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Five Very Different Compressors, One Dryer Question
Refrigerated air dryers for the shops, mills, buildings and labs that run on small and mid-size compressors
This section covers the air systems that do not belong to a production line: a cabinet shop on a 10 hp piston compressor, a weaving mill on 500 hp of rotary screws feeding air-jet looms, a building's pneumatic thermostats on a 2 hp duplex unit in the boiler room, a dental practice on an oil-free twin-head compressor, and an electronics floor or laboratory on oil-free scrolls. The flows run from 5 SCFM to 2,500, the compressors from lubricated piston units with no aftercooler to oil-free machines with a class 1 oil specification, and in every case the question is the same: how dry, how clean, and which dryer.
For most of them a refrigerated dryer holding a 38°F pressure dew point (ISO 8573-1 class 4) is the answer, because the air is used indoors at room temperature and nothing it touches is colder than 38°F. The exceptions matter and are named on each page: pneumatic control lines that pass through unheated spaces, hospital medical air under NFPA 99, and purge or dry-box air in a laboratory, all of which need desiccant drying that a refrigerated unit cannot reach.
- Small piston compressors without an aftercooler discharge 180°F air and take a high-inlet-temperature dryer, not a standard one
- Oil-free compressors still need a 0.01 micron afterfilter for particles and, where vapor matters, an activated carbon stage
- A 1701 bypass valve and a wall bracket turn a 7 to 35 SCFM dryer into a serviceable installation in a closet or mechanical room
Five Applications
Find Your Building or Shop Below
Each page names the air-using equipment, the damage water and oil do to it, the compressor and flow typical of the trade, the dew point and ISO class to specify, where the dryer and filters go, and whether cycling or non-cycling drying fits the hours the compressor runs. The table beneath the tiles gives the short answer for each.
Small Compressors and Hot Inlet Air
Three of the five trades on this page run on reciprocating compressors, and most small piston units have no aftercooler: the air leaves the head at 180 to 200°F and reaches the dryer little cooler if the pipe is short. A standard refrigerated dryer is rated for 100°F inlet air and loses a third of its capacity at 120°F, so on that duty it is either the high-inlet-temperature type, the HTR Series at 180°F and the HRHT Series at 200°F, or a standard dryer fed through a receiver large enough to cool the air first.
The high-inlet dryers deliver a 50°F pressure dew point, class 6, rather than the 38°F of the standard line. In a shop or building held at 65 to 75°F that is still 15 to 25°F of margin, and it is the honest figure to design to. Where a lower dew point is needed, the receiver-then-standard-dryer route gives 38°F.
Where Refrigerated Drying Stops
A refrigerated dryer cannot go below about 35 to 38°F without freezing its own condensate, and that fixes its limits. Pneumatic control mains that cross an unheated attic or feed rooftop units in a northern winter need a class 2 dew point of -40°F. Hospital medical air under NFPA 99 must be dried below the frost point at line pressure with an alarm at 35°F, which puts the duplex medical-air plant on desiccant dryers. Laboratory purge gas for spectrometers and dry-box air are the same story.
Everywhere else in these buildings, which is most of the air, the refrigerated dryer is the right tool: shop tools, dental handpieces, textile looms, pick-and-place machines and the utility air that runs sterilizers and building controls. The refrigerated versus desiccant page draws the line in detail, and the dew point and ISO classes page explains the class numbers used in the table above.
Compressor, Flow, Dew Point and Dryer by Trade
Typical figures for one shop, mill, building or practice. Refrigerated dew points are pressure dew points at 100 psig.
| APPLICATION | TYPICAL COMPRESSOR | FLOW TO DRY | DEW POINT NEEDED | DRYER THAT FITS |
| Woodworking | 5 to 10 hp piston, or a 15 to 50 hp screw in a cabinet plant | 20–230 SCFM | 38°F class 4; the 50°F class 6 of a high-inlet dryer is fine for a shop at 65°F | High-inlet-temperature (HTR, HRHT) on a piston unit; 5 to 50 and 65 to 250 SCFM on a screw |
| Textiles | 100 to 500 hp of rotary screws, often several | 450–2,500 SCFM | 38°F class 4; oil class 1–2 for air-jet looms | 1,600 to 4,200 SCFM non-cycling; 2,500 to 12,500 SCFM cycling |
| HVAC controls | 1 to 5 hp duplex piston in a mechanical room | 5–20 SCFM | 38°F class 4 indoors; class 2 desiccant where lines cross unheated space | 5 to 50 SCFM non-cycling with a 0.01 micron filter and carbon stage |
| Medical & dental | 1 to 5 hp oil-free piston (dental); 7.5 to 30 hp for support air | 5–40 SCFM | 38°F class 4 for dental and utility air; NFPA 99 medical air is desiccant-dried below 32°F | 5 to 50 SCFM non-cycling with fine filtration |
| Electronics & labs | 5 to 30 hp oil-free scroll | 15–120 SCFM | 38°F class 4, oil class 1; class 2 desiccant for purge and dry-box air | 5 to 50 SCFM and 20 to 250 SCFM cycling |
Drying air in buildings, shops and labs
Low enough that the coldest pipe in the run stays above it. An attic or rooftop in a northern winter can sit far below freezing, so those lines take a class 2 dew point of -40°F from a desiccant dryer; a 38°F refrigerated dryer would leave them condensing and freezing at the first restrictor. Lines that stay inside the heated envelope are served by the refrigerated dryer at the duplex compressor, with an oil-removal filter behind it.
Most do, and the spray gun is usually how they find out. Nailers and random-orbit sanders tolerate some water, though their drivers and motors rust; a finish gun does not, and water spots appear in lacquer on the first humid day. A small piston compressor without an aftercooler needs the high-inlet-temperature type, whose 50°F dew point is fine in a shop at 65°F. Keep the dryer clear of sanding dust, which packs condenser fins.
For the handpieces, air-water syringes and chair valves, yes: those want clean, dry air at room temperature, and a 38°F dew point leaves nothing to condense in the delivery unit. The compressor is usually oil-free, so the filters guard against wear particles and room vapors rather than lubricant. Keep this separate from piped hospital medical air under NFPA 99, which carries its own desiccant drying and alarm requirements and is a different system.
Big, and steady. A weaving room on 100 to 500 hp of screws draws 450 to 2,500 SCFM around the clock, because every pick is inserted with an air jet. That steady load suits large non-cycling dryers, though a mill who's compressors run part-loaded overnight may justify cycling sizes. Oil class 1 to 2 matters as much as dew point, since oil carried by the jets stains yarn and shows up later as a fault in the cloth.
On the larger systems. A lab floor on a 20 to 30 hp multi-scroll package uses air in bursts, then sits nearly idle overnight and at weekends, and a cycling dryer stops its refrigeration compressor through those hours. A single small scroll under 50 SCFM is different: the non-cycling dryer draws so little that the premium never comes back. Purge and dry-box air still take a class 2 desiccant stage on a branch of their own.
It coats the tiny restrictors, bleed ports and bellows in thermostats and actuators, so setpoints drift and dampers hunt, and the only cure for a contaminated control system is cleaning or replacing every device on it. That is why control air from a lubricated duplex compressor gets a 0.01 micron coalescer and an activated carbon stage after the dryer. Those two filters cost little next to a building full of fouled controllers.
