Questions and answers from Conor Logan's Car Park Ventilation webinar

Posted by Conor Logan on 17/09/13 13:36


Car Park Ventilation:  Q&A

First of all a big thank you to those who attended my webinar last Friday on 'Car Park Ventilation'. If you missed it or would like to watch it again, a recorded version is available here. Many questions were asked after the presentation and I decided to post them here for everybody’s benefit.

 1. Can you please elaborate on the ways to test and commission an induction fan system? 

That is a very good question! There are a number of things that need to be done during the testing and commissioning of such a system, quite apart from the normal electrical tests that you would do with any fan system.  I do not want to go into any detail on those normal routines here, because they are fairly common for any extract system. 

You do need to prove the extract rate you are getting is what you said it would be, so we would normally carry out either a pitot scan or any scan of the extract duct to prove you are getting the extract volume rate that you have designed.

In addition, in order to check that the analysis that has been carried out to show the performance of the system is going to be adequate to keep the space clear of smoke, we would possibly apply a smoke test. However that is relatively rare with smoke clearance systems: it is more usual with smoke control systems and (depending on the size of the car park and the objectives of the systems) that would consist of providing smoke generators around the space allowing a quantity of smoke to develop to activate the system to show the car park can be cleared of smoke quite efficiently.  We tend to apply a full smoke test in that manner to around one in ten car parks.

2. We are looking at a mixed use retail event with an undercover semi-enclosed car park.  Can we use a smoke control system with induction fans to compensate for the omission of sprinkler systems?

Yes - potentially you can, and we have done that in the UK.  Clearly you cannot show that the impulse ventilation can extinguish the fire, but you can demonstrate very good access conditions for the fire brigade to get in and put the fire out. So if the purpose of the sprinklers is to improve fire access, then that can be achieved with a smoke control system, but I would suggest that it would have to be a substantial car park to make that beneficial as the extract rates and the complexity of the system go up quite significantly. 

3. How would fire risk assessors assess the effectiveness and compliance of venting systems? 

I think that goes back to the commissioning and the approvals process.  If you do have a computational fluid dynamics model of the car park, certainly you should make sure that you have no stagnant areas within the car park space, so you have good air movement and good air velocities within the space. 

If the CFD analysis has shown some tracer particles around the car park, and most CFD packages allow this facility, they should show that you have good mixing and you do not have a build up of any traces in any one location.  Likewise, once the system is installed, you can check that the fans are of the correct duty that have been used in the simulation, and that the extract rate is performing at the duty specified. If you do either velocity or volume flow checks at the extract points, this will demonstrate that you are getting an effective system.

4. Will the slides be available on the website?

5. Should an impulse system always be reversible?  What if the car park is on a sloping site?

No it would not ordinarily be reversible. Occasionally we have had naturally ventilated car parks where we have used reversible impulse fans to direct fumes or smoke out of one side or the other, depending on which direction the wind is blowing, and that has enabled us to provide an effective system without a central extract plant. 

There are some instances where it would not necessarily need to be reversible to achieve that objective, but in terms of the design in those instances you should consider all possible wind directions and the possibility that the wind may override the performance of the impulse fans. 

6. You stated a 10m distance where visibility up to the source is absent.  At what height off the floor is this measured?  Is it correct that a light omitting source is assumed? 

10m visibility is specified within BS7346 part 7.  It does not specify at what height that would be, but normally if you are looking at facilities for firefighting you would say a minimum of 1.5m - 1.75m from floor level.  If you are looking for firefighter access, then yes, I would think it is fairly sensible to assume a light omitting source because they are trying to locate the fire and deal with the fire effectively, so a light omitting source would be a safe assumption.

7. Are you suggesting that a CFD model is always created for an impulse system? 

No, I would not say that is a given: there is not always the requirement for a CFD model if it is a straight forward car park with a relatively good distribution, relatively narrow extract at one end, inlet only at the opposite end and with a few impulse fans in between. 

Quite often you can fairly safely predict what the airflow patterns are going to be within a space like that and there certainly is a case for not carrying out CFD in that type of application for smoke clearance or general ventilation purposes, but anything more complex then I would suggest that you need to look at something other than CFD.

8. How do you determine the number of jet fans needed in a car park?

The initial estimate is based on the plan area of the car park.  As I explained in the presentation, assuming we are using jet fans, if one jet fan covers 400m2 usually at a height of between 2.5 and 3m, then quite clearly one fan every 400m2 would give you the number of fans you need to cover your car park. 

Likewise with the 100 Newton induction fans, one fan every 1,000m2 would give you a safe initial estimate of the numbers, and then any others to cover any dead spots within the car park would need to be added in and that is more or less something you would do from experience. Anything more complex would need CFD modeling. 

9. What is the major criterion in selecting jet fans? Air volume or thrust?

Normally it’s thrust.  Depending on the air velocity, a 50 Newton fan is usually going to cover around 30m of throw down the car park and 400m2 is fairly safe. We have done velocity traces and scans of our fans and we have a rough idea of their coverage, but we cannot vouch for all kinds of products.

10. Servicing periods – is this dependent on the size of the car park and the recorded usage of the fans?

In England and Wales normally the guidance would come from BS 9999 which recommends a weekly test and obviously any faults reported need to be dealt with from that test.  That would obviously be a fairly limited test.  We’d recommend a quarterly full test, and then an annual maintenance service where all the manufacturer’s recommendations should be taken into account.

11. Would we need to carry out a cold smoke test to test the installed induction system in a car park with a smoke control system of some 800 cars? 

That very much depends on the view of the approving authorities and the complexities of the system.  The cold smoke test only shows you so much.  It only shows you how you can dilute the smoke within the space: although it is not going to give you a comparative test against anything significant, it will give you a good steer as to how well the system can perform in a real situation. Although a cold smoke test looks quite impressive, it does not indicate how hot smoke might behave.  We at Colt use software to size car park extract or ventilation systems, but that’s generally for internal use only. 

12. What type of smoke test can we use to commission an induction system as logically we cannot do a live fire test in an operational car park?  

Correct, we generally use cosmetic smoke for those purposes.

13. What CFD program was used to undertake Colt’s modeling? 

Generally we use FDS (Fire Dynamic Simulator) at Colt, though occasionally we use Autodesk Simulation depending on the source of the car park data.

14. Can I use Colt induction fans for smoke clearance from a shopping mall?  What is the maximum smoke zone area allowed? 

I generally would not recommend using them within the mall because what you are looking to do is keep the smoke above people’s heads so they can evacuate safely.  You will find that induction fans tend to blow smoke in a particular direction horizontally along the space, so you might find that at one end of the mall you have a smoke logged area, albeit you have clear conditions upstream of the fans, so it would not be a recommended application.

Conor Logan Conor Logan is a Technical Manager of 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: Smoke Control, CFD, Smoke ventilation, Car Park Ventilation, CPD