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Quotes - Engineering - SalesWhat a 15 to 50 hp screw compressor asks of its dryer
Steady flow, warm air, long hours
A 15 to 50 hp screw compressor delivers 65 to 225 SCFM, which puts every one of them in the same dryer band. This page turns the horsepower on the compressor nameplate into a dryer size and points at the series that cover it.
Rotary screw compressors in this class run loaded for most of the shift, deliver air 15 to 20°F above room temperature through their aftercooler, and are usually the only compressor in the building. The dryer therefore sees a continuous, warm, full-flow duty, and it is sized to the compressor's whole output.
15 to 20 hp
- 65 to 90 SCFM delivered
- Dryer size 85 to 105 SCFM
- Small shops, body shops, one-line plants
25 to 30 hp
- 105 to 130 SCFM delivered
- Dryer size 125 to 150 SCFM
- Machine shops, cabinet shops, packaging lines
40 hp
- 170 to 180 SCFM delivered
- Dryer size 185 to 200 SCFM
- Plants with a second shift or a paint line
50 hp
- 210 to 230 SCFM delivered
- Dryer size 250 SCFM
- Small central plants, bottling, food packaging
Horsepower to dryer size
Delivered flow at 100 to 125 psig for a typical rotary screw (4.3 to 4.6 SCFM per hp). Dryer sizes are at the standard rating point; correct for inlet and ambient temperature before ordering.
| COMPRESSOR HP | TYPICAL DELIVERY | DRYER SIZE AT 100°F | DRYER SIZE, HOT ROOM (110°F) | DRYER BAND |
| 15 hp | 65 to 70 SCFM | 75 to 85 SCFM | 100 to 105 SCFM | 65 to 250 SCFM |
| 20 hp | 85 to 90 SCFM | 100 to 105 SCFM | 125 SCFM | 65 to 250 SCFM |
| 25 hp | 105 to 110 SCFM | 125 SCFM | 150 SCFM | 20 to 250 SCFM cycling |
| 30 hp | 125 to 130 SCFM | 150 SCFM | 165 to 185 SCFM | 20 to 250 SCFM cycling |
| 40 hp | 170 to 180 SCFM | 185 to 200 SCFM | 230 to 250 SCFM | 65 to 250 SCFM |
| 50 hp | 210 to 230 SCFM | 250 SCFM | 300 SCFM | 300 to 1,250 SCFM |
Non-cycling: the compressor runs loaded all shift
When the screw is loaded 60 percent of the time or more, a non-cycling dryer costs nothing extra to run and has the fewest parts.
- DXR Series: Direct-expansion non-cycling dryer in 27 sizes from 10 to 4,200 SCFM; the widest single range on the site, with a full data sheet behind every model.
- HPRB Series: Current small non-cycling dryer, 7 to 50 SCFM, with low-GWP R513A refrigerant, a 3-in-1 heat exchanger and a built-in separator on the larger sizes.
- HPRN Series: Non-cycling refrigerated dryer from 75 to 1,200 SCFM for plant air on a single compressor; previous generation, cross-referenced to current models.
Cycling: nights, weekends and variable demand
When the plant idles overnight with the compressor still up, a cycling dryer stops its refrigeration compressor and rides on the thermal mass, cutting dryer power 50 to 80 percent.
- FLXA Series: Current cycling dryer, 75 to 2,000 SCFM, air- or water-cooled, with low-GWP refrigerants and a heat-transfer cycling control that follows the load.
- TMC Series: Thermal-mass cycling dryer from 30 to 635 SCFM; stores cold in a thermal mass so the refrigeration compressor rests when air demand drops.
- NXC Series: Cycling dryer in 19 sizes from 20 to 2,000 SCFM with a dry thermal mass and up to 80 percent power savings at part load.
Reading the compressor before sizing the dryer
Three numbers off the compressor package decide the dryer: delivered SCFM, discharge temperature and hours loaded.
Delivered flow is on the nameplate or the data sheet, and it beats the horsepower rule every time; two 30 hp machines from different makers can differ by 15 SCFM. Discharge temperature after the package aftercooler is normally room temperture plus 15 to 20°F, so a compressor in a 95°F room delivers 110 to 115°F air and the dryer must be corrected for it. Hours loaded decide cycling versus non-cycling.
Pipe the dryer after the receiver, not before it. The receiver knocks out the bulk water and smooths the flow the dryer sees; a dryer piped straight off a modulating screw sees the compressor's full output every time it loads. Put the 1 micron prefilter between receiver and dryer and the 0.01 micron oil-removal filter after the dryer, both sized for the compressor's full output.
A 25 hp screw in a 100°F compressor room is the 125 SCFM dryer; the same compressor in a 110°F room is the 150.
Size a dryer for my compressor
Choosing the dryer for a 15 to 50 hp screw
Because its delivered 210 to 230 SCFM already sits near the 250 SCFM top of the band. Correct a 230 SCFM machine for a 110°F compressor room, 230 / 0.88, and the duty is about 261 SCFM, so the order becomes the 300 SCFM dryer from the next band up. A 50 hp in a well-ventilated room stays in this band; the summer temperature of the room, not the horsepower, makes the call.
The compressor. Both housings pass the full delivered flow every time the screw loads, so a filter sized to a smaller dryer, or picked by pipe size alone, adds pressure drop the compressor pays for all shift. Size the 1 micron prefilter and the 0.01 micron afterfilter for the compressor's whole output. When the dryer is upsized for a hot room the filters need not follow, since their rating depends on flow and pressure, not on room temperature.
Often, yes. A 25 hp screw on a single shift frequently stays running through the evening at light load, and a non-cycling dryer draws full power the whole time. A cycling unit in the 125 SCFM size stops its refrigeration compressor when the load falls and cuts dryer power 50 to 80 percent over those hours. Check the controller's loaded hours; below about 60 percent the case is strong.
A second shift raises the loaded hours, which pushes the choice toward a non-cycling dryer since there is little idle time left to save on. A paint line changes the filtration, not the dryer size: the booth branch takes its own coalescer and activated carbon element after the dryer. Size the dryer for the 170 to 180 SCFM the 40 hp makes, corrected for the room it sits in.
