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Home » Smoke exhaust – it’s serious: Smoke duct

Smoke exhaust – it’s serious: Smoke duct

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By Ron Burns

Smoke shafts are undoubtedly the only method of extracting smoke from multilevel buildings.

Ron Burns - Bio

Although a somewhat bold statement, other options do exist: discharging through a vertical façade with a powered extraction system — effective; yet, unsightly. Not too many architects would cherish the idea of louvres scattered along the façade of the building they have painstakingly designed. Opening windows just do not meet the requirements; there are too many uncontrollable variables to consider. The beauty of the smoke shaft is the concealment. Tucked away inside the building, unseen, the letting agent will shudder at the thought of losing the lettable floor area, approximately 5m2 per smoke zone — a decent trade-off for safety in my opinion.

The installation of galvanised sheet metal duct in the masonry-constructed shaft provides the designer with an effective, reliable, and fit-for-purpose method of extracting smoke from the smoke zone. Although not wanting to replicate the EN 12101 code verbatim, reference to the definitions used in the code is important. Terminology, when accurately used, prevents any ambiguity in discussions. It is easy to appreciate the significance of the duct systems; an entire part has been written on the subject. The definitions are extracted from EN 12101-7:2011. For reference purposes, I will only refer to the page numbers when referencing the definitions:

3.12 smoke control duct – horizontal
Smoke control duct which passes horizontally through vertical walls.
[p. 9]

3.12 smoke control duct – vertical
Smoke control duct which passes vertically through horizontal floors.
[p. 10]

Each duct section is no different; as a component, as a certified smoke extraction fan, or fire door. The requirement is for each certified component to be individually labeled. No label equals no certification. The code identifies each certified component with the following identifier:

EI 60 (Ve; Ho) S (500; 1 000; 1 500)

Where:

EI 60 refers to the integrity and insulation class (in accordance to EN 123501-4;
(Ve; Ho) refers to the suitability for vertical or horizontal use;
S refers to the satisfaction of an extra restriction on leakage;
(500; 1000; 1500) refers to the tested negative pressure characteristics achieved.

The single most defining difference between the vertical and horizontal duct is the reaction of the flanges and the deflection of the duct at elevated temperatures. In the horizontal plane, the duct rests on a series of hangers with the largest stress on the bottom flange. The vertical duct experiences a compounding load, the load is transferred vertically from one flange to the other. The duct supports are critical for vertical installations. Transferring of vertical loads place unnecessary stress on the metal at test temperatures, resulting in deflection and subsequent leakage. Every effort needs to be taken to ensure no load is transferred between duct sections.

Once the individual components are tested, they need to be assembled and then, as a system of interconnected components, inspected and performance tested on site, ensuring the system performs in accordance with the overall code requirements. Installing components that only marginally meet the requirements, results in system performance difficulties. The seriousness of the inspection is such that it is a requirement to inspect the duct installation annually.

3.14 smoke control duct – single compartment
Smoke control ducts, built from more than one smoke control duct section, for use within single compartment applications designed to transport smoke and/or hot gases away from the source of a fire.
NOTE: may also have a dual function as a normal air-conditioning duct.
[p. 10]

Single compartment duct relates to the duct installed to a powered extraction fan within the smoke zone that the system is extracting from. Should the designer be installing a powered system in a major tenant store, within a shopping mall, the extraction fan is connected to duct components between the fan and the discharge louvre in the external façade of the building. This duct is referred to as a single compartment duct. This duct will not need to be tested for insulation class. The insulation class refers to the ability of the duct to radiate heat. The extraction fan and duct are contained within the smoke zone, which is already at elevated temperatures. There is no possibility of the duct radiating heat and igniting any additional fuel source; the space is already in a fire condition. Single compartment smoke control ducts will be subject to the standard time temperature heating curve levelling out at a maximum of 600°C.

3.13 smoke control duct – multi-compartment fire resisting
Fire resisting smoke control ducts, built from more than one smoke control duct section, for use in multi-compartment applications designed to transport smoke and/or hot gases away from the source of a fire.

NOTE: may also have a dual function as a normal air-conditioning duct.
[p. 9]

Expanding the comments from the single compartment above. The store in discussion may be a multi-zone configuration. If it is the designer’s intention to install a single extraction fan connected to a duct system that exhausts from both smoke zones, it is possible that the smoke is transported from the smoke zone in a fire condition, requiring extraction through the smoke zone that is not in a fire condition. The duct may now not radiate heat. There are specific radiation and integrity levels that need to be maintained to prevent the duct becoming a source of ignition and/or pre-heating any fuel source. This is important to bear in mind when specifying duct that is required to travel multiple smoke zones and through building shafts. During the construction phase, additional equipment and services may be installed in the shaft. Ignition of these products may result in the promulgation of fire, which can result in the loss of the entire building.

