Book - School of Science and Technology

Criteria which affect human comfort ± definitions 7Table 1.3 Typical metabolic rate and heat generation for an adult humanActivityMetabolicrate (met)Heat generation(W)Sleeping 0.7 75Sitting 1.0 105Standing 1.2 125Typical office work 1.2 125Walking (1.3 m/s) 1.6 165Heavy machine work 3.0 315outside air is now preferred. In many instances the outside air volume may have entereda building by infiltration, in which case it is usually referred to in terms of air changes orroom volumes per hour but, when handled by some form of mechanical equipment, this israted in either m 3 /s or, if realism prevails and decimal places are avoided, in litre/s.MovementIt is not always well understood that air movement within a room is a positive rather thana negative effect. The source will be the position and velocity of admission since thelocation of an opening for removal has virtually no effect upon distribution. Air movementis measured in terms of air velocity (m/s) and must be selected within the limits ofdraughts at one extreme, and of stagnation at the other.Activity and clothingThe interaction between human comfort, activity and clothing has been mentioned earlier.In most instances, the variables derive from the nature of the enclosure and thepurpose for which it is to be occupied and thus allow for a group classification with likesituations.Human activity is graded according to the level of physical exertion which is entailedand to the body area, male or female, ranging from a heat output of about 75 W whensleeping, through to in excess of 900 W for some sports activity. Table 1.3 gives sometypical rates of heat generation, calculated using a body surface area of 1:8m 2 , representingthe average for an adult. As to clothing, this is graded according to insulation value,the unit adopted being the clo. Unity on this scale represents a traditional business suit,with a value of 0:155 m 2 K/W:zero in the scale is a minimum swim suit and light summerwear has a value of about 0.5 clo. Typically, for people dressed in a traditional suit, a risein activity rate equivalent to 0.1 met corresponds to a reduction of 0.6 K in the dryresultant temperature to maintain a similar comfort level.Air purityPollution can derive from sources outside a building or as a result of contaminantsgenerated within it. In the former case, if air enters the building by simple infiltration,then dusts and fume particles, probably mainly carbonaceous in urban areas, nitrousoxides (NO x ) and sulphorous oxides (SO x ) will enter with it.Table 1.4 gives examples of outside levels of air quality; the values for the four airpollutants are typical annual average concentrations. Mechanical plant, used for ventilationsupply is usually provided with air filtration equipment but this, as a result of