The presence of storage receivers in a compressed air system has many benefits. The additional storage helps with compressor control, making the system pressure more stable and slowing down the compressor load/unload cycles — reducing compressor power consumption. But there is often confusion as to where to put the primary storage located near the compressors. Should the receivers be “wet,” located before any system air dryers, or “dry,” storing already dried and filtered compressed air.
Locating the storage tanks on the wet side of the air dryers and filters provides a location that allows the compressed air to cool and drop any free water and lubricant in the quiet zone inside the vessel. This takes some of the moisture and contaminate load off the air dryer and its associated filters, resulting in better compressed air quality.
In addition to this, the compressor “sees” the whole effective storage volume rather than having to “look” through the pressure differentials formed by the air dryer and filters. This increases the effectiveness of the storage volume.
But locating all the system storage on the wet side of the dryer has a disadvantage; if there are any large system events that would use significant amounts of compressed air (that exceed the capacity of the air compressor and dryer), there could be problems. Because air dryers are typically sized to closely match the air compressor size, and high flow system events would draw air from the air compressor and the wet storage receiver, the air dryer capacity may be exceeded, allowing moisture through to the system, reducing air quality.
This problem can be avoided by placing the storage receiver on the dry side of the system. Then, during plant peaks, the already dried air is used as a buffer instead, keeping the flow through the air dryer to only the output of the air compressor. But, because there is a pressure differential across the dryer and filter, the dry tank loses its effectiveness in reducing compressor load/unload cycle frequency.
For example, if the dryer pressure loss is 4 psi and the loss across the filters is 3 psi, the total 7 psi pressure loss will reduce the working pressure band at the dry receiver. This means the receiver “looks like” a smaller receiver to the compressor by a factor of 7 divided by 10, making it 70% less effective in helping reduce compressor power through reduced cycle frequency.
Many experts have found the solution to this dilemma, install both wet and dry receivers, and operate the compressors with remote sensing. Having a wet receiver sized to about 30% of the total storage capacity and a dry receiver making up the final required volume is recommended. Placing remote sensing on the compressors, either through secondary pressure transducers or by using a compressor controller keeps the plant storage effective, allowing the compressor to see the full storage capacity, and saves energy during average conditions.
How big should the storage be? Try to install between 5 to 10 gallons of storage capacity for each cfm output of your largest load/unload compressor. For a 25-hp compressor producing 100 cfm, this would be a total capacity of between 500 and 1,000 gallons.