On Sat, 17 Feb 2007 22:23:53 GMT, "Edwin Pawlowski"
wrote:


"mm" wrote in message What about air?

If you have a heated building and open an overhead door 12 to 16 feet
high, for trucks, how fast will the cold air outside pour into the
building, and how fast will the hot air nearer the ceiling be forced
out.

I was recently at such a building, when it was below freezing out, and
I was close to the door and every time the door was opened, I'd feel a
bit of cold air, but not as much as I expected, and when the door was
shut, the building seemed back to normal very quickly. If it had been
full of water, in a twentieth of the time it took a car to drive in or
out, all the water would have run out. Does air not flow even at
1/20th the speed of water? What am I missing?

You need something to move the air. When you open the door, there is
already air on both sides so there is little exchange. If there is a
breeze, or temperature differences, there will be some air movement, but it
is not like spilling water onto a table. To prove this, put a divider in a
container of water. Color the water on one side,let it stand and be very
still, then pull up the divider. It will eventually mix, but not very fast.
Just as some cocktails are poured in layers.

On a windy day, open the door and you will feel more air, but if you have
openings at the other end of the building, no matter how large, you will get
a draft through the building. At one of our buildings on a breezy day, I
can tell when a big door is opened as far as 1000 feet away. On a calm day,
you don't feel anything even 50 feet away.


Air movement in side a closed or semiclosed area is extremely complex.
Although air densitytemperature gradients play an important role, air
mixing is highly dependent on the existing HVAC and placement of
furniture or equipment in the room. Although I could not find the
report in a short imte, I have seen a Department of Energy study from
quite a few years ago that discussed the result of smoke tests in a
variety of room configuratoins. What amazed me is that in some parts
of rooms with what might be consdered good circulation, there were
dead air spots that persisted for very long periods.