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Quotes - Engineering - SalesAir Dryers for Reciprocating Piston Compressors
A piston compressor is the opposite of a screw in every way that matters to a dryer. It delivers 3.5 to 4 SCFM per horsepower in pulses, it cycles on and off against a receiver rather than running loaded, and most units below 30 hp have no aftercooler, so the air leaving the head is 180 to 200°F. A standard refrigerated dryer is rated for 100°F air and derates to half its nameplate or less at those temperatures.
Two routes fix that. The high-inlet-temperature series accept 180 to 200°F air directly and are the natural match for shop and service-bay compressors; or the piston unit gets an aftercooler and a standard dryer follows. The pages below size both routes by horsepower.
Pick the horsepower band
Each page sizes both the high-inlet-temperature route and the aftercooled route
2 to 5 HP: Garage, farm and small-shop compressors delivering 7 to 20 SCFM. The 25 SCFM high-inlet-temperature dryer covers all of them straight off the tank; the 7 to 25 SCFM plug-in dryers fit where the air has been cooled.
7.5 to 10 HP: The two-stage 80 gallon shop compressor class, 25 to 40 SCFM. High-inlet-temperature dryers at 35 to 60 SCFM, or the 42 to 50 SCFM standard sizes behind an aftercooler.
15 to 30 HP: Duplex and industrial piston units delivering 50 to 120 SCFM, usually aftercooled. The 65 to 150 SCFM band plus the top of the high-inlet-temperature range at 100 to 125 SCFM.
High-inlet-temperature dryer
- Takes 180 to 200°F air at the tank
- 50°F pressure dew point, ISO class 6
- Tops out at 125 SCFM
Aftercooler plus standard
- Discharge cooled to room plus 15 to 20°F
- 38°F dew point for paint and instruments
- Sized at 3.5 to 4 SCFM per hp
Three differences
Why a piston compressor needs its own sizing rules
1.
Hot air: 180 to 200°F at the discharge
Without an aftercooler the head discharge runs 180 to 200°F, and even after a receiver and 20 ft of pipe it often reaches the dryer at 120 to 130°F, where a standard unit is derated to 55 to 65 percent.
The high-inlet-temperature dryers are rated at 180 or 200°F inlet and deliver a 50°F pressure dew point, ISO 8573-1 class 6.
2.
Less flow: 3.5 to 4 SCFM per horsepower
A 5 hp piston delivers 17 to 20 SCFM, a 10 hp 35 to 40, a 30 hp 105 to 120.
Two-stage units at 150 to 175 psig help the dryer: at 150 psig it carries 15 percent more than its 100 psig rating.
3.
Intermittent duty: on, off, on
A piston compressor runs to a cut-out pressure and stops, so the dryer sees full compressor output for a few minutes and nothing for the next few.
The receiver and its drain do the bulk water removal, and a timed drain wastes a large share of a small compressor's output; fit a zero-loss drain on the tank.
Choosing between a high-inlet-temperature dryer and an aftercooler
Measure the air temperature where the dryer will connect, with the compressor running at its duty cycle on a warm day. Below about 115°F a standard dryer one size up handles it; between 115 and 130°F a standard dryer two sizes up or a high-inlet-temperature unit; above 130°F only the high-inlet-temperature dryer or an aftercooler will do.
The two routes give different dew points: 50°F from a high-inlet-temperature dryer, 38°F from an aftercooled standard dryer. Paint, powder and instrument air want the 38°F route; tools, lifts and blow-off are served by either.
The high-inlet-temperature route
The current series are 115 V plug-in units from 25 to 125 SCFM with a built-in separator, a zero-loss drain and an exchanger sized for 180 or 200°F air. A high-temperature particulate filter rated 450°F goes ahead of the dryer to catch the carbon and oil a hot piston head throws downstream; ordinary filter elements are limited to about 150°F.
The previous-generation high-inlet series in 20 to 125 SCFM sizes cross-references to the current 180 and 200°F lines by flow.
High inlet temperature pages
The aftercooler route
An air-cooled aftercooler on the compressor discharge brings 190°F air to within 15 to 20°F of the room, and a separator behind it catches the water that condenses. From there the receiver, a 1 micron prefilter and a standard 38°F dryer sized at 3.5 to 4 SCFM per hp complete the set.
This is the route for a piston compressor feeding a paint booth or a dental office, and for any 15 to 30 hp unit whose flow exceeds the 125 SCFM top of the high-inlet-temperature range.
Choosing a dryer route for a piston unit
Strap a surface probe to the bare pipe at the planned dryer inlet, then let the compressor run its normal duty cycle on a warm afternoon. Take the highest value after several on-off cycles, not the first reading after a cold start. The reading picks the route: under 115°F a standard dryer a size larger copes, while over 130°F it takes the high-inlet-temperature unit or an aftercooler.
A hot piston head throws carbon and oil downstream, and at 180 to 200°F ordinary filter elements, limited to about 150°F, soften and fail. The 450°F high-temperature filter takes that debris out before it coats the dryer's exchanger. Skip it and the exchanger fouls, pressure drop climbs and the dew point drifts upward over months, long before anyone thinks to suspect the dryer.
Not if the receiver sits between them. The tank absorbs the pulses and the start-stop surges, so the dryer sees full compressor flow for a few minutes and then rest, which a refrigerated unit handles well. The damage comes from a dryer piped straight to the head with no tank, or a tank who's drain has failed and sends slugs of water downstream each time the compressor restarts.
On a shop unit that only feeds tools and lifts, usually not; a high-inlet-temperature dryer at the tank is simpler and a 50°F dew point serves those uses. Add the aftercooler when the air goes to a paint gun, powder booth or instrument line that needs 38°F, or when the flow is beyond the 125 SCFM top of the high-inlet range. A separator after the aftercooler is part of the job.
Mostly, yes. Each older high-inlet model maps by its SCFM rating onto the current 180 or 200°F line, so the replacement is picked from the old unit's rating and the compressor it serves. Check the inlet temperature it actually sees today, though; a compressor that has lost its aftercooler or moved into a hotter room may now want the 200°F model.
