Is carpark exhaust air obnoxious?

[paulkeating]

Here in Australia, our main ventilation code applies extra requirements for the siting of exhausts from what they term "objectionable" or "obnoxious" systems (ie discharge point must be vertical and at minimum 5 m/s and cannot terminate any lower than 3m above the roof if it is a flat roof, or 1m if it is a pitched roof). However, it doesn't say if carpark exhaust air is to be considered objectionable. And seing as we have lots of underground carparks with mechanical exhaust, well this leads to all sorts of discussions - developers want it treated as NOT objectionable (so they can save roof space for the swimming pool or a larger penthouse!!) but some city councils argue the contrary case and say that it carpark exhaust will not be "properly" dispersed if it isn't vertically at the roof at 5 m/s ect etc. What rules have you guys got elsewhere? Any Code websites that may tell me more?

 

[ChasBean1]

Paul, does the exhaust ventilate continuously or is it based on CO? CO can creep up to maybe 50-100 ppm during short stints in a garage (e.g., if everyone tends to start their car and leave within the same time period) but the exhaust concentration would be well diluted by the time it reached an intake, so long as it is reasonably spaced. By this I would mean maybe 25-75 feet.

I think that no matter what dilution model you use, even to dilute diesel exhaust (what percentage of the cars are diesel?) down to its odor threshold, the exhaust point should not require the stack height and velocity you mention so long as it is reasonably spaced from any intakes.

If you took your building out of the ground, parking garage and all, and put it on flat grade without exhaust ventilation, where would the exhaust go? To me, this is an argument to discharge it back onto the street where cars go in and out, as long as there are not any intakes near it.

Our code (US - MA) has fairly complex equations for source to receptor distances based on dilution - I don't have it nearby at the moment. But I would interpret "objectionable" or "obnoxious" exhaust as being a more concentrated stream such as sewer gas, direct combustion exhaust, fuel tank venting, etc. This exhaust stream consists primarily of ambient air mixed with traces of combustion exhaust.

Do you agree? Best regards, -CB

 

[paulkeating]

Chas,

Thanks for that reply - I'll look at it in more detail later on. essentially my question is on of legislation and so on, not so much a question of science.

In a nutshell, at the end of a project, I have to sign a certificate which says that the HVAC systems (including some fire/smoke systems) have been designed in accordance with the Building Code of Australia (BCA). The BCA says (and I paraphrase for the sake of brevity) that I must design ventilation systems either (a) to provide a safe and healthy environment or (b) in accordance with AS.1668.

Now, 1668 is the code that talks about obnoxious and all that and whilst it says thatsomething that contains asphyxiant or carcinogeni or offensive etc etc etc components is "obnoxious", it doesn't say whether or not it deems the concentrations of these things in carpark exhaust to be bad enough to warrant the "get that exhaust as far away as possible from all windows and intakes etc etc" approach.

So, from a legislation point of view, I'm in a quandry re 1668 - how can I say if a system complies with it when it itself is not explicit?

So I take the alterantive tack and try to second guess what it WOULD say if it WAS explicit, hence the need for a bit of research on legislation elsewhere (and not so much the science, which is the same here as it is elsewhere, last time i checked!!)

So I'm looking for the LEGAL obligations imposed on the siting of carpark exhausts.

Paul

 

[ad77]

The latest AS1668.2, 2002, gives a bit more information on Type A & Type B effluents (with some additional commentary). But, does not clarify the matter outright.

However, the 2002 edition has not been adopted by the BCA in its latest ammendment.

I suggest that you develop an engineering solution which you are comfortable with and approach the approval authority (whether it be council or a private building certifier). They usually are guided by the design engineer on these matters and are open to interpretation. Which in this case would be appear to be appropriate.

I have seen both low level and roof discharges adopted as accepted solutions.

Good luck.

 

[paulkeating]

Thanks Anthony - and where is it you work?

I'm aware of the AS.1668:2002 issue re not being gazetted yet and yes, I have thought about going down the BCA96 alternative solution path however it's only really ever going to be a subjective matter unless we do wind tunnel test and so on, and that's way off the scale of the problem. Standards have finaly today told me that they consider carpark exhaust as NOT being obnoxious (apparently I asked the same question back in 93 or sdo when I was at Arups!!)

With regard to PCAs and Council, I don't think they ever adjudicate - they don't say whether or not your alternative solution makes sense or not, at best all they will do is check whether or not you have gone through the correct sequence of steps in preparing your argument. Ultimately, under self-certification, you could submit an argument that the earth is flat and as long as you've followed the right structure in your argument, well council will accept your solution!!

