Smoke and environmental ventilation of multi-storey buildings using shafts webinar Q&A

Posted by Paul Compton on 21/07/15 11:30

pan-peninsula-coltshaft1Here is a copy of the questions and answers from our recent webinar on smoke and environmental ventilation of multi-storey buildings using shafts, slightly edited for clarity.

During the webinar I introduced our new whitepaper on the subject, which you can download here.

There is also a recording of the webinar available here.

Where planning conditions on projects in London apply, where plant is to be mounted on the roof, is it possible for smoke fans to be fitted within the shaft?

Yes this is possible. There needs to be sufficient space between the top of the shaft and the ceiling of the highest floor that you are ventilating. If you are ventilating the top floor with a shaft, that limits the available distance for fans. These are normally run and standby in series, so the required depth can be fairly great 

To deal with this the top storey can be ventilated separately with a natural ventilator on the roof of the lobby or corridor.

You won’t be able to provide day to day ventilation via the fans, because they will be too noisy as you won’t have room to fit the necessary attenuation. You can only do this if you have a smoke-only system.

Is there a maximum distance from the shaft to the end of the communal corridor (i.e. dead end distance), which if you exceeded you could not get the CFD to pass?

If you have an ADB compliant corridor, then you are allowed a 7.5m dead end anywhere. You can then put the shaft anywhere you like.

If you have an extended corridor, then we would recommend that you place the shafts as close as possible to the ends of the corridor.

The general guidance is that this distance should not exceed 7.5m of the end of the corridor, because there will be area beyond the end of the shaft which will stay smokier for longer. The assumption is that if you have an ADB compliant corridor, then you only have a 7.5m corridor which may well be smoky anyway, so that you won’t be any worse in that 7.5m than you would be if it was a compliant corridor. While 7.5m is an absolute maximum, I recommend that you make this distance as small as possible, as this will give you the best system.

Is the 3m2 shaft a geometric or aerodynamic free area?

It is the geometric area, the physical size of the shaft. The 1.5m2 shaft is also the geometric area, but the 1m2 or 1.5m2 for the ventilators or grilles etc. can be taken either as a geometric or aerodynamic free area, whichever is the most advantageous.

What material should a shaft be constructed of?

It needs to be fire rated, in just the same way as any riser passing through the building would, to protect against the vertical spread of fire. It can be cast concrete, builders work / blockwork, plastered or fire board – so long as it has the correct size, it is reasonably smooth, it provides the required fire resistance and it is reasonably smoke-sealed.

Particularly where a mechanical system is installed into a relatively tall building, if the shaft is too leaky, you will find that at the bottom of the shaft the extract rate falls off because of the leakage from the shaft.

What are the requirements for the maintenance of variable speed fans to check that they operate within the required parameters?

This should comprise an annual test and check. This would involve running the fans, to check that they can run at full speed with the doors open and that with the doors closed  there is a pressure differential which is in the order of around 15-30 Pa. It is a simple check. 

What are the design requirements to ensure a high degree of reliability with inverters?

The run and standby fans should each have their own inverter. Normally a fire-fighter’s switch would be provided as a bypass so that the fire-fighters can actually bypass the inverter to a direct contact to provide full speed. They would do this once they have opened the door into the lobby or corridor, so that there is no issue with pressure differentials because the door is open.

To what extent can the relative humidity be allowed to increase with the cooling of a corridor with an evaporative cooling system?

There will be an increase of humidity but generally this won’t induce discomfort or mould etc. The actual allowable value will depend on the location and the design of the system and this is always subject to a design on a project by project basis.

What is the maximum temperature that the extract fans can tolerate? Is it possible not to supply sprinklers for cooling purposes?

Such fans are normally rated at 300C and certainly sprinklers would not be required to protect the fans. The reason for this is that we are not taking smoke directly from the plume; we taking smoke which has come from the fire and which has been through a door and done a lot of mixing.

We have never had a situation where the CFD has shown that the temperature within the smoke extract shaft will be 300C or higher. We normally get conditions around 50-200C. A 300C fan is perfectly acceptable and readily available: anyway, this is the normal rating for fans used for smoke control in the UK.

