Desiccant (adsorption) dryer: how it works, dew point and sizing
The adsorption dryer — also called a desiccant dryer — dries compressed air by binding water vapour to the surface of a solid desiccant. It reaches a pressure dew point (PDP) of −40 °C to −70 °C, where a refrigerated dryer tops out around +3 °C. It is the required choice as soon as piping is exposed to freezing or the process needs very dry air (instrumentation, ISO 8573-1 Class 1 and 2). In return, it consumes purge air to regenerate and demands careful upstream filtration.
Adsorption or refrigeration: which for which need?
The two families do not play in the same dew-point range, nor at the same operating cost.
| Refrigerated dryer | Desiccant dryer | |
|---|---|---|
| Typical dew point | ≈ +3 °C (Class 4) | −40 °C (Class 2) to −70 °C (Class 1) |
| Below freezing | No | Yes |
| Energy / maintenance cost | Lowest | Higher (purge air + desiccant) |
| Flow loss | None | ≈ 15% as purge air (heatless) |
| Typical use | General industry, heated shop | Freezing, outdoors, instrument air, critical processes |
For the detailed trade-off, see Refrigerated or desiccant dryer: how to choose and the Choosing a refrigerated dryer article.
How a desiccant dryer works
The dryer has two towers filled with desiccant. At any moment, one tower dries the air while the other regenerates; the towers swap roles at a fixed interval.
- Drying. Humid compressed air, already stripped of liquid water by a separator and pre-filter, flows up through the active tower. The desiccant adsorbs the water vapour (surface binding — not to be confused with absorption) and lowers the dew point. Dry air exits the top, passes a check valve, then a dust post-filter, on to the network.
- Regeneration. A fraction of the dry air produced is drawn off at the outlet, depressurised to atmospheric pressure (purge air), then routed top-down through the saturated tower. This dry, expanded air picks up the moisture released by the desiccant and carries it outside through a muffler.
- Switchover. After a set time, the regenerated tower is slowly re-pressurised, then the roles reverse: the fresh tower dries, the saturated tower regenerates.
This is the principle of heatless regeneration: regeneration uses only depressurised dry air, with no heating. Simple and reliable, it is the most common method for small to medium flows; in return it consumes purge air continuously.
Dew point and ISO 8573-1 classes
The target dew point sets the desiccant type and the purge air required.
| Pressure dew point (PDP) | ISO 8573-1 class | Typical desiccant |
|---|---|---|
| −40 °C (−40 °F) | Class 2 | Activated alumina (standard) |
| −70 °C (−94 °F) | Class 1 | High-performance desiccant / molecular sieve |
The lower the target dew point, the larger the dryer must be for the same flow (see sizing below) and the more purge air it uses. For the dew-point concept and the quality classes, see Compressed-air dew point and ISO 8573-1 air quality.
Mandatory filtration, upstream and downstream
A desiccant dryer is sensitive to liquid water and oil: they foul the desiccant, raise the pressure drop and degrade the dew point. The filtration chain is not optional.
- Upstream — water separator. A high-efficiency water separator with a condensate drain removes entrained liquid water. Most makers may void the dryer warranty if this separator is not installed.
- Upstream — coalescing pre-filter. A 0.01 µm (XA grade) coalescing pre-filter removes the oil aerosol and fine droplets before the desiccant inlet.
- Downstream — dust post-filter. A 1 µm dust filter catches desiccant fines that could migrate into the network. Activated alumina is considered a harmful dust: use appropriate protection when replacing desiccant.
For the hardware, see Line filters and water separators and Condensate treatment.
How much air does regeneration consume?
For a heatless dryer, purge air is about 15% of the treated flow (at 100 psig, −40 °C dew point). This dry air vented to atmosphere is the main cost item of a desiccant dryer.
Two levers reduce the bill:
- Demand-based regulation — a controller that measures the dew point (or load) shortens the purge cycle when demand is low, instead of purging at a fixed interval.
- Correct sizing — a dryer neither too small (drifting dew point) nor too large (purge paid for nothing).
Sizing a desiccant dryer
A dryer’s rated capacity is given at reference conditions (often 100 psig, ~35 °C, −40 °C PDP). The actual flow is corrected by three factors, supplied in the model’s datasheet:
Corrected flow = inlet flow ÷ (PCF × TCF × DCF)
- PCF — pressure factor. Below the reference pressure, the factor drops under 1 and a larger dryer is needed.
- TCF — inlet-temperature factor. The warmer the air, the lower the factor.
- DCF — dew-point factor. Targeting −70 °C instead of −40 °C lowers the factor (≈ 0.70) and forces a larger dryer.
You then pick the model whose capacity exceeds the corrected flow.
Worked example
Factor values shown for illustration only — always use the correction tables from the maker of the chosen model.
| Condition | Value | Factor |
|---|---|---|
| Inlet flow | 15 SCFM | — |
| Inlet pressure | 6 barg (87 psig) | PCF = 0.87 |
| Inlet temperature | 25 °C (77 °F) | TCF = 1.06 |
| Target dew point | −70 °C (−94 °F) | DCF = 0.70 |
Corrected flow = 15 ÷ (0.87 × 1.06 × 0.70) = 15 ÷ 0.645 = ≈ 23.2 SCFM
Answer: you need a dryer of at least 23 SCFM at these conditions. Targeting a −70 °C PDP instead of −40 °C raises the requirement from 15 to over 23 SCFM. To estimate your flow and pressure losses, see the compressed-air calculator.
Mistakes to avoid
- Installing the dryer without a water separator and coalescing pre-filter upstream (flooded desiccant, voided warranty).
- Undersizing by forgetting the dew-point factor (DCF) when targeting −70 °C.
- Skipping the dust post-filter downstream (desiccant fines in the network).
- Choosing adsorption when a refrigerated dryer would do (≈ +3 °C in a heated shop): you then pay for purge air needlessly.
- Forgetting that purge air reduces the usable flow: the compressor sizing must account for it.
- Handling desiccant without protection during a change-out (harmful dust).
Further reading
- Choosing a refrigerated dryer — purchase and rental
- Compressed-air dew point
- ISO 8573-1 air quality
- Water in the compressed-air network — diagnosis
- See our models: Desiccant dryers
Onyx M3 helps you select the right dryer, even without a purchase on the site. Contact us.