HVAC location of exhaust grilles

[Vince79]

Hi everyone,

I need your help again.

I'm having trouble about locating my exhaust grilles in the room I'm designing the HVAC for.

I'm worry about the air stratification because of the temperature differential (ie hot air is lighter than cold air)

If the air I'm supplying to the room cools the air inside most of the time, then should I locate the exhaust grilles at the highest level possible?

Because if the HVAC supplies cold air, then it will have the tendency to "stay" near the floor so by putting an exhaust grille near the ceiling, it will force that air to "go up".

Am I right?

 

[Ziggypump]

That's a pretty common design standard, for me at least. Try to keep the supply and exhaust grilles as far apart as necessary.

Is the room going to be positively or negatively pressurized? That may help determine the answer as well.

 

[Vince79]

Thanks for your quick answer. The room is going to be positively pressurized.

I should have said that the airflow is supplied by textile duct diffusers (but I don't know if it changes anything).

 

[tys90]

In general, yes, that would be the correct location. In areas like hazardous prep rooms in pharmacies and operating rooms in hospitals, you want the air to travel in one direction, almost always ceiling to floor. You don't want turbulence stirring up particles in these rooms and airflows are so high that stratification is not a problem.

 

[Zesti]

If the air change rate is less than 10...20 times/hour, then the location of the exhaust grilles does not matter much.
The air movement in the room will be turbulent and mixing of supply air and room air will be the dominant effect.

If it is laminar flow you want in the room (cleanroom, operating room) then you have higher air change rates and you want the exhaust to be located low.

The supply of the air is what counts if the air change rate is relatively low and the flow is turbulent.

The effect of an exhaust grille will not be noticeable at a short distance from the grille. Do the math yourself: calculate the air velocity through the surface of an imaginary half-sphere with a radius of 0,5 metres.
This velocity will be low and so of no consequence to the actual air flow patterns in the room.
Supply grilles located flush in the ceiling often make use of the coanda effect to make the air travel further away from the grille and mix better.

Or, burn a candle and try sucking it out instead of blowing it out...

 

[Vince79]

Ok, well thank you Zesti for your answer. It is very clear.

The air change rate is just 3 volumes per hour so I won't have to worry about the location of the exhaust grilles.

Thanks everybody

 

[Drazen]

Zesti is correct in general, but in your specific case, textile all-along duct diffusers together with very small air exchange probably leads to very low supply air velocities even at outlet plane.

That can create sort of laminary pattern with very strong influence of thermal rise power, so I would strongly suggest that you put exhaust high, and supply should be preferably low, so thermal rise and your equipment will "work together" to drive air quantities through the room.

 

[Zesti]

If you only have 3 air changes per hour then a textile duct does not seem the right choice to me.

I would choose supply grilles that can still generate some mixing of supply air and room air.

A textile duct will not be able to do this at this low exchange rate, as Drazen already mentioned. You need a textile duct when you have to supply lots of air at a relatively large temperature difference in which case draft might be an issue with regular grilles.

Unless you have other resasons for choosing a textile duct, I would consider choosing supply grilles.