Synthetic comfort scales
Many attempts have been made, with mixed success, to correlate the four environmental factors (dry-bulb, velocity, mean radiant temperature and relative humidity) that contribute to the comfort of humans by influencing bodily thermal equilibrium. Several scales of comfort have been proposed and used in air conditioning: effective temperature, new effective temperature, standard effective temperature, and resultant temperature.
Yaglou (1923, 1947) and Yaglou and Miller (1924, 1925) using a survey of the responses of subjects to a relatively shortterm exposure in different environments founded the scale of effective temperature, defined as the temperature of still, saturated air that gives a feeling of comfort similar to that of another combination of the three relevant environmental variables. It was considered to over emphasise the influence of humidity and more recently Rohles and Nevins (1971), Rohles (1973, 1974) and Gagge et al. (1971) it has been recognised that with a longer term occupancy of an environment the importance of humidity is less when the body is in comfortable thermal equilibrium. A new scale of effective temperature has been proposed by ASHRAE (1997), expressed in terms of operative temperature (see equation (4.13)) and combining the effects of dry-bulb temperature, mean radiant temperature and humidity into a single index. Two such combinations could be compared, but only if they had the same air velocity. Furthermore, clothing and activity must be defined. These difficulties have prompted ASHRAE (1997) to offer a Standard Effective Temperature, defined as ‘the equivalent temperature of an isothermal environment at 50 per cent relative humidity in which a subject, while wearing clothing standardised for the activity concerned, has the same heat stress (skin temperature) and skin wettedness as in the actual environment’.
Figure 4.1 illustrates the ASHRAE summer and winter comfort zones, based on ASHRAE Standard 55-1992. It differs from the original in that the abscissa in the figure is expressed in terms of dry-bulb temperature, instead of operative temperature. The justification for this is that operative temperature is not used in the UK. The two temperatures are related by equation (4.13).
Figure 4.1 refers to clothing having insulation values of between 0.5 and 0.9 clo for summer and winter conditions, respectively, with a sedentary activity. The nearly vertical temperature lines, separating the winter and summer comfort zones, correspond to lines of constant standard effective temperature within which it is seen how comfort is affected by humidity. The lower level of comfort is defined by a dew-point of 2°C. The upper levels
10 15 16.7 20 25
Dry-bulb temperature, °C
Fig. 4.1 ASHRAE comfort zones for summer and winter (1997). People are assumed to be wearing suitable summer or winter clothing for the appropriate season. The original ASHRAE figure uses operative temperature as the abscissa but the above figure uses dry-bulb temperature because operative temperature, although similar to resultant temperature, is not used in the UK.
See equations (4.11), (4.12), and (4.13).
Of humidity are theoretical and based on limited data but Nevins et al. (1975) recommended that relative humidity should not exceed 60 per cent, to prevent warm discomfort. The shape of the upper bounds of the comfort zones are corroborated by experimental data for an 80 per cent comfort acceptability level.
Dry resultant temperature (resultant temperature), developed by Missenard (1933 and 1935), is commonly used in Europe and, for most practical purposes, is defined by
‘res = [(T’rm — 273) + y/2 (4.11)
Where tres is the dry resultant temperature (°C) and Trm is the mean radiant temperature (K).
More exactly it is obtained from the reading of a thermometer at the centre of a blackened copper sphere of 100 mm diameter or, if the mean radiant temperature is known, by the equation
_ (rm -273) + faVT0^ res 1 + VlOv
In which v is the air velocity (m s ‘) and ra is the dry-bulb temperature (°C).
Operative temperature, t0, is defined by
To = [hr(Trm — 273) + hct. Mhr + hc) (4.13)
In which hr and hc are the radiative and convective heat transfer coefficients, respectively.
In virtually still air, by putting v = 0.1 m s-1, equation (4.12) degenerates to equation (4.11),
The form commonly used.
Posted in Engineering Fifth Edition