Car park ventilation webinar Q&A

Posted by Conor Logan on 23/06/15 11:30

car_park_ventAt our recent webinar on car park ventilation, I received lots of excellent questions during the Q&A section. Here you can see my answers to these questions, slightly edited for clarity.

During the webinar I introduced our new whitepaper on designing car park ventilation systems, which you can download here.

There is also a recording of the webinar available.

What should the size be of one smoke zone in a car park using jet fans?

For a smoke control system, we would try to limit the notional control zones (they are control zones rather than smoke control zones) to 2-3000 m2. This depends on how far from inlet to the exhaust, so may not be possible to limit to a fixed m2 area. This depends on the design and the objectives of the system.

How is the pressure loss calculation in a road tunnel actually done?

With all road tunnel calculations, you need to calculate the resistance path of the tunnel including any obstructions and changes in direction, as well as the losses at the entrance and exit. You then size jet fan arrangements to suit.

On what basis should you design the impulse fans for the car park?

If it is a smoke clearance scheme, then all we are trying to do is to create the most efficient route from the inlet source to extract in order to maximise the dilution effect through the introduction of fresh air.

Will the car park ceiling play role in the calculations if the ceiling is not straight?

With tunnel ventilation, the effect of beams or downstands can be either positive or negative, depending on their orientation. Downstands can certainly hold smoke back. If you look at the small video clip of the Shires tunnel, you can see that the change of height of the tunnel is useful in holding the smoke back as the velocity of the air increases as it turns under the step. On the other hand longitudinal barriers are useful for channelling smoke towards an extract position. 

Will impulse systems help in maintaining the tenable environment on day to day basis, because CO is heavier and will stay at floor level? If this is not the case then do we have to install two different systems?

Because of the downward thrust of impulse systems we get a lot more mixing of the air, with the result that stratified layers of fumes is avoided, So while CO or other gases are more dense than air, because of the mixing effect there are no stratified layers being exhausted as would be the case with ducted systems.

Are there standards for EC fans when used for smoke extract?

There are no F300 classifications for EC motors. My comment there was relating to using dedicated fume exhaust fans, in which case we could then switch to EC motors for the fume only and then have dedicated smoke extract fans which are only used for emergencies.

EC motors are less reliable under mains disturbance, when for example changing over to a generator: how is this unreliability catered for?

If they are not being used for emergency purposes, as set above in question 6, then we would not be concerned about this.

Can an issue arise where low ambient temperature might cause the smoke to cool and make it less buoyant within the space?

With car park ventilation schemes we are not relying so much on the buoyancy of the smoke as with other types of smoke ventilation schemes. If we are dealing with a fire, then the temperatures are relative to the ambient air, in which case the buoyancy pressures are very similar to the ambient air conditions, and the colder it is the more buoyant the hot smoke will be. However mixing will start to cool the smoke more rapidly. Once the smoke is entrained into the airflow, we are not looking for buoyant smoke, just for it to be transferred from the inlet source to the extract, using the momentum created by the impulse fans.

For enclosed car parks, why do the extract points need to be split 50% at high level and 50% at low level?

This is for ducted systems, where we have issues with stratification: if you have denser than air gases such as petrol vapours or CO fumes or any other stratified gases forming at low level, then the ducted extract systems can pick this up as they have high and low level extract points. Smoke will be removed in a fire mode at high level, whereas low level extract points would pick up fume that stratifies at low level.

Which CFD package do you use to carry out the modelling?

This depends on the project type. We use Fire Dynamics Simulator for smoke clearance and smoke control projects, but also Autodesk Simulation for isothermal projects (e.g. non-fire related projects where for example we have to calculate what fume will do). We also use CFX on occasion for fume ventilation.

What redundancy allowance should be made for the number of fans?

With smoke clearance schemes, if it is a plain 6 or 10 ACH rate, then there is the requirement for 50% redundancy for the extract system, but no requirement for redundancy for the induction fans. For smoke control schemes, then it is common to go with n+1 redundancy. 

With Liverpool One we had n+1 for the extract plant. There was a more complex arrangement for the Cyclone induction fans: where we were operating 6 such fans for any zone, they were operating at 80% of their full capacity. This meant that if we lost one of those fans, then we would bring a fan that was further away to operate at 100% and so giving a similar sort of performance to a fan within the zone.

On the FDS slides and slice files, some areas showed no colour at all (i.e. some areas were transparent where as they would normally be red by default), did these areas have a visibility of 30m?

Yes, visibility in these areas would be 30m or better.

At the beginning of the presentation you mentioned a naturally ventilated car park with louvres with 2.5 % free area and 3 ACH mechanical ventilation: is this only with impulse fans or does that require extract fans as well to obtain the 3 ACH?

We have designed some projects for 3 ACH just using impulse fans. You still need a good degree of openings on both sides of the car park to allow this to be achieved. It can be done without extract if the car park layout allows it.

Is VSD control in case of fire recommended?

