Last week I attended the Advanced Management of Compressed Air Systems seminar put on by the Compressed Air Challenge. For those unfamiliar with the Compressed Air Challenge, it’s an organization focused on delivering reliable and sustainable compressed air that has maximized efficiency. Many of the industry’s best practices are preached, if not mandated, and the ultimate goal is to reduce compressed air use as much as possible. This fits in line with EXAIR products, their design for maximum efficiency, and the recurring ability of our customers to reduce their compressed air use by using our products.
The “advanced” seminar dives into compressed air system profiles, explores the math and theory behind system design, explains the various types of system controls, and shows how to balance compressed air supply and demand. These things are great not only on their inherent value, but also because when Brian Farno, Russ Bowman, and I attended the Fundamentals of Compressed Air Systems seminar, we kept raising our hands asking questions that were “too advanced”. The material presented here answered many of those questions, and sparked a few new ones.
One of the questions that came to me during the training had to do with the capacitance of a compressed air system. When storing the energy of a compressed air system in a receiver tank, there has to be a pressure gradient in order for there to be energy storage. If a receiver tank has the same inlet and outlet pressure, it is merely part of the system plumbing and provides no benefit to the system when demand peaks. So I thought to myself, “if a pressure drop is needed across a receiver tank to achieve system capacitance, and the capacitance of the system is related to the value of that differential, a system could theoretically be supplied enough compressed air volume with the right pressure specs”.
So, I looked to the formula used for sizing a receiver tank.
V = (T x (C – R) x Pa)/P1-P2
V = Receiver volume in cubic feet
T = Time of the event in minutes (amount of time for which the receiver tank must be able to provide compressed air at the needed rate)
C = Intermittent demand amount (how much flow or “Q”) in CFM
R = Flow into tank during event (through needle valve, spare air in system, etc.) in CFM
Pa = Absolute atmospheric pressure (14.7 PSIA)
P1 = Initial receiver tank pressure (in PSI)
P2 = Final receiver tank pressure (in PSI)
Ok, nothing new there. First grade stuff. Plugging in some theoretical values we could say:
T = 1 minute
C = 50 cubic feet per minute
R = 0 cubic feet per minute. In this example we’ll assume there is no residual compressed air flow and that the receiver tank must deliver all the airflow for the duration of the event.
Pa = 14.7
P1 = 100 PSIG
P2 = 90 PSIG
Using these values, the volume calculates to be 73.5 cubic feet. But, most receiver tanks are sized in gallons so we can multiply by 7.48 to get the figure in gallons. (7.48 gallons = 1 cubic foot) This yields an approximate value of 550 gallons. In plain terms, for the application above, we would need a 550 gallon receiver tank with an inlet pressure of 100 PSIG and an outlet pressure of 90 PSIG to provide compressed airflow over the needed (1) minute duration.
That’s a big tank.
Now, back to my thought on pressure differentials – if we increase the ΔP, we can decrease the size of the receiver tank. Let’s say the inlet pressure to the receiver tank can be as high as 130 PSIG (a wet tank, in line before any filters or dryers). This will quadruple the pressure differential and reduce the size of the tank by 75% to 138 gallons. Great!
Well, great for a new system, but what about one already in place? What if the application needs 50 CFM of compressed air flow for 1 minute, and the shop already has a 175 gallon tank. We can work the equation in reverse to determine the necessary pressure differential that will ensure the system has enough capacitance to sustain the event (approximately 32 PSI). It’s good to know the math.
As a whole, the seminar was a great success and the presenters proved why they’re experts in the field of compressed air. We’re not too shabby here at EXAIR either. If you have an application need, give us a call.