What is a condensation dehumidifier?

A condensing dehumidifier is a type of dehumidification system that removes water vapour from the air by cooling it to the point where moisture condenses into liquid water.

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How a condensing dehumidifier works

A condensing dehumidifier removes moisture from the air by cooling it below its dew point, causing water vapour to condense into liquid water.

The amount of moisture air can hold depends on its temperature. Warm air can contain significantly more water vapour than cold air. For example, at 20°C, air can hold up to 14.7 g of water per kilogram of dry air, whereas at 5°C, the maximum is only 5.4 g/kg.

Imagine air at 20°C with an absolute humidity of 10 g/kg and a relative humidity of 69%. As this air is cooled, it eventually reaches its dew point at around 14°C. Below this temperature, the air can no longer retain all its moisture, so excess water vapour condenses into liquid. By the time the air reaches 5°C, approximately 5 g of water per kilogram of dry air has condensed and been removed.

This is the same principle that causes spectacles to fog up when moving from a cold outdoor environment into a warm room. The cold lenses cool the surrounding air, causing moisture to condense on the surface.

A condensing dehumidifier applies this natural process in a controlled way. A fan draws humid air across a cold evaporator coil. As the air cools, moisture condenses on the coil and is collected in a condensate tray or drained away.

The cold surface is created by a closed refrigeration circuit operating on the heat pump principle. After the moisture has been removed, the dry air passes through the condenser, where it is reheated before being returned to the room. Because the air is warmer again, it can absorb more moisture from the surrounding environment, allowing the dehumidification process to continue.

Condensing dehumidifiers are most effective in environments with medium to high temperatures and high relative humidity, making them an ideal choice for applications such as indoor swimming pools, warehouses, commercial buildings and many industrial processes.

Sizing a condensing dehumidifier

Once the required dehumidification capacity has been calculated for the application, the appropriate condensing dehumidifier can be selected using the manufacturer's performance diagram.

The capacity of a condensing dehumidifier depends heavily on the operating conditions. Performance increases as the air temperature rises but decreases significantly at lower temperatures, as colder air contains less moisture.

Most manufacturers specify a nominal dehumidification capacity under standard test conditions, typically 30°C and 80% RH, and sometimes 27°C and 60% RH. In some cases, only the maximum capacity at 35°C and 80% RH is stated. While these values provide a useful reference, they rarely reflect the actual operating conditions of a specific application.

To determine whether a dehumidifier can deliver the required capacity under design conditions, the manufacturer's performance diagram should always be consulted. These diagrams show how the dehumidification capacity varies with air temperature and relative humidity, making it possible to assess the unit's actual performance.

For example, a condensing dehumidifier may be rated at 40 litres per 24 hours under standard conditions of 30°C and 80% RH. If the application requires the removal of 20 litres per 24 hours at 20°C and 60% RH, the actual capacity under these operating conditions must be verified using the manufacturer's performance diagram.

The example performance diagram below illustrates how the dehumidification capacity can be determined for the required design conditions.

Check operating limits

Condensing dehumidifiers are generally suitable for operation at relative humidity levels of up to approximately 45% and temperatures of approximately 5–36 °C. If conditions differ, contact the manufacturer or use an adsorption dehumidifier.

Rise in temperature

Temperature rise: All condensing dehumidifiers release the waste heat from the refrigeration circuit, the fan motors and the internal electrical components into the room air, which – depending on the size of the unit – can result in anything from a minimal rise in room temperature to an enormous additional heat load. This effect is completely negligible in many applications or even desirable (e.g. in swimming pool areas), but can lead to problems in certain applications. The temperature aspect must therefore be taken into account, particularly in the case of large systems and temperature-sensitive applications.

Indoor air quality

Condensing dehumidifiers are generally suitable for use under normal indoor air conditions; in other words, they are not designed for use with aggressive or pollutant-laden air. Substances such as chlorine and ozone can corrode components and cause the units to fail within a short space of time. Swimming pool dehumidifiers are therefore protected against chlorine as standard through special measures. Some manufacturers are also able to supply specialised units for commercial use, featuring special protective coatings against, for example, acid or salty air.

Installation

Condensing dehumidifiers are available in portable or fixed versions. Portable units are almost exclusively operated with free exhaust, whilst large industrial dehumidifiers can also be connected to a ventilation duct network. In this case, care must be taken to ensure that sufficient pressure is available.