HUMIDITY CONTROL
Desiccant vs condensing dehumidifi ers
Dave Marshall-George, sales director at Condair, compares dehumidifi er technologies and gives an overview on best practise dehumidifi er selection.
T
here are two main types of commercial dehumidifi er – condensing and desiccant. They both lower humidity but do so in quite diff erent ways, which frequently determines when you should use one instead of the other. A condensing dehumidifi er operates using the basic principle
of creating a cold surface upon which moisture from the air will condense. It incorporates the type of refrigeration circuit that you would fi nd in a fridge or AC unit. Air is drawn into the dehumidifi er with a fan and meets the cold coil evaporator element of the fridge circuit. Its temperature is reduced below dew point, causing condensation to form on the coil, from where it drips to drain or into a water tank for disposal. The cold air is then heated with the condenser part of the fridge circuit before being returned to the room, drier and slightly warmer than it entered the dehumidifi er. A desiccant dehumidifi er operates using the adsorption
properties of a rotating desiccant wheel, a bit like a sponge literally soaking up moisture directly from the air. Air is drawn into the dehumidifi er and passes through a slowly revolving desiccant wheel. The desiccant absorbs moisture and the dried air is returned to the room. In order to allow the desiccant wheel to indefi nitely absorb moisture, the wheel passes through a regeneration area, where it is heated by a secondary hot airstream. A heater, frequently electric, heats this secondary airstream to around 120°C before passing it through the wheel.
The hot air absorbs the moisture from the desiccant and is then vented externally.
So a desiccant dehumidifi er has two airstreams – a process
airstream and a regeneration airstream, which exhausts the moisture collected from the process airstream. As the freshly regenerated part of the wheel is hot, it carries some residual heat when it rotates back into the process airstream. Therefore, as well as drying the process airstream, the desiccant wheel also provides some heat.
Selection
As a condensing dehumidifi er relies on temperature to dry the air, the technology is most effi cient when the atmosphere is warm. The warmer the room, the greater the condensation eff ect on the dehumidifi er’s cold evaporator coil. This makes this type of technology ideal for applications like swimming pools, where the air is typically very warm and moist. Especially, as some condensing dehumidifi ers can incorporate heat recovery systems, which return the heat generated from the drying process to heat the pool water. If the atmosphere requiring dehumidifi cation is lower than
around 20°C, frequently desiccant dehumidifi ers start to become a more effi cient solution. Their effi ciency of operation is not temperature dependent, so they perform well in both warm and cold environments. However, this fl exibility comes at a cost, as they do use more energy than condensing systems, due to the heating required to regenerate the desiccant wheel. In comparison, a condensing system would typically consume 0.5-1.5kW to remove one kilo of water, while a desiccant would consume 1-3kW for the same capacity. It is possible for a desiccant dehumidifi er to incorporate a gas or hot water heat exchanger, which can reduce the cost of this energy consumption.
Due to the need to vent the regeneration airfl ow externally, to get rid of the hot wet air coming off the desiccant wheel, desiccant dehumidifi ers can also be more complicated to install. A condensing dehumidifi er can dry an atmosphere with only an electrical supply and a drain. A desiccant dehumidifi er will always need ducting to vent the regeneration airfl ow away from the area being dried. With the higher energy costs and the more complex installation requirements involved in desiccant dehumidifi cation, if a project’s environmental conditions suit a condensing dehumidifi er, it will normally be the most cost eff ective solution. However, temperature isn’t the only determining factor. Condensing dehumidifi ers are very eff ective at maintaining
12 September 2024 •
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