocating air compressors:
- Reduce the pulsations from the compressor discharge
- Provide condensate removal through condensation and settling
- Eliminate short cycling of the compressor controls
Starting in the 1960’s with the universal acceptance of rotary screw technology (both oil-flooded and oil-free), the reciprocating air compressor has virtually been replaced within the general industrial market. The demise of the market for reciprocating air compressors has led to a misunderstanding of when and how to apply control storage. This lack of understanding seems to have coincided with two major advantages inherent in rotary screw design – the elimination of pulsations at the compressor discharge and the development of alternative control schemes such as modulation, variable displacement and variable speed.
Over the last 30 years, there has been a perception that control storage is no longer required, as its function has been replaced by the flexibility of the modern compressor controls. Therefore, the question becomes “Is control storage an outdated concept?” The answer is no, control storage is not an outdated concept; however its role has become more application specific. The discussion now becomes “Where should control storage be applied and is it a required component of an air compressor system regardless of control type?” While there is not a clear yes or no answer, there are a few general principles that can be applied when taking into account the four basic types of compressor controls offered on rotary screw air compressors:
- Modulation Control
- Load/No Load Control
- Variable Displacement
- Variable Speed Drive (VSD)
Rotary screw compressors with modulation control do not necessarily require control storage. While modulation control is not the most efficient control at partial compressor loads, its smooth control reaction to changes in compressor load minimise the need for control storage. The only exception would be very low demand loads, where the compressor would operate for a sustained period of time outside of the modulation range. It should be noted that in reality this is a very uncommon occurrence.
Load/No Load (On-Line/Off-Line) Control
Load/no load (also referred to as on-line/off-line) control, which is predominant in oil-free and some manufacturers of oil-flooded rotary screws, does require control storage. Without the proper amount to control storage, the short cycling of the controls will occur causing premature wear and failure of the compressor control system. This is exhibited in rapid loading and unloading of the air compressors at less than full load conditions, sometimes misdiagnosed as a high load condition. Most of the load/no load controls are set at a 7-10 psig differential, which is monitored at the compressor discharge. When a load/no load compressor is piped into a compressed air distribution system, the control differential of the air compressor is reduced by the pressure drop across the clean-up equipment. Essentially any pressure drop created by filters, dryers and piping will subtract from the controls differential. This could easily result in a true controls differential of 2-7 psig instead of the normal 10 psig and result in a short cycling in the compressor controls. Properly applied control storage will create a buffer between the sensing point of the compressor controls and clean-up equipment providing time to smooth out the compressor controls reaction. Optimally, a control storage ratio of 3-5 gal./cfm output is recommended.
Variable displacement controls are similar to modulation in the fact that as long as the plant load stays within the operational range of this type of control, then control storage is not necessarily required. However, it needs to be understood that the control range of a variable displacement control is narrower than that of a modulation control. Variable displacement control will only operate at loads of 50% or higher. Below 50%, the compressor operation will revert to a load/no load or modulation control. Should the compressor operate below the 50% level, control storage (3-5 gal./cfm output) should be applied.
Variable Speed Drive (VSD)
Variable speed drive controls are also similar to both modulation and variable displacement in that as long as the compressor operation is within the operational range of the VSD, a case can be made that control storage provides little benefit. As with the case of the variable displacement control, the operator must be aware of the turndown capability of the VSD, which can vary from manufacturer to manufacturer and even within models of a manufacturer’s product line. If the compressor operates below the turndown range of a VSD compressor, then control storage should be applied (3-5 gal./cfm output).
The rationale of utilising control storage solely as a liquid knockout tank on compressor control schemes that otherwise would not normally require control storage is problematic. In order for control storage to be effective in condensate removal, sufficient cooling and a reduction in velocity of the compressed air would have to take place. While some condensate will be discharged from the system at this location, if the sole function is just condensate removal, the cost of high quality separators and condensate drains on the compressor would be more effective and will produce a much better ROI.
As is the case with all types of storage, knowledge of the compressed air system is required to maximise the value it would add to a compressed air system. Without application knowledge, the benefits of storage can be greatly compromised. Control storage is a tool that can significantly improve the reliability and operation of an air compressor system, but needs to be properly evaluated. It is not possible to cover all the aspects regarding the application of control storage within the contents of this article. There are other specific applications that can arise in the design of compressed air systems that may require the use of control storage.
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