Fluid dynamics question

[Dax Liniere]

Hey gang,

I have an HVAC system and I'd like to add a filter box to the fresh air inlet. I'm going to fold and weld the box from galvanised sheet and I was hoping to get some advice from someone about the take-offs.

I can get plain (square or flat) take-offs for the input and output of the box or I can use swept take-offs. The filter will be around 300x300mm (TBD).

My question is, would there be an impact on efficiency using the (cheaper) plain take-offs for one of both sides of the filter box? My gut tells me that the swept part might only need to go on one side, but I don't have the fluid mechanics fundamentals to confirm or deny my suspicion.

These are the 2 products:

http://www.has-store.co.uk/images/stories/virtuemart/product/ilflattakeoff.jpg
http://www.has-store.co.uk/images/stories/virtuemart/product/tbsflatshoe.jpg

Help from an experience professional would be great!

All the best,
Dax.

[Billy]

It might be worth having a look at this,

https://www.gingerybookstore.com/product36.html :D

While it's specific to the USA, so it's all in imperial measurements, it contains one of the more concise and clear explanations of the formula's.

Wishing that this had been available when i was at Uni. :D

[Dax Liniere Puzzle Factory]

Cheers Billy.

[Scott Young]

I am now curious about 2 different parts of this thread.

1st up, what kind of HVAC system? I live in Canada and this seems like a very tiny filter to feed a house. The filter on our furnace is about 450mm x 600mm and about an inch thick. We change it every 3 months, having 2 cats. Before our cats we got away with changing it every 6 months.

2nd, is that the same gingery that designed the gingery machine set? One of our members made the set, foundry, drill press, horozontal mill, lathe and shaper. The shaper never really worked well but the others are quite good. 

Thanks

On Fri., Feb. 5, 2021, 4:32 p.m. Dax Liniere Puzzle Factory, <d...@puzzlefactory.uk> wrote:

Cheers Billy.

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[Dax Liniere]

Hi there,

It's commercial premises HVAC and the fresh air inlet that feeds the MHRV is ø250mm. This is where the filters would be fitted, so they're not cleaning the air for recirculation as it sounds like from your application.
Currently the only filtration on the inlet is inside the MHRV and it's about the same surface area.
I wanted to see if I could reduce the frequency of having to clean those filters, so wanted to fit a pre-filter.

If anyone has any information on this I'd great appreciate the input. (flat vs swept take-offs)

All the best,
Dax.

[Erica Calogero]

Hey Dax,

If no CFD ppl pitch in, I might be able to put you in touch with a mechanical engineer who may be able to advise.

Best,

Erica.

[Scott Young]

Ok,

I will apply a disclaimer, I am not an engineer, nor an HVAC installer or tinbasher. I am an electrician who used to build race cars and currently works on small engines.

Adding a pre filter will add flow restriction. The trick is trying to reduce that as much as possible. The easiest way to do this is maximize your filter size. A wider/taller/deeper filter is always better for flow and service life. In an engine airbox, the simple solution is to build a large box that holds the airfilter, then have the intake port inside the box with a velocity stack to avoid surface turbulence. See attached image.

I would go for the largest filter commonly available, use the longest swept take foo to get to the filter that you can, and have a plain square airbox after the filter with a velocity stack inside. 

To view this discussion on the web, visit https://groups.google.com/d/msgid/london-hack-space/835bf77e-5840-4c55-918b-6af14bb0c189n%40googlegroups.com.

[Billy]

"2nd, is that the same gingery that designed the gingery machine set? One of our members made the set, foundry, drill press, horozontal mill, lathe and shaper. The shaper never really worked well but the others are quite good. "

Yes! :D

We built the foundry, and used it to make some of the components of the mini-lathe we used to have.

https://wiki.london.hackspace.org.uk/view/Project:GingeryFurnace :D

I still want to build the rest of the set. :D

I'd be interested in hearing about your friend's experiences. :D

Please pass on my contact details to them.

[Dax Liniere]

Thanks Nevj,

I guess I'll need to give up on the idea of using pre-fab takeoffs. :/ Though I have no idea how I would bend a cone to a rectangle with my workshop tools available. I built myself a pan brake capable of folding up to 1.7m long sheet, but conical seems a bit extravagant.

As far as filter size goes, I don't have HEAPS of free space to install the box inline. The flatter the better, so I might go with something like 300x600mm. I also considered putting the filter on an angle so I can maximise surface area. I might be able to squeeze a 400 or 450x600mm filter in if I angle it right.

Thanks so much for your help, really appreciate it.

All the best,
Dax.

[Des Quilty]

You mentioned welding galvanised sheet...I presume you know that welding galvanised steel will give off nasty poisonous gasses? ...

[Dax Liniere]

Hey Des,

Thanks for the email and thanks for thinking of me. Yes, I'm aware of the arsenic. I always wear a half-face respirator with 3M 2138 P3 filters whenever I'm welding anything at all and this would be no different.

I've got pretty good airflow in my workshop and I figure that since I would only be spot welding (moderate amount of concentrated heat), that there wouldn't be much released.

All the best,
Dax.

[Toby Catlin]

It the zinc oxide that can cause you serious problems.

https://en.m.wikipedia.org/wiki/Metal_fume_fever

I don't know if your respirator is up to the job but this was one of the key safety points on the welding course I went on. He advised to never weld galvanised and if you must grind away as much zinc as possible.

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[Des Quilty]

Wearing a good filter mask, ventilation, grinding away the zinc are all good recommendations.

And there is also the possibility of using other joining techniques, perhaps the humble rivet might do the job....

[Dax Liniere Puzzle Factory]

Fun fact! Did you know that drinking milk before being exposed to zinc fumes minimises the symptoms? Crazy, right?
But yes, always wear a suitably-rated respirator.

When it comes to respirators, having a beard is another matter, though.. :/

Dax Liniere
Record producer | Mixer | Engineer

London, UK

Phone: +44 (0)747 0747 750
www.puzzlefactory.uk
www.MixDirection.com

We are 100% carbon neutral

UK company number: 10187067

[Scott Young]

Having lots of copper in your diet also helps.

Metal fume fury is what you get from zinc poisoning. It is throughly unpleasant. 0/10 would not recommend. 

On the plus side, it doesn't have any permanent side effects and as long as you are not doing it frequently or in large quantities it isnt a massive issue. It is really more of a chronic issue for industrial workers. I have used a spot welder for small galvanized tin work, just kept the door open and ventilation going and didnt have any issues. I accidentally added zinc to a pot of molten aluminum in my early casting experiments and found myself with the shakes and nausia for about a day.

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[Dax Liniere]

Hey Scott,

Ahh, that's very interesting about the copper. I guess the zinc is bonding to it in some way. It's interesting how chemistry in the body works.

Cheers!
Dax.