Compressed Air Storage: A Critical Component for Efficiency Gains
With today’s ever increasing emphasis on improving manufacturing efficiency and reliability, all aspects of the manufacturing process are being examined. Energy audits of manufacturing plants have readily shown that compressed air systems rank very high in potential for efficiency gains. In particular, one area of the compressed air system has consistently proven to be a critical component in achieving this goal – Compressed Air Storage.
When applied correctly, compressed air storage has been an excellent tool in the effort to increase the efficiency, performance and reliability of a compressed air system. It has the potential to supply large amounts of mass flow at an increased rate, which in turn reduces the reaction time required of the system components to demand events. This results in the ability to stabilize system pressure and reduce online power, thus lowering operational and maintenance costs.
Despite these facts, this is still one of the most misunderstood and misapplied aspects of a compressed air system. Here, we address some of the more common misconceptions surrounding compressed air storage and how to apply it.
Myth #1 Useful storage can be created by simply adding storage tanks.
Installing air receivers in a compressed air system without defining the purpose and expected results is a prescription for failure. Poorly applied compressed air storage is very common and while it may not negatively impact the operation of a compressed air system, it will most certainly result in little if any efficiency or performance gains. In order to maximize the effectiveness of compressed air storage, it is essential to measure and verify the performance characteristics of the compressed air system from both the supply side and the demand side. Simply adding additional storage volume, such as an air receiver without the intentional creation of a useful differential would create very little storage and serve no useful purpose. For example, a 10,000-gallon air receiver with only a 2 psi differential would only create 180 cfm of useful storage, while a 10 psi differential would create 900 cfm of useful storage for the same size receiver.
Myth #2 The more compressed air storage the better.
There is some truth to this statement since excessive compressed air storage is very seldom detrimental to the operation of a compressed air system. In fact, excessive storage applied to a compressed air system has the potential to reduce pressure fluctuations by increasing the reaction time available to the supply side of the compressed air system in relation to demand side requirements. However, the downside to over sizing storage in a compressed air system is the increased costs, which will invariably extend the pay-back period and may result in an unfeasible project. As stated earlier, it is always beneficial to measure and verify the performance characteristics of your compressed air system in order to maximize the benefits and improve the return on investment. This can best be accomplished by having a quality compressed air system audit performed.
Myth #3 System piping replaces the need for air receivers.
System piping (headers, sub-headers and drop legs) provide very little useful storage and is not nearly as cost effective as utilizing air receivers. For example, as you can see from the table included, 1,000 ft of 4“ piping has the volume equivalent of a 400-gallon air receiver. Obviously, it is far more cost effective to install a 400-gallon air receiver rather than 1,000 feet of 4” piping. It is also very difficult to create a meaningful pressure differential across system piping, when you must rely only on the natural pressure drop to create this differential. As mentioned previously, the small pressure differential that can be created results in very little if any meaningful storage. Air receivers by contrast, when coupled with Demand Regulation, provide a great opportunity to maximize useful storage by allowing a much greater pressure differential to be created.
Myth #4 Certain types of air compressors do not require storage.
First of all, it is necessary to differentiate between the storage requirements of a particular compressor controls verses that of the compressed air system. When addressing the needs of a particular compressor control, it is true that some controls require less storage than others. For example, when operating at high load levels, some compressor controls such as modulation, variable displacement, and variable speed benefit very little from what is considered control or wet storage. Control or wet storage is considered storage between the compressor discharge and any clean up equipment. However, when these same types of controls operate at lower load levels (certainly below their turndown), control (wet) storage becomes critical to preventing short cycling. Compressor controls such as load/no load do require control (wet) storage, at virtually all load levels to reduce the possibility of short cycling, excessive oil carryover and to increase the efficiency of operation.
When addressing the needs of the compressed air system, the fact is that the compressed air system will always operate more efficiently and reliably with storage properly sized and applied. The permissive of the compressor control and the compressed air system requirements are different and should be considered separately when considering the implementation of compressed air storage.
This article is the first in a series where we will discuss aspects of the creation and application of compressed air storage.
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