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Quotes - Engineering - SalesWhat the largest screw compressors ask of a dryer
Flanged flows, heat by the tens of kilowatts and a header that never stops
A 300 hp screw delivers about 1,300 to 1,400 SCFM, a 500 hp machine 2,150 to 2,300, and above that the dryer usually serves two or more compressors on a common header. This page takes horsepower and compressor count to a dryer duty from 1,600 to 12,500 SCFM.
From 300 hp up the compressors are the plant utility: 8,000 hours a year, often two or three machines sharing a header under lead/lag control or with a variable-speed trim unit, and behind them a dryer that is a 4 to 12 in flanged vessel drawing 8 to 50 kW. Redundancy, water cooling and part-load efficiency become design questions rather than options.
300 to 350 hp
- 1,290 to 1,610 SCFM delivered
- Dryer size 1,500 to 2,000 SCFM
- One large machine, 4 in flanged dryer
400 to 500 hp
- 1,720 to 2,300 SCFM delivered
- Dryer size 2,000 to 2,500 SCFM
- Single or twin compressors, 6 in flanges
600 to 750 hp installed
- 2,580 to 3,450 SCFM when all run
- Dryer size 3,000 to 3,500 SCFM
- Two or three compressors, air- or water-cooled
1,000 hp and up
- 4,300 SCFM and more on the header
- Modular dryers to 12,500 SCFM
- Water-cooled skids in 1,500 to 2,000 SCFM modules
Horsepower to dryer size, 300 hp and up
Delivered flow at 100 to 125 psig at 4.3 to 4.6 SCFM per hp; for several compressors the flow is the sum of the machines that can run together, not the installed total. The hot-room column applies the 0.88 factor for a 110°F compressor room.
| COMPRESSORS | DELIVERED FLOW | DRYER SIZE AT 100°F | DRYER SIZE, HOT ROOM (110°F) | DRYER BAND |
| One 300 hp | 1,290 to 1,380 SCFM | 1,500 to 1,600 SCFM | 1,600 to 1,800 SCFM | 1,600 to 4,200 SCFM |
| One 350 hp | 1,505 to 1,610 SCFM | 1,750 to 1,800 SCFM | 2,000 SCFM | 1,600 to 2,000 SCFM cycling |
| One 400 hp | 1,720 to 1,840 SCFM | 2,000 SCFM | 2,200 to 2,400 SCFM | 1,600 to 4,200 SCFM |
| One 500 hp | 2,150 to 2,300 SCFM | 2,400 to 2,500 SCFM | 2,600 to 3,000 SCFM | 2,500 to 12,500 SCFM |
| Two 300 hp, both duty | 2,580 to 2,760 SCFM | 3,000 SCFM | 3,150 to 3,500 SCFM | 1,600 to 4,200 SCFM |
| Four 250 hp, three duty plus standby | 3,225 to 3,450 SCFM | 3,500 to 3,750 SCFM | 4,000 to 4,200 SCFM | 2,500 to 12,500 SCFM |
Non-cycling for a header that is loaded day and night
A continuous-process plant whose compressors sit above 70 percent load gets little from cycling, and a large direct-expansion dryer with a hot-gas bypass is the simplest machine to run: flanged sizes to 4,200 SCFM, plus 2,500 and 3,000 SCFM units with a low-pressure-drop exchanger.
- 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.
- HPRplus Series: Large non-cycling dryer for 2,500 and 3,000 SCFM central plants with digital controls and a low-pressure-drop exchanger.
Modulating and modular dryers for plants that swing
Where the header falls to half load on nights and weekends, digital-scroll and variable-speed dryers follow it continuously and phase-change cycling units rest between loads. Above 3,000 SCFM the water-cooled skids gang 1,500 and 2,000 SCFM modules on one header.
- 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.
- FLX Series: Modular water-cooled cycling dryer skid for 3,000 to 12,000 SCFM, built from paired 1,500 and 2,000 SCFM modules.
- HES Series: Energy-saving digital-scroll dryer from 2,500 to 12,500 SCFM for central plants, modular above 3,750 SCFM.
- AES Series: Digital-scroll cycling dryer from 600 to 10,000 SCFM in air- and water-cooled builds, matching refrigeration capacity to demand continuously.
- VDR Series: Variable-speed dryer from 2,600 to 8,450 SCFM who's inverter-driven compressor tracks plant load; air- and water-cooled versions.
Installing a dryer above 1,600 SCFM
At this scale the dryer is a flanged vessel on a housekeeping pad with its own cooling water, a bypass header and a drain manifold, and the decisions are made on the piping drawing.
Connections run from 4 in ANSI flanges at 1,600 SCFM through 6 in at 2,500 to 3,000 and 8 to 12 in on the modular skids, so the dryer sets the header size in the compressor room. An air-cooled unit at 2,500 SCFM draws close to 20 kW and rejects that, plus the heat it pulls out of the air, into the room; from about 2,000 SCFM most plants go water-cooled, which needs 85°F water at the data-sheet flow and a way to keep it flowing when the tower is down.
Give the dryer a full-size bypass so the compressors keep the plant alive while it is serviced, and consider two dryers at 60 percent of the header flow each instead of one at 100 percent when the plant cannot tolerate wet air for a day. Filter housings at this size are ASME vessels holding several elements; a zero-loss drain on each, on the wet receiver and on the dryer separator keeps the header dry.
Two 300 hp screws both loaded are a 3,000 SCFM duty at 100°F; if one of them is a standby, the dryer is sized for 1,500 to 1,600 SCFM, not 3,000.
Size a dryer for my compressor room
Specifying a dryer above 1,600 SCFM
It sits right at the crossover. The machine delivers 1,505 to 1,610 SCFM and takes the 1,750 to 1,800 SCFM dryer in a ventilated room, just below the 2,000 SCFM point where most plants switch to water. Let that room reach 110°F and the corrected order becomes the 2,000 SCFM size, rejecting its heat into air that is already too warm. If tower water already serves the compressor, the water-cooled build settles the question.
It loses its condenser, trips on high refrigerant pressure, and the plant runs on undried air. Plan for it with a tower-down alarm, a city-water or chiller backup behind a switchover valve, or an air-cooled unit kept as standby on the bypass. The data sheet gives the water flow at 85°F; check that the backup source can deliver that flow at the temperature it actually has.
Where a wet day is costly, yes. Two units each sized at 60 percent of header flow keep most of the plant dry while one is serviced, and on light-load nights one can be shut down entirely. The price is a second set of filters, drains and valves, and more room on the housekeeping pad. Plants that can tolerate a day of wet air usually take the single dryer and a full-size bypass.
Because on a service day the bypass carries the whole plant. A reduced valve around a 4 in flanged dryer throttles the full header flow, pressure at the far end of the header sags, and the compressors climb toward their unload setpoint trying to hold it. Build the bypass spools and valves at the dryer's own flange size, 4 in at 1,600 SCFM and 6 in at 2,500 to 3,000, so taking the dryer out of service costs dry air and nothing else.
From the flow the compressors can make together, 4,300 SCFM and more at this size, built up from 1,500 and 2,000 SCFM modules on one header. Because the skid is a cycling design, a light night lets most of its refrigeration rest while the trim load passes through. Add one module beyond the peak and any module can be serviced without wetting the plant, which buys the redundancy of a second dryer without a second full-size vessel and its piping.
