Book - School of Science and Technology

Air change and ventilation allowanceFor application to the air change concept and in order to work in units consistent withthose used for conduction heat loss through the building fabric, the term ventilationallowance is now used, this being related to the air change rate (N):N(1:219 1000)/3600 ˆ (0:339 N) J/s m 3 K ' (N/3) W/m 3 KAir infiltration 47Table 2.9 lists the commonly accepted rates of air change for various building types, togetherwith the associated ventilation allowances. It should be noted that the values listed in thistable are confined to the task of assessing natural air infiltration for heat loss calculations.They do not necessarily represent desirable ventilation rates for the comfort of occupants.Infiltration through window cracksTurning now to the more specific method of calculation referred to previously, it shouldbe understood that this is not an alternative to the air-change method, which can beapplied generally to buildings other than those for which it has been developed. The twoinfluences considered in this method are that due to wind pressure and that due to thechimney or stack effect in a tall building: each of these effects will be dealt with in rathermore detail in Chapter 13. In the present context, research* has concluded that windpressure is the dominant factor, that due to stack effect being small in comparison exceptin very tall buildings and other unusual circumstances.Air flow through cracks may be evaluated using the following general expression:Q ˆ C(P) nwhereQ ˆ air volume flow rate per metre run of window-opening joint (litre/s)C ˆ window air flow coefficient (litre/m s)n ˆ flow exponent, representing type of opening.P ˆ pressure difference across the window (Pa)The air flow coefficient depends upon the character of the window, being 0.1 whereweather stripping has been applied and 0.2 where it has not. The exponent has beenevaluated as 0.5 for large openings and 0.66 for cracks around windows and doors.In strict terms, solutions from this equation are, however, applicable only to a buildingwith an open plan form, air entering on one side having free access to a similar escaperoute on the other. In instances where the building has many internal partitions which willimpede the cross-flow, then the building infiltration rate overall may be only 40% of thecalculated value. A typical figure for the generality of buildings might be 70% of thatproduced by the equation.The pressure difference P is a function of the prevailing wind speed which will varyaccording to the terrain surrounding the building and the height of the windows aboveground. Wind speed data published by the Meterological Office relate to a height of 10 mabove ground in open country but may be corrected for other situations by use of theexpression:V ˆ V m k s z a* Air Infiltration and Ventilation Centre, An Application Guide ± Air Infiltration Calculation Techniques,June 1986.