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Quotes - Engineering - SalesAir Dryers for Rotary Screw Compressors
A rotary screw is the easiest compressor to size a dryer for. It delivers a steady 4.3 to 4.6 SCFM per horsepower at 100 to 125 psig, its aftercooler brings the discharge down to room temperature plus 15 to 20°F, and it runs loaded for long stretches. Horsepower therefore maps almost directly onto a flow band: 5 to 10 hp is the 5 to 50 SCFM band, 15 to 50 hp is 65 to 250 SCFM, 60 to 250 hp is 300 to 1,250 SCFM, and 300 hp and up is 1,600 SCFM and beyond.
What the horsepower does not tell you is the compressor room temperature, the state of the aftercooler and the hours a day the machine is loaded. Those decide the correction factor and whether a cycling or non-cycling dryer pays.
Pick the horsepower band
Each page turns nameplate horsepower into a dryer size, a flow band and the series that cover it
5 to 10 HP: Small tank-mounted screws delivering 21 to 46 SCFM. Plug-in 115 V dryers from 25 to 50 SCFM cover them, with the 65 SCFM size held back for hot rooms.
15 to 50 HP: The most common industrial compressor sizes, 65 to 230 SCFM, all inside the 65 to 250 SCFM band. Where a 25 hp screw becomes a 125 SCFM dryer and a 50 hp a 250.
60 to 250 HP: Plant compressors delivering 260 to 1,150 SCFM into the 300 to 1,250 SCFM band, where non-cycling, cycling and water-cooled start to matter to the power bill.
300 HP and Up: Single 300 to 500 hp machines and multi-compressor rooms from 1,600 SCFM up: large non-cycling units, modular skids and modulating dryers.
Non-cycling dryer
- Refrigeration runs all the time
- Hot-gas bypass holds the evaporator
- Fits a screw loaded 60 percent or more
Cycling dryer
- Stores cold in a mass or glycol loop
- Stops its compressor as load drops
- Cuts dryer power 50 to 80 percent
Three numbers
What a screw compressor hands its dryer
1.
Flow: 4.3 to 4.6 SCFM per horsepower
A 25 hp screw delivers 108 to 115 SCFM, a 100 hp machine 430 to 460. The data sheet beats the rule; two 50 hp compressors from different builders can differ by 20 SCFM.
At 125 psig the dryer carries 8 percent more than its 100 psig rating, at 80 psig 8 percent less.
2.
Temperature: room plus 15 to 20°F
The aftercooler leaves the air 15 to 20°F above the room, so a 95°F compressor room delivers 110 to 115°F air and derates a standard dryer to about 80 percent.
A fouled aftercooler pushes the inlet to 120°F and the factor to 0.65.
3.
Hours: loaded, unloaded, stopped
A screw loaded 60 percent of the shift or more on two or three shifts is non-cycling territory.
One shift, idle nights or a variable-speed compressor at half output favor a cycling dryer, which stops its refrigeration compressor as the load drops and cuts dryer power 50 to 80 percent.
Reading a rotary screw package before choosing the dryer
Three things on the package matter more than the nameplate horsepower: the delivered flow and pressure on the data plate, the temperature of the air leaving the aftercooler, and the controller's load history. Most screw controllers log loaded hours against run hours, and that ratio is the cycling-versus-non-cycling decision made for you.
Then look at the room. A screw throws its motor and oil-cooler heat into the space around it, and a dryer parked beside it breathes that air through its condenser, often 10°F above the plant floor.
Non-cycling or cycling behind a screw
A non-cycling dryer runs its refrigeration compressor continuously and holds the evaporator just above freezing with a hot-gas bypass: lowest first cost, fewest parts, a steady 38°F dew point. Behind a screw loaded most of the day its power is simply part of the base load.
A cycling dryer stores cold in a thermal mass, a glycol loop or the exchanger itself and shuts its refrigeration compressor off until the mass warms, so behind a screw that unloads at night, dryer power falls with the missing load.
Piping the dryer into a screw package
The order that works is compressor, aftercooler, receiver, 1 micron prefilter, dryer, 0.01 micron oil-removal afterfilter, header. The receiver smooths the load steps of a load/no-load screw and drops the bulk water; the prefilter keeps compressor oil and rust out of the dryer's exchanger.
Match the connections to the compressor discharge, and fit a bypass so the dryer can be isolated for service with the compressor still running.
Matching a dryer to a screw package
As a dryer that seems to have shrunk. A clean aftercooler hands over air 15 to 20°F above the room; once its fins load with dust and oil mist, the discharge can reach 120°F and the dryer carries only 0.65 of its rating. The dew point creeps up in summer, water reaches the header and the dryer gets blamed. Check the aftercooler outlet temperature before condemning the dryer.
The peak flow is the same either way, because the dryer sees the compressor's full output whenever it loads. What differs is the pattern. A load/no-load screw hands the dryer full flow and then nothing, which a receiver smooths and a cycling dryer exploits. A modulating or variable-speed machine sits at partial flow for long periods, which is where a cycling dryer's power savings are largest.
Divide loaded hours by run hours from the controller over a representative month. At 60 percent or more across two or three shifts the dryer works hard all day and a non-cycling unit is the economical choice. A much lower ratio, or a plant idle on nights and weekends with the compressor left running, is where a cycling dryer earns its premium by resting along with the load.
The receiver does two jobs the dryer cannot. It drops the bulk liquid the aftercooler condensed, so the dryer's separator only has mist to deal with, and it blends the load/no-load steps into a flow the exchanger can follow. A dryer piped between the compressor and the tank meets a slug of water and a fresh surge of hot air on every load cycle, and its dew point swings with them. Only a dry receiver belongs after the dryer.
The dryer gains a little. At 125 psig a refrigerated dryer carries 8 percent more than its 100 psig rating, since air at higher pressure brings less vapor in with each unit of free air. The compressor loses more: raising discharge pressure cuts delivered flow and adds power, so check the new delivered SCFM on the data sheet. Turning pressure down to 80 psig costs the dryer 8 percent.
Not automatically. A separate dryer outlives a compressor or two, so check it against the new machine's delivered SCFM, its aftercooler outlet temperature and how it loads. A new variable-speed screw replacing a fixed-speed one often makes the existing non-cycling dryer the wrong machine on energy, even when its capacity is fine. If the flow rises past the dryer's corrected rating, the dryer goes too.
