Designing Car Park Ventilation Systems

Designing car park ventilationsystems

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Colt whitepaper - Designing car park ventilation systems

This whitepaper covers:

An explanation of the basic legislativerequirements and how these areachieved using impulse ventilation

Application of impulse ventilation, both for carbon monoxide and forsmoke clearance or smoke control

The advantages of impulse ventilationover traditional ducted extractsystems

Use of CFD

How impulse ventilation can beused to control smoke movement, allowing smoke control to be used, aspartofafirestrategy,tocompensatefor the relaxation of other legislativerequirements, e.g. travel distances

A case history of a particular project where travel distances were relaxed using impulse ventilation designed for smoke control. An explanation of how this project was validated using CFD andlivefiretests

Control systems.

1. Traditional ventilation for car parks

Traditionally, underground car parks and other enclosed car parks have generally been poorly lit, damp, of low spec construction and perhaps with a bit of an odd smell about them.

To cap this, where mechanical ventilation has been required, this has been tended to be through a ducted system with extract points at high and low level. If this approach is taken, depending on the size of the car park, the extract requirement can be very large and therefore the ductwork needs to be equally large in size and extending all over the car park down to ground level. These systems take up a lot of room, causing headroom problems for vehicles and getting in the way of pedestrians, as well as reducing the number of available parking spaces.

Summary

Enclosed or underground car parks normally require ventilation systems to assist firefighting operations. These systems generally also prevent the build-up of carbon monoxide during normal day to day use of the car park.

2. The new look of car parks

However things have moved on: more and more often people are realising that because most people arrive at a shopping centre or a company by car these days, it is a good idea to help them form a good first impression of the underground car park, which is the first thing that they will see.

So therefore car parks these days are much lighter, more open, much more pleasant places to be. One significant contributor to this is the change away from traditional mechanical ventilation to impulse ventilation systems. Impulse units are small fans which direct fumes and in the event of a fire, smoke, towards a single extract position, thereby getting rid of all the internal ductwork and result in a much cleaner, tidier appearance.

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3. The legislative requirements

In England and Wales the requirements for ventilation in car parks come from approved Document B and approved Document F to the Building Regulations. Scotland and Northern Ireland have similar regulations. They set out requirements for car park ventilation, giving options for natural or mechanical ventilation.

a) Open sided and naturally ventilated car parks

Certainly if the car park is above ground then normally it is best to provide natural ventilation, because compared to mechanical ventilation it is simpler, cheaper to procure and there are no running costs. Anyway such car parks are regarded as having a higher level of fire resistance.

An open sided car park is defined as one where there are natural ventilation openings in the walls which equate to at least 5% of the floor area. So if a car park measures 1,000m2 then 50m2 of ventilation area will be needed. There is a further requirement that the ventilation area should be split, so that at least a quarter of it is down each of two opposing sides and reasonably evenly spaced so it creates cross flow through the car park. The other 50% of the ventilation can be put in wherever you can find room for it.

If the 5% cannot be achieved then there is an option for 2.5% of ventilation area and that works in exactly the same way in that a quarter of it needs to be provided down each of two sides. If there is 2.5%, then while this is regarded as totally satisfactory for smoke ventilation, it is not regarded as adequate for day to day fume ventilation. Therefore a small mechanical system providing 3 air changes an hour (ACH) to supplement the natural ventilation is needed.

b) Mechanically ventilated car parks

Normally an underground car park wont have any natural ventilation at all. In this instance there are requirements for mechanical ventilation. 10 ACH is required in fire mode and 6 ACH for general ventilation. Again the requirements are for a system designed to ensure that we get a reasonable ventilation flow throughout the car park.

In terms of general day to day ventilation all of those requirements are actually deemed to satisfy the requirement to achieve a maximum carbon monoxide concentration of 30 parts per million (ppm) averaged over 8 hours, leading to an environment which is both pleasant and safe.


