The Economics of Pressure Drop

You saw the headline and kept reading. So you must be truly interested. Unlike the professors of so many eye glazing economics courses, I am practical minded. So let’s talk about using compressed air more efficiently. Most of us have seen the “pay me now or pay me later” commercial. This holds true to many things, and compressed air is no exception.

A major use of compressed air is as a power medium, to transfer energy from one place to another in what is many times a safer and more convenient form. The energy transfer occurs when you allow pressurized air to return to atmospheric pressure. This returning of air to its “natural state” pushes pistons, rotors, abrasive blast media, paint, and valves. In short, the air accomplishes work.

With compressed air, as with any energy form, it is important to minimize losses. In electricity, energy loss it is normally called line loss. In pneumatics, losses come in various forms. Leaks and pressure drop are two big areas of loss. It’s easy to understand why a leaking pipe or fitting constitutes energy loss. Pressure drop is not quite as obvious, as you cannot hear it.

Let’s step back and recall what energy is. Available energy is determined by the formula below. Stephen Hawking once said that for every equation one includes in an article, readership drops by 50%. Here is to hoping that is not true:

W = P x V

In this case, W is foot-pounds of work, P is pounds per square foot (to keep units consistent) and V is volume in cubic feet. Therefore, for every pound of pressure loss in a 100 PSIG system, 1% of the available work energy goes away.

Now here’s where we should apply this lesson. Compressed air filters restrict air flow through the system. That filters cause pressure loss is unavoidable. So before selecting a compressed air filter, you should learn how much pressure drop it will cause. Most manufacturers publish pressure drop data. I’ve seen too many customers purchase a small undersized filter because it’s cheaper. Often an operator will select a filter by matching pipe connections and not consider pressure drop. There is nothing wrong with matching pipe size and filter connections, as long as you’ve also considered the effect of pressure drop. If you don’t take pressure drop into consideration when sizing a filter, well, here is where economics comes into play.

Let’s assume it takes 100 horsepower to make 500 SCFM of compressed air, which is about 75 Kwh. If 2% of this is lost through pressure drop, that amounts to 1.5 Kwh, or enough to run your average toaster with a little juice left to spare. Not a big deal...  Until this loss goes on for a year, adding up to 8760 hours for a total of 13,140 Kwh.   If the electricity rate is 7.5¢ per Kwh, the cost of pressure drop, measured by power cost alone, is $985.50. You see my point—pressure drop costs real money.

This is why changing filter elements more rather than less frequently saves money. Running a set of coalescing elements until the pressure differential goes to 10 PSID instead of replacing the elements at 5 PSID means when conditions are between 5 and 10 PSID, you have an average loss of 7.5 PSIG over that period. If this period is 3 months, the additional energy lost is 2.5% of the compressor’s input energy. At $.075 per Kwh, this loss is $303.75. That’s more than enough to pay for several typical 500 SCFM coalescing filter elements.

The take away lesson here is that you can't save money by stalling element replacement, even while running up greater and greater pressure loss. Pressure drop is no different than a leak, as it prevents the energy produced by the compressor from reaching its target.

The moral of the story is:

1. If you are on the borderline when sizing a filter, go up a size. It will pay off later.

2. When the pressure differential gauge is in the red area or greater than 5 PSID, get a new element, no matter what the accountants say.

3. There is more than one moral.

Another approach is to use a mist eliminator type of element, which relies on a different physical process than a coalescer. It uses diffusion, and does not need velocity (translated as pressure drop) to operate.

These are more expensive initially, but have great operating economics over an extended time period because initial pressure drop is very low and element life is warranted to be 10 years before there is an excessive pressure drop and the element needs replacement. This saves money not only from low pressure loss, but from the time and expense (parts and labor) of element replacement. It is another dimension to the “pay me now or later” principle.

We have charts and spreadsheets that you may use to fill in and present to bosses and customers. Let us know and they will be on their way.