Please explain the power supply requirements for mechanical extract systems

BS 8519 requires both a maintained primary and secondary supply with an automatic transfer switch between the two that provides the maximum possible security for the supply to the units.

The type of secondary system depends on the application: for instance it can be a generator system or a second mains supply system.

What is the maximum allowable velocity in a ColtShaft?

We design on the pressure drop down the shaft, not the velocity, in such a way that we limit that pressure drop: we should achieve a reasonable balance between the flow rate when we open the top damper and the flow rate when we open the bottom damper. Because of this factor, the taller the shaft is, the lower the velocity we want.

What kind of air change rates is used to select a fan for summer overheating?

We do not design on an air change rate. We look at the heat generated by the pipes in the corridor and then design to maintain a reasonable pressure differential.

What is the maximum allowance velocity for dampers in a ColtShaft system?

This depends on the product and system used. We tend size a conventional damper on 5 m/s for use with a mechanical system such as ColtShaft.

What the maximum length of corridor we have designed for use with a mechanical extract system?

We have designed systems up to 27m, though many are around 15-25m. Guidance suggests that 30m is the maximum that would be accepted by Building Control. This is because where you have a corridor divided by fire doors, then the maximum distance allowed between these is 30m, so this fairly well accepted. Technically I think we could get a larger distance to work but it would not be approved.

What is the allowable maximum velocity in a pressurisation shaft?

This comes down to achieving a balance between the power that is needed in the fan and keeping the duct size to a reasonable minimum. Typically 10 m/s might be the design velocity, but this will be influenced by the overall design and priorities.

If smoke ventilation used for day to day ventilation, is inlet an issue as regards air quality?

Generally not, as air inlet and extract are normally at roof level, where the air quality is likely to be high. However if a system has low level inlet, then this can indeed be a problem. If there are any dirty discharges from the rest of the building then these do need to be kept a reasonable distance away from the inlets for the day to day ventilation for the stairs or corridor.

What allowance should be made for leakage when determining the pressure regulation control for leakage in the corridor itself and the control of the variable speed fan?

In this instance we are not making an allowance at all, because system just runs at whatever speed is necessary. So if the corridor is very leaky, you may find that the fan may run at 40-50% of full speed; if the corridor is well sealed, the fan may run at 5-10% of full speed. This happens automatically so there is no allowance in terms of the design.

How do you make sure that opening and closing of doors do actually close so as to maintain the negative pressure in the lobby?

Door closers do the work of closing the door, but unfortunately they are variable in terms of the amount of power they provide, depending on how they are set up, how big the door is and so on.

Although we design for a notional -25 Pa in the lobby, we adjust the lobby pressure at commissioning to allow us to ensure that the doors will close under that negative pressure.

On occasions, where there are very large doors, we have had to get these adjusted to get the right balance between the two. But the relative pressure is not a very critical value: if it goes too high, problems with door opening and closing can occur, but it can go down to a low value without issues occurring.

If combining smoke and environmental control, do you need to supply independent fans in any situation, i.e. one dedicated to smoke and the other to environmental control?

Yes, you can do this, but you do not have to. In general you can use a smoke control fan with an inverter to reduce the volume flow rate and attenuators to keep the noise down and to reduce the power, or a separate dedicated environmental fan. Which you use comes down to the geometry of the building, in particular what space you have available.

What are the requirements for the periodic inspection and maintenance of AOVs?

Generally they should be maintained once a year. They should also have a weekly test to ensure that they are still working, since they are life safety systems, normally along with the fire alarm test.

Whitepaper - Smoke and environmental ventilation of multi-storey buildings using shafts

Download the whitepaper which looks at the various types and applications of shafts.

Webinar - Smoke and environmental ventilation of multi-storey buildings using shafts

If you missed our recent CPD-accredited webinar on this topic, you can watch a recording here.

Paul Compton Paul Compton is Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.

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Topics: Webinar, Smoke ventilation, Smoke shafts