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Car park type

Open sided Naturally Mechanicallyventilated ventilated

ADB 5%* 2.5%* 10ACH Fire Safety

ADF 5% 2.5% + 3ACH 6ACHGeneral Vent

* Of which at least 50% should be split equally between two opposing walls to provide

crossflow ventilation

ADF limits CO concentrations to 30ppm over 8 hours and 90ppm over 15 minutes.

4. How does impulse ventilation work?

Impulse ventilation systems move the air throughout the car park and ensure that the air goes where we want it to go, rather than where it would flow to if left to its own devices. It is a simple method of ventilation that uses the thrust created by small fans that discharge air at a high velocity to create a large air movement at low velocity.

An impulse fan is suspended from the ceiling and arranged to provide an air movement to meet the design requirements. The airflow is normally angled downwards in such a way that there is mixing throughout the height of the car park. As that jet of air leaves the fan, then the turbulence in the air starts mixing it with the ambient air around it so that as one moves further and further away from the impulse fan then the flow of a jet from the fan spreads both vertically and horizontally and creates a relatively low air velocity across quite a large area. So with a few well-positioned impulse fans based around a building, the flow of air through that building can be controlled very efficiently.

For example a fan providing an airflow of 1m3/s with a discharge velocity of 20m/s can result in a general air movement of about 10m3/s at 1m/s as the airflow from the fan spreads out.

This is a Computational Fluid Dynamics (CFD) image of an impulse fan showing how the flow of air mixes down the car park. CFD is a means of providing a 3 dimensional visualisation of the flow of air around structures. In this case red is fast air movement, green and blue is slower air movement. The angled thrust from the fan is clearly demonstrated and shows that there is no longer a need to have extract at high and low level as the air is fully mixed downstream of the fan.

An extract system is still required, but extraction is only needed from a single point in the car park with the smoke directed towards it by the impulse fans.


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5. The design approaches: (a) smoke clearance

It is important to understand that there are two different smoke ventilation design approaches available and to understand what can be achieved with each.

A smoke clearance system clears smoke in the case of a fire. Traditional or impulse ventilation are suitable for smoke clearance systems.

Smoke clearance is intended purely to assist the fire service by clearing smoke once the fire has been brought under control and returning the car park to use. It is not intended to achieve any clear conditions within the car park and generally will result in complete smoke logging of the car park.

Smoke clearance is not going to help people escape and it is not certainly going to compensate for any extended travel distances. All it really does is to provide a means of getting smoke out of the car park after the fire and ensuring that the smoke in the car park is kept to a reasonably low temperature, thereby reducing the risk of flashover.

6. The design approaches: (b) smoke control

As an alternative to smoke clearance, it may be desirable to design a smoke control system: going beyond the requirements of the Building Regulations, this will actually keep part of the car park clear of smoke either to aid escape or to aid the Fire Service and provide an alternative to sprinklers. In countries which also require relatively small fire compartments in car parks, then smoke control systems can also provide an alternative if the Regulatory Authorities agree.

In other kinds of buildings smoke control is usually provided by main-taining the smoke layer well above head height. Due to the low ceiling height in most car parks it is not usually possible to achieve the vertical separation between the clear air and the smoky layer above as is done with more traditional systems. Instead, control of the smoke is achieved by creating a clear area upstream of the fire location whilst allowing the area downstream to smoke log. This makes it suitable for firefighting by providing fire fighters with clear air in which to approach the fire and enabling them to locate the fire more easily. Generally, this can only be achieved with impulse ventilation.

As regards the above statement that a smoke control system can be installed as an alternative to sprinklers, in fact in most of the UK sprinklers are not required in the majority of car parks. These tend to be designed for car parks which are a bit special, perhaps because they have got stacked systems, or because they are attached to shopping centres where sprinkler systems are needed.

The image shows the velocity vectors produced by the CFD representation of a Colt Cyclone 100 fan unit

7. Design guidance: BS 7346-7

As set out above, in England and Wales Approved Documents B (ADB) and F (ADF) provide recommendations for the ventilation of car parks.

