What is a condensation dehumidifier?

How a condensation dehumidifier works

The colder the air, the less moisture it can hold. At 20°C, this would be a maximum of 14.7 grams of water per kilogram of dry air. At 5°C, the corresponding maximum value is just 5.4 grams of water per kilogram of dry air.

Let us now take, for example, air conditions with a temperature of 20°C, an absolute humidity of 10g per kg of dry air and a relative humidity of 69%. As the air cools, the dew point line – and thus the maximum amount of water vapour that can be absorbed – would be reached from 14°C onwards. At a temperature of 5°C, the air would have released a total of 5 grams of water per kilogram of dry air, i.e. it would have condensed.

The phenomenon of water condensation caused by rapid cooling of the air is particularly familiar to spectacle wearers. If you enter a warm room in winter with cold spectacle lenses, the air cools down abruptly on contact and water condenses. The lenses fog up. In a condensation dehumidifier, the natural principle of moisture condensation on cold surfaces is mechanically generated by a cooling circuit. A fan draws the air to be dehumidified over a cold surface consisting of many fins, where water condenses and drips into a condensate tray.

The low temperature on the cooling fins is generated by a closed refrigeration circuit based on the heat pump principle. The cooled and dehumidified air is then reheated in the condenser and, due to the higher temperature, can once again absorb moisture from the environment. The use of condensation dehumidifiers is particularly recommended for applications with high relative humidity levels combined with medium to high temperatures, such as in indoor swimming pools.

sizing Once the required dehumidification capacity for the specific application has been calculated, the appropriate dehumidifier can be selected using its performance chart. The capacity and efficiency of condensation-type air dehumidifiers increase as the temperature rises and decrease significantly as the temperature falls. The technical documentation usually only specifies standard performance values at 30 °C and 80% RH, and occasionally at 27 °C and 60% RH. Often, only the maximum possible dehumidification capacity at 35 °C and 80% RH is specified. These figures are often not meaningful enough in relation to the specific application to enable an assessment of whether the selected unit can actually deliver the required dehumidification capacity under design conditions. Most manufacturers provide performance diagrams for this purpose, which can be used to determine the actual dehumidification capacity under design conditions with sufficient accuracy. Example: a condensation dehumidifier is specified in the documentation as having a dehumidification capacity of 40 l/24h at 30 °C and 80% RH. According to the calculation, 20 l/24h should be dehumidified at 20 °C and 60% RH. The actual dehumidification capacity under the desired conditions can be read from the manufacturer’s performance diagram shown opposite as follows: Check operating limits Condensation dehumidifiers are generally suitable for operation up to approx. 45% RH and a temperature of approx. 5–36 °C. If conditions differ, contact the manufacturer or use an adsorption dehumidifier. Temperature increase Temperature increase: All condensation 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 lead to a minimal increase in room temperature or even a significant 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 large systems and temperature-sensitive applications. Indoor air quality Condensation dehumidifiers are generally suitable for operation under normal indoor air conditions, i.e. not for aggressive or pollutant-laden air. Substances such as chlorine and ozone can corrode components and destroy the units within a short time. Swimming pool dehumidifiers are therefore already protected against chlorine as standard through special measures. Some manufacturers are also able to supply specialised units for commercial use with special protective coatings against, for example, acid or salty air. Installation Condensation dehumidifiers are available in portable or stationary versions. Mobile units are almost exclusively operated with free exhaust, whilst large industrial dehumidifiers can also be connected to a ventilation duct network. In this case, sufficient available pressure must be ensured.