The choice of inside design conditions

For a person to feel comfortable it appears that the following conditions are desirable:

(1) The air temperature should be higher than the mean radiant temperature in summer, but lower in winter.

(2) The average air velocity in the room should probably not exceed 0.15 m s-1 in an air conditioned room but higher velocities may be acceptable with air temperatures greater than 26°C.

(3) Relative humidity should desirably lie between about 45 per cent and 60 per cent.

(4) Relative humidity should never exceed 70 per cent.

(5) The dew point should never be less than 2°C.

(6) The temperature difference between the feet and the head should be as small as possible, normally not exceeding 1.5°C and never more than 3°C.

(7) Floor temperatures should not be greater than 26°C when people are standing and probably not less than 17°C.

(8) The radiant temperature asymmetry should not be more than 5°C vertically or 10°C horizontally.

(9) The carbon dioxide content should not exceed about 0.1 per cent.

As explained earlier, not all these variables are directly amenable to regulation and no air conditioning system is able to achieve control over all of them. The two most important are dry-bulb temperature and air velocity, with mean radiant temperature of slightly less importance. Of these a comfort air conditioning system can only exercise direct automatic control over the dry-bulb temperature. A suitable choice of air velocity may be achieved by proper attention to the system of air distribution and acceptable values of mean radiant temperature should result from co-operation between the design engineer and the architect, aiming to eliminate objectionable radiant effects from sunlit windows in summer, cold windows in winter, cold exposed floors or walls, and excessive radiation from light fittings.

The choice of inside comfort design conditions for an air conditioned room or building depends on the physiological considerations already debated and on economic factors. The designer then examines the outside design state, the clothing worn by the occupants, their rate of working and the period of occupancy. An air temperature of 22°C or 22 °C with about 50 per cent saturation (the same as relative humidity as far as comfort is concerned) is a comfortable choice for long-term occupancy by normally clothed, sedentary people in

The UK but the humidity can be allowed to rise to 60 per cent (or even a little more) or to fall towards 40 per cent, under conditions of peak summer heat gains if psychrometric, commercial or other practical considerations warrant it. (It is possible that as high as 23°C dry-bulb may be quite tolerable in this country if the clothing worn is appropriate, when there is sufficient air movement and the mean radiant temperature is less than the air temperature. In the author’s view more than 23°C is usually too high as a design choice in the UK. As summer passes and heat gains diminish to become heat losses under winter design conditions outside, the inside dry-bulb may be allowed to fall to 20°C and the humidity also towards a value corresponding to 2°C dew point. (There is some evidence, according to Green (1981), that the incidence of the common cold, and absenteeism, diminishes in winter if people live or work in buildings where the relative humidity is kept in the vicinity of 50 per cent.)

There is economic sense in not adopting too low a temperature inside during summer when air conditioning a space such as a shop, or the foyer of a theatre, where the occupancy (by the patrons) is relatively short term. For example, a foyer at 25°C when it is 28°C outside gives an immediate impression (fading with time) of comfort upon entry. On the other hand, an auditorium should be at 22°C or 22 °C because the audience is present for several hours during which 25°C would be noticeably too high with English customs of dress. There is a lower limit to the temperature of a conditioned space, set by winter design weather and the activity and clothing of the occupants. Thus in the UK the inside temperature should be 20°C (or perhaps 21°C) while the temperature outside rises from its winter design value to about 20°C. As the outside temperature rises from 20°C to about 28°C, the inside temperature should follow from 20°C or 21°C to 22°C or 22 °C. Apart from the change in outside temperature, variation in the inside temperature between 20°C and 22j°C (or even 23°C) may occur because of changes in the heat gains from internal sources or by solar radiation through windows. Such temperature variations should be quite acceptable and be under thermostatic control by the system. It is quite unnecessary to specify tight tolerances over automatic temperature control for comfort conditioning, apart from being needlessly expensive. Less than 20°C dry-bulb will usually be uncomfortably cool in an air conditioned building, at any time, because of the fairly high air change rates (5 to 20 per hour) that prevail.

In a warm or tropical climate, it is neither comfortable nor economically desirable to provide temperatures as low as those used in a temperate climate. It is customary in such circumstances to select a dry-bulb about 5° to 20°C less than the outside design value, with a preferred inside maximum of 25°C dry-bulb, coupled with a humidity of about 50 per cent to 60 per cent. However, there are occasions, even in such climates, where dry-bulbs of 22° to 23°, with humidities of 50 per cent to 60 per cent, are maintained for long-term occupancy.

Posted in Air Conditioning Engineering

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