Quick answer: For dew points below about 5 °C, a desiccant dehumidifier is the usual choice; for −20 °C and below, the design becomes a system: a tight room, pre-cooled process air, a rotor sized for the moisture left after pre-cooling, sometimes a purge sector or a second rotor, and very careful control of leakage and people. Specify the leaving-air dew point at a stated inlet condition and airflow, not just a removal rate in L/day. In low dew-point rooms, air leaking in through doors and gaps is usually a far bigger load than the process itself.
Quick answer
- Specify: target dew point, inlet condition (summer worst case), process airflow, leakage.
- Single rotor: often to about −20 to −40 °C dew point with pre-cooling; lower needs purge or two-stage designs.
- Room tightness and air locks matter as much as the dehumidifier.
Typical dew-point targets
| Application | Typical target |
|---|---|
| Cold store anterooms, loading docks | Dew point below the cold surfaces (often below 0 °C) to stop frost and ice |
| Pharmaceutical tablets, powders, capsules | Often 15–30 % RH at room temperature (dew point around −5 to +5 °C) |
| Confectionery, chocolate, gelatin | Often 30–40 % RH |
| Plastics resin drying (hopper air) | Very low dew points, often around −40 °C |
| Lithium battery dry rooms | Commonly −35 to −60 °C dew point or lower, by process step |
| Military, archive, wind turbine and substation preservation | Moderate RH (for example below 40–50 %) in cold conditions |
Your product or process specification sets the actual target.
Why leakage dominates at low dew points
The drier the room, the more each cubic metre of leaking air matters. Moisture content of air at different dew points:
| Dew point | Moisture content |
|---|---|
| 10 °C (normal room) | about 7.7 g/kg |
| −20 °C | about 0.63 g/kg |
| −40 °C | about 0.08 g/kg |
| −60 °C | about 0.007 g/kg |
Worked example: a 500 m³ dry room held at a −40 °C dew point. If outside air at 20 °C / 50 % RH (about 7.3 g/kg) leaks in at only 0.5 air changes per hour (250 m³/h, about 300 kg/h): 300 × (7.3 − 0.08) ÷ 1,000 ≈ 2.2 kg/h of water.
Each person working in the room adds roughly 0.05–0.1 kg/h. So the leakage of a slightly loose room can outweigh many people and the process. Dry rooms are built with sealed panels, air locks, positive pressure and controlled material entry.
System building blocks
| Element | Role |
|---|---|
| Pre-cooling coil | Removes most of the water from warm, humid make-up air before the rotor; lowers the rotor's load |
| Desiccant rotor | Removes the remaining water to the target dew point |
| Purge sector | Cools the rotor before it re-enters the process stream, improving low dew-point performance |
| Second rotor (two-stage) | For the lowest dew points |
| Post-cooling | Removes heat added by adsorption and reactivation carry-over |
| Reactivation heater | Electric, steam, hot water or gas; the main energy user |
| Air locks and positive pressure | Keep humid air out |
See how a desiccant dehumidifier works and rotor structure and seals.
Energy
Most energy goes into reactivation and pre-cooling. To keep it under control:
- tighten the room first;
- recirculate room air and add only the make-up air needed for pressure and people;
- use waste heat for reactivation where possible;
- control reactivation heat to the actual load.
See desiccant dehumidifier power consumption.
RFQ checklist
- Target dew point (and RH/temperature) in the room and at the supply
- Worst-case outdoor condition for make-up air
- Room volume, construction, door types, air locks
- Number of people and processes releasing moisture
- Make-up and recirculation airflow
- Reactivation energy source available
- Space, power, controls and alarms
- How performance will be accepted (dew point at stated points, over a stated period)
Our desiccant dehumidifiers (RY series, 93–3,000 m³/h process air) cover moderate low-dew-point duties; at 20 °C / 60 % RH inlet, for example, the RY-1000M's rated removal corresponds to a leaving dew point of roughly −6 °C. Lower dew points need pre-cooling or a multi-stage system.
FAQ
Frequently asked questions
What dew point can a desiccant dehumidifier reach?
Typically well below 0 °C; with pre-cooling, purge sectors or two-stage rotors, down to −40 to −60 °C or lower.
Why is my dry room not reaching its dew point?
Usually leakage: doors, gaps, people or materials bringing moisture in. Check room tightness and pressure before adding capacity.
Do I need pre-cooling before a desiccant rotor?
For warm, humid make-up air and low dew-point targets, yes; it removes most of the water cheaply and lets the rotor reach a lower dew point.
Sources
- ASHRAE Handbook—HVAC Systems and Equipment, Desiccant dehumidification and pressure-drying equipment.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