However, in recognition of the fact that previously there was nothing to provide guidance for the approving authorities regarding impulse ventilation systems, BS 7346-7 was published in 2006 to cover ventilation in car parks. This provides recommendations for impulse ventilation systems alongside further details on traditional systems, as well as detailing the approaches to smoke control and day to day ventilation.


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8. System requirements for naturalventilation

As set out above, where natural ventilation is being considered, then either 2.5% or 5% of the floor area is needed as natural openings. In ADF there is an unusual definition of the area of those openings, called the equivalent area. The actual definition is: A measure of aerodynamic performance of a ventilator. It is the area of a sharp edged orifice which air would pass at the same volume flow rate, under an identical applied pressure difference, as the opening under comparison.

This takes into account any obstructions to the airflow such as louvres, grilles or even

plants, but it is not a simple definition! What it basically means is that the aerodynamic free area of the ventilation openings is divided by 0.6 thereby resulting in the equivalent area. This means that the aerodynamic free area of the opening needs to be known, which should be relatively straightforward if the ventilator is something like a louvre system, which the manufacturer can normally provide. If something like a wire mesh screen is involved then there is usually generic information available from a CIBSE Guide or other sources which again will tell you what the typical aerodynamic coefficient will be. However if the ventilation openings are a bit special, the only way to find out that aerodynamic performance is, is to actually test it.

9. System requirements for smoke clearance using mechanicalsystems

ADB sets out some additional requirements for smoke clearance using mechanical ventilation systems above what is needed for natural ventilation:

a) The system should be designed to run in two parts each capable ofextracting 50% of the required extract rate. What this means is thatif one fan fails then there is still at least 50% of the airflow. Soalthough it is possible to have more than 2 fans if the design of thecar park demands it, the minimum number is 2.

b) Each part to have an independent power supply to operate in theevent of mains power failure.

c) Extract points to be arranged such that 50% are at high level, 50%at low level. With the traditional ducted systems this was achieved byhaving the ductwork with grilles at high level and then havingdropper ducts coming down to floor level, again with grilles in them.

With an impulse system there is no ductwork system. Instead thereis a large extract point somewhere in the car park, normally floor toceiling, so effectively there is half the extract at high level and halfat low level just by providing that single extract point. There is alsothe benefit that the impulse fans themselves provide a lot of mixingbetween high and low level, so again throughout the car park airmixing is good.

d) Fans to be rated to run at 300OC for a minimum of 60 minutes andductwork to have a melting point not less than 800OC. Any fan ratedat F300 to EN 12101-3 (the standard for smoke extract fans) willmeet that requirement. The ductwork melting point of 800OC mightseem an odd requirement since standard galvanised steel ductworkwont melt at that temperature, so any ductwork that is actuallyneeded within the car park area can just be standard galvanised steelduct to DW144. Obviously once the ducting goes outside of the carpark area then the ductwork may then need to be fire rateddepending upon where it is running.

10. Other considerations when designing in mechanicalventilation systems

Traditional ducted systems cause a number of issues in terms of ceiling height. Obviously a minimum clear height is needed for vehicles to travel through the car park. Against this, in an underground car park the soffits should generally be as low as possible to minimise dig out costs. Such soffits very often have drop down beams to help support the structure above, and therefore there is a fairly limited area which is available for ductwork to pass under these beams. The low level inlets that are needed tend to impede parking bays and they often require barriers for protection, so to conclude such ducted systems are a bit of a nuisance within a car park.

This is probably the main reason why impulse ventilation has largely taken over from ductwork. There are various other reasons why impulse ventilation systems are generally better and why it is uncommon these days for a traditional ducted system to be installed unless there is a particular benefit in terms of the particular layout of the car park.

Impulse ventilation removes the need for ductwork and low level intakes, providing:

A safer, lighter environment Improved security for CCTV A potential increase in number of parking bays Easier and quicker installation less storage required on site Fewer clashes with other work packages Lower power consumption due to less resistance on main

extract fans Lower height constraints leading to possible reduced dig out costs.

