Humidity control
www.heatingandventilating.net
Dessicant vs condensing dehumidifiers
Dave Marshall-George, sales director at Condair, compares dehumidifier technologies and gives an overview on best practise dehumidifier selection
T
here are two main types of commercial dehumidifier – condensing and desiccant. They both lower humidity but do so in quite
different ways, which frequently determines when you should use one instead of the other. A condensing dehumidifier 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 find in a fridge or AC unit. Air is drawn into the dehumidifier 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 dehumidifier. A desiccant dehumidifier 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 dehumidifier 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 indefinitely 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 dehumidifier 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 dehumidifier relies on temperature to dry the air, the technology is most efficient when the atmosphere is warm. The warmer the room, the greater the condensation effect on the dehumidifier’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 dehumidifiers can incorporate heat recovery systems, which return the heat generated from the drying process to heat the pool water. If the atmosphere requiring dehumidification
is lower than around 20°C, frequently desiccant dehumidifiers start to become a more efficient solution. Their efficiency of operation is not temperature dependent, so they perform well in both warm and cold environments. However, this flexibility 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 dehumidifier to incorporate a gas or hot
14 July 2021
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