Carpark Natural Ventilation

[Joseph156]

Hello from a first time poster from Australia

I am an HVAC design engineer currently working on a project where there are a number of large open deck carparks. Our definition of open deck is generally an above ground carpark that is substantially open on all sides. Our local ventilation standards have diagrams that define the various perimeter wall opening scenarios that allow the carpark to be considered as naturally ventilated. These diagrams show the minimum perimeter wall free area requirements for number of scenarios – two opposite open sides, one open side and two adjacent open sides. Very, very generally 50% free area of the perimeter wall is required for natural ventilation. Normally we would apply these various diagrams to the carpark geometry to establish the extent of natural ventilation.

In this instance the perimeter walls need to have a very high concrete upturns to stop cars crashing though and to comply with safety requirements (mitigate people from climbing over upstands). As a result, the design is below the general 50% free area requirement- closer to 40/45%. Instinctively, we feel that with essentially all 4 sides open this should be more than adequate to provide natural ventilation to the carpark even with less than 50% open are perimeter walls. As a counterpoint the Fire Engineer had advised that the minimum free area for passive smoke management was only 30%. We currently have proposed the use of impulse / jet fans in these carparks but would rather rely on natural ventilation for obvious environmental reasons.

Can anyone advise of any published methods / models where natural ventilation to carparks can be assessed in open 3 sided and four sided scenarios.

Much Appreciated

PS: We have approached a number of “expert” consultancies and they were very reluctant to commit to modelling outside of our Ventilation Standard.

 

[317069]

it is not about feeling or even engineering, it is about code and legal issue for the future, local code says minimum 50% of free area, it is clear, you have 50%, no problem, you have 49.99% you are against the local code, use code definition not your company or client definition. you can talk with the local authority, they are the only one who can decide for such issue

 

[Joseph156]

Thanks for the reply 317069.....................

However our codes allow for engineered solutions as an alternative to prescriptive applications of the codes. Our design practices here routinely apply alternative solutions to HVAC design - as long as there is solid justification.

This is especially true with carpark ventilation systems. The diagrams I spoke of above are dated from 1979 so engineering and modelling has moved on from there.

So what I seek is some form of model / diagrams / rules that would apply to a four sided open deck carparks

 

[317069]

Joseph56..........................................................................
Engineering principals are the same in 79 and present, there is nothing called 3 sides or 4 sides natural ventilation, there is a flow rate should be ensured to a space by a natural or mechanical mean.

 

[Joseph156]

Yes but

Engineering capability has advanced in leaps and bounds since 1979 - I am especially referring to CFD ventilation analysis. TBH I have not designed a (prescriptive) code compliant carpark ventilation system for over a decade. They have always been alternative engineered solutions using CFD analysis results as the acceptance criteria, this included two carparks over 60,000 sqm. When I called upon my usual go-to CFD consultants (including ARUP - of all people) they were a little shy in tackling such an project. Which is surprising

 

[willard3]

Call the CFD guy to model this.

Engineering principles haven't changed much since 1979.

 

[Joseph156]

When I asked the CFD consultants about natural ventilation they were reluctant to commit as CFD in carparks is reliant on a driver – that primarily being velocity of the ventilation and air mass transfer. Whereas natural ventilation is far more nuanced and complex as it will be influenced by internal / external temperatures, entropy, external wind (speed/direction) and external topography, and how each of those attributes interact with each other. On reflection it is not surprising that the CFD consultants are not willing to risk their PI on such complexity.

When I first started in the industry we ventilated carparks based on a blanket minimum of 4AC (how’s that for considered engineering ??). Then we moved to a table based on no. of cars and usage types (commercial, retail, etc). Then we moved a more complex formula that looked car numbers & movements, usage types, lane lengths, exiting and entering conditions. Nowadays we use CFD analysis based on the expected fleet age (to estimate emissions), plug in the traffic engineer’s report for traffic movements, look at the expected exposure times for the likely occupants to establish the acceptance criteria. We can then play around with the air quantities and distribution to optimise the ventilation system design. So all in all I do think Engineering has moved on from 1979. And during all this time of significant engineering advancement the same Natural Ventilation diagrams have been in our Standard. There is something inherently wrong with this.

One of my past designs was a for a very large carpark for a retail centre. It was over 70,000 sqm, with a ~500,000 l/s exhaust system. The exhaust was arranged in a “donut” in the centre with the make-up being essentially natural on all four sides even though the carpark was partially underground. All controlled via a zoned CO system. The distances between the openings and the grilles in some cases was well over 200m. When we examined the usage of this ventilation system over ten years operation we find that it hardly ever runs and this is one of the busiest retail centres in the country. What tells us is that this carpark is predominately naturally ventilated even though geometry of this carpark is well in excess of those natural ventilation models.

In this environmentally conscience world I do think that providing passive systems is where the application of innovative HVAC engineering should be heading. I am quite surprised that in this instance there seems to a lack of information in this area. I do not find unthinking blanket adherence to prescriptive Codes to be real engineering.