dirty registers

Brad White_9

Well, Tim, I did write it for a science teacher like he asked. Guess I went a little overboard. Ooops.

:)

Brad

Joe_72

dirty vent registers

How would you describe to a science teacher why dust would attach itself to the cieling tiles by the register. I believe it has something to do with tatic electricity but I need a good explanation.

Timco

Positive charge of the dust particles from friction as they move through the vent, and a grounded register? Opposites attract? I like the static idea, and am pretty sure the friction charges it.

Tim

Brad White_9

Coanda Effect

Airborne ambient dust is the deposited material.

The ceiling register (diffuser) when it releases air at velocity creates a slight negative pressure in the wake of the moving air. (Think of "drafting", being pulled behind an 18-wheeler on the highway as one analogy).

Into this wake is induced room-temperature air, essential for mixing to equilibrium. And with that air, dust.

The initial discharge into the room is cold air at AC temperatures (55F more or less). The immediate effect is a cooling of the ceiling discharge area below the dewpoint of the room. The discharge air also hugs the ceiling, sort of a surface tension effect, the essential part of the Coanda effect.

This causes a field of moisture on the ceiling to which clings airborne dust entrained from ambient room air. It sticks to the ceiling via the moisture.

Notice rooms where smoking is allowed and how much worse it is, to see an extreme example.

To alleviate this, some manufacturers offer "smudge rings", perimeter dropped surfaces that allow the air to travel further away for better mixing before resuming ceiling travel.

Hope this helps.

Brad

Timco

What he said...

T

oilwizard_2

thanks

That sounds good. I was also tol dthat the air in the room is a littl emore humid/moist then the discharge air so this causes the dust to stick more due to the static electricity that is made and in the air. It is kind of the same principle as you may find above baseboard heates and such. There is no heat exchanger to worry about due to this i shydro air system. Again, thanks for the help.
Joe

Brad White_9

Similar principles

Static electricity is quite probably a factor but a relatively small one compared to the moving force of air. Key is, what are the respective charges of the particles and is the ceiling negative and the particles positive, all that. Definitely a factor for if the particles repelled the effect would be less. Still present but less to what degree I do not know.

The "Baseboard Streaking" phenomenon (not to be confused with your Slant-Fin or Argo running naked down the street) is also similar in origin.

Ever been in an old church where you can "read" the lath, plaster and stud lines in dust on the wall above a radiator?

That is due to the colder surfaces harboring slight moisture giving adhesion to the convective dust rising from the radiator. Same church, different pew as one might say