The loss of heat from the body and the feeling of individual comfort in a given environment is much affected by the clothing worn and in a room with a mixed population of men and women wearing different garb, comfort for everyone may be almost impossible to achieve. There also appears to be a seasonal pattern in the clothing worn, according to Berglund
(1980), which prevails even if the temperature of the working environment is virtually constant throughout the year, lighter garments of less thermal insulation value being worn in the summer.
The unit used to describe the thermal insulating quality of the clothing worn is the clo with a physical value of 0.155 m2 K W-1. Table 4.2 lists some thermal resistances proposed by Berglund (1980) for individual items, to be combined by equation (4.5), according to McCullough and Jones (1984).
/cl0 = 0.835 I/Clui +0.161 (4.5)
Where /clui is the effective thermal insulation of garment i, and /clo is the thermal insulation of the total clothing ensemble.
Table 4.2 Thermal resistances for some items of clothing according to Berglund (1980)
(a) Deduct 10% if sleeveless or short sleeved.
(b) Add 5% if below knee length; deduct 5% if above knee length.
It is reckoned by ASHRAE (1997) that total clo-values cannot be estimated to be better than 20 per cent accuracy and this should be borne in mind. ASHRAE (1997) quote clo — values that are a little less than the figures given in Table 4.2.
Estimate the insulation value of the clothing of a man dressed as follows:
(a) T-shirt, underpants, light-weight trousers, ankle socks and shoes,
(b) Sleeveless singlet, underpants, long-sleeved shirt, heavy-weight trousers, jacket and waistcoat, knee-length socks and shoes.
(i) Using Table 4.2 the sum of the individual items of clothing is 0.09 + 0.05 + 0.26 + 0.04 + 0.04 = 0.48 and by equation (4.5) we have 0.835 x 0.48 + 0.161 = 0.56 clo.
(ii) The sum of the individual items is 0.06 + 0.05 + 0.22 + 0.32 + 0.49 + 0.29 + 0.10 + 0.04 = 1.57 and by equation (4.5) we have 0.835 x 1.57 + 0.161 = 1.47 clo.
Similar calculations for the light and heavy extremes of women’s clothing yield figures of about 0.6 and 1.3 clo. We might therefore conclude that 1 clo represents average clothing for a man and perhaps 0.9 clo for a woman. It is not surprising that achieving satisfactory conditions of comfort for an air conditioned room with a mixed population sometimes proves difficult. It has been suggested by Berglund (1980) that the comfort of a clothed individual corresponds to a decrease in ambient dry-bulb of about 0.5°C for each clothing increase of 0.1 clo but this can only be true over a limited range of temperature. For sedentary workers a realistic lower limit for a period of more than one hour is about 18.5°C, provided the air movement is imperceptible, as it might be in a room that was only heated and not mechanically ventilated or air conditioned. In air conditioned rooms with typical air change rates of 5 to 20 per hour air movement is not imperceptible at such a low temperature, no matter how well the air distribution system is designed. For air conditioned rooms the realistic lower limit is 20°C in the UK and even then it will be unsatisfactorily cool for some of the occupants.
The intensity of air turbulence (Tu) is relevant to a sensation of draught and is defined by ASHRAE (1997) and Fanger (1987) as
Tu = 100(Vsd/V) (4.6)
Where Vsd is the standard deviation of the local air velocity, measured by an omnidirectional anemometer with a time constant of 0.2 s, and V is the mean air velocity in m s-1. The value of Tu is used in equation (4.7) to predict the percentage of people dissatisfied (PD) because of the presence of the draught:
PD = (34 — *„)(V — 0.05)°’62(0.37VTu + 3.14) (4.7)
Where t. d is the dry-bulb temperature of the air.
The equation is relevant for 20°C < t. d < 26°C and for 0.05 < V < 0.5 m s-1, according to ASHRAE (1997). As an example, this reference shows that, for an air temperature of 22°C with a mean air speed of 0.25 m s’1 and a turbulence intensity of 2 or 3 per cent, it is still likely that 15 per cent of the people will be dissatisfied. Fanger et al. (1988) have shown that discomfort depends also on the frequency of fluctuation of the draught, people being particularly sensitive to the range from 0.3 to 0.6 Hz.
Posted in Engineering Fifth Edition