Purge is a percentage of the wrong number
A heatless regenerative dryer dries the offline tower by bleeding already dried air backwards through it, and the data sheet quotes that bleed as a share of the flow entering the dryer. Read it as a share of what the shop gets and the arithmetic comes out low.
At 15 percent purge and 100 cfm needed downstream, the dryer inlet has to be 117.6 cfm and the purge is 17.6 cfm. That is 17.6 percent on top of what the shop uses, not 15. At 20 kW per 100 cfm the purge alone draws 3.53 kW, and over 4,000 loaded hours at 14 cents that is 1,977 dollars a year of electricity turned into a hiss at the muffler.
None of which makes a heatless dryer the wrong choice. It has no heater, no moving parts to speak of and it reaches dew points a refrigerated dryer cannot approach. It just does it by spending air, and the air has a price.
Every psi of differential is charged on the whole flow
The filters and the dryer take pressure, and the compressor has to make it up by discharging higher. Working the compression from the pressure ratio raised to 0.286, going from 114.7 psia to 122.7 psia — that is 8 psi of differential on a 100 psi header — costs 4.38 percent more energy. Over the 23.5 kW the machine is drawing to make the inlet flow, that is 1.03 kW, 4,120 kWh and 577 dollars a year.
The per psi figure works out at 0.55 percent, which is where the old rule of about one percent per two psi comes from. The rule is close over normal shop pressures and it drifts at the edges, which is why the page computes it rather than applying it.
What loaded elements cost
Filter elements start at some differential and climb as they load. Going from 3 psi when new to 8 psi now is 5 psi of pure loading, and on these numbers it is the difference between 220 dollars and 577 dollars a year — 357 dollars, or about 30 dollars a month for as long as it stays that way.
That is not an argument for a change interval. When an element must be replaced is set by the differential its manufacturer states and by what the air downstream is doing, and this page has no view on either. It is an argument for reading the gauges, which most shops have and few people look at.
Both together
At the defaults, making 117.6 cfm at the base pressure costs 13,176 dollars of electricity a year. The purge adds 1,977 and the differential adds 577, so the treatment train is running at 19.4 percent on top of the air bill. A refrigerated dryer would replace most of the purge cost with its own electricity, which is why the form has a field for that too — the comparison is only fair if both machines are charged for what they actually use.
Where the model gives out
The compression penalty uses single stage adiabatic work from atmospheric, which is a clean and conservative way to get the shape of the relationship. A two stage machine with intercooling does slightly better than the model says; a variable speed drive changes the picture differently again, because it responds to pressure by changing speed rather than by cycling. For a specific machine, the performance curve from the manufacturer is the real answer, and it will not be far from this over the few psi that a filter differential represents.
What is not approximate is the direction. Avoidable differential is charged against every cubic foot that passes, all year, whether anybody is using it or not.
Questions people ask
How much air does a desiccant dryer use for purge?
It depends on the type and it is on the data sheet, which is why the page asks rather than assumes. Heatless regenerative dryers use the most, heated and blower purge types use much less air and some electricity instead, and refrigerated dryers use no purge air at all and run a refrigeration circuit. Whatever your figure is, remember it is quoted against the dryer inlet flow: 15 percent purge means the compressor makes 17.6 percent more air than the shop uses.
How much does one psi of pressure drop cost?
On the defaults here, about 0.55 percent of the compressor energy per psi, which comes to roughly 72 dollars a year per psi on a machine drawing 23.5 kW for 4,000 hours at 14 cents. That is where the familiar rule of about one percent per two psi comes from. The page computes it from the compression work ratio rather than applying the rule, because the rule drifts at pressures away from the middle of the usual range.
Is it cheaper to run a refrigerated dryer than a desiccant one?
In energy terms usually yes, because it spends its own electricity rather than throwing away compressed air, and compressed air is expensive electricity. But they do not do the same job — a refrigerated dryer works above freezing and a desiccant dryer reaches dew points far below it. Which one belongs in a line is decided by what the air is used for, not by a running cost comparison. Put both machines into this page with their own purge and power figures if you want the costs side by side.
When should I change my filter elements?
At whatever differential the manufacturer states for the element, and sooner if what the air is doing downstream demands it. This page will not tell you, and a payback sum is the wrong tool for that decision. What it does tell you is what the extra differential is costing while you decide, which at 5 psi of loading on these numbers is about 30 dollars a month.
Why does the purge cost more than I expected?
Two reasons stack. The percentage is against the dryer inlet rather than the delivered flow, so it is larger than it looks. And purge air is fully made air — compressed, cooled, dried and then vented, carrying the full specific power of the machine with it. At 20 kW per 100 cfm, 17.6 cfm of purge is 3.5 kW running continuously whenever the compressor is loaded, which is a small motor left on all year.