1-805-484-2992
Quotes - Engineering - SalesHow a regenerative twin tower dries a plant
Each dryer has two vessels of activated alumina. Air flows upward through the on-line tower, where the desiccant adsorbs water vapor, while the second tower is depressurized and regenerated. A timer or a demand control switches them, so the outlet holds its dew point with no break in flow. The heatless frames are built for a 4.8 second bed contact time, with stainless diffusers that spread the air across the whole bed.
What separates the three types is the energy spent regenerating the off-line tower. A heatless dryer spends compressed air. An externally heated dryer spends electricity on a heater and a smaller amount of air. A blower purge dryer spends more electricity, on a heater and a blower that pushes ambient air through the bed, and at design conditions uses no compressed air at all. Heatless and blower purge units are rated class 2 at -40°F; the externally heated design is rated class 3 at -4°F as standard and reaches class 2 in cold weather or with an optional ambient air booster package.
Ratings are at 100 psig, 100°F saturated inlet and 100°F ambient. All three types handle 150 psig. Heatless units offer a 250 psig option, and on those frames the minimum inlet pressure goes from 60 to 120 psig.
Request a Quote
Three ways to regenerate the off-line tower
Listed from the type that spends the most compressed air on regeneration to the type that spends the least.
Heatless: The simplest twin tower and the widest range: 18 frames from 40 to 5,400 SCFM with no heater to maintain. One version has a demand purge control that reads bed temperature, a touchscreen and a 30-day data log; the other runs a fixed 10-minute timer for plain -40°F duty. 40 to 5,400 SCFM, inlet to 140°F Class 1, 2 or 3 by cycle on the demand version Pressure drop under 5 psi
40 to 5,400 SCFM · HHX, HHE Series
Externally Heated: A heater warms the regeneration air so less compressed air is needed, and an energy management controller that senses temperature and humidity sets the purge. Rated for 100 percent duty cycle across 13 models from 300 to 3,200 SCFM. Class 3 at -4°F standard; -40°F in cold weather or with the booster option Heater 4.5 to 40 kW, full-load average 2.0 to 21.4 kW Up to 15% less purge than heatless
300 to 3,200 SCFM · HPD Series
Blower Purge: A heater and a dedicated blower regenerate the off-line tower with ambient air, taking compressed air out of the regeneration loop at design conditions. The pick for large, steady loads where every SCFM of purge shows up on the power bill. 500 to 4,300 SCFM in 13 models Class 2, -40°F; inlet 40 to 120°F Full-load average 10 to 70 kW
500 to 4,300 SCFM · HBP Series
Purge on a heatless dryerAbout 75 SCFM vented from a 500 SCFM unit to regenerate its beds, the load heated and blower purge designs exist to cut.
Heatless, heated and blower purge side by side
Published figures for each type at the standard rating conditions.
| COMPARED | HEATLESS | EXTERNALLY HEATED | BLOWER PURGE |
| Flow range | 40 to 5,400 SCFM | 300 to 3,200 SCFM | 500 to 4,300 SCFM |
| Standard dew point | -40°F, class 2; -100°F or -4°F by cycle | -4°F, class 3; -40°F with option or in winter | -40°F, class 2 |
| Regeneration energy | Dried purge air, about 15% of flow | Heater plus reduced purge | Heater and ambient blower; no purge at design conditions |
| Electrical load | Controls only | Full-load average 2.0 to 21.4 kW | Full-load average 10 to 70 kW |
| Pressure and inlet | 150 psig, 250 optional; inlet to 140°F | 150 psig; inlet 40 to 120°F | 150 psig; inlet 40 to 120°F |
| Weight range | 365 to 12,000 lb | 1,400 to 9,000 lb | 1,866 to 12,350 lb |
Sizing a twin tower dryer
Size on the flow at the dryer's actual inlet pressure and temperature, not the compressor nameplate alone. Heatless capacity is corrected by a published factor that runs from 0.65 at 60 psig to 1.52 at 250 psig, so a dryer on a 60 psig system carries about two-thirds of its rated flow. Inlet air above 100°F holds more water per SCFM and pushes the selection up a size.
Put a refrigerated dryer ahead of the towers wherever the rest of the plant already needs one; the beds then see a fraction of the water and the alumina lasts longer. The 0.01 micron coalescing prefilter is not optional, and a 1 micron afterfilter keeps desiccant dust out of downstream valves.
Call and talk it through with an engineer: 1-805-484-2992
Running and siting a twin tower dryer
Three to five years is the usual change interval for alumina that sees clean air at or below its rated flow. Oil is the common killer: aerosol that gets past a neglected coalescer coats the beads and never comes off in regeneration. Liquid slugs from a failed prefilter drain do similar damage, and a tower brought up to pressure too fast lifts and grinds the bed into dust, which shows first as climbing pressure drop across the afterfilter. A dew point that drifts up while the valves switch on time and the purge flows normally usually means the bed itself is spent.
Usually as a dew point that holds for a while and then climbs steadily, because one tower has stayed on line far past its cycle. Listen at the exhaust as well. A muffler that never goes quiet before a changeover points to an exhaust valve that will not close, and the dryer is then venting dried air to atmosphere around the clock. A tower gauge that never falls to zero during regeneration is the opposite fault: that tower is not being stripped at all. Rule out the valves before ordering desiccant; a new charge will not cure either problem.
Because alumina stands up to the pressure swing and to the occasional splash of liquid water far better than gel does. A twin tower depressurizes and refills each bed several times an hour for years, and ordinary silica gel beads crack when liquid water reaches them, then break down into fines that plug the afterfilter. Gel holds more water at moderate humidity, which is why it suits a single tower that is simply recharged when spent. Molecular sieve, the third common desiccant, is the choice where very low dew points matter more than cost.
Heatless frames can, with protection; heated and blower purge units only where the site never drops below 40°F, the floor of their ambient rating. Heatless dryers tolerate ambient air up to 120°F and can be specified with low-ambient protection and a NEMA 4 enclosure, but the parts that freeze first are outside the dryer: the prefilter drain, its bowl and any condensate line leading from them. A shelter that stays above freezing, with the drain line inside it, is the usual answer. In hot climates shade the towers, because direct sun pushes the whole assembly toward its ambient limit.
