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Quotes - Engineering - SalesWhat a small screw compressor asks of its dryer
Modest flow, full-time warm air, a shop that goes home at night
A 5 to 10 hp rotary screw delivers 21 to 46 SCFM at 100 to 125 psig, usually from a tank-mounted package in a shop that runs one shift. The dryers that fit are 115 V units from 25 to 50 SCFM; this page shows which size goes with which compressor and what pushes the answer up to the 65 SCFM class.
Small screws load and unload many times an hour and keep the discharge near room temperature plus 15 to 20°F through their aftercooler. The dryer sees 100 to 115°F air at the compressor's full output whenever it loads, in a shop that is often idle from evening to morning.
5 hp
- 21 to 23 SCFM delivered
- Dryer size 25 SCFM; 32 in a hot room
- Cabinet shops, small service bays, sign shops
7.5 hp
- 32 to 35 SCFM delivered
- Dryer size 42 SCFM; 50 in a hot room
- Two-bay auto shops, small fabricators
10 hp
- 43 to 46 SCFM delivered
- Dryer size 50 SCFM; 55 to 65 in a hot room
- Machine shops with two or three CNC machines
Two small screws
- Two 5 hp on one receiver: 42 to 46 SCFM together
- Dryer size 50 SCFM after the receiver
- One dryer only if both can run at once
Horsepower to dryer size, 5 to 10 hp
Delivered flow at 100 to 125 psig for a single-stage oil-flooded screw at 4.3 to 4.6 SCFM per hp. Dryer sizes at the rating point of 100 psig, 100°F inlet and 100°F ambient; the hot-room column applies the 0.88 factor for a 110°F compressor space.
| COMPRESSOR | TYPICAL DELIVERY | DRYER SIZE AT 100°F | DRYER SIZE, HOT ROOM (110°F) | DRYER BAND |
| 5 hp | 21 to 23 SCFM | 25 SCFM | 30 to 32 SCFM | 5 to 50 SCFM |
| 7.5 hp | 32 to 35 SCFM | 42 SCFM | 50 SCFM | 5 to 50 SCFM |
| 10 hp | 43 to 46 SCFM | 50 SCFM | 55 to 65 SCFM | 5 to 50 SCFM or 65 to 250 SCFM |
| 10 hp at 80 psig | 43 to 46 SCFM | 50 SCFM (factor 0.92) | 65 SCFM | 65 to 250 SCFM |
| 10 hp variable speed | 15 to 46 SCFM | 50 SCFM cycling | 55 SCFM cycling | 20 to 250 SCFM cycling |
| Two 5 hp, one receiver | 42 to 46 SCFM | 50 SCFM | 55 to 65 SCFM | 20 to 250 SCFM cycling |
Plug-in non-cycling: one small screw, one shop
At 25 to 50 SCFM a non-cycling dryer draws 0.3 to 0.9 kW, so running it continuously costs a few dollars a month. It plugs into 115 V, connects at 1/2 in NPT and hangs on a wall bracket beside the compressor.
- 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.
- 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.
Small cycling dryers: the compressor idles more than it works
Where a 10 hp screw feeds a shop that draws air in bursts, or a variable-speed machine spends the day at 15 SCFM, a 30 to 55 SCFM cycling dryer shuts its refrigeration compressor off between loads and holds the dew point from its thermal mass.
- 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.
Installing a dryer on a tank-mounted screw
Most 5 to 10 hp screws arrive as a package on an 80 or 120 gallon receiver, and the dryer goes downstream of that tank, never between the compressor and the tank.
The receiver drops the water the aftercooler condensed and turns the compressor's load/unload steps into a steady draw. Take the dryer feed from the receiver outlet through a 1 micron prefilter, then the dryer, then a 0.01 micron oil-removal afterfilter, with a bypass around all three so the shop keeps running while an element is changed. Up to 35 SCFM the bypass is a single brass body with three positions; above that use a three-valve manifold.
Give the condenser room to breathe: a wall bracket with 6 in of clearance behind the unit keeps it out of the compressor's exhaust. The dryer, the receiver and the filter bowls each need a drain with somewhere to go, and a zero-loss drain on the receiver stops it blowing shop air into the floor drain every ten minutes.
A 7.5 hp screw in a 100°F shop is the 42 SCFM dryer; the same package in a 110°F back room, or one re-set to 80 psig, is the 50.
Size a dryer for my small screw
A dryer for a 5 to 10 hp shop screw
Lower pressure means a larger volume of air through the same exchanger. At 80 psig a dryer carries only 0.92 of its 100 psig rating, so a 50 SCFM dryer on a 10 hp screw delivering 43 to 46 SCFM loses its margin and passes water on the busiest stretches of the day. Either restore the pressure or take the 65 SCFM size, as the table on this page shows.
It depends on the dryer size. Up to 35 SCFM a single brass body with three positions handles the dryer and both filters, which covers the 5 hp package. At the 42 and 50 SCFM sizes for 7.5 and 10 hp screws, use a three-valve manifold. Either way, the bypass goes around prefilter, dryer and afterfilter together so the shop keeps its air during an element change.
It changes the energy case, not the size. A 10 hp variable-speed screw can deliver anything from 15 to 46 SCFM, and the dryer still has to carry the top of that range, so the 50 SCFM size stays. Because the compressor spends most of the day well below full speed, a cycling dryer that idles its refrigeration compressor matches it better than a non-cycling unit running flat out.
