The amount of moisture
in a given volume of air is most often stated in terms of its relative humidity
(RH). This is a measure of how much water vapour there is in the air sample
compared to its saturated state. Completely dry air would have a relative
humidity of 0%. Air which is saturated would have a relative humidity of 100%.
For human comfort the relative humidity of the air in a room should be between
40 and 70% RH. If the air is below 40% RH the air will feel dry and lead to discomfort
through dry eyes and throats. It is also known that the risk of static shocks
and problems with VDU screens increases in dry atmospheres. Relative humidity
above 70% result in discomfort due to clamminess and overheating. This is
because the body's normal mechanism for cooling itself down, sweating cannot
operate effectively in a humid environment. Prolonged relative humidity above
80% can lead to mould growth in buildings.
In addition to human
comfort, some industries require stable relative humidity for the production
and storage of materials without degradation. Examples are the high relative humidity
required in the textile industry, typically 65% in wool processing and 75% RH
in cottons, to avoid problems such as electrostatic build up and yarns breaking.
50-55%RH is required in the print industry to prevent sheet papers curling and breaks
in newspaper webs. At the other end of the scale, low relative humidity are
required by some industries such as in car panel manufacture to avoid corrosion.
Low relative humidity
occur when cold outside air is brought into the building and is heated. For
example the relative humidity of outside air at 0oC and 90%RH drops to 23%RH
when heated to 20oC. The problems associated with this can be overcome by
adding moisture to the airstream (humidifying it). High relative humidity occur
when warm summertime air is cooled or in spaces with open bodies of water such
as swimming pools. Problems associated with high relative humidity can be avoided
by removing moisture from the airstream (dehumidifying it). Dehumidification is
discussed in the next section.
This section will discuss
methods for humidifying a space. Humidification systems are categorized by the way
they deliver water vapour to the air in a room. The two categories are direct
and indirect humidification.
Direct
humidification is used in industrial situations and involves adding moisture
directly into the air of the room in which humidification is required. Indirect
humidification is used in buildings with central air conditioning systems. The
air is humidified within the air handling unit and is then delivered to the
room using ducting.
