CPD PROGRAMME | VENTILATION zone would be designed in the range 0.15m.s-1 to 0.25m.s-1. Below 0.05m.s-1 the air is considered stagnant and not appropriate for sedentary occupation. However, as with so many rules in building services engineering, it is important to consider the potential benefits of stretching these limits. For example, in cooling applications as illustrated in the chart in Figure 2, based on ISO7730,2 where the curves indicate a constant level of comfort (for office-type applications) a higher air velocity may be used to offset the warmth sensation caused by increased temperature so reducing the potential cooling requirement. This will be significantly affected by the mean radiant temperature, r when r is relatively low (the higher curves), a larger increase in air velocity is needed to maintain comfort as air temperatures rise. Fluctuations in air velocity will impact occupant comfort, and for cooling applications may be assessed by determining the turbulence intensity (Tu) (see boxout, Assessing turbulence and draught). As Tu increases, the acceptable mean air velocity for comfort will reduce. For air conditioned and mechanically ventilated buildings, the draught rating (DR) combines air velocity, temperature and turbulence intensity into a discomfort factor that is often used in conjunction with predicted percentage of dissatisfied (PPD) to provide an assessment of comfort conditions in a space. A DR value of approximately 15% is typically considered acceptable for people undertaking light, mainly sedentary activity (with any draught being assumed at neck level). The significance of perturbations in air velocity (that is, air speed and direction) can potentially transform a comfort zone into one that generates occupant complaints. For example, as taken from CIBSE Guide A,3 if the temperature of air passing over the body is 23C and the turbulence intensity is 60%, a draught rating of 15% corresponds to an air velocity of 0.14ms1. However, if the turbulence intensity is only 10%, the limiting velocity for comfort is increased to 0.23ms1. For comfort, it is recommended3 that there should be not more than 3K difference in air temperature between ankles and head. However, if air velocities are higher at floor level than across the upper part of the body, CIBSE recommends that this could be increased to a maximum temperature gradient of 2K for each metre height. To characterise the moving plume of air emanating from a supply ATD, as illustrated in the high-level example of Figure 3, there are several terms that are reasonably universal in application, although quoted typical values can vary between Figure 2: Air velocity required to offset increased temperature (Source: Based on ISO 7730) Figure 3: The top section of a room with a high-level wall-mounted supply air diffuser manufacturers. (BS EN 127924 provides an extensive list of standard terminology.) The primary air leaves a supply ATD at an outlet velocity, ms1, which will be determined by the total pressure (velocity pressure + static pressure) at the rear face of the ATD and the geometry of the outlet. The velocity of the air in the room is often described by considering envelopes known as an isovels (as in Figure 3) that indicate a boundary line linking points of common air velocity. As air leaves an ATD, room air is entrained and mixed into the air stream, so reducing the velocity of the moving mass of air while increasing the total volume. After the air leaves from the ATD the flow tends to drop or rise depending on its temperature relative to the room air. As the velocity and inertia of the total air reduces, its trajectory will become increasingly influenced by gravitational forces. In a horizontal supply, the air stream will tend to curve upwards if the incoming air is warmer than room air and downwards if cooler. The velocity of the air will reduce more swiftly if the air has a greater spread as it is introduced into the space, and the air will increasingly diverge (typically with5 an included angle of between 20-24) as it entrains surrounding air, so increasing the total air volume. The terminal velocity is when it is assumed that the inertia has reduced to such a point that gravitational forces begin to dominate, typical considered as 0.25m.s-1 although, as reported by Legg,6 this can be a rather arbitrary value. It is variously Figure 4: Adaptable air diffuser employing adjustable air nozzles (Source: Swegon) 74 December 2022 www.cibsejournal.com CIBSE Dec 22 pp73-76 CPD 208 Swegon.indd 74 25/11/2022 16:27