Desiccant compressed air dryers

01

Choose the regeneration type by purge loss and running cost

Atlas Copco explains that regenerative desiccant dryers hold adsorbent in two towers: compressed air dries in one while the other regenerates by depressurising and passing dried purge air through the bed. Heatless dryers use only compressed air as purge, and Atlas Copco puts their average purge air consumption at about 16–20% of rated flow capacity; heated types reduce purge air, and heated blower purge dryers use ambient air from a blower over an external heater. Over 10 years it puts the heated blower purge dryer's total cost of ownership about 40% below the heatless type, because purge losses make up about 80% of a heatless dryer's TCO. [1]

Atlas Copco suggests heatless dryers suit low running hours or backup duty, and on balance ranks heated blower purge with zero-purge cooling first on economy; its BD+ range guarantees a pressure dew point of −40 °C and can reach −70 °C. [1] Denair's externally heated purge desiccant dryers list pressure dew points from −20 to −70 °C, activated alumina or molecular sieve desiccant, automatic control with alarms and noise of 72 dB(A) or less. [2]

The heatless principle goes back to Charles W. Skarstrom's US patent 2,944,627 (filed 1958-02-12, published 1960-07-12; original assignee listed as Exxon Research and Engineering). One bed adsorbs at feed pressure while the other is opened to a lower pressure and back-washed in counterflow by part of the dried product, so desorption "may be accomplished without addition of heat from an outside source". In its drying example, air with about 4,000 ppm moisture was fed at about 40 psig, a cycle timer switched the two adsorbers at three-minute intervals, and the dried product flow was about 1.0 standard cubic foot per minute; for drying the patent prefers a cycle time of about one to five minutes, and names activated alumina for removing water vapour and carbon dioxide together. [3]

Editorial calculation: a heatless dryer rated at 300 l/s that purges 16–20% of its rating uses about 48–60 l/s of compressed air for regeneration, which the compressors must produce on top of the plant's demand.

Give the supplier the compressed air flow (normal and peak), working pressure, inlet temperature and ambient, required pressure dew point, oil and particle limits (pre- and after-filtration), running hours, available power and whether purge air, heat or a blower may be used, and the space and noise limits.

02

Check pressure drop and dew-point dependent switching

Atlas Copco stresses that a dryer's internal pressure drop raises the compressor discharge pressure and energy cost, and in extreme cases output loss through the dryer can reach 20%; it advises choosing desiccant dryers with a very low pressure drop and cites a figure below 0.2 bar. Dew-point dependent switching uses a dew-point sensor to delay tower changeover until the bed is loaded, extending cycle time, which it says can generate energy savings of up to 90%. [1]

Atlas Copco notes regeneration cycles are based on time, dew point or both, and that for installations needing a negative dew point adsorption dryers are regarded as the only valuable solution. [1] Editorial note: ask for the dryer's pressure drop and dew point at your actual flow, inlet temperature and pressure, not only at the rating conditions.

03

Compare dryer types by dew point and energy per 100 cfm

The US Department of Energy's sourcebook Improving Compressed Air System Performance compares dryer types on dew point and energy, including the pressure drop through the dryer. Single-tower deliquescent dryers suppress the pressure dew point by 15–50 °F below the inlet temperature and need about 0.2 kW per 100 cfm, excluding replacement desiccant. Refrigerant dryers give a pressure dew point of 35–39 °F, with a pressure drop of 3–5 psid, and need about 0.79 kW per 100 cfm. Twin-tower desiccant dryers are rated at a pressure dew point of −40 °F; a pressure-swing type uses purge air of 10–18% of its rating, heated types use less purge but add heater energy, and the approximate requirement is 2.0–3.0 kW per 100 cfm. The sourcebook notes that dryer ratings use the 'three 100s' inlet conditions (100 psig, 100 °F inlet air, 100 °F ambient), that higher inlet temperature or lower pressure reduces rated capacity, that cutting saturated air temperature by 20 °F roughly halves its moisture content, and that drying beyond the needed dew point wastes energy. [4]

Editorial calculation: a dryer rated at 300 l/s handles about 636 cfm, so a twin-tower desiccant dryer would need about 13–19 kW against about 5 kW for a refrigerant dryer at the same flow; −40 °F equals −40 °C, and 35–39 °F is about 2–4 °C. If your end use only needs a dew point a few degrees above freezing, the desiccant dryer's extra 8–14 kW buys nothing; specify the dew point the process needs and ask for flow correction factors at your inlet temperature and pressure. [4] [1]

04

Denair's heated purge listing and what to ask any dryer supplier

Denair is the one supplier linked here; its card points to its externally heated purge dryer page.

Ask any supplier for the dryer type, rated flow and its reference conditions, pressure dew point, purge or heater power, pressure drop, desiccant type and change interval, control and alarms, and filter grades supplied.

05

Towers, desiccant, heater or blower, valves and filters

The heater or blower row is the one that changes with the regeneration type chosen above.

06

Flow, dew point, purge and pressure drop at your inlet conditions

Rated flow only means something with its inlet conditions attached; the DOE sourcebook notes that higher inlet temperature or lower pressure reduces rated capacity. [4]

07

Send the air, quality and running data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Air: [flow; pressure; inlet temperature; ambient]
Quality: [pressure dew point; oil; particles]
Running: [hours per year; load profile]
Regeneration: [heatless; heated; blower]
Control: [dew-point switching; alarms]
Site: [power; space; noise]

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