Environmental influences on comfort
From the foregoing it can be inferred that the body maintains a thermal equilibrium with the environment by heat exchanges involving evaporation (about 25 per cent), radiation (about 45 per cent) and convection (about 30 per cent), referred to an environment of approximately 18.5°C and 50 per cent relative humidity for a normally clothed, sedentary person. The figure for evaporation covers respiration and insensible perspiration, without sweating. It further follows that there are four properties of the environment that influence comfort by modifying the contributions of these three modes of heat transfer:
(i) dry-bulb temperature (affecting evaporation and convection),
(ii) relative humidity (affecting evaporation only),
(iii) air velocity (affecting evaporation and convection) and
(iv) mean radiant temperature (affecting radiation only).
Items (i) and (ii) may be directly under the control of the air conditioning system although the influence of humidity at comfortable temperatures and air velocities is considered less important today than in the past. Item (iii) is a consequence of the design of the air distribution system and is most important in air conditioning because of the associated high air change rates. It is unusual for item (iv) to be regarded as controllable even though many systems have been designed, installed and successfully commissioned making use of a chilled ceiling in conjunction with a dehumidified auxiliary air supply, but these only give partial regulation of the mean radiant temperature because the ceiling capacity is commonly controlled from room air temperature. It is not possible therefore to specify a comfortable environment in terms of a single physical variable, such as dry-bulb temperature (although this may be the single most important factor): instead, account should be taken of as many as possible of the variables over which the system has a say, usually dry-bulb temperature and air velocity, plus humidity to a restricted extent.
In the UK dry-bulb temperatures of between 20°C and 23°C are often regarded as satisfactory for long-term occupancy and a sedentary activity. For shorter term, transient occupancy higher temperatures are sometimes adopted, up to the outside summer design value. In warmer climates, where outside temperatures may be 10°C or more higher than in the UK, inside dry-bulbs for sedentary occupations are 25°C to 26°C, on a long-term basis but even so values as low as 22°C, depending on the circumstances, are sometimes necessary, commercial considerations permitting. The human body is tolerant of considerable variation in humidity and Green (1979) has shown that humidities corresponding to dew points between 1,7°C and 16.7°C are satisfactory in terms of comfort, skin dryness, respiratory health and mould growth. Such dew points give humidities approximately between 30 per cent and 80 per cent saturation at 20°C and between about 26 per cent and 72 per cent at 22°C. ANSI/ASHRAE Standard 55-1992 relates to an environment where 80 per cent of sedentary or slightly active people are comfortable and adopts a slightly different view of Green’s results. Account is taken of seasonal change of clothing in summer and winter and the 18°C and 20°C wet-bulb lines, rather than 16.7°C dew point, related to the upper bounds of winter and summer moisture content. Figure 4.11 illustrates a current view of ASHRAE (1997). Standard 55-1992 also considers that in order to decrease discomfort due to low humidity the dew point should not be less than 3°C. The Standard’s view on an upper limit for humidity is controversial and suggests that dew points exceeding 20°C are associated with unacceptable air quality (see section 4.10). It points out that in warm conditions discomfort increases with humidity and skin wettedness may then become a problem, being related to sweat on the skin and clothing. An index of skin wettedness has been explored by Gagge et al. (1969a and 1969b) and by Gonzalez et al. (1978).
The influence of humidity is small when the body is in comfort equilibrium. Although the work ofYaglou and Miller (1925), leading to the original concept of effective temperature (see section 4.8), seemed to show that humidity was significant, it was based on the very short-term (a few minutes) exposure of subjects to comparative environments and Yaglou’s later work (1947), and that of Koch et al. (1960), Nevins et al. (1966) and McNall et al. (1971) substantiate the current view that humidity is not very significant in a condition of thermal balance. However, high humidities may pose health risks. See section 4.11.
