and Cooling technology Page 6
Fig. 4 Correlation
between air temperature t, absolute humidity x and relative humidity phi
Relative humidity also plays
a certain role in comfort. This is because warmth in humans created by the metabolic
process is partly dissipated through water evaporation from the skin surface.
At a room temperature of 20°C, heat dissipation through evaporation plays an admittedly
minor role. Humidity also has no great influence as a factor within this range.
This means that a person in a room with a room temperature of 20°C will hardly
notice a difference between 35 % and 65 % relative humidity.
Experiments have shown that a normally clothed person at rest in central Europe
will begin to perspire at 25°C where the humidity is at 60%. At 50% this will
first occur at 28°C. Productivity sinks to nil where the air and room temperature
is 37°C and humidity 100 %. Under these conditions the body has no possibility
to rid itself of the steadily increasing body heat, whether through radiation,
convection or perspiration. When determining the upper comfort limit, one should
set the relative humidity lower, in direct relation to the height of the air temperature.
To summarise the influence of air humidity on human comfort, we can say that at
a normal temperature of 20 ... 22°C humidity should be kept within a range between
35 ...65 %, while this choice should take Fig. 5 into account for higher room
temperatures. It is, however, important to remember that not only the air temperature
of a room is decisive for relative humidity, but that physical activity and air
movement also influence comfort. With increased physical activity, the relative
humidity in relation to normal values applying to a person at rest must be reduced,
while a lethargic sensation first appears, in conditions of increasing air movement,
at a higher level of relative humidity than normal values indicate.