Fire Rated Ductworks – An Important Passive Fire Protection Solution for Smoke and Air Management in a Building

Promat 25/11/2018

fire rated ductwork
ducts
duct protection
smoke extraction ductworks
smoke extractors
fire rated ventilation ductworks

Ductworks are very important elements of fire safety systems in buildings. Depending on its role, we can distinguish two main types: smoke extraction ductworks and ventilation ductworks.

smoke extract ductwork

Smoke Extraction Ductworks

smoke extract ductwork

Smoke extraction ducts are used to remove smoke from buildings, to enable emergency evacuation of the occupants as well as improved firefighting. 

They are mainly used in large compartment, car parking, high-rise building, kitchen and atriums in commercial, office and residential buildings. Especially height compartments (typically greater than 15m) are vulnerable to spreading smoke and present a challenge for fire protection engineers because their decreases the effectiveness of automatic sprinkler systems.

The ductworks are required to conduct the smoke outside from the building, even using fan assisted systems. If a smoke extraction ductwork is wholly contained within a fire compartment, it must at least be capable of resisting the anticipated smoke temperatures generated during the development of a fire. If the ductwork penetrates a fire resisting barrier (such as a fire rated partition or ceilings), it must also be capable of providing the same fire resistance of the barrier.

From an effective smoke removal point of view, it is necessary to subject the room in question to a detailed analysis in terms of the fire propagation velocity, floor surface area, and height.

Read about testing conditions >

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fire rated ductwork ventilation

Fire Rated Ventilation Ductworks

fire rated ductwork ventilation

Fire rated ducts are used to guarantee the fire compartmentation. More precisely, fire rated ventilation ducts can avoid fire and heat spread between two compartments.

It is possible to prevent fire spread from one fire compartment to another, along stairways, rooms and general access corridors, only if all building materials and structural elements share a common fire classification and fire resistance rating.

Compartment walls and floors must have a certain fire resistance, which means that the performance criteria of load-bearing capacity (stability), integrity and insulation have been met for a duration of 30 to 240 minutes. It is therefore extremely important that, where compartmentation boundaries are penetrated by services, the fire separation criteria for the penetrated elements are maintained and, in particular, that ducting should not become a conduit along which fires, or hot smoke, may spread to other areas.

The view on risk of smoke spread in ventilations systems varies per country. In some countries smoke spread via the ventilation system is not considered to be a problem at all, in other countries smoke dampers or fire/smoke dampers are always required where there are more than one fire compartment supplied by the same system. Finally, there are countries where the regulations in addition to fire/smoke dampers also allow performance-based design.

In any case, the fire performance of a duct which penetrates a fire resisting/separating element must requires careful consideration by specifiers. Typical non-fire rated steel ducts fail to meet internationally accepted fire protection requirements. They heat and deform rapidly from the effects of fire. Under specific circumstances they can even accelerate the spread of flame and smoke.

Read about testing conditions >

What is the best ductwork system for my project?

Choosing the right ductwork system is difficult due to complexity of the duct construction (passing through different fire compartments), function of the system in the ambient and fire conditions.

Promat offers a wide range of solutions for both ventilation and smoke extraction and specific kitchen extract – for different operating pressures, sizes, orientations, configurations, even for partial fire exposures. The solutions developed by our technical experts can meet any fire resistance requirement from 30 minutes (E – or EI 30) up to 180 minutes (E – EI 180). For special cases 240 minutes integrity solutions are also available.

Traditionally, all ductwork used to be fabricated from steel. It required to be encased in a fire protection system when passing through a compartment wall or floor without the aid of a fire damper, or when used as part of a smoke extraction system. Promat has developed self-supporting systems without a steel liner, for example PROMATECT®-L500 and DURASTEEL ® which can match the leakage and air flow performance of steel ducts both for air duct and smoke extraction ducts.

Main advantages:

  • No steel sheet ducts are required
  • Single layer construction
  • Dimensionally stable and moisture resistant
  • Easy to cut to size, ex factory or on site
  • Lightweight shaped sections
  • Serviceable smooth surface, robust
  • Tested according to the most severe international standards
  • Supporting hangers, supports and their fixings can be unprotected
  • Big dimensions (maximum dimensions are all classified by authorized laboratories)
  • Roughness factor of the surface substantially identical to steel

The acoustic performances and the room temperature thermal insulation of Promat ductworks can be even improved using additional layer of mineral wool and/or plasterboards.

Promat products can be used also for cladding of existing steel ducts with PROMATECT®-H boards and Promat®-WRAP glass fiber fabric.

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Do you have more questions about ducts or not sure which type of ductworks you should choose for your project?

