In the seventh and penultimate article in our series about designing smoke shaft systems, we discuss power requirements.
Natural shafts
One of the major advantages of natural systems is the very low power requirement. Most systems use 24V ventilators and run off standard 230V single phase supplies, with standby power provided by a battery back up unit which transforms and rectifies the incoming power to charge the batteries and drive the ventilators. The batteries then provide a temporary source of power in case of loss of mains supply. This is ideal for residential buildings that may not have a 3 phase supply.
Mechanical shafts
Most smoke extract fans require a 400V, 3 phase power supply, and typically have a motor power between 3kW and 7.5kW.
To minimise the power required:
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avoid complex ductwork
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size equipment generously
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use turning vanes in duct bends.
Just think of it as a normal duct system that you’re trying to limit the SFP (specific fan power) on.
To save roof space, a run + standby fan in series is the default option. Putting the fans in parallel takes up more space and is generally more costly, but it can reduce the power required as “windmilling” losses in series fans can be significant.
Where a system is reversible, use of a fully reversible fan may be the obvious choice, but these fans are inefficient and power hungry. A standard fan will produce something like 60% of normal air flow when running backwards, so, as long as the corridor has sufficient leakage area, this might be a more energy efficient option.
Power supplies
A maintained supply is always needed for a mechanical shaft system. The primary supply is invariably from the mains but the secondary supply may be from a generator, a UPS or, in some cases, a second mains supply. In some cases these might be shared with other emergency equipment, but in residential buildings and smaller commercial buildings it’s quite common for the secondary power supply to be required just for the smoke control system.
Because the secondary supply is serving a smoke control system, it should be CE marked to EN 12101-10, but I’m not aware of any generator or UPS supplier who has products certified to the standard.
Smoke shafts v pressurisation webinar
Watch out for the final post in this blog series, where we’ll look at dual purpose shaft systems. In the meantime, watch my CPD accredited webinar on smoke shafts v pressurisation, where I discuss:
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Features and benefits of both systems
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Common problems with both systems
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How to choose which option is best for your project
Paul Compton is Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.