11. The types of ventilation equipment available

Probably the most common type of impulse fan is a circular unit with a central axial flow fan with attenuators either side of it, so it is a very simple unit. The Colt Jetstream shown on the right is designed to be aesthetically pleasing and it has specially shaped attenuators to keep the diameter to a minimum and to match the fan. It is also quite common for a standard circular attenuator to be placed on the end of a standard smoke temperature rated axial fan in order to reduce cost, though this is not so aesthetically pleasing. Those sorts of fans generally are typically rated at 50 Newtons thrust; typically they are about 400mm in diameter, and one of those units will cover an area of about 400m2 and will have a throw of about 20 30m. If the fan was to be tested in a wide open space then the throw would actually be more than this but of course the car park has columns, cars and all sorts of other obstructions so the throw is limited by these.

The alternative is what is has become known as an induction fan. The Colt Cyclone shown on the right is an alternative to the Jetstream and has the advantage in that we can provide higher rated units, typically rated at 75 or 100 Newtons. They are also shallower, which can be an advantage if the designer is aiming to keep the height of the car park to a minimum, and they are also significantly shorter, so if there are a lot of down stand beams relatively closely spaced, then it is much easier to install these than it is to install a standard impulse fan. But basically both types of fan do exactly the same job and it is simply a case of choosing which one is going to be the most convenient and most cost effective for the particular project.


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The main extract fans are also needed and two typical examples are shown here. Shown on the left is a typical system for a small car park there is just an axial fan with podded circular attenuators either side of it. Larger car parks typically require much bigger fans and rectangular attenuators are very often needed to provide the attenuation that we need to keep noise down to reasonably low levels as shown on the right. CIBSE Guide Volume A: 1999, Environmental Design, sets out a recommended maximum noise level of NR 55 within the car park itself. There is no single guideline for the limitation of external noise, since the attenuation requirements will depend upon the local environmental health requirements. Typically systems are designed for about 55 dB(A) at 3m, though some systems will need to be higher, some will need to be lower.

12. Designing smoke clearance systems

The design of smoke clearance systems is relatively simple. The extract is sized to provide at least 6 ACH to all levels at the same time and 10 ACH for the largest level in smoke mode. The assumption there is that there is only going to be one fire and it is only going to be on one level, therefore we need to provide our 10 ACH extract from that level.

The impulse fans are located generally over roadways so that parked cars are not going to impede the airflow too much, and they are generally laid out to ensure that we dont get any stagnant areas within the car park. The actual layout is basically determined by the skill and experience of the engineer: there are no particular guidelines to inform the designer exactly where to put the ventilators. Instead the layout is


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13. Smoke clearance systems and the Bristol car park tests

When impulse ventilation was initially introduced within car parks, there wasnt really much guidance or knowledge of the systems and it was almost a leap of faith that the systems actually would comply with Building Regulations.

Because of this Colt carried out some test work in 2004. The purpose of the tests was to provide a comparison between the performance of a traditional ducted system and impulse systems in the same car park. The car park was 50m x 30m x 2.9m high with a well distributed extract system and inlet coming through the ramp.

We modified the existing ducted ventilation system so that we had the option of extracting either through the distributed ductwork or at a single extract point located top centre in the diagram, and installed some impulse and induction fans and checked the performance of those.

Three tests were carried out using the same approach with each test so that a comparison could be made between each of the three systems.

based around the rule of thumb that typically one 150 Newton unit is needed per 400m2. This provides a rough number of units and then the locations of the units are selected to match the geometry of the car park.

Because the design is done by skill and experience rather than by calculation, then generally Building Control wants to see a CFD model to confirm that the system actually works very well. However in a simple car park and with an experienced engineer the CFD model is largely unnecessary but it is often the only way that Building Control approval is going to be secured, so it generally needs to be done.

The car park was filled with smoke for approximately 4 minutes using smoke generators and warm air heaters to give the smoke a degree of buoyancy. When the generators were stopped the visibility was down to approximately 10m.