Air movement is very important and is related to air temperature and the part of the body blown upon. Air distribution terminals are generally selected to give a maximum centreline velocity for the air jet of 0.25 m s-1 at the end of its throw when about to enter the occupied zone. According to Fanger (1972 and 1987) small changes in air velocity are important, especially between 0.1 and 0.3 m s’“1. For sedentary activities he quotes the relationships given in Table 4.3 for the comfort equilibrium of lightly clothed people.
Table 4.3 Air velocity and comfort
Air velocity, m s_1 0.1 0.2 0.25 0.3 0.35
Dry-bulb temperature, 0°C 25 26.8 26.9 27.1 27.2
In the practical case of an air conditioned room in the UK some departure from these temperatures might be expected because of uncertainty as to what actually constitutes the part of the body relevant to the air velocity and the variation in the thermal insulation of the clothing worn in a mixed population, referred to earlier.
An air distribution performance index was proposed by Miller and Nash (1971) and Nevins et al. (1974) that relates comfort to measured values of effective draught temperature, fed, and air velocity within the occupied space:
Fed =(f* — f’r)-7.65(vx -0.152) (4.8)
Tx is the local air temperature, vx a local air velocity and tr the mean room dry-bulb temperature. The air distribution performance index is the percentage of positions in the occupied zone, to a specified standard of measurement, where red is between -1.7°C and +1.1°C and vx is less than 0.35 m s_1. An 80 per cent value is regarded as very satisfactory.
There appears to be no minimum value of air movement necessary for comfort according to ASHRAE (1989) but an upper practical limit is 0.8 m s“1, above which loose papers start to fly. Small fluctuations in air velocity, with a cool temperature, may cause complaints of draught, as the earlier mention of draught intensity (equation (4.6)) defined.
Non-uniformity in the environment seems most important for the head-to-feet temperature gradient and this should not normally exceed 1.5 K and should never be more than 3 K from ISO 7730 (1995). Discomfort at the feet depends on the footwear as well as the floor temperature. Nevins et al. (1964) and Springer et al. (1966) found that values up to 29°C with a three hour exposure were acceptable to walking and sitting subjects. (The author’s experience is that 26°C may cause complaint in an ambient dry-bulb of 22°C, particularly for people who are standing.) For cold floors, a lower limit of 17°C to 18°C has been suggested by Nevins and Flinner (1958).
During winter, when heating, the mean radiant temperature should be higher than the dry-bulb but, in summer, the reverse is true and a lower mean radiant temperature is pleasant with a higher dry-bulb temperature. People are more tolerant of lateral than vertical asymmetry in thermal radiation and so excessive radiation is best not directed on the tops of heads. Asymmetry in thermal radiation can be expressed in terms of the plane radiant temperature difference. See McIntyre (1976). The plane radiant temperature is defined as the mean radiant temperature of half the room, with respect to a small, one-sided elemental plane area. If there is a difference between opposing half-room, mean radiant temperatures of this sort it is termed the ‘radiant temperature asymmetry’. According to McIntyre (1976) and Olsen and Thorshauge (1979), the limiting differences for comfort with asymmetric radiation are 5 K vertically and 10 K horizontally. The maximum difference between two opposite plane radiant temperatures is sometimes called the vector radiant temperature. In practical terms the three most common sources of radiant asymmetry provoking local discoMfOrt are: cooling near windows in winter (complicated by natural infiltration and convective downdraught and sometimes loosely termed cold radiation), heating from overhead lighting and short-wave solar radiation through unshaded glazing.
The presence and proximity of other people can be regarded as an environmental influence on comfort. Fanger (1972) reported experimental evidence that crowding had little effect on the physiological response of subjects occupying floor areas and volumes as little as 0.8 m2 and 2.0 m3, respectively, reciprocal radiation being unimportant under warm conditions but, in cooler circumstances, the mean radiant temperature was increased enough to require a reduction in the dry-bulb in a crowded room. In conditions of extreme crowding the boundary layers of people intermingle (e. g. in rush-hour transport), suppressing heat transfer by convection and needing a lower dry-bulb for comfort.
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