Use Control (Wet) Storage for an Efficient Compressed Air System

When evaluating a compressed air system and the proper use of storage, many people misunderstand control storage. Many people refer to control storage as wet, or primary storage. However, the term control storage is more reflective of its main function – to maximize the operation of the compressor control.

For the purpose of this post, we will define this type of storage as any storage created between the air compressor discharge and before any cleanup equipment. This equipment includes filters and air dryers.

Control storage differs from demand (dry) storage in one particular way. Demand storage is created after cleanup equipment and before demand regulation. Basically, the types of storage differ in the storage location and use. It is not that unusual in compressed air system design to use both types of storage.

This post focuses mostly on rotary screw use. The rotary screw air compressor has nearly entirely replaced reciprocating compressors as the best choice in industrial applications. Please note that systems using reciprocating and rotary screw air compressors together must use this type of storage.

Why is Control Storage Used in Reciprocating Air Compressors?

The reciprocating air compressor was once the air compressor of choice and used extensively in industrial plants. Control storage has always been a necessary part of reciprocating air compressor use regardless of the system size. The reasons for this are:

  • Reduced pulsations from the compressor discharge
  • Provided condensate removal through condensation and settling
  • Elimination of short cycling of the compressor controls

When rotary screw compressors began to hit the market at large in the 1960’s, the reciprocating air compressor was nearly entirely replaced. The demise of the market for reciprocating compressors led to a misunderstanding of when and how to apply control storage.

How the Rotary Screw Compressor Created a Misunderstanding

The demise of control storage occurred as a result of two significant rotary screw advantages.

  • The elimination of pulsations at the compressor discharge.
  • The development of alternative control schemes such as modulation, variable displacement, and variable speed.

Many people believe that control storage is no longer required, as its function has been replaced by the flexibility of the modern compressor controls.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.

Where Should Control Storage Be Used?

There are a few principles that can be used to help you decide if you need to use control storage. These principles are based on the four basic types of compressor controls offered on rotary screw air compressors.

Modulation Controls

Rotary screw compressors with modulation control do not necessarily need control storage. While modulation control is not the most efficient control at partial compressor loads, its smooth control reaction to changes in compressor load minimizes the need for 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. Please note that this happens rarely.

Load/No Load (On-line/Off-line Control)

Load/no load (also referred to as on-line/off-line) control does require control storage. Without the proper amount of control storage, the short cycling of the controls will occur. The result is premature wear and failure of the compressor control system. Signs that this is occurring include rapid loading and unloading of the air compressors at less than full load conditions. People often misdiagnose this 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. In addition, this may result in short cycling in the compressor controls.

Properly applied control storage will create a buffer between the sensing point of the compressor controls and cleanup equipment providing time to smooth out the compressor controls reaction. Optimally, we recommend a control storage ratio of 3-5 gal./cfm output.

Variable Displacement

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, 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, you should apply control storage (3-5 gal./cfm output).

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 reason for utilizing this storage solely as a liquid knockout tank on compressor control schemes that otherwise would not normally require control storage is problematic. In order for the 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.


Control storage is a tool that can significantly improve the reliability and operation of an air compressor system but needs to be properly evaluated. Use the tips above to help you decide whether this storage option is right for you. As always, if you have any questions related to control storage, please contact us by email or phone.