In each case, the time taken to clear the smoke from the car park was measured, giving a comparative measure of the performance of each system.

The results demonstrate that the impulse ventilation and the induction ventilation systems were both much quicker to clear the smoke than the traditional ducted system. These tests provided confidence that these newer kinds of systems do actually comply with Building Regulations and are as good as or better than traditional ducted systems.

System Time to see Time toend wall clearance

Traditional 27 minutes 42 minutes

Jetstream 19 minutes 33 minutes

Cyclone 17 minutes 28 minutes

14. Designing smoke control systems

With a smoke control system the design becomes much more complex: there are many more calculations involved in it and much more engineering. As described above, the purpose of such systems can include extending travel distances, avoiding the need for sprinklers or getting rid of the requirement for compartmentation of the car park.

The first thing we need to know is how big the fire is likely to be. There are three standard steady state fire sizes set out in BS 7346-7, based on test work which is being done around the world:

Sprinklered: 14m perimeter 4000 kWUnsprinklered: 20m perimeter 8000 kWSprinklered 2 tier stack: 14m perimeter 6000kW

The smoke and heat from a car fire rises vertically to the ceiling and spreads out rapidly in what is termed a ceiling jet in concentric circles out until something stops it.

As the smoke spreads in these concentric circles, the further away it gets from the fire the lower the velocity becomes, so by providing an opposing velocity the smoke flow can be stopped at pretty much any distance from the fire, and the smoke can be pushed back as long as there is enough velocity. Sufficient thrust needs to be employed to push the ceiling jet in the opposite direction using air flow from the impulse ventilation system.


Thus in general terms the design of the smoke control system is based around working out what velocity is needed over what area and then ensuring that the system can provide that velocity over that area. Having provided that velocity, it is important to ensure that the smoke which we are pushing towards the extract point is then totally extracted and does not re-circulate and come back behind the fans.

Thus such systems do not work on a simple 10 ACH of extraction, as would be required with a smoke clearance system. Sufficient smoke extract would be needed to match the flow rate that we achieve and generally that smoke extract rate is going to be significantly greater than it is with a smoke clearance system.

15. Smoke control systems for narrow car parks

The principle for a narrow car park is shown here. By knowing what the velocity of the smoke flow is going to be, the impulse fans can push the smoke, in this case from right to left. The guidance sets out that if the design is to enable fire fighter access, then the aim should be to keep areas that are more than 10m away from the fire clear of smoke, allowing clear approach to within 10m of the fire. The entire width of the car park does not need to be kept clear of smoke, but there should be just sufficient space available from the fire service entrance point to wherever the fire is so that they can see where the fire is and fight it.

16. Smoke control systems for wide car parks or servicetunnels

With wide car parks it becomes impracticable to achieve a velocity across the whole cross section area because the mass flows would need to be ridiculously high. So then it is necessary to zone the car park so that we can detect where in the car park the fire is and provide ventilation and then only over a limited width to limit the ventilation flow rates.

Below is an example of a system which we designed for a tunnel which is connected to a car park.

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With smaller car parks smoke is pushed away from fire fighting access, allowing clear approach to within 10m of the fire in order to assist fire fighting.

Highcross Shopping Centre car park road tunnel


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Case history: ventilation for the service tunnel at the Highcross Shopping Centre in Leicester

A road tunnel has been built to provide an exit route from the car park. The tunnel is approximately 125m long with the road sloping down towards a central horizontal section and passing under the main retail development.

Due to the single direction of traffic flow, there is an increased risk that an incident in the tunnel would block traffic flow, meaning that those trapped behind it would most probably have to evacuate on foot, while those ahead of it could continue to drive out of the tunnel. The approach chosen was to use Colt Cyclone fans to stop smoke from flowing upstream and direct it towards the tunnel exit, preventing the smoke from affecting the evacuation of the occupants whose vehicles are trapped.

