How Much?

In order to select the correct capacity dehumidifier, individual moisture loads require calculating. Whilst this is not an exact science, with experience a close estimation is possible.

The guide below provides examples of some common moisture loads. For further help, the on-line calculator is available here:-

1. Determine what climate is needed

How is airborne moisture causing a problem? Is it the relative humidity or the dewpoint of air that requires control? Is air temperature an important factor and will this also require control? What is the ideal climatic condition required to solve the particular problem? Will the system also require humidification during colder months?

2. Moisture load from mechanical ventilation

In humidity controlled spaces, mechanical ventilation (extract & supply) should be kept to the absolute minimum necessary for human occupancy, fume extract or that required to maintain a positive or negative pressure in the room.

Whenever possible incoming fresh air should always be dealt with at the dehumidifier air inlet.

3. Natural infiltration

Unless a space is perfectly sealed there will always be some natural infiltration of fresh air into that space. Whilst using fresh air to produce a slight overpressure may help to reduce this infiltration, because of the behaviour of water vapour it is unlikely to eliminate it completely.

The total volume and surface area of the space or building influences the natural air change rate (ac/h). A large, well sealed building, such as a PVC hall, may have an air change rate as low as 0.05 ac/h, while a small room of brick & plaster construction could be in excess of 1.0 ac/h.

Normally the infiltration air is outdoor air so the summer design moisture content needs to be determined. In UK, the summer design moisture content used varies between 11.0 g/kg & 16.0 g/kg depending on the nature of the project.

mV = 1.2 • V • (X1-X2) / 1000

where mV = moisture load from ventilation [kg/h]
V = total ventilation [m³/h]
X1 = water content for fresh air, in most cases outdoor air [g/kg]
X2 = water content for the room air [g/kg]

3. Moisture load from people

People give out moisture to the room through respiration and perspiration. At rest this may be as low as 0.05 kg/h, whilst an overweight adult male working hard will give out around 0.3 kg/h.

4. Moisture load from water surface

Evaporation of water into air is influenced by the following factors.

a) Temperature of the water.
b) Relative humidity and temperature of the air
c) Water surface disturbance
d) Air movement across the water surface
e) Surface area of the water

As a guide:-

mW = s • A • (X1 - X2) / 1000

where mW = moisture load from water surface [kg/h]
s = coefficient for vaporisation, normally 25 [kg/(m² h)]
A = open water area [m²]
X1 = water content for saturated air at the water temperature [g/kg]
x2 = water content for the room air [g/kg]

5. Moisture load evaporated from product

It is impossible to know the exact moisture loads from all products as there are so many variables involved. A rough estimation is normally calculated and then tested using a small pilot system.

6. Moisture load by diffusion

There is some moisture load by diffusion through building fabrics (permeance). In modern buildings, this is usually negligible when compared to other moisture loads. Some older building however, use materials that have an unacceptable permeance level and are therefore not suitable for humidity controlled environments unless they are first sealed (eg: cement/asbestos corrugated roofing).