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How is the fire resistance performance of ductwork tested?

In the global market, there are 3 commonly used standards to define the fire resistance of ductwork, also known as fire rated ductwork.

The 3 standards are European Standards (EN), British Standards (BS) and American Standards (ASTM).

In the Middle East region, the most commonly accepted standard is the British Standard BS476: part 24 (1987). 

This standard defines the way of executing the fire test and also how to interpret the different criteria.

The standard split the different fire rated ductwork in 2 big groups

  1. Duct type A – fire is outside the duct
  2. Duct type B – Fire is inside the duct
duct A internal fire duct B external fire
duct A internal fire duct B external fire

The fire resistance of ductwork needs to be expressed in minutes of duration of heating until failure occurs according to one or more of the following criteria:

  1. Stability
  2. Integrity
  3. Insulation

Stability

If the duct passes the stability criteria in the case of duct type A the whole duct doesn’t collapse as well the parts inside or outside the furnace.
If the duct passes the stability criteria in the case of duct type B the duct part outside the furnace doesn’t collapse. This means that the type B duct can collapse inside the furnace (or in practice inside the compartment with fire)

Integrity

Both ducts, type A and B, will pass the integrity criteria if during the fire test there is no formation of cracks, holes or other openings outside the furnace through which flames or hot gases can pass the duct.

Insulation

The ducts will pass the insulation criteria if they measured temperature rise of the ducts are not exceeding the maximum allowed temperature rise. There are 2 criteria:

  1. the average temperature increase is maximum 140°C
  2. the maximum temperature increase on a single point is maximum 180°

What performance do I need for my ductwork?

As we saw in the previous chapter we test fire rated ductwork according to different requirements, but which requirements are needed in a building.

Every building in the region has ductwork, but every ductwork can have a different purpose. Think about smoke extraction ducts, ventilation ducts, pressurization ducts, kitchen extract ducts. 

This different kind of ducts requires a different set of performance, to make a building safe for people.

For example some common situations.:

Ventilation ducts

A ventilation duct needs to be tested according to a type A and type B duct. There is a risk, depending on where the fire is located in the building, that the duct needs to protect the fire from outside to inside (duct type A) or from inside to outside (duct type B).

Next to this, it is up to the designer/ consultant to prescribe if the ductwork needs to fulfill only stability and integrity or if additional insulation criteria are needed.

(note: if the ductwork is located in escape corridors or closer than 500 mm next to combustible material or person the insulation criteria is mandatory) 

Smoke extraction ducts

A smoke extraction duct (like ducting in a parking garage) needs to be basically tested according to a type B duct. The main purpose is to extract smoke and hot gases from the compartment with the fire, so the fire is inside the duct.

But only a type B duct is not enough, there is a mandatory additional requirement that should be proven by a fire test. The mandatory requirement is that the duct maintains a minimal cross-sectional area of 75% during the whole test. In short, if the cross-sectional area of the duct is 1 m², during the test the duct can deform but it should not become smaller than 0.75 m² of area. This allows the duct to keep his performance of extracting smoke and hot gasses.

When the smoke extraction duct is also travelling through other parts of the building than there is an also a risk that the duct needs to protect the fire from outside to inside (duct type A).

Next to this is up to the designer/ consultant to prescribe if the ductwork needs to fulfill only stability and integrity or if additional insulation criteria are needed.

(note: if the ductwork is located in escape corridors or closer than 500 mm next to combustible material or person the insulation criteria is mandatory) 

Kitchen extract ducts

A kitchen extraction duct needs to be basically tested according to a type B duct. The main purpose is to go extract smoke and hot gases from the compartment during normal use of the kitchen but also during a fire, so the fire is inside the duct.

When the smoke extraction duct is also travelling through other parts of the building than there is an also a risk that the duct needs to protect the fire from outside to inside (duct type A).

Next, to this, the designer/ consultant doesn’t have the flexibility to only describe stability and integrity. In the situation of kitchen extract ducts, the insulation criteria are mandatory. 

During the normal use of the kitchen extract duct, the inside of the duct will collect oil and grease. These components are highly flammable and combustible. The insulation criteria of a duct avoid heating up the oil and grease, this means there is no ignition inside the duct by only temperature increase.

Pressurization ducts

A pressurization duct needs to be tested according to a type A. The purpose of the duct is to increase the pressure in a safety route/ escape route, because of this there is only a risk of fire outside the duct.

Next to this is up to the designer/ consultant to prescribe if the ductwork needs to fulfil only stability and integrity or if additional insulation criteria are needed.

(note: if the ductwork is located in escape corridors or closer than 500 mm next to combustible material or person the insulation criteria is mandatory)