Paging Brad White!

mtfallsmikey

The Coanda Effect.

Brad White

Oh dear....

The Coanda effect as it affects me in my work, is usually discussed and applied to air diffusers.

The Coanda effect itself is the tendency for a moving airstream to cling to an adjacent surface. An aircraft wing is one example.

For an HVAC/Diffuser example, when a ceiling air diffuser discharges air along the ceiling plane, there is a tendency for that airstream to hug the ceiling. This in turn carries it further away from the diffuser before falling to the lower parts of the room.

A secondary effect with this is to have "secondary" room-temperature air entrained into that moving airstream which at once mixes with it to equalize and absorb room heat ("why we do air conditioning") and this in turn minimizes drafts because when the air does eventually fall, the temperature is elevated somewhat and has traveled further out into the room. Still cooler than the room, but by less.

By contrast, the "lack of Coanda Effect" could be demonstrated/illustrated by a sidewall diffuser set down some distance from the ceiling, or by a "ceiling diffuser" set on an extended neck below the ceiling.

When air is so discharged and there is not extended surface on which to cling, the discharged air tends to drop or "dump" directly onto occupants. (This is especially pronounced with VAV systems when the air volume and velocities drop below critical levels.)

When such low-elevation ceiling diffusers are applied, an extended panel is usually provided to give at least somewhat of a surface for the Coanda Effect to take place. Better than nothing.

Another tip for inducing a Coanda Effect when using sidewall grilles or diffusers is to aim them upward (assuming a foot or so below the ceiling. This allows the ceiling to act as the final diffuser, much as it should.

How was that?

There is also the Anaconda Effect, basically a squeeze when you run out of time.

mtfallsmikey

Right on!

The subject came up the other day whilst I was watching a Trane VAV Webinar...I guess the secondary effect in my case is initiated in the parallel VAV boxes that I have around the perimeter of my building, which injects air from the return plenum. (??)

Brad White

Parallel VAV

If you mean fan powered or induction boxes, yes, the return air ceiling plenum is a reservoir of "free reheat". Heat from lights is recovered as a first stage of heating. It is not "injected" per se, but is drawn in under negative fan pressure (in a fan-powered box) or induced (in an induction box, into the Venturi, another effect we can discuss...)

The Coanda Effect as we have been discussing happens on the room-side though. Not above the ceiling, at least in a way that benefits you.

The use of plenum returns in this way is not new and has been used since the 1960's if not earlier. Personally, I think it is going to go away is it creates negative pressure imbalances within the building. Also as lighting becomes more efficient, there is less waste heat to harvest. But it really is probably here to stay because it has a lower first cost with less return ductwork.

mtfallsmikey

Thanks!

Also, all of my fan-powered VAV boxes have reheat, but all light fixtures are T8,but I'm sure there is still a great amount of heat to be harvested. (especially since my tenants have overloaded things with additional electronics!). Since I'm exposed to a fair amount of new office building construction, unfortunately the ceiling plenum concept is and will be used for a while longer, or at least until developers/building owners are willing to spend the "green" money...don't get me started again on "value engineering"!

Brad White

\"Value Engineering\"

is neither....