Outdoor air requirements

When resting quietly with a metabolic rate of 88 W an average man’s oxygen consumption is only 0.006 litres s_1 but to provide this the breathing rate must be about 0.09 litres s-1 according to Haldane and Priestly (1905). Oxygen consumption is directly proportional to metabolic rate so for exhausting effort and a metabolic rate of 1055 W the oxygen consumption is 0.07 litres s"1 and the breathing rate reaches 1.15 litres s’1. The amount of fresh air needed to sustain life is thus very small, extending from about 0.1 litres s_1 to 1.2 litres s_1.

It is highly desirable to keep the quantity of fresh air handled by an air conditioning plant to a minimum for economic reasons and to conserve energy but this is not dictated by the requirement for breathing purposes. Enough air from outside must be supplied to reduce the accumulation of body odours, other smells and the pollution from smoking and other sources to a socially acceptable level. It is further necessary to dilute the concentration of carbon dioxide to a maximum acceptable value of 0.1 per cent by the introduction of fresh air with an average concentration of about 0.031 to 0.035 per cent (see section 2.2), but this is not as significant as the need to deal with odours when establishing the minimum fresh air required. As little as 5 litres s_1 for each person may be acceptable for a banking hall or a church where there is a large volume with no smoking but as much as 43 litres s_1 may be wanted in rooms where smoking is heavy. This topic is also dealt with in chapter 16.

In the past it has been thought by Dessauer (1931), Yaglou et al. (1932) and Gagge and Moriyama (1935) that air containing molecules with a high negative ion content exhibited indefinable qualities of freshness that some subjects found desirable, but subsequent work by Brandt (1933), Herrington (1935), Herrington and Kuh (1938) and Yaglou (1935) and earlier Yaglou et al. (1933), has failed to support this. Current, reputable, scientific evidence indicates that the ion content of air has no measurable effect on human beings in buildings. If any influence is produced by the ions present it must be exceedingly subtle.

The current outside air supply rates recommended by the CIBSE (1999) are given in Table 4.5.

An addendum to the ASHRAE Standard 62n-1999 (Ventilation for Acceptable Indoor Air Quality) proposes what appear to be smaller rates, modified according to the floor area and the use of the occupied space.

The proposals by Fanger (1988), to calculate the pollution balance in an occupied space (see section 16.1), have not found general acceptance in Europe or the UK. Results have been inconsistent and the supply rates of outside air so determined have proved unrealistically large.

Table 4.5 Recommended outdoor air supply rates for sedentary occupants

Level of smoking

Proportion of occupants who smoke (%)

Outdoor air supply rate (litre s-1 person"’)

No smoking

0

8

Some smoking

25

16

Heavy smoking

45

24

Very heavy smoking

75

36

Reproduced by kind permission of the CIBSE from Guide A, Environmental Design (1999).

Posted in Air Conditioning Engineering


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