Air Handling Units for multistorey buildings in Brisbane City 1

[geopat69]

hi All

I am not a mechanical engineer and work in an office building in Queen street, brisbane. i have a couple of novice questions for my own education... so forgive my ingorance. My questions are related to air handling units (which are located in a plant room, on each floor of the office building).

1) Firstly, do all air handling units have two - stage filter?

2) As I understand, typical AHUs draw in some outside air (I think 20%?)... So assuming a typical office, how good are the filters at removing pollutants from this outside air?

3) I read that there are several Australia standards related to AHU filters and the selection of filters. How does one determine the grade of filter... i mean surely there is a guide? Specifically, i would be interested in knowing what grade (G1, G2 etcc) of filter a typical office building in brisbane city would have.?

4) Finally, assuming the worst case (i.e lowest grade) of filter that would be used in brisbane city , can someone advise how small are the particles from the outside air it will remove?

regards

geopat

 

[Nekoto]

Have you read this:

AS 1324 Air filters for use in general ventilation and airconditioning.

 

[geopat69]

hi There.

Thanks for that response. But that code does not address any of the specific questions

 

[CMCCJRS]

Hi geopat69

My responses to your queries are below

1) Firstly, do all air handling units have two - stage filter?
Not always but generally in your building they would. We couldn't be certain with out seeing the plantroom or a drawing though

2) As I understand, typical AHUs draw in some outside air (I think 20%?)... So assuming a typical office, how good are the filters at removing pollutants from this outside air?
Yes they do generally draw in outside air. The effectiveness of the filters will vary. They are never 100% perfect and it will depend on how old the system is, how often it is maintained and what grade of filter the unit has.

3) I read that there are several Australia standards related to AHU filters and the selection of filters. How does one determine the grade of filter... i mean surely there is a guide? Specifically, i would be interested in knowing what grade (G1, G2 etcc) of filter a typical office building in brisbane city would have.?
The typical arrangement for something built now would generally be a G4 panel filter for debris and dirt followed by a F7 bag filter for dust and particles.

4) Finally, assuming the worst case (i.e lowest grade) of filter that would be used in brisbane city , can someone advise how small are the particles from the outside air it will remove?
An F7 filter is expected to achieve an average filtration efficiency of between 80-90% for particles down to 0.6 microns (µm) when tested to AS1324. From the attached image you can see that this good for fumes and dust etc.

There is a lot of discussion in the industry at the moment regarding the effectiveness of building ventilation systems and also the risk of aerosolized virus particles transferring through the air conditioning system. I'd be happy to talk with you directly on this if you wanted to discuss more.

Thanks

 

[geopat69]

hi CMCCJRS

That was an excellent explaination to a novice like myself. Thank you for being patient and explaining so well.

If i could ask one final question in regard to your comment about some AHUs possibly only having one filter...

If that were the case (ie one filter only), would they be fitted with a bag F filter only, or, a G Panel filter only?

I suspect the G4 panel does not have a micrin rating of particles size removed?


***Also, any clarification on which types of buildings would not require 2 stage filtration would be appreciated. My building happens to be a 25 storey concrete building (open plan cubicals) constructed in mid-eighties.. i suspect the AHU may be the original ones from then.


thankyou again

 

[CMCCJRS]

Hi Geopat69

No worries on the explanation. Happy to help.

There is no one standard that determines which buildings have what extent of filtration. For example, there is no filtration requirement in the Property Council of Australia Office Grading standards. The filtration amount is determined by the design engineers and AHU manufacturers to meet what they think is required to keep dust and debris out of the AHU and the duct system.

I think in the mid 80s there wouldn't have even been a filtration standard to meet. However it's likely that this instance would have a 2 stage filtration set up as this is a typical arrangement for an AHU system for an office building of that size. A two stage filter arrangement on an office building that size would be typical going back to 50s/60s so I would anticipate that you have it in your case.

Just to answer your question, a G4 panel filter is only approx 60% efficient for 10micron particles.

Cheers

 

[geopat69]

CMCCJRS you have been a great help. thank you for helping.

Its a good point you mentioned regarding the filters for the AHU unit from the eighties. i suspect since that time, the filters would heve been replaced dozens of times and at the very least, i suppose a modern day G4 inserted (although not confirmed). Not sure about the second filter level... but from what i see from your reply above... even an F7 is quite impressive at the filter level it achieves.

 

[SAK9]

Modern AHUS require two stage filtration typically G3 plus F7 because they use more efficient cooling coils. The more efficient cooling coils have closer fin spacing and therefore require better filtration so as not to get clogged with dust in between the space between fins. As for the possibility of virus spread through air in my view it would be far better to increase the ventilation rate in office buildings than trying to improve filter efficiency. One of the ways to achieve this would be to increase the design temperature band wider and allow the AHU to operate on economy cycle more.