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Quotes - Engineering - SalesCycling vs Non-Cycling Refrigerated Air Dryers
Both kinds of dryer are sized the same way and both deliver a 38°F dew point at full load. The difference is what they do for the many hours a plant runs below full load. A non-cycling dryer keeps its refrigeration compressor running whatever the air flow and dumps the unneeded cooling through a hot-gas bypass valve, so it draws its nameplate power at 20 percent load as surely as at 100. A cycling dryer stores cold in a glycol loop, a phase-change material or the exchanger mass, and switches its compressor off until that store warms up.
That single difference decides the running cost. The rest of this page puts kilowatt-hours on it for a 500 SCFM dryer across four duty profiles.
How each technology behaves at part load
Non-cycling: the DXR Series, the HPRB Series and the large HPRplus units. Power is flat, the dew point is the steadiest of any type, and the machine has the fewest parts and the lowest price. Thermal-mass cycling: the TMC Series with a glycol loop, the FLXA Series with a phase-change reservoir and the NXC Series with a dry mass. The compressor rests whenever the store is cold and power tracks the air load, which is where the published figure of up to 80 percent savings at part load comes from.
Digital scroll: the AES Series from 600 SCFM and the HES Series from 2,500 SCFM, whose compressor modulates from 0 to 100 percent instead of stopping and starting. Variable speed: the VDR Series from 2,600 SCFM, with inverter-driven compressors that follow the load continuously. Both suit central plants where the load moves all day and the dew point must not wander.
Where the payback comes from
A cycling dryer costs more than a non-cycling dryer of the same capacity, and at full load around the clock it saves nothing, because the compressor never gets to rest. The saving is the product of the dryer's full-load kilowatts, the hours it is powered, and how far below full load the plant sits during those hours. A plant that leaves the air system up all year but draws real air for 4,000 hours is the ideal case, and there the cycling dryer cuts its own power by more than half.
Payback is the price premium divided by the annual saving at your electricity rate. We quote both machines on every inquiry above 100 SCFM, and we say when the non-cycling unit is the better buy.
Cold banked in glycol, a phase-change reservoir or the exchanger's own metal, which keeps chilling the air while the refrigeration compressor is off and is recharged when it restarts.
Worked example: 500 SCFM, two shifts, five days
Dryer powered all year, compressor loaded 4,160 hours
- Non-cycling DXR 0500 NA draws 3.2 kW for 8,760 hours: about 28,000 kWh a year
- Thermal-mass TMC 0510 N draws 3.7 kW at full load, averaging about 35 percent of it across the year: about 11,300 kWh
- Saving about 16,700 kWh, roughly 2,000 dollars a year at 12 cents per kWh
- Same 500 SCFM plant loaded 100 percent around the clock: saving nil
Reading a cycling dryer's data sheet
What the rating, the kW and the savings claim each mean
- SCFM is rated at the same 100 psig, 100°F, 100°F point as a non-cycling dryer
- The kW figure is full-load draw; part-load draw falls with the air load
- Savings of up to 80 percent apply at low load, not at full load
- Dew point is class 4 to 5: it moves a few degrees across each cycle
Annual dryer power by duty profile, 500 SCFM class
Non-cycling taken as the 3.2 kW DXR 0500 NA at its nameplate draw whenever powered; cycling taken as the 3.7 kW TMC 0510 N drawing in proportion to the average air load. Figures are rounded estimates for comparison, not guarantees.
