Compressed air systems are significant investments for business owners, and they require a full understanding of which factors can affect performance. Before making the purchase, consider one of the most important aspects: the size of the system’s storage tank in relation to your needs.
Appropriately sized storage capacity is vital to the overall stability, reliability and efficiency of a compressed air system. For example, by properly sizing storage in a load-unload compressor, not only can the compressor avoid short cycling, but it can also reduce the system’s overall energy use.
Times have changed, and the old rule of thumb of one gallon of storage per CFM output is no longer the norm when sizing. Often, two gallons of storage per CFM output is suggested. But Mike Chudy, Ingersoll Rand Product Manager at John Henry Foster, believes that, typically, much more storage capacity is needed for maximum efficiency.
“During the hundreds of audits that John Henry Foster has performed, we often find that two gallons per CFM doesn’t even scratch the surface,” says Chudy. “We’re looking to define the system capacitance and the number of cycles per CFM output of the system. We now see that we may need as much as 10-20 gallons of storage capacity (in gallons) per CFM output. What we’re trying to do with properly sized storage is to reduce the overall online horsepower in the system and size it for the worst-case scenario in regards to max flow.”
Properly defining system capacitance helps to determine what the ratio of gallons per cfm is required. Capacitance is rated in cubic feet per psi (pounds per square inch) and is defined as the volume of air required in the system to move pressure +/- 1 psi. Knowing capacitance will accurately tell you the amount of storage necessary to facilitate additions or subtractions of equipment tied to the compressed air system and how they might affect overall efficiency.
If peak flows only occurs for a short duration of time, you can make up that short duration with extra capacity.
What this means for buyers is that by sizing the storage tank correctly and requiring additional capacity, the size of the air compressor can be reduced. This in turn reduces the overall purchase price; it reduces the overhead maintenance costs; and more importantly, it reduces the long-term electricity expenses.
A well-sized storage tank provides stability. When it is properly sized and used in conjunction with a large-demand expander or flow controller, it takes any fluctuation of pressure out of the equation. The only way it can work in conjunction and maintain a steady set pressure is with enough storage capacity.
Maximum storage can also help reduce short cycling on a load-unload style fixed-speed air compressor. With a variable-speed drive, enough storage helps flatten out the peaks and valleys that are seen when systems are trying to maintain the plant’s demand.
Another way to stabilize plant pressure is to add a pressure control with properly sized storage. For example, run a compressor and a tank up to 110-115 PSI, then regulate down at the pressure control valve to 100 PSI. Now you have a 15-PSI bandwidth of additional storage ready to tackle any event.
“What this does is help stabilize plant pressure, which is also good for reliability and quality of whatever product the plant is producing downstream,” says Chudy. “It also helps in the case of a failure. The main compressor is able to start a backup machine before the plant ever realizes there is a problem.”
Once system capacitance is defined, you can determine how pressure control will add to the overall efficiency and reliability of your system. We’ll tackle this in Part II.
Whether your compressed air system is a small, portable tank or an industrial-sized compressor used in manufacturing applications, choosing the correct size storage tank deserves much consideration. As you weigh your options, evaluate your current CFM requirements and forecast your future demands. By assessing this information and properly sizing your system, you have the power to save time, long-term costs and more.
Author: Megan O’Neill