Dehumidifiers

The relative humidity of a sample of air can be reduced using two principle techniques; cooling below the dew point temperature and chemical adsorption.

Dew Point Dehumidification. Humid air contains invisible water vapour. This only becomes visible when it changes back to liquid water. This phase change is achieved by cooling the humid air until condensation starts to occur. This is what happens when air in a room touches a cold window. It becomes chilled and condensation forms on the cold glass surface. However, this only happens if the glass is cold enough. The threshold temperature below which condensation occurs is called the dew point temperature.

Dehumidification of air occurs in the same way. The air is made to pass over a cold coil in the air handling unit which is below the dew point temperature of the air. This causes some of the water vapour in the air to condense out onto the coil where it is drained away. In some dehumidification applications the condensate can be collected and re used. One example is in swimming pool dehumidification where the condensate is used to top up the pool to offset the use of some of the mains water which must be purchased.

Dehumidification by chilling is an energy intensive process since the air must be reheated to bring it back up to comfort temperatures. One way of achieving this efficiently is to use heat pump dehumidification. Both coils of the heat pump are placed in the ducting as shown in below figure.

The first coil the air meets is the evaporator coil. This is cold and removes water from the air by condensation. The air then passes over the condenser coil of the heat pump which re heats the air using energy which, in a simple cooling situation, would go to waste. Both sensible and latent heat are recovered in this process.

Desiccant Dehumidification involves the removal of water vapour from the air by chemical adsorption. The humid airstream (below figure) is passed over a surface which is coated with a desiccant chemical such as silica gel. This removes water vapour from the airstream. The gel would quickly become saturated and unable to remove further water from the airstream. It must, therefore, be reactivated by heating. The desiccant chemical coats the tubes of a desiccant wheel. The lower part of this wheel absorbs moisture out of the airstream.

This section then rotates into a new section of ducting where warm air drives off the moisture and re generates the wheel. The now dry section of wheel rotates back into the humid airstream to continue the drying process.