The alternative would have been to use a ducted extract system

Larger projects must use a zonal system: in a fire, the use of selective fans upstream of the fire forces the smoke towards the extract point

from the tunnel, but it was considered that this would not be practical due to the large volume of smoke to be extracted. This chosen smoke control design was modelled and verified using (CFD) to prove its ability to operate against very onerous wind conditions.

The design is based on a two car fire. In fire mode, the ventilation system is designed to move smoke in the same direction as the traffic flow. This means that vehicles downstream of the fire can continue driving, and leave the tunnel. Occupants of vehicles trapped upstream of the fire are protected from the smoke so that they can be evacuated safely, on foot, as necessary. Fire fighters attending the scene can approach the fire in relatively smoke-free conditions to within approximately 10m of the fire location within the tunnel, as required by BS 7346-7, even when there is a significant opposing wind (modelled at 8 m/s) and a smoke from an 8 MW car fire. No extract plant is required.

See www.coltinfo.co.uk/highcross.html for further info.

17. Detailed design considerations with smoke control systemsfor wide car parks

With a larger car park then it is not quite as easy because there arent the walls there to constrain the fire. The image below is of a typical quite large car park. There is an extract point in the bottom right, air inlet through the ramp at the top right, and at the top left there are some fans providing inlet air which are basically a secondary extract point which can be reversed to provide inlet. This is because in this instance, depending upon where the fire is in the building, we can either extract top left to bottom right or bottom right to top left.

In this case the car fire is within the red area in the middle of the car park and the blue areas are effectively clear of smoke. Rather than creating airflow across the whole width of the car park, the use of selective fans combined with appropriately located extract points enables the spread of smoke to be limited to a particular area of the car park. This makes it very easy for the Fire Service to come in and get close to the fire and start fighting the fire in clear conditions. In this case the system was designed to avoid the need for sprinklers which was a requirement for this particular car park in Scotland.

When designing this sort of system there is a set of basic guidelines that should be followed.

1. The system must be linked to a zoned fire detection system so onlythe zone containing the fire activates. It is important for the systemto know exactly where the fire is to operate the correct fans andso there is more onus on the detection system. So, it is necessaryto identify the zones which are going to be used, and theres a needto link to a smoke detection system which is zoned so that only thezone containing the fire will activate.

2. Typically a smoke detection zone size of up to 2,000m2 and a smoke control zone size of up to 3,000m2 are used, because if the fire is on the edge of the detection zone then the smoke is going to spread a little bit.

3. The extract rate must equal the bulk air movement, not 10 or 12 ACH.

4. It is necessary to achieve the required velocity across the cross- sectional area of the zone some lateral spread should be expected.

5. If the scheme is to permit fire-fighting, the aim is to limit the smoke to maintain 10m visibility everywhere that the Fire Service needs for entrance until they get within 10m of the fire.

6. If the scheme is to aid people to escape, then the aim is to ensurethat there is a safe escape time and generally this is achieved bylimiting the smoke to a relatively small area, so that people canescape from that area and then have as much time as they need toescape from the car park in generally clear conditions.

Case history: a smoke control system in a large car park at Liverpool One

See www.coltinfo.co.uk/paradise-street.html for further info.

Liverpool One is a very large retail development in the centre of Liverpool with a large 4 storey enclosed car park. Above the car park is Chevasse Park, a green open space, into which the designers werent permitted to put escape stairs. Therefore extended travel distances were required which in turn required a smoke control system within the car park, which assists both escape and fire-fighting.

The car park has four levels of parking, the largest of which is about 300m x 100m and each of these levels has about 30 induction fans. Typically each 100 Newton induction fan will cover about 1000m2 and in this case the area is 300 x 100m = 30,000m2 so in fact around 1 fan per 1,000m2 has been used roughly the same as would be required in a smoke clearance system. So having a smoke control system doesnt necessarily increase the number of impulse or induction fans that are needed but it does significantly increase the extract required.

