In this article, the second in our series about designing smoke shaft systems, we will look at the choice of natural vs mechanical shafts.
Smoke shafts in multi-storey buildings take up potentially valuable space, so keeping the shafts as small as possible is beneficial and has led to mechanical shafts becoming a popular choice.
But are they always the best choice? We look at some of the issues.
Natural shafts
Prescriptive limits
For natural shaft systems there are prescriptive minimum shaft areas (3.0m2 and 1.5m2) and vent areas (1.5m2 and 1m2 respectively) and maximum aspect ratios (2:1 ratio between the length and width). There are also limits on the permitted deviation from a straight vertical shaft arrangement and the location of the shaft termination.
Key benefits
These prescriptive limits are the main reasons for considering mechanical shafts, but if you can live with them, natural shafts have significant benefits:
- No fans. The primary benefit is that no fans are needed, together with their cabling and controls and, of course, their standby power requirements.
- Low noise. The systems are virtually silent, although they can still be a source of noise transmission in noisy areas, such as under flight paths.
- Low costs. Purchasing and operating costs are low, with a very small power requirement and limited equipment to be tested and maintained.
- No roof top plant. Little roof space is required, simply room for the termination at the top of the shaft.
Mechanical shafts
Key benefits
Mechanical shafts have 3 major benefits:
- Size. They can be small
- Flexibility. They can be more flexible in layout
- More ventilation. They can provide improved ventilation to compensate for extended travel distances (but that’s for another blog)
Smaller shafts save space
A smaller shaft can save a lot of space in a tall building. Typically shaft requirements vary between about 0.5m2 and 1.2m2, depending upon flow rate and building height. We often quote a typical 0.6m2, simply because this shaft size is always suitable for buildings up to 30m height, covering the majority of UK buildings.
Shape matters
Smaller vents
The vents opening into the shaft can also be smaller, making them less obtrusive.
Mind the pressure drop
Flexibility is a key benefit, the addition of fans allowing the system to have a much higher pressure drop, allowing bends, horizontal duct runs and less favourable termination locations. A key point to remember, if taking advantage of this, is that systems don’t normally include VCD for balancing, so it is important that the difference in total pressure drop between flow through the top and bottom vents is kept reasonably low to avoid excessive differences in extract rates. Thus the actual builder’s work shaft still needs to be as straight and unobstructed as possible.
Potential disadvantage: fixed flow
One potential disadvantage of mechanical systems is that they provide a fixed flow regardless of fire size, whereas a natural system will provide a higher flow rate as the fire size and heat output increases. The common design fire size for residential and sprinklered commercial fires is 2.5MW and most systems are designed to operate correctly against this fire. However in unsprinklered commercial buildings the fire size can be much greater. BR368 recommends 12MW for an unsprinklered open plan office fire. While a natural shaft system will cope with this without change, a mechanical shaft system would be overwhelmed unless upgraded. Colt offer such an upgrade as a standard option.
Conclusions
Well, as you might have expected, there’s no clear winner here. You have a choice of two alternative systems with differing features and benefits to allow you the flexibility to incorporate the most suitable one into your building.
Smoke shafts v pressurisation webinar
Watch out for further posts in this blog series, where we’ll look at other aspects of shaft system design. In the meantime, register for our upcoming webinar on smoke shafts v pressurisation, where I will cover:
- Features and benefits of both systems
- Common problems with both systems
- How to choose which option is best for your project
The webinar is on June 27th at 12:30pm UK time and is CPD accredited. There will be the chance to ask me questions at the end of the session and for those who register but can't make the seminar at the last minute, the session will be recorded and made available online.
Read the other blogs in this series...
Paul Compton is a Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.
