The storage of compressed air in receiver tanks is very important to consider when excellent system energy efficiency is the goal. Among other things, compressed air that is stored in system air receivers can be used to increase the efficiency of the running air compressors and to help stabilize system pressure so lower average pressures can be used. It also can help the system to ride out large compressed air demands or major system events that might cause transient low pressure issues.
How much storage capacity does a system need? This is a common question and is typically subject to a number of different industry rules of thumb. Led by the efforts and recommendations of Compressed Air Challenge, many in the industry are now recommending a storage capacity of between 5 and 10 gallons per cfm for compressed air systems. Applying these sizes reduces the compressor cycle frequency substantially and saves significant power.
Storage capacity and load/unload control
In the past, the recommended storage receiver size was 1 gallon per each cubic foot per minute (cfm) of the system air compressor capacity. For example, if the compressor was a 100 hp, 400 cfm rated unit, then a storage receiver sized at about 400 gallons was used. This sizing guideline was common when most lubricated screw air compressors were running in inefficient modulation mode pressure control. (Modulation control is where the compressor inlet valve is progressively closed off by the local control system to control the output flow. In modulation mode, the power consumption only reduces 3 percent for each 10 percent reduction in flow.)
Over the years, load/unload control mode has become the most common control method because it is a more efficient way to run lubricated screw compressors than modulation mode at loads that are lower than the full compressor capacity. Load/unload controlled compressors cycle between loaded and unloaded condition at part loads. In load/unload control mode, the theoretical ideal power turn down is about 7 percent for each 10 percent reduction in flow. But with lubricated screw compressors, this turn down is affected by load/unload cycle frequency.
The characteristic cycle frequency of a load/unload compressor depends on the width of the compressor pressure band, the volume of the effective system storage, and the loading percentage of the compressor. The higher the cycle frequency, the less efficient the compressor runs at part loads, especially if it is a lubricant injected compressor. The changes to the power vs. flow characteristics with changing storage receiver sizes are shown in Figure 1.
The load/unload state of the compressor is controlled by the setting in compressor pressure controller, usually set with a band which is 10 psi wide. If the old "1 gallon per cfm" rule is applied to a compressor with a 10 psi wide setting, the expected theoretical efficiency gain due to switching from modulation to load/unload control would not materialize because the load/unload cycle frequency is too high, as seen on the top dark blue curve in Figure 1. This problem has led compressor suppliers to upgrade the industry rule of thumb to "2 gallons per cfm". Increasing the capacity to this level reduces the cycle frequency by a factor of 2 but the compressors still have marginal efficiency at some part loads.
In examining Figure 1, if a compressor with 100 kW full load power and 1 gallon per cfm of storage capacity was operating at 40% load, it would consume about 82 kW. If that same compressor was working with 3 gallons per cfm of storage, the power consumption would reduce to about 68 kW for energy savings of 17%. This is still significantly different than the expected 55 kW power consumption for 33% savings if the ideal compressor curve is used to calculate the expected savings.