In this instance there are a number of extract points on the north side of the car park which provide a maximum of 200m3 per second and supply points on the south side at fixed locations providing a little larger supply. In this particular case 10 ACH would have equated to about 150m3 per second, so there is not a massive difference between the smoke clearance and smoke control requirements. Naturally if the car park was half as wide or half as long, say 150m, then we still would have needed 200m3 per second extract but 10 ACH would have equated to only 75m3 per second, so the dimensions of any car park have a significant impact on the design.


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The above image is a drawing showing the basic layout of the car park and the general direction of flow. There were two key design objectives:

During the escape phase (0 to 10 minutes)smoke needed to be kept within a limitedand relatively narrow zone, maximum 50mwide and with a plan area of 2-3,000m2, toensure that if anybody that got caught withinit they didnt have to travel more than 25mto be out of the smoke and in a safe areawhere they could then escape at theirleisure.

During the fire-fighting phase (10 to 30minutes) smoke should not encroachbackwards too far, so that the Fire Servicecould actually approach the fire from therear in relatively clear conditions to within10m of the seat of the fire, to assist firefighting in lieu of sprinklers.

North side extract positions

South side supply air

Flow direction

CFD modelling at Liverpool One

In this case the CFD modelling predicted smoke movement and temperatures throughout the car park. In this case because a time-dependent fire rather than a fixed fire size was required, we modelled a growing fire based upon a single fire initially growing up to 4 MW, decaying and then spreading to 2 adjacent fires and peaking at about 9 MW after 20 minutes.

Quite a few different scenarios were modelled in the car parks because different layouts and different floors were under consideration. In the above illustration the fire is shown in red, the little green area is an escape staircase. This was regarded as being quite a difficult condition because the fire was just behind the staircase, which would tend to push the smoke wider, and if a 50m width of smoke could be achieved here then obviously 50m would be achieved in the areas without that kind of obstruction. In this case 6 cyclone fans were positioned in the area behind the fire.


Outline of one of the models showing the extract, supply air, cyclone fan locations and fire position

Visibility at 10 minutes

The above image shows visibility at 10 minutes into the fire and it can be observed that the smoke there was kept within the 50m limit.

Visibility at 20 minutes

At 20 minutes into the fire the smoke hadnt actually spread very far laterally but it had spread backwards at high level, against the flow of the fans.

Visibility at 20 minutes: horizontal section

On the other hand the horizontal section shows that there is a clear layer underneath the smoke at and behind the fire, and therefore there was plenty of access for the Fire Service to get close to the fire with no problems at all.

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Typical fire location

Extract

Fire

Cyclone fans

50m widthindicator

Almost clear

Densesmoke

50m widthindicator

Clear air for fireservice access

Smokelayer

Empirical tests at BRE for Liverpool One

However the above were purely CFD models and because this was such a high profile and large project the authorities were quite concerned as to whether the CFD modelled real life realistically. So to prove that the CFD would model real life Colt built a relatively large scale model of the car park at BRE in Middlesbrough.

Obviously it would not be possible to model either a 300 x 100m car park or a 9 megawatt fire, so a relatively small space was used and the fire scaled down to 1 MW. The test rig was built out of scaffolding and covered by a canvas which was open at either side and at either end. It did not matter that only the Cyclone impulse fans and no extract fans were being used since the objective was to prove that the CFD worked rather than that the system worked.

However it was necessary to conduct the testing only on fairly still days in order to get the test rig to work properly. The rig had natural openings at either end which caused problems during the tests with changing wind directions - in one test, the wind was able to reverse the flow against the thrust of the Cyclone fans. This clearly demonstrated the importance of careful attention to possible adverse wind pressures when designing smoke control systems.


Middlesbrough fire test rig

Middlesbrough Fire Test: Visibility at a height of 2m Middlesbrough Fire Test: 3D visualisation of soot density

So before the test rig was built and tested a CFD model was run which showed where smoke would be expected to flow and with what density. The above image shows that quite dark smoke was expected downstream of the fire, but also that there would be some re-circulation just from the end of the rig so that some light smoke would be re-circulated within that area. This was borne out in the empirical tests.