Fire resistance tests shall be carried out by accredited testing facilities. Once again, the focal point relates to the correct certification. A certificate that is not issued through an accredited test house caries no weight when evaluating product failure. Localised pre-testing as a cost-saving measure may provide satisfactory results; however, this is cannot replace an accredited certificate.

Smoke control duct shall be subject to a furnace-based fire resistance test. Testing allows the manufacturer the resource and ability to evaluate the smoke control duct sections. These results allow evaluation of deflection and structural stability at elevated temperatures. Duct hangers and associated sundry components have an overall performance effect on the system when the components are installed. Accumulative leakage points may render the system inadequate and not fit for purpose. Remedial work to completed duct installations that failed inspection and performance testing created unnecessary pressure on contractors. Installation under pressurised installation timelines create unnecessary additional costs to a project.

The installation of the duct run requires careful consideration when selecting the duct size and installation position. The typical installation of a duct hard against the soffit is unlikely to allow for the fixing of the flanges and duct clamps, resulting in a system that fails inspection and testing. A point worth remembering: although tested and certified products are installed, installation that does not allow adequate sealing renders the smoke duct installation non-compliant. The practical restrictions of securing the flanges with the associated fixings requires careful diligence in the design phase.

Duct systems installed as part of smoke ventilation systems in South Africa generally fall short of the code requirements.

Smoke duct systems need to limit clean air leakage into the system. Extraction capacity consumed by clean (non-contaminated) air places the smoke zone under pressure to maintain the designed clear layer and smoke plume temperature. The inverse challenge applies to the effect of low leakage on the positive pressure side of the duct system. Smoke dumped on the discharge side of the fan is as detrimental to the extraction system as leakage on the suction side.

Below is listed a series of quoted extractions that are self-explanatory. Installing EN 12101-7 duct is not sufficient to ensure a functional fit-for-purpose system. It is paramount to adhere to several other considerations. Having the correct component is just the starting point.

8.1 Product specification
Full details of the hanging system and any protection used shall be described.


Full details and specifications of any components shall be described.
[p. 21] 

8.2 Installation information
The manufacturer shall provide appropriate installation details that shall include at least information for:

a) fixing and installation: hangers and positions, penetration seals to be used.

[p. 21]

8.3 Maintenance information
The manufacturer shall provide appropriate maintenance information for the smoke control duct section that shall include at least:

a) inspection and maintenance procedures;

NOTE: Regular testing/inspection should be undertaken to meet regulatory requirements, or at intervals not exceeding 12 months. A comprehensive example of the above procedure is given in Annex A.

b) the recommended frequency of operational checks;
c) recommended checks to establish the effects of corrosion.

[p. 22]

The broad-based checkpoints for smoke extraction duct are to provide a duct that is able to function under the following conditions:

  1. at a temperature of 600°C: not deflect more than 10% accumulative on the internal surface (internal bracing and deflection prevention required);
  2. less than 10m3/h per 1m2 of internal surface area leakage.

Caution
During the testing of the individual components, if the duct inside the furnace collapses, so that it can be judged as not being able to maintain its smoke extraction or fire resistance function, this shall be regarded as failure under the criterion of mechanical stability.

The code is relatively silent relating to the gaskets and duct sealants. Naturally, these commodity products need performance testing and should withstand the testing regime at 600°C. Failure of these components have a direct effect on the leakage characteristics. The same applies to the stiffening and hanging system requirements. Stiffeners and hangers that fail, create stress on the flange system, creating duct failure and consequential leakage.

I leave you with these thoughts: duct systems installed as part of smoke ventilation systems in South Africa generally fall short of the code requirements. Long duct runs installed and sealed using standard air-conditioning sealants are going to fail once exposed to elevated temperatures. The sealants will liquefy and drip, exposing flange gaps. The result is inadequate extraction and the inability of the extraction system to maintain the clear layer.

In single compartment extraction systems, engineering principles are seldom applied, fans are bolted onto plates, which in turn are bolted onto walls with little thought to the excessive pressure drop created between the fan and the discharge louvre, resulting in a lower extraction rate — the consequences of insufficient extraction facilitating flooded smoke reservoirs and uncontrolled smoke spread. It is time to look deeper into the extraction systems.