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Air change ratings

The simplest method of determining the ventilation rate required is to make use of the accumulated experience of the industry expressed in a table of air change rates. The volume in cubic metres (m3) of the space to be ventilated is calculated and multiplied by the number of air changes per hour to give the ventilation rate in m3 per hour. Division by 3.6 converts this to litres per second.

Ventilation rate (litre/s) = Volume (m³) x air changes per hour / 3.6

The table below gives a guide to the number of air changes generally recommended. Provided air inlets and outlets are properly sited, the tabulated values will be sufficient for air movement requirements. They will also meet requirements for pollution and moisture removal, in typical examples of the accommodation indicated, and heat removal requirements in temperate climates without heat sources.

However, particular installations should always be checked against the following questions:

• Are there specific bylaws or other legal requirements applicable in the locality?
• Is the space expected to become so crowded that a higher ventilation rate might be obtained by calculating on a per person basis?
• Are there abnormal sources of heat or fumes?
• Is it desirable to provide a higher ventilation rate for summer cooling?

Ventilation rates per person

Each human being produces their personal quota of heat, water vapour, carbon dioxide and body odour. The ventilation requirement for the last item will cover the needs of the other three during the heating season. It is not sufficient merely to ensure an acceptable average freshness of atmosphere throughout the room; if people are crowded together by shortage of space the average odour concentration must be reduced. This means that the fresh air supply per person must increase with the number of people in a room, and consequently the air change rate must increase. Recommended minimum fresh air supply.

Recommended minimum fresh air supply

3m3

These rates should be increased by a third if smoking is permitted or if there are doubts about standards of personal hygiene. If minimum legal requirements are specified, they are generally about a third less. In contrast, as little as 1 litre/s will suffice to meet the strictly physiological needs of one person for oxygen supply and carbon dioxide limitation.

Further calculations on air changes per hour will come later.

Consideration of dust and odours in a normal office or workroom ventilation job can usually be ignored when preparing the scheme, as the rate of ventilation needed to give comfortable conditions based on temperature, humidity and air movement - 6 to 10 ACH - would be considerably higher than that required to keep dust and odours down to a comfortable level - about 1 to 2 ACH.

Incidentally, it is always advisable when dealing with a fairly crowded space (e.g. auditoria, dance halls) to give a quick check for the minimum air volume required by allowing 28m3/h per person. This is to prevent body odours from becoming a nuisance. Also the carbon dioxide content of the air, gradually increasing due to the CO2 exhaled from the lungs, should be prevented from exceeding 10 parts per 10,000. This calculation is commonly used as the maximum CO2 concentration desirable in a closed occupied space.

Before you begin to think that this is getting rather complicated, be reassured that in practice if we design a normal ventilation scheme to deal with temperature rise and air movement, the remaining factors of humidity and purity (concentration of CO2, odours and dust) are taken care of. There are exceptions to this in the case of humidity and dust, but they usually occur in the more specialised applications where the requirements are either for full air conditioning with dehumidification by refrigeration for humidity control, or for specific dust removal.