Paul

 

[ChasBean1]

Paul, as a followup to previous post, our code indicates:

S = 0.04 Q^.5 [D^.5 - (V/2)]

S is separation distance in feet
Q is the exhaust air volume in CFM
D is the dilution factor (code says 25 for garages)
V is exhaust air velocity in feet per minute based on direction; negative if directed toward source, positive if directed away, 500 fpm if hot gas exhausting up. Use V = 0 if it's not a directional exhaust (like if it's a mushroom ventilator)

(the V part is a little confusing to me - the numbers go crazy if you use the V)

So per our code, if I have a 3,000 cfm garage exhaust my allowed distance to the nearest intake is 11 feet.

This is crazy to me, but it tells me that codes are not well developed in this area and you're left with the engineering of it. You mentioned that "Standards have finaly today told me that they consider carpark exhaust as NOT being obnoxious..." says one person. Can you get it in writing?

Anyway, best of luck, -CB

 

[ad77]

Paul,

It is interesting that Standards Australia have been asked the question regarding 'objectionable' / 'obnoxious' nature of car park discharges numerous times and have not clarified the answer in the latest edition of the standard. I am positive you are not the first person to ponder this matter, it has bothered me numerous times.

Finally SA have the same terminology and have dropped the confusion between 'objectionable' / 'obnoxious'!

I have seen some very subjective and qualitative reasoning (especially in the design of smoke hazard management systems claiming to use fire engineering principles) which, the design engineer had no issues presenting and the building certifier had no problems accepting. Most of the time it is based on rhetorical anecdotal evidence with little scientific / engineering justification.

By the way I used to work as a consulting engineer in Melbourne & Sydney and now have moved to the dark side - contracting.

 

[paulkeating]

ChasB,
Thanks for that piece of code - I'll apply it to a few situations here and see what it throws up.

Anthony,
I too have come over to the dark side - my days at Arups and Lincolne Scott and a few minor players in between erased all respect that I used to have for the quality and ethics of consultants and I decided it was time to earn my crust an honest way and had a spell at Environ (until Tyco killed it) and now run the engineering at JL Williams. Game-keeper turned poacher, rather than the usual reverse, and it annoys some consultants to deal with me, I think it's because I don't afford them the automatic respect that they think is their traditional right!!

 

[ad77]

paul,

its been along time. however, something you stated in one of your posts stuck with me.

how did you get in contact with standards australia to get a response on your particular question????

 

[paulkeating]

Anthony - yes, it has been a while. The guy to speak to at Standards is Boris Krastev - I'll dig out his direct line but you can also drop him a mail - in many respects, the email is the way to go because he can then simply farm your query out to the various members of the appropriate committee. Try him at Boris.Krastev@standards.com.au . I've made more than one suggestion to him re stuff that is unclear and could warrant a bit a of re-writing, and he's generally very happy to receive suggestions from industry.

And Chasbean, I know this particular post is a bit parochial ... sorry that it's not relevant to the good ole US of A ....

 

[ad77]

thanks for the prompt response

 

[friartuck]

This answer might be a bit late, but anyway, In the UK we ventilate to 6C with the system running on low duty continuosly until initiated by a fire alarm or CO build up. The high rate is 10AC.

The exhaust is not classified necesarily as contaminated since the CO level is deemed to be controlled to under the EH40 TLV value, however the local codes may disagree. The main concern is in fact the fire condition. The exhaust has to be dischrged to a safe area i.e. high level or away from escape routes.

Also ductwork has to be fire rated.

Drapes

 

[paulkeating]

Thanks for that, mate. My problem (which has since been resolved) was more one of legal/administrative detail rather than one of science and common sense. I finally got a letter from our standards body here confirming that their codes assumed that carpark exhaust discharges are "not obnoxious" (again, based on their own definition of obnoxious). I was collecting information from overseas just in case they judged otherwise and I had to go back to them to argue a contrary viewpoint.

Interesting that your rules keep your carpark vent running on low speed - ours allows us to STOP the ventilation IF you meet certain limits re CO levels (used to be CO and NO2 but is now CO only, probably becuase of the decrease in the number of diesel vehicles in our typical of vehicles). So we monitor the CO constantly, run time-weighted averages, and if you are consistently below various benchmarks, well they allow you to stop the ventilation - it's an energy-saving thing ... - works well in large underground carparks with very low usage factors and where you have a huge ventilation system that even on low speed will still easily deal with the emissions from the one or two car movements an hour that you might have ...