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Paul Compton explains a technique to reduce the valuable space in a building that is usually needed for a smoke shaft.
All buildings over 18m to an occupied top floor require one or more firefighting cores – voids containing stairs, lifts, and lobbies with hose connections to allow fire fighters access to any floor. These cores must be kept free from smoke, and this is achieved by ventilation – either openable windows or, if they are away from outside walls, via a pressurisation system or shaft system (typically standard or BRE shafts).
Pressurisation systems
Pressurisation is a technique for protecting escape routes against the ingress of deadly smoke. This is achieved by maintaining the pressure within the escape route at a higher level than in the adjacent spaces. A pressurisation system comprises two main components:
– Supply air (where air is injected into the area that is to be protected).
– Air release (air and smoke are released from the adjoining fire area).
Combining these two elements creates a positive pressure difference that prevents lobbies and staircases from filling with smoke. Pressurisation systems meet the recommendations of Approved Document B, BS 5588-4 Code of practice for smoke control in protected escape routes using pressurisation, and BS 5588-5 Code of practice for fire fighting stairs and lifts.
Two requirements must be met to ensure the pressurisation system is effective. These are maintaining a:
– Pressure difference for a closed-door condition to overcome buoyancy pressure generated by the hot smoke layer, expansion of the gases in the compartment due to heating, stack pressure and wind pressure.
– Velocity for an open-door condition to hold back the smoke on the fire floor when the door onto the fire floor is open.
However, getting the right balance for a pressurisation system demands meticulous design. Insufficient pressure difference across a closed door will allow the passage of smoke into the protected space, while excess pressure will impede door opening and hence escape.
Conventional smoke shaft systems
Traditional smoke shafts offer an alternative. The trouble with them is that they tend to take up a large amount of valuable floor space, reducing the useable area and constraining the architect in terms of the creativity of the design. A standard shaft, for example, requires a 1.5 square metre automatic opening ventilator unit that opens into a 3 square metre shaft, which is open at the top and bottom. A BRE shaft also requires a 3 square metre shaft, but with an opening at the top of the shaft only. Each level of the building is provided with a 1.5 square metre fire damper that opens into the shaft.
There is an alternative, however. The Colt shaft is similar to a BRE shaft but uses a fan system to draw smoke away from the firefighting core. This means it requires a shaft of only 0.6 square metres with a damper size between 1.5 square metres and 0.8 square metres, representing up to an 80% saving in floor space.
But there are two common problems associated with mechanical extraction. First, as the lobby is fire rated, the area of ventilation into it is usually small. So even a small amount of extract will cause a high negative pressure in the lobby, which could make doors difficult to open. Secondly, negative pressure could cause smoke to be drawn into the lobby from the fire compartment.
The Colt shaft overcomes excessive negative pressures – without compromising the integrity of the stairs and lobby – by automatically reducing the ventilation rate when the lobby doors are closed. It does this via a pressure sensor linked into the control system that varies the fan speed. With all doors open, the fan runs at full speed to extract smoke discharging from the accommodation. With all doors closed, the fan runs at minimum speed to help mop up smoke leaking past the closed door. In intermediate conditions, the fan speed modulates to ensure adequate ventilation without excessive depressurisation.
Colt has conducted a series of smoke tests, observed by the London Fire Brigade, which demonstrates that the system maintains the intended depressurisation level. The tests also show that the system reacts quickly to pressure changes; smoke clears quickly from the lobby when the accommodation or stair doors are opened; and the lobby is kept clear of smoke once the accommodation door is closed. With the door to accommodation closed, a typical 5m2 lobby will clear totally within 15 to 20 seconds of opening the stair door.
For a copy of the short report describing these tests, contact Alex Howard on 0239 2491 951 or email: customer.services@uk.coltgroup. Colt would be happy to arrange a viewing of these tests on video and to go through the report.
HOW IT IS DONE
The Colt Shaft comprises:
– Duty and standby extract fans rated at 300 degrees C for 2 hours
– Colt Seefire duct termination and stairwell ventilator
– Ducting to link the builder’s work shaft (by others) to fans and duct termination
– A motorised smoke damper and grille to each lobby or Doorman door actuator for each lobby
– Main control panel plus mimic panel for fire service use
– Pressure sensor to each lobby
– An optional fire detector for accommodation close to each lobby.
The Colt Shaft is designed to meet the recommendations of Approved Document B and BS5588 Part 5 for smoke ventilation in fire-fighting cores.