Dehumidification: why a humidity window costs cooling, heating and space

01Humidity is the underestimated load

With ventilation and air-conditioning systems, temperature is the first thing that comes to mind. In cleanrooms, laboratories and many production rooms, however, a narrow humidity window is required – and this window determines the size of the cooling coil and the cooling generation, the need for reheat and the operating costs. The reason lies in physics: to remove water vapour from the air, it must condense, and in doing so the latent heat of vaporisation is released again. In energy terms, one gram of water per kilogram of air corresponds to a temperature change of around 2.5 kelvin.

02The fallacy: relative humidity as the target quantity

Relative humidity describes how far the air is from saturation – and this depends strongly on temperature. If air is heated without a change in moisture content, the relative humidity falls, even though not a single gram of water has been removed. A specification such as “45 % ± 5 %” at a permissible 20 to 24 °C therefore allows a moisture content of between around 5.8 and 9.3 g/kg. In control terms, that is something completely different from a fixed dew point band. The dew point describes the quantity of water independently of temperature and can be measured directly.

03The principle: cool, dehumidify, reheat

If the outdoor air is more humid than the desired supply air, there is no way around dehumidification. The air is cooled down to its dew point, then further along the saturation line – producing condensate in the process. Only below the target dew point has enough water been removed. Because the air is then too cold, it is reheated to supply air temperature.

04What this means in figures

A fictitious calculation example: assume that an air handling unit delivers around 10 000 m³/h of outdoor air at 32 °C and 40 % relative humidity and is to supply air at 22 °C and 45 %. The cooler must provide around 110 kW for this, of which about 36 kW is for dehumidification alone. Around 41 kW of reheat is then required, and around 52 litres of condensate are produced per hour.

33 %of the cooling capacity is attributable in the example to dehumidification alone

For very dry targets, such as a dew point below 4 to 5 °C, the coil surface would have to be cooled to or below 0 °C – the coil ices up. Desiccant dehumidification then takes over: a desiccant wheel binds the water vapour and is regenerated with hot air. This requires a lot of heat, but is particularly worthwhile when waste heat is available.

05What you should define for your project

• Specify the humidity window as dew point or moisture content, not just as relative humidity
• Justify tolerances on the basis of the process – every narrowing costs cooling and reheat
• Examine heat recovery between cooler and reheater
• Include part load and the winter case in the calculation, not just the summer design point
• Provide humidity sensors with a calibration plan and a suitable position

Made early, these specifications prevent oversized systems – and rooms whose humidity never becomes stable in operation.

Takeaway

A humidity window is an energy decision: specify the dew point, not just the relative humidity, and set the tolerance as tight as necessary, not as tight as possible.

SEDC AG · Article from our internal training series “Technik Praxis”, abridged for clients and users.