| DUTY PROFILE | HOURS POWERED | AVERAGE AIR LOAD | NON-CYCLING | CYCLING | SAVING |
| Three shifts, seven days, compressor loaded flat out | 8,760 | 100 percent | 28,000 kWh | 32,400 kWh | none: non-cycling is the right machine |
| Three shifts, weekends idle, dryer left powered | 8,760 | 60 percent | 28,000 kWh | 19,400 kWh | 8,600 kWh, 31 percent |
| Two shifts, five days, dryer left powered | 8,760 | 35 percent | 28,000 kWh | 11,300 kWh | 16,700 kWh, 60 percent |
| One shift, dryer switched off with the compressor | 2,080 | 50 percent | 6,700 kWh | 3,800 kWh | 2,900 kWh, 43 percent |
Log the powered hours
Count the nights and weekends the dryer stays switched on
Estimate average load
The honest fraction of compressor output in use
Work both kWh totals
Full-load kW times hours, scaled for the cycling unit
Divide the premium
Price difference over the annual saving gives payback
Thermal-mass, digital-scroll and non-cycling dryers
Choosing by duty profile
Write down the hours the dryer is powered each year and, honestly, the average fraction of the compressor's output the plant uses during them. Under 60 percent average load, or any profile that leaves the dryer powered through nights and weekends, favors cycling. A plant loaded 90 percent or more whenever it is running is a non-cycling duty, and the cheaper machine is also the right one. To 2,000 SCFM the thermal-mass units are the usual cycling choice; from 600 SCFM the digital-scroll AES Series competes, and above 2,500 SCFM the AES, HES and VDR Series are the options.
The hot-gas bypass valve
A non-cycling dryer would freeze its own condensate at low load if the evaporator kept getting colder, so a hot-gas bypass valve feeds hot refrigerant back to the evaporator inlet to hold it just above 32°F. It is a reliable, simple control, and it is also the reason the compressor never stops: the cooling is made and then thrown away rather than not made at all.
Mistakes in the comparison
Comparing full-load kilowatts, which favors the non-cycling dryer, when the plant never runs at full load. Assuming the savings claim applies at every load. Forgetting the hours the compressor merely idles, when a non-cycling unit is still drawing its full rate. Buying a cycling dryer for a compressor that runs flat out on three shifts. And sizing a cycling dryer smaller because it stores cold: the store covers minutes, not the next load cycle, so the corrected duty stands.
Dew point across the cycle
A thermal-mass dryer lets the store warm a few degrees before the compressor restarts, so the outlet dew point drifts between roughly 35 and 45°F, which ISO 8573-1 counts as class 4 to 5 and which no tool, cylinder or paint gun will notice. Digital-scroll and variable-speed units modulate rather than stop and hold within a degree or two. Where a process is sensitive to a 10°F swing, say so and we select the AES or VDR Series or a non-cycling unit.
Deciding when stored cold pays for itself
Its refrigeration compressor never stops. To keep the evaporator from dropping below freezing when air flow is light, a hot-gas bypass valve returns hot refrigerant to the evaporator and holds it just above 32°F. Cooling the plant does not need is produced and then canceled, so the nameplate kilowatts are drawn whether the header is busy or idle. That same simple control is the reason the outlet dew point barely moves.
Multiply the cycling unit's full-load kilowatts by the hours it is powered and by the average fraction of compressor output the plant uses, then set that against the non-cycling unit's kilowatts times the same hours. For the 500 SCFM example on two shifts, five days, with the dryer left on, the result is about 11,300 kWh against 28,000. Divide the price difference by the saving at your electricity rate to get the payback.
Not across a whole year. That figure applies at low load, when stored cold carries the dryer through long spells with it's compressor off. A two-shift, five-day plant that leaves the dryer powered averages nearer 35 percent load, and the worked figures show a saving of about 60 percent. A plant loaded flat out on three shifts saves nothing, so judge any savings claim against your own hours and load.
No. The outlet drifts between roughly 35 and 45°F as the store warms and the compressor restarts, and pipe inside a heated building rarely gets cold enough for that air to condense. The swing only matters to a process that reacts to a 10°F change in moisture. Those duties are better served by a digital-scroll or variable-speed unit, which modulates instead of stopping, or by a non-cycling dryer.
If the plant truly uses no air overnight, yes, and doing so shrinks the case for cycling: on the single-shift profile the non-cycling unit uses about 6,700 kWh a year and the cycling unit 3,800. Power the dryer up before the compressor loads so the refrigeration circuit is already cold when air arrives. A dryer started with air flowing through it passes wet air for its first few minutes.
