BS 5588-4:1998
10 Design procedures 10.1 Pressurization systems 10.1.1 General NOTE The following paragraphs are intended to illustrate the general design principles involved and may be adapted to suit other applications.
Where a stair is intended for firefighting purposes it is more appropriate to carry out the firefighting design procedure before that for means of escape. Information regarding air leakage areas for typical forms of construction is given in Annex D. Guidance regarding the calculation of effective leakage areas for flow paths in series and in parallel is given in 14.1. 10.1.2 Design for means of escape Evaluate effective leakage areas for the following flow paths at each storey (see 14.1): — from stair to simple lobby to accommodation; — from stair direct to the external air; — from accommodation to the external air; — from lift well to the lobby; — from lift well direct to the external air; — from lobby to accommodation. NOTE 1 In existing buildings the leakage areas will be highly dependent upon the quality of the workmanship and the nature of the structure, hence the actual leakage values may vary considerably from assumed design values. Effective leakage areas should, if possible, be evaluated by an on-site airflow measurement. Only where this is impractical should assumed values be used.
Estimate the air leakage rate via each flow path from the pressurized shaft at the design pressurization level with all stair, lift and lobby doors closed (see 14.1). NOTE 2 For simplicity, the leakage area from the accommodation may be assumed to be large relative to the leakage area of the lobby and stair doors.
Sum all air leakage rates to give the theoretical air supply rate. To give the total required air supply rate, multiply this value by a factor of at least 1.5 to take account of uncertainties in identified leakage paths (see Note 4). Determine air release requirements in accordance with 9.6 and 14.3. NOTE 3 Calculating the air release requirements may be ignored if the firefighting design procedure has previously been carried out.
Determine the required air supply rate in accordance with the respective class of system (see 5.2, 5.3, 5.4, 5.5 and 5.6). The calculation should also take account of leakage via closed doors, lobbies and accommodation, and to other identified leakage paths from the shaft. NOTE 4 The anticipated leakage via all paths other than the open door should be multiplied by the factor of at least 1.5. Where: a) the air required to satisfy the airflow criteria is in excess of the factored air supply required to satisfy the pressurization criteria, i.e. air for the airflow criteria > 1.5 × the air required for pressurizing the shaft; and b) the fan performance characteristic would ensure that the needed pressure is available in the shaft with the unidentified leakage paths having been considered; then this 1.5 factor need not be applied.
Compare the required airflow rates for the closed and the open door conditions, and select the higher as the total required air supply rate. Determine the excess air supply rate with all doors closed, i.e. the total air supply rate minus air lost through leakage with all doors closed. Calculate the nominal area of overpressure relief vents required to relieve excess air supply from the shaft (see 9.7 and 14.3.5). NOTE 5 The lift well should not require overpressure relief vents if the supply system is initially set up so as to achieve the required pressurization level with all doors closed.
© BSI 8 December 2004
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