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Quotes - Engineering - SalesSizing a Refrigerated Air Dryer by Flow
Every refrigerated dryer is rated in SCFM at one set of conditions: 100 psig inlet pressure, 100°F inlet air, 100°F ambient, delivering a 38°F pressure dew point. Sizing means finding the flow the dryer will actually see, converting it to that rating point, and picking the first model above the corrected number.
The flow to use is the compressor's delivered output, not the plant's average consumption. A dryer sized to average use is undersized every time the compressor loads.
Start from the compressor, not the tools
The dryer sits between the compressor and everything else, so the most air it can ever see is what the compressor delivers. Read that from the compressor data sheet as ACFM or SCFM at its discharge pressure. If the data sheet is gone, the rules of thumb are 4 to 5 SCFM per horsepower for a rotary screw and 3 to 4 for a piston compressor at 100 psig.
Where several compressors feed one header, size the dryer for the sum of the ones that can run at the same time. Where compressors are in different rooms, one dryer per compressor is usually simpler than a long wet header to a single dryer.
SCFM, ACFM and the rating point
SCFM is flow referred to standard conditions (14.7 psia, 68°F, dry); ACFM is the actual volume at line pressure and temperature. A compressor rated 100 ACFM at inlet delivers about 100 SCFM of free air, and that is the number the dryer tables want. A dryer's SCFM rating at 100 psig is its capacity for that free-air flow arriving at 100 psig.
Pressure matters because the same mass of air occupies less volume at higher pressure, so the dryer's exchanger handles it more easily: a dryer rated 100 SCFM at 100 psig handles about 115 SCFM at 150 psig and only 80 at 60 psig. The pressure correction page has the factors.
Where the sized dryer is
Standard cubic feet per minute: flow stated as free air at 14.7 psia and 68°F, dry, so compressors and dryers can be compared whatever pressure they run at.
Worked example: 30 hp screw, warm room
Nameplate to model number in four steps
- Compressor delivers 128 SCFM at 110 psig (data sheet)
- Inlet air 110°F, room 105°F in July: correction factor 0.80 x 0.88
- Corrected duty 128 / (0.80 x 0.88) = 182 SCFM
- Model chosen: the 185 SCFM size, not the 130
Reading a dryer rating
What the SCFM figure on the nameplate assumes
- 100 psig at the dryer inlet
- 100°F compressed air entering
- 100°F air around the condenser (or 85°F cooling water)
- 38°F pressure dew point at the outlet
Compressor horsepower to dryer duty
Delivered flow at 100 psig for typical compressors, and the dryer duty after a common set of corrections (110°F inlet, 100°F ambient, 100 psig: factor 0.80).
| COMPRESSOR | DELIVERED FLOW | DRYER DUTY, RATING POINT | DRYER DUTY, CORRECTED | FLOW BAND |
| 5 hp piston | 16 to 20 SCFM | 20 SCFM | 25 SCFM | 5 to 50 SCFM |
| 10 hp screw | 40 to 45 SCFM | 50 SCFM | 55 to 60 SCFM | 65 to 250 SCFM |
| 25 hp screw | 105 to 110 SCFM | 125 SCFM | 135 to 140 SCFM | 65 to 250 SCFM |
| 50 hp screw | 210 to 230 SCFM | 250 SCFM | 265 to 290 SCFM | 300 to 1,250 SCFM |
| 100 hp screw | 440 to 470 SCFM | 500 SCFM | 550 to 600 SCFM | 300 to 1,250 SCFM |
| 250 hp screw | 1,100 to 1,200 SCFM | 1,250 SCFM | 1,400 to 1,500 SCFM | 1,600 to 4,200 SCFM |
Read delivered flow
Compressor data sheet, or 4 to 5 SCFM per hp on a screw
Measure the conditions
Inlet pressure, inlet air temperature, room temperature
Divide by the factors
Flow over the product of the three corrections
Pick the next size up
First model at or above the duty, never rounded down
Dryers sized by these rules
The method
Write down the compressor's delivered flow. Measure or estimate the three conditions the rating assumes: inlet pressure, inlet air temperature, ambient temperature. Look up the correction factor for each on the sizing pages, multiply them together, and divide the flow by the product. The result is the rating-point SCFM the dryer must carry; pick the first model at or above it. Never round down: a dryer at 105 percent of capacity loses its dew point, and a lost dew point on a hot afternoon is exactly the failure this whole exercise exists to prevent.
How much margin
The corrections already carry the conditions, so a further margin of 10 percent is enough to cover a compressor running a little above nameplate and an aftercooler that is a year past its last cleaning. Oversizing beyond that costs money and, on a non-cycling dryer, buys nothing. If the plant is about to add a compressor, size for the future one now.
Common mistakes
Sizing to average consumption instead of compressor output; forgetting that a piston compressor without an aftercooler delivers 180°F air, which no standard dryer is rated for; reading the compressor's inlet CFM as delivered SCFM; and taking the ambient from the plant floor when the dryer lives in a compressor room ten degrees hotter. Each one produces a dryer that looks right on paper and passes water in August.
When flow varies
A plant that runs at 40 percent of compressor capacity for most of the day still needs the dryer sized for 100 percent, because the compressor delivers 100 percent every time it loads. What varies is the energy the dryer uses, and that is the argument for a cycling dryer: it sizes the same, but its refrigeration compressor stops when the load drops. The cycling versus non-cycling page shows where the payback is.
From compressor output to dryer duty
Usually yes. A long header carrying undried air between rooms or buildings collects condensate in every low point, and a dryer at the far end recieves slugs of water along with air that has cooled and rewarmed on the way. One dryer per compressor, sized to that machine's delivered flow and its own room's conditions, keeps the wet section short. Where the compressors share a room, one dryer sized for those that run together is simpler.
Start from about 16 to 20 SCFM delivered, at 3 to 4 SCFM per horsepower. With the common correction for 110°F inlet air that becomes a 25 SCFM duty, inside the 5 to 50 SCFM band. The bigger question on a piston machine is discharge temperature: without an aftercooler the air leaves at about 180°F, which no standard dryer accepts, so confirm one is fitted before trusting these figures.
No. Displacement on a piston compressor is the swept volume of the cylinders, and the machine delivers noticeably less once valve and clearance losses are counted. Look for a figure labeled free air delivered, or FAD, at the working pressure, since that is what the dryer sees. If only displacement is printed, cross-check with 3 to 4 SCFM per horsepower for a piston or 4 to 5 for a screw, and size on the more conservative result.
Only if it can run alongside the others. A true standby that starts only when the lead machine fails adds nothing, and the dryer is sized for the largest set of compressors that can run together. Check the sequencer before deciding, because many are set to bring the lag machine on at peak demand, and then the dryer sees both. Two 25 hp screws on that setting make a 50 hp duty, about 210 to 230 SCFM delivered.
