Supply and Demand, in economics, defines the relationship between the volume of commodities that sellers want to exchange for a certain amount of currency, and the volume of said commodities that buyers are willing to exchange a certain (and sometimes different) amount of currency for. The best chance for an ideal condition is when the volume produced is the same as the volume being consumed. That typically means both sides are amenable to the same certain amount of currency, and everyone’s happy: the sellers are making a fair profit, and the buyers are paying a fair price. Thing is, that’s a difficult balance to maintain. The reasons for imbalances are often debated and usually contentious, and I have no intention of going in to them here. I only brought up the subject to draw an analogy to the difficulties in maintaining a “supply and demand” balance in the design & operation of compressed air systems.
Just like in economics, we have to consider both sides – supply AND demand – to best maintain this balance. Also like in economics, there are numerous factors…on both sides…but the two most critical factors are:
- Compressor capacity control (supply side)
- System storage (demand side)
My colleague Jordan Shouse wrote an excellent blog on “Air Compressor Motors and Controls, Working Together”, outlining the ‘supply side’ variables, allowing me to concentrate my efforts today on system storage. Distribution piping makes up a certain amount of this, and another great blog from another colleague, Tyler Daniel – “Intelligent Compressed Air: Distribution Piping and Pressure Drop” – gets me off the hook for THAT part of the discussion today.
We can consider the air capacity of system piping to be fixed for the purposes of this discussion, so our “variable” will be the capacity of storage tanks. Let’s start with the reasons for the need for system storage: Strategically placed point-of-use air receivers provide stored energy for intermittent demands. This enables the compressed air system to handle fluctuating loads, efficiently & reliably. It also minimizes impact (e.g., sudden and often detrimental drops) on the system pressure.
Next, we’ll look at location. There are a couple of common options to consider:
- The intermittent demand. Installing a receiver here will provide enough air for short duration, high consumption events, protecting the rest of the system from pressure excursions. Dedicating the receiver to this application will mean isolating it from the rest of the system with a check valve (so it only supplies the load in question) and a needle valve (so recharging the receiver itself, between the intermittent uses, doesn’t adversely affect total system pressure).
- The critical load(s). Instead of using stored air for the intermittent load, you can also use it for the important loads you’re trying to protect. All sorts of machinery with pneumatic components can “crash” if a nearby intermittent demand starts up & “steals” their air. You’ll use a check valve (same as above), but using a needle valve to throttle the air flow that recharges the receiver risks “starving” the critical load. Don’t do that unless there’s a really good (and likely really specific) reason for it.
Finally, we’re going to do some math, so we know how big this receiver has to be. Here’s the equation we use to do that:
Let’s calculate the receiver size needed to supply an intermittent load of 400 SCFM (C) @80psig (P2), that’ll run for one minute (T). You can use data specific to your system to come up with a value for (Cap) but here I’m going to assume we want the receiver to be able to handle the whole thing, so Cap = 0. I’m also going to assume we’re at sea level, so Pa = 14.7psia and that our compressor’s discharge pressure (the pressure at which the receiver can be charged to) is 120psig (P1):
That’s an awfully big tank. Now, let’s calculate the receiver size needed to protect a critical load that uses 55 SCFM @60psig, and that due to the system design, we can count on 25 SCFM @120psig from the compressor:
This is a much more manageable size, in fact, our 60 Gallon Receiver Tank (Model 9500-60) would be ideal. It’s 20″ in diameter and just over 50″ tall, so it doesn’t take up a lot of floor space. It comes with a drain valve and connections for compressed air flow in & out, pressure gauge, relief valve, etc.
Now, the above example is a completely hypothetical situation, and I purposely chose exaggerated values to show that there can indeed be a clear “winner” in the choice between the two installation points. “Your mileage may vary,” as the car folks say. If you have a situation like this, and would like help in finding the solution that makes the most sense, give me a call.
Russ Bowman, CCASS
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Cover image courtesy of: Tennessee Valley Authority; SVG version by Tomia, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons