Smoke Control by Pressurisation

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By: J.A. WILD, C.ENG; F.I.MECH.E. November 1998 (Third Edition)

© Copyright 2000 Fläkt Woods Limted England.






Chapter One

Smoke Control By Pressurisation - Basics


Chapter Two

Why Pressurisation?


Chapter Three

The Pressurisation System

9 - 16

Chapter Four

Air Requirements of a Pressurisation System

17- 20

Chapter Five

Fan Selections

21- 27

Chapter Six Appendix One

Fans for Pressurisation Systems References

28 - 29 30

ACKNOWLEDGEMENTS The author wishes to acknowledge the assistance he received from Mr Cyril Moss, KG Smoke Dispersals Ltd and Mr E Gorden Butcher of Fire Check Consultants with the preparation of this paper. 3


Fans for Pressurisation Systems

SUMMARY There are basically two main methods for controlling smoke in buildings to prevent it contaminating escape routes - by Ventilation and by Pressurisation. Fans for Fire Smoke Venting (Ref 1) examines the motivation behind the increased use of Powered Ventilators for the exhausting of hot smoke from fires and determines their requirements and specifications. This paper undertakes a similar task to determine the requirements and specification for FANS IN PRESSURISATION SYSTEMS, based on the requirements of BS5588 Part4:1998.



Smoke Control By Pressurisation - Basics
1.1 BASIC PRINCIPLES Fire induced forces create pressure differences across doors etc, which allow smoke to flow through any gaps present. By altering these pressure differences we can control the movement of smoke. The two BASIC PRINCIPLES of smoke control were defined by JH KLOTE (Ref 2) as:a) Airflow can control smoke movement if the average VELOCITY is of sufficient magnitude. b) A PRESSURE difference across a barrier can act to control smoke movement. Although the second of these principles can be taken as a special case of the first, when dealing with them as an engineering problem it is easier to take the two separately viz:Large Gaps- Open Door etc - VELOCITY a) Door Velocity Thomas’ correlation (Ref 3) estimates that a 2.4 MW fire will cause smoke to flow through a 0.9 metre wide opening at 3-4 m/ sec. BS5588 Part 4:1978 requires this for permanent openings. In practice a velocity of this magnitude is virtually impossible to achieve in the type of buildings using Pressurisation for Smoke Control, and some compromise is necessary. Doors are considered as only being opened intermittently and hence VELOCITIES can be reduced. Table 1 shows VELOCITIES between 0.75 m/sec and 2.0 m/sec. BS5588 Part 4:1978 specified up to 0.75 m/ sec during the Escape period. BS5588 Part 5:1991 - 2.0 m/sec for Fire Fighting. b) Number of Open Doors Variations in this parameter have the greatest effect on fan size. Again table 1 shows much variation from ONE EFFECTIVE DOOR OPEN specified by BS5588 Part 4 1978 to FOUR EFFECTIVE DOORS OPEN required by the CANADIAN STANDARD - N.B.C.C. 1990. However the combination of DOOR VELOCITY and DOORS OPEN likely to produce the greatest air supply requirement is that specified by BS5588 Part 5 1991 - 2.0 m/sec with TWO EFFECTIVE DOORS OPEN. 1.3 RECENT DEVELOPMENTS BS5588 Part 4 - 1978 was revised and reissued during April 1998 The revised Code Practice designated BS5588: Part 4: 1998, retains the 5

Small Gaps - Closed Door etc - PRESSURE These, then are the BASIC PRINCIPLES which control the design and ultimately the satisfactory functioning of a PRESSURISATION SYSTEM for SMOKE CONTROL. They formed the basis of the British Standard Codes of Practice - BS5588 Parts 4:1978 and 5:1991, and the same elements can be seen in many of the various national standards listed in TABLE 1. 1.2 SUPPLY AIR PARAMETERS The two parameters which have the greatest effect on the size of the supply fan are:a) The AIR VELOCITY...
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