For smoke clearance systems, the latest version of BS 7346-7 states that the fans can be operated in an emergency service with “fire mode inverters” without a direct on line bypass. The Smoke Control Association is writing guidance on what is needed along with Gambica, the trade association for such devices, as there is no clear definition of what “fire mode inverters” actually means and what parameters should be locked out when we go into emergency service mode.

Where should CO detectors be located?

Because CO is toxic, such detectors should be at breathing height on the columns, say 1.5 mm FFL. The spread depends on the manufacturer’s recommendations, but seems usually to be 400m2 in plan area.

Can we use these kinds of fan technologies in other applications apart from car parks, for example to enhance the extract from an AHU unit?

Yes indeed. The energy efficiencies we’ve looked at, e.g. EC fans, reducing pressure drops, being more proactive in control systems, can be applied to all ventilation systems.

How do you split a car park larger than 3000 m2 into smaller smoke zones? Can you use curtains?

The smoke control zones are infact notional control zones, so these are zones where we are detecting smoke and operating the fans, so we are not expecting the smoke necessarily to be limited to those areas. These are the zones where if a fire occurred, we would look to operate fans. If you have a 100m wide car park, and fire is one side and extract on the other, then all we are trying to do it to control the lateral spread. It is difficult to move the smoke to specific areas and a lot depends on the controls.

Does the smoke control industry have any good case studies of jet fan systems being used in anger by the fire service during a fire incident? It could be extremely useful for the industry, building owners and the fire service to understand the benefits and capabilities of these systems.

There are not many. We would like to see more of them!

What is the maximum allowable velocity at an air intake?

We would size the system so that the velocity to limited to 1-1.5 m/s in day to day mode for fume extract. For example we don’t want it to become excessive across entrance ramps where people may be wanting to access ticket machines – tickets could be blown away! The design velocity will tend to be increased to 3-5 m/s in smoke extract mode.

How effective is 10 air changes per hour in respect of the car park size?

It is relatively effective, as it provides a good turnover of air, clearing a moderate sized car park within 20-40 minutes if the source of the smoke is removed. This depends on the layout of the car park. It is a notional clearance rate and not based on any scientific approach.

Is an 8 MW fire size adequate in a tunnel when vehicles much larger than cars are present?

In the Shires case history we discussed, the tunnel had a very low clearance at the low point and so no larger vehicles could travel through it, so 8 MW was OK. On other projects, such as with service tunnels where smoke control systems are used, the design has often been based on 20-30 MW to deal with lorry fires.

Do you generally monitor the NO2 levels as well as CO?

Generally not, as the requirement for CO monitoring is defined by ADF as it is potentially lethal. Nitrous oxides are unpleasant and may have some long term health issues for asthmatic people, for example, but there is no lethal aspect to the build-up of such gases. In addition there is very little guidance on the threshold levels for these types of gases.

Is there a fire resistance rating requirement for ducting systems? Do we need to cover the ducts with fire rated materials?

This depends on where the ductwork is going and what its role is. If a duct passes through another compartment before it leads to the outside, where there is the possibility of fire spreading from one compartment to another, then yes it would need to be fire rated. Whether it would need to be insulated or not in addition depends on what other combustibles are adjacent to it and whether they pass through an escape route. If there is a fire strategy for the building, this would normally identify the requirements.

Are currently designed fire sizes appropriate to the type of vehicle and methods of construction right now and also in the future?

It is very difficult to predict what fire loads for cars will be appropriate in the future as the materials and fuel types are changing. There are some very dramatic videos of LPG cars on fire, and some countries prevent LPG cars from using underground car parks. We don’t have much specific guidance. BRE tests from just a few years ago suggested that an 8 MW fire load for a car could be low compared to the peak of around 16 MW. This 16 MW peak was very brief and took some time to achieve, with 10-15 minutes of low heat output followed by a short peak of 16 MW. 8 MW is a good test to check if the design is likely to be effective or not.

What should the velocity of air extract through extract louvres be?

General design guidance sets out that if it is a cladding or performance louvre, the velocity of air should not exceed 2.5 m/s, which equates to a resistance of approximately 25 Pa, depending on the louvre type. Above that then it is likely that there would be regenerated noise. I would recommend that this should be dropped to 1-15 m/s for day to day usage.

How about electric cars in the future?

Different types represent different risks, and we expect some guidance from BRE or DCLG in due course. There are no restrictions in the UK at the moment. The fire load will indeed be different.

Whitepaper - Designing car park ventilation systems

Download the whitepaper which explains the basic legislative requirements needed for a car park and how these are achieved with impulse ventilation, with reference to CFD analysis and case studies.

Webinar - Car park ventilation

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

Conor Logan Conor Logan is a Technical Manager at Colt UK, Smoke and Climate Control Division. Conor designs innovative smoke control and HVAC systems and is also Chairman of the Smoke Control Association.

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Topics: Webinar, Car Park Ventilation