1. Environmental conditions
Human comfort:During cooling in air conditioning applications, we know air reaches a dew point temperature. That means vapour is removed from the air and condensation is realised. If the air temperature becomes well below human comfort, it increases evaporation from the nose membrane and throat and causes skin and hair to dry out. So the relative humidity in areas for human comfort should be above 20 percent and less than 60 percent.
Process control and material storage:The following factors are controlled in the space for processing or storage of certain products: regain; rate of chemical reaction; rate of bio-chemical reaction; rate of crystallization; product accuracy and uniformity; corrosion; rusting and abrasion; static electricity; cleanliness; and product formability.
Static electricity:Electrostatic charges are generated when materials of high electrical resistance move against each other. The accumulated charges may result in sparks; difficulty in handling sheets of paper; clinging of dust to oppositely charged objects; and explosive gases. If moisture content of the air is increased the charges can be controlled.
Prevention and treatment of diseases:At 50 percent relative humidity, the mortality rate of certain organisms is highest and the influenza virus loses much of its virulence. The mortality rate decreases both above and below 50 percent relative humidity. Operating theatres in heath facilities are critical.
- Sound absorption is higher in lower humidity, above 15 percent and below 20 percent. Remember humidity control becomes effective if the temperature is well controlled.
Normally in certain countries we experience low humidities during winter. The extent of adding moisture to the building will depend on its walls, roof and other elements to prevent or tolerate condensation. The visible condensation may result in deterioration of the surface finishes, mould growth and a reduction of visibility through windows.
During the building and air conditioning design process, the relative humidity should be taken into consideration.
To be continued in the next issue with part two.
 RACA, ASHRAE