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Quotes - Engineering - SalesPoint-of-Use Refrigerated Air Dryers
A point-of-use dryer is sized to the machine it serves, not to the compressor. It sits on the drop to one CNC cell, one paint booth or one lab bench, takes air that has already cooled to room temperature in the header, and only has to carry the flow that drop draws: 5 to 30 SCFM for a lab or dental office, 10 to 60 for a single machine, 30 to 100 for a paint booth. That makes it the smallest dryer on the site, a 115 V plug-in unit with 3/8 to 1 in connections.
The reasons to dry at the point of use are practical: there is no central dryer, the header runs outdoors and re-condenses, or one process needs cleaner air than the rest of the plant. The three pages below turn each duty into a flow, a dryer size and the filters that belong with it.
Pick the point-of-use duty
Each page turns the process into a flow, a dryer size and the filtration that goes with it
Single Machine: One CNC cell, packaging machine, press or blast cabinet drawing 10 to 60 SCFM from a wet header. Sized from the machine's peak draw with a 7 to 65 SCFM dryer and a two-stage filter set.
Paint Booth: Spray guns, sanders and supplied-air hoods drawing 30 to 100 SCFM. A 38°F dew point plus 0.01 micron oil removal and activated carbon, so the finish has no water spots or fisheyes.
Lab & Dental: Instrument air and dental operatories drawing 5 to 30 SCFM from small oil-free compressors. The 7 to 32 SCFM plug-in dryers, and the high-inlet-temperature units where the oil-free head runs hot.
Dryer at the drop
- Sized to one machine's peak draw
- Catches water a cold header recondenses
- 115 V plug-in on a wall bracket
Central dryer only
- Sized to the whole compressor output
- Dry only at its own outlet
- A second unit at the drop adds little
Three rules
Sizing a dryer to a machine instead of a compressor
1.
Flow is what the drop draws at its peak
Take the peak from the machine's data sheet, not the average: a machine that averages 8 SCFM may pull 30 during a blow-off or a cylinder stroke, and the dryer sees the peak.
Where the sheet is missing, a 3/16 in blast nozzle at 80 psig draws 45 to 50 SCFM, a spray gun 10 to 15, a dental chair 1 to 2.
2.
Inlet is room temperature, pressure is line pressure
Air at a drop has cooled to the plant's temperature, so the inlet correction is usually nil and the dryer is sized close to nameplate.
Line pressure at the machine is often 80 to 90 psig, which derates the dryer by 8 percent; a drop regulated below 80 psig should have the dryer upstream of the regulator.
3.
Filters decide the air quality, the dryer decides the water
A refrigerated dryer delivers a 38°F pressure dew point and nothing else. Oil, oil vapor and particles are the job of a 1 micron prefilter, a 0.01 micron coalescer and, for paint or breathing air, an activated carbon element after the dryer.
A process that needs air drier than 38°F takes a desiccant dryer downstream.
When a point-of-use dryer is the right answer
Where a central dryer exists and the header stays indoors, a second refrigerated dryer at the machine adds little; both deliver about 38°F, so the second is mostly a filter housing with a compressor attached. The point-of-use dryer earns its place when the central dryer is absent, undersized or a long way off.
It also earns its place on a header that leaves the building. Air dried to 38°F and piped across a yard at 20°F condenses again, and a dryer at the far end, indoors, takes that water back out before the machine sees it.
Sizing from the process rather than the nameplate
A 10 hp compressor feeding a whole shop may deliver 40 SCFM, but the one CNC machine on the drop draws 12, and the dryer is the 15 SCFM size, not the 42. A small dryer on a small load also holds its dew point better than a large dryer idling at a tenth of its rating, and draws 0.15 kW instead of 0.5.
The trap is the peak. Blow-off nozzles, air knives and large cylinders draw many times the machine's average for a few seconds, and a dryer sized to the average passes wet air during every one of them. Size to the highest flow the machine can sustain for a minute.
Filters, bypass and mounting at the drop
A point-of-use set is dryer, 1 micron prefilter, 0.01 micron afterfilter and, for paint or instruments, a carbon element, all on one wall bracket with a single-body bypass valve so the machine keeps running through an element change. The whole set for a 25 SCFM drop fits in a space 3 ft wide.
Every bowl and the dryer separator need a drain, and in a clean room, a clinic or a food area the discharge is piped to a suitable point rather than dripped on the floor.
Drying air at one drop instead of the plant
The rule is the dew point. Air leaving a refrigerated dryer has a pressure dew point of about 38°F, so any stretch of header colder than that condenses water out of it again: a run across a yard at 20°F, a loading dock with the doors open, an unheated mezzanine in January. Put the point-of-use dryer indoors at the far end of that cold run, where the machine takes its air.
Usually, yes. A large non-cycling unit idling at a tenth of its rating spends its time on hot-gas bypass and can let the dew point drift, while a unit matched to the drop works near its design point all shift. The small one also draws about 0.15 kW against 0.5 for the oversized unit, and its filters and bypass cost less because they are sized for the drop.
Find out first why the machine sees water. If the central dryer is undersized for today's plant load, adding a dryer at one drop treats a symptom and every other machine stays wet. If the header stays indoors and the central dryer is right, the culprit is probaly oil and droplets that a missing 0.01 micron coalescer would catch. Dry at the drop when the header runs cold or the machine is far from the compressor room.
