How a Desiccant Dehumidifier Works

Quick answer: A rotary desiccant dehumidifier dries air by adsorption instead of condensation. A slowly turning wheel (rotor) coated with a desiccant such as silica gel sits in two air streams. In the larger process air sector, the desiccant attracts water vapour and the air leaves dry and warmer. In a smaller reactivation sector, heated air drives the water out of the desiccant and is exhausted outdoors as warm, humid air. Because the desiccant attracts water at any temperature, the unit keeps working in cold air and can reach dew points well below freezing, where a refrigerant coil would ice up.

Quick answer

• Process air: through about three-quarters of the wheel; leaves dry and warm.
• Reactivation air: heated, through the rest; leaves wet, ducted outdoors.
• No drain needed: the water leaves as vapour in the reactivation exhaust.

The parts

Step by step through the wheel

1. Adsorption: moist process air passes through the honeycomb channels. Water vapour is held on the desiccant's huge internal surface. Adsorption releases heat, so the dry air leaves warmer than it entered.
2. Rotation: the loaded part of the wheel turns slowly into the reactivation sector.
3. Reactivation: hot, dry reactivation air flows through in the opposite direction, heats the desiccant and carries the water away.
4. Cooling and return: the dried, warm part of the wheel turns back into the process sector, ready to adsorb again.

Worked example from our RY-1000M data

The RY-1000M is rated at 180 L/day at 20 °C / 60 % RH with 1,000 m³/h of process air.

• Inlet air at 20 °C / 60 % RH holds about 8.7 g of water per kg of dry air (dew point about 12 °C).
• Removal: 180 L/day = 7.5 kg/h. Process air: about 1,200 kg/h.
• Moisture removed per kg of air: 7.5 ÷ 1,200 ≈ 6.2 g/kg.
• Leaving air: about 2.5 g/kg, a dew point of roughly −6 °C, from inlet air with a dew point of about 12 °C.

That is the strength of desiccant drying: a single pass takes the dew point far below what a refrigerant coil can reach without icing. The full range is on our desiccant dehumidifier page.

Why it reaches low dew points

A refrigerant dehumidifier can only dry air to about the temperature of its coil, and the coil cannot go much below 0 °C without ice. A desiccant has a lower vapour pressure at its surface than the air even at low temperatures, so it keeps pulling water out. That is why desiccant units are used for:

• cold stores and unheated warehouses;
• low dew-point processes such as battery dry rooms, pharmaceuticals and powders;
• water damage drying in cold buildings;
• wind turbines, substations and other cold, remote equipment.

Energy

Most of the energy goes into the reactivation heater. Rated removal divided by rated power across our RY range gives about 0.6–1.0 L/kWh at 20 °C / 60 % RH, lower than refrigerant units in warm air but often the only practical option in cold air. Using waste heat, steam or hot water for reactivation cuts running cost; the US DOE has reviewed thermally activated desiccant technology (US DOE). See desiccant dehumidifier power consumption.

Installation essentials

• Duct the reactivation exhaust outdoors, sloped away from the unit; it carries all the removed water.
• Take reactivation intake air from a point away from the exhaust.
• Filter both air streams.
• Plan for the warm process air: add cooling if the room has a temperature limit.
• Seal the room: desiccant systems are most valuable when leakage is controlled.

Common mistakes

• Comparing desiccant L/day (rated at 20 °C / 60 % RH) directly with refrigerant L/day (rated at 30 °C / 80 % RH).
• Exhausting reactivation air into the building.
• Forgetting the reactivation power supply or heat source.
• Ignoring filters: dust and oil mist shorten rotor life.

For selection, see how to choose a desiccant dehumidifier, rotor structure and refrigerant vs desiccant.

Sources

• ASHRAE Handbook—HVAC Systems and Equipment, Desiccant dehumidification and pressure-drying equipment.
• US DOE, Thermally activated desiccant technology status report.