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Quotes - Engineering - SalesWhat a multi-compressor room asks of its dryer
A flow that steps, a room that is warmer than the plant and a sequencer that decides the peak
With several compressors the flow through the dryer steps up and down as machines load, unload and stop, and the peak is set by the sequencer: lead/lag pairs, a base-load machine plus a variable-speed trim unit, or everything on together at the shift start. Read the peak from the controller's load history, then apply the room's summer temperature to the dryer.
Two compressors, lead/lag
- 2 x 50 hp with one standby: 215 to 230 SCFM
- Dryer size 250 SCFM
- Both running at peak: 430 to 460 SCFM, the 500
Base load plus trim
- 100 hp fixed plus 50 hp VSD: 645 to 690 SCFM
- Dryer size 800 SCFM
- Trim machine counted at full output
Three duty plus one standby
- 3 x 100 hp running: 1,290 to 1,380 SCFM
- Dryer size 1,500 to 1,600 SCFM
- Standby excluded unless the sequencer can start it too
Large plant, all duty
- 4 x 250 hp: 4,300 to 4,600 SCFM
- Dryer size 5,000 to 5,250 SCFM
- Modular or twin dryers for redundancy
Compressor room to dryer size
Rotary screws at 4.3 to 4.6 SCFM per hp; only the machines that run together at peak are summed. Dryer sizes at the rating point; the hot-room column applies the 0.88 factor for a compressor room at 110°F.
| COMPRESSORS THAT RUN TOGETHER | COMBINED FLOW | DRYER SIZE AT 100°F | DRYER SIZE, HOT ROOM (110°F) | DRYER BAND |
| 2 x 50 hp, one on standby | 215 to 230 SCFM | 250 SCFM | 265 to 300 SCFM | 65 to 250 SCFM |
| 2 x 50 hp, both duty | 430 to 460 SCFM | 500 SCFM | 550 to 600 SCFM | 300 to 1,250 SCFM cycling |
| 100 hp base plus 50 hp VSD trim | 645 to 690 SCFM | 800 SCFM | 800 to 850 SCFM | 300 to 1,250 SCFM |
| 3 x 100 hp duty, 1 standby | 1,290 to 1,380 SCFM | 1,500 to 1,600 SCFM | 1,600 to 1,800 SCFM | 1,600 to 2,000 SCFM |
| 2 x 200 hp plus 100 hp VSD, all duty | 2,150 to 2,300 SCFM | 2,400 to 2,500 SCFM | 2,600 to 3,000 SCFM | 1,600 to 4,200 SCFM |
| 4 x 250 hp, all duty | 4,300 to 4,600 SCFM | 5,000 to 5,250 SCFM | 5,250 to 6,000 SCFM | 2,500 to 12,500 SCFM |
One dryer after the common receiver
The standard arrangement: every compressor discharges through its aftercooler into the wet receiver, and one dryer sized for the combined peak takes the receiver outlet. Above 2,500 SCFM the digital-scroll, variable-speed and modular units give the turndown a multi-compressor room needs when only the trim machine is running.
- 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.
- 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 whose inverter-driven compressor tracks plant load; air- and water-cooled versions.
One dryer per compressor
Each package gets a dryer matched to its own output, 250 SCFM behind a 50 hp, 500 behind a 100 hp, with a check valve before the header. Cycling dryers suit this arrangement because each one rests whenever its compressor is off, and a compressor can be added later with its own dryer.
- 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.
- 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.
- 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.
Arranging the dryer in a multi-compressor room
The wet receiver is the meeting point: everything upstream of it belongs to the compressors, everything downstream is one air system with one set of drying and filtration.
A single dryer after the receiver, with a full-size three-valve bypass, is the arrangement most plants end up with, and the receiver is what makes it work: it drops the bulk water every aftercooler produced and blends the step changes of four machines loading and unloading into a flow the dryer can follow. Size the filter housings for the combined peak, and give the dryer a dry receiver downstream so a plant surge is served from storage rather than by overrunning the dryer.
Where wet air for a day is unacceptable, split the duty across two dryers at 60 percent each on common inlet and outlet manifolds, so either one carries the base-load compressor alone. One dryer per compressor is the answer when the machines sit in different rooms or were installed at different times; each set then needs its own bypass, drains and check valve.
Three 100 hp compressors with one on standby are a 1,500 to 1,600 SCFM duty, not 2,000; if the sequencer can bring the standby on during a peak, size for all three and buy the 2,000.
Size a dryer for my compressor room
Multiple compressor dryer questions
Only if both can run loaded at the same time. Two 75 hp screws together deliver 650 to 690 SCFM and need the 800 SCFM dryer; if the second is a standby that runs only when the first is down, the duty is 325 to 345 SCFM and the 400 SCFM dryer is correct.
At its maximum output, added to the base-load machines that run with it. The trim unit's average may be half of that, which is an argument for a cycling or modulating dryer, but the dryer still has to carry the peak when the trim machine is at full speed.
One dryer after the common receiver costs less and is easier to filter and drain; one per compressor suits machines in separate rooms or bought at different times. Two dryers in parallel on one receiver is the middle path for plants that cannot run wet while a dryer is serviced.
Either a dryer in each room sized for its own compressors, with check valves where the two lines meet, or one dryer at the point where the lines join, sized for both rooms together. The single dryer needs a receiver ahead of it at the junction to catch the water that condenses in 200 ft of pipe.
If the fourth machine runs together with the other three at peak, the combined flow rises and the dryer must carry it; a dryer bought with 10 to 20 percent margin often already does. If the fourth is a standby or replaces an older unit, the peak is unchanged and the dryer stays.
