MODULE III
Air-conditioning and Cooling technology Page 6

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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 20C, 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 20C 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 25C where the humidity is at 60%. At 50% this will first occur at 28C. Productivity sinks to nil where the air and room temperature is 37C 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 ... 22C 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.


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