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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
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
5 to 10 hp screw compressor dryer questions
A 7.5 hp screw delivers 32 to 35 SCFM at 100 to 125 psig, so the 42 SCFM dryer is the match in a normal shop. Take the 50 SCFM size if the room passes 105°F in summer or the package pressure has been turned down to 80 psig.
After the tank, on the wall or the floor beside it, with the prefilter between the tank outlet and the dryer inlet. Piping the dryer between the compressor and the tank makes it see every load step at full aftercooler temperature and loses the water separation the tank gives for free.
Rarely on energy alone; a 25 SCFM non-cycling dryer draws under 0.3 kW. The case for a cycling unit at this size is a variable-speed compressor or a shop with long idle periods where a 30 SCFM cycling dryer comes at much the same first cost.
The 25 to 50 SCFM dryers run on 115 V single phase and draw 0.3 to 0.9 kW, so they normally get their own 15 A receptacle. A 5 to 10 hp screw is usually 230 V single phase or 460 V three phase, which the dryer cannot use.
The dryer is fed 45 SCFM every time the compressor loads, nearly twice its rating. The evaporator cannot hold 38°F at that flow, the dew point drifts toward 60°F, and pressure drop across the small exchanger climbs to several psi, so the shop gets wet air and less of it.
