Pressurisation ductwork
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Pressurisation ductwork is a fire protection system subject to stringent bounding. It is used to supply a steady stream of fresh air to any area of refuge or designated emergency evacuation or egress route.
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[edit] Purpose
- To ensure a positive pressure environment of clean outside air, free from smoke to enable people to either hold out until rescued or to escape with a minimum of dangerous smoke exposure.
[edit] Requirements
Typically, pressurisation ductwork is subject to demonstrable product certification on the basis of fire testing as per ISO 6944. In the United States, additional hose-stream testing is required to achieve product certification, as the system includes not just a section of ductwork tested in a full scale floor furnace, but also a firestop, which must survive the hose.
Pressurisation systems are evaluated for exterior fire exposure. Grease ducts, on the other hand, are evaluated for interior fire exposures.
[edit] Systems
There are two means of providing fire-resistance rated ductwork:
- 1. an inherently fire-resistant duct, made from DuraSteel or proprietary factory assembled chimney pipes, which tend to be made of sheet metal shells filled with mixtures of rockwool, or ceramic fibre and silicon dioxide
- or
- 2. ordinary ductwork equipped with exterior fireproofing materials, such as blanket rockwool, ceramic fibre or intumescent paint.
[edit] History
[edit] Grandfathering
Before the advent of realistic 3-D full scale fire testing as per ISO 6944, the most common method to fireproof ductwork in North America, in particular, was the use of drywall shaftwall systems. This became legally indefensible in the 1990's after the first ISO 6944 test was run at Underwriters' Laboratories of Canada, an affiliate of Underwriters Laboratories, which resulted in the first certification listing for an inherently fire-resistant duct made from DuraSteel. Drywall systems were only tested as flat walls and a three-dimensional solution surrounding a real duct with four corners was never done, yet widely accepted in construction, in the absence of a realistic solution.
The first test was followed by a plethora of tests and listings most notably for ceramic fibre blankets, applied externally to the ducting by insulators.
In Europe, where ISO 6944 originated, lower cost rockwool systems, calcium silicate and sodium silicate bound and pressed vermiculite as well as the proprietary DuraSteel systems had been in use decades earlier.
[edit] Technical Competition
As is typical in all passive fire protection, users of man made mineral fibres (mmmf) have criticism concerning the safe use of such fibres levelled at them, despite the use of foils etc., which are easy to breach.
Users of intumescents are subject to criticism concerning the longevity of the intumescing property of the active ingredients within their paints.
Inherently fire-resistant ducts are subject to criticism for their mass, being that both fireproofing AND ductwork are lifted and hung up and fastened at once.
No system is absolutely perfect - each have strong points and weak points.
[edit] Trade Competition
In the case of inherently fire-resistant ductwork, the sheet metal worker does all the work. In the case of ordinary ductwork with exterior fireproofing, the duct is built by the sheet metal worker and the insulation is usually installed by the insulation trade. Contractors who engage in firestopping as well as insulation work are both likely candidates to execute such work in construction. Designers of systems for product certification must take the labour into account for both trades in order to evaluate the competitiveness of any system.
[edit] See also
- Fire protection
- Passive fire protection
- Duct (HVAC)
- Smoke exhaust ductwork
- Grease duct
- Emergency evacuation
- Area of refuge
- Fireproofing
- Firestop
- Underwriters Laboratories
