MVHR Basics

[Ed Davies]

10²?

 

But won't the allowable flow velocity be lower with a long narrow opening than, say, a round pipe?

[Jeremy Harris]

  On 14/02/2019 at 11:33, Ed Davies said:

10²?

 

But won't the allowable flow velocity be lower with a long narrow opening than, say, a round pipe?

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I think it depends on the Reynolds number of the slot, and whether the flow velocity over the length of the slot is enough to trip the airflow from laminar to turbulent (essentially whether the critical Reynolds number is exceeded).  My gut feeling is that 2.5m/s is still below the velocity needed to trip the flow through a slot like this, though I could have a go at doing a bit of math to see if this is likely to be the case or not (not really done anything at such low air speeds before).

 

[Ed Davies]

Yep, it's not obvious. I'd just be a bit cautious about using a fixed velocity irrespective of the shape of the opening. It's something I wondered about back when I was thinking of doing my own MVHR exchanger but didn't come to any useful conclusions.

[Moonshine]

  On 14/02/2019 at 09:28, Bitpipe said:

IRC, he said that low frequency sounds are very hard to prevent.

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Yes, particularly in timber frame, which basically doesn't have the same mass per unit area as masonry that is effective at insulating against low frequency. It is also really dependent on the detailing of the design, and workmanship. Sound insulation of timber framing can be a complex matter that i won't go into in a MVHR thread.

 

@PeterW a solid sealed core door won't so much 'transmit' the sound, but impinge its transmission more than a hollow core door.

 

FD30 fire doors, which are solid core with seals at the jambs but not the threshold (small gap, though if a true fire door / smoke requirement couldn't have the gap) will probably achieve Rw 25 dB, and if a threshold seal around Rw 30 dB.

 

Lorient have a very good database of how there acoustic seals perform based on different door / blank manufacturers.

 

http://www.lorientuk.com/acousticsearch

 

Building regs for entry doors in Rw 29 dB, so i can't really see what would have the need for internal door sets of >=Rw 25 dB, @Sjk what is your sound insulation needed for, kids play room, music room, adult dungeon?

[Jeremy Harris]

  On 14/02/2019 at 11:47, Ed Davies said:

Yep, it's not obvious. I'd just be a bit cautious about using a fixed velocity irrespective of the shape of the opening. It's something I wondered about back when I was thinking of doing my own MVHR exchanger but didn't come to any useful conclusions.

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Just done some quick estimates using some old experimental work from the 1950's.  Looks like the critical Re for flow in a slot could be a bit lower than I'd assumed.  I'd guessed that Re would need to be around 10,000, but it looks like I was out by a lot; the critical Re for flow in a slot of the same order of general dimensions as that under a door looks to be around 2,500 or thereabouts.  For a 30mm wide door that means the flow velocity needs to be below about 1.2m/s to be reasonably confident that the flow will remain laminar.  That's assuming that it's primarily the flow tripping from laminar to turbulent that is the primary cause of noise (I suspect it is, based on the hours I've spent inside wind tunnels years ago)

[Bitpipe]

  On 14/02/2019 at 11:33, Ed Davies said:

10²?

 

But won't the allowable flow velocity be lower with a long narrow opening than, say, a round pipe?

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This is why I got fired from NASA. Apparently getting 'quite close' to Mars is not good enough...

[jack]

  On 14/02/2019 at 13:05, Bitpipe said:

Apparently getting 'quite close' to Mars is not good enough...

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Especially when you were aiming for the moon.

[Gone West]

  On 14/02/2019 at 09:08, JSHarris said:

I did also post that this would probably be OK in a bigger room:

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It would be interesting to know how the shape as well as the size of the room affects the supply and extract interaction. Our kitchen/diner is only 6m x 3.2m.

[Jeremy Harris]

As long as the supply and extract ducts are a reasonably long distance apart I doubt there is much real interaction.  The air velocity coming from the terminal(s) is pretty low and will slow down a great deal within a couple of metres from the terminal, so for a room with a cross section of 3.2m x 2.5m, with a single supply terminal running at 8l/s then very roughly the flow velocity a couple of metres or more across the room will probably be down to around 0.001m/s or so.  This is slower than sort of typical air movement velocities associated with micro convection within a body of air.

[Ed Davies]

 

  On 14/02/2019 at 12:02, JSHarris said:

For a 30mm wide door that means the flow velocity needs to be below about 1.2m/s to be reasonably confident that the flow will remain laminar. 

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30"? 760 mm? But, yes, that's the sort of difference I was expecting. The differences in air leakage between a round hole and a long narrow slot of the same area are similar orders of magnitude.

[Jeremy Harris]

  On 14/02/2019 at 15:57, Ed Davies said:

 

 

30"? 760 mm? But, yes, that's the sort of difference I was expecting. The differences in air leakage between a round hole and a long narrow slot of the same area are similar orders of magnitude.

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Sorry, I meant 30mm in the sense of Re, i.e. the thickness of the door being L.  Should have written "thick" rather than "wide", but my brain was still in Re equation mode, and thinking of the view perpendicular to the direction of air flow.