A BRE-endorsed video clip of the tests is available from www.coltinfo.co.uk/paradise-street.html.

Next, lets turn to consideration of controls - an essential part of any system. For car park ventilation, this subject can be split into two parts, for day to day usage and for fire.

The rig was 30m wide, 35m long and 3m high with a fire on one side basically so that it could be considered as a 2 MW fire, as the test was only looking at one half of it. A fire was set in a burnt out car shell using a tray of diesel fuel sized to provide a heat output of 1 MW. A typical car fire is reckoned to be about 4 MW peak output, but 1 MW was deemed large enough to provide a useful simulation without being a danger to those involved.

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Middlesbrough Fire Test

35m long

30m wide

Fire

Cyclone fans

Dense smokedownstream

of fire

Light smokerecirculated Light smoke

upstream

18. Control systems: day to day usage

The Building Regulations mention 6 ACH for day to day ventilation, an air change rate that is designed to ensure that 30 ppm of CO is not exceeded in quite a heavily used car park. However most car parks are actually not used that heavily for quite large periods of time so if the system is run at 6 ACH then an awful lot of energy is wasted and a lot of unnecessary noise is created.

Its therefore become common practice instead to use CO detection throughout the car park and to use the output from this to decide how at what rate to run the system.

Typically a fairly simple design schedule might be as shown in the table above. If there are very low CO levels then the main extract fans would provide 3 ACH and the impulse fans wouldnt be run at all. If CO levels increased then the impulse fans would be switched on, and if they continue to increase up to 30 ppm then the main extract fans would provide the full 6 ACH, again with the impulse fans running at low speed. This provides a reasonably efficient system that uses a minimum energy but still provides a good level of safety.

The reason that the entire system is not switched off if there are very low CO levels is that there is always a slight concern that if cars are within the car park there could be a petrol tank failure or carburettor leak of petrol, which could potentially cause a flammable atmosphere and therefore to avoid that risk just a minimum background ventilation rate is generally kept going through the car park the entire time. However if the car park is going to be totally empty at night then there is no reason then why the system should not be totally switched off.


19.Controlsystems:usageinafire

The system would normally be activated into fire mode via the smoke detection system in the case of a fire, though sometimes it is done manually by the Fire Fighters override switch. Generally once the system goes into fire mode all the fans are switched over from their day to day mode, and the extract fans run at high speed to provide the 10 ACH.

If the car park is multi-storey at that point the dampers will be closed on all levels except on the fire floor. The impulse fans are not switched on for a few minutes, since in the early stages of the fire there is probably not an awful lot of smoke and what smoke there is will be at fairly high level, because as the smoke is warm it rises and spreads under the ceiling, so it is not a particular risk to people who are escaping for the first few minutes. During that period people escape, and then the impulse fans are started again. They will have the effect of dispersing that smoke but they will also spread it between high and low level so they do actually tend to smoke-log the car park more quickly than if no impulse fans were running. However that adverse effect is more than compensated for by the fact that we are much reducing smoke temperatures and greatly increasing the safety within the car park.

20. Whats next?

Traditional ducted ventilation systems have been almost completely replaced by impulse or induction type systems since their introduction about 10 years ago. They have enabled designers to do more and more: for instance it has been possible to cut out the need for ventilation in the lobbies to the stairs by virtue of showing that negative pressure is provided in the car park outside by using an impulse ventilation system, and there is widespread use of them for means of escape as well as fire-fighting applications. No doubt these systems will continue to evolve to provide further useful applications.

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Main extract Jetstream/Cyclonefans fans

Low CO 3 ACH Off (below 15ppm)

Intermediate CO 3 ACH Low speed(15 to 30ppm)

High CO 6 ACH Low speed(30ppm and over)

About ColtSince 1931 Colt has been harnessing the natural elements to provide healthy, comfortable and safe working and living conditions in buildings. Colt is a specialist in smoke control, climate control and HVAC systems, industrial ventilation and solar shading, with a presence in more than 50 countries.

Designing Car Park Ventilation Systems

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