> Ventilation is secured by air being forced by a fan into a elongated plastic
> tube; it having holes being of the same diameter distributed along its length.
> Via. these holes air is pushed and results theoretically in air being
circulated
> around the hive bodies.
> In reality, there appears to be unequal air distribution - holes in the 3
metre
> section nearest the fan have little volumes of air passing out, whilst those
> at the far end of the tube release the majority of the air. Those hives in
areas
> of limited air supply have been found to be weak, dead, whilst those in areas
> of better supply fared well.
I don't winter bees inside, but I can help with the
basic ventilation problem, as it comes up often in
greenhouse designs and industrial venting, and is
a classic "practical physics" problem posed to
students in exams written by sadistic post-doc
instructors.
As you found out the hard way, equally-sized holes
spaced equally along a tube just do not work.
Assuming that the holes you have are both spaced evenly
along the tubing and are of equal size, you should get
roughly twice the amount or airflow AND twice the velocity
from the holes at the "far end" of the main tube that you
do from the holes nearest the fan.
Twice the airflow and twice the velocity means that only
a small fraction of the air was coming out the end nearest
the fan. I'm not sure what the people who put up the tube
might have been thinking. Even if they were thinking about
water and pipes, the majority of the water exits an evenly
perforated pipe nearest the source, so evenly-spaced holes
would not have been "correct" for that model, either. But
lucky for you they did not design it "thinking of water",
because such a system would have been "backwards", and
might have killed ALL the colonies.
If you measure the airflow with thin strips of ribbon
taped above each hole, (note that borrowing some of
your wife's hair ribbons for this purpose can be fatal,
so go raid the Christmas ribbon instead) you can see the
problem clearly, and "tune" your system with nothing more
than a roll of duct tape. (That's why they call it "duct tape"!)
If you use duct tape to cover some of the holes, you
can even things out, so that each open hole along the
tube will cause equal lengths of ribbon to wave and
flap in the same manner and at the same rate. Start
by covering every 8th hole on the 1/2 the tube furthest
from the fan, then every 4th hole, and so on, like this:
x = closed
o = open
1) o o o o o o o x o o o o o o o
2) o o o x o o o x o o o x o o o
3) o o x o o x o o x o o x o o x
4) x o x o x o x o x o x o x o x
5) x x o x x o x x o x x o x x o
6) x x x o x x x o x x x o x x x
7) x x x x o x x x x o x x x x o
You may need to increase the spacing of the taped
holes as you get closer to the fan to get an even
airflow from each hole. For example, you may find
that the far end needs only every 3rd hole open,
while the middle needs to have every other hole open.
Play around with it, but recall that the further you
are from the fan, the more holes should be covered
between open holes.
When you are done, and have nicely waving ribbons,
you can then do some math, and find out if you
really have an "optimal" system.
If you measure the size of your duct (the pipe), and
then add up the size of each hole, you can find any
basic error in the design:
Number of open holes = N
Area of one hole = S
Cross-sectional tube area = A
Ratio = X
(( N * S ) / A ) = X
You want a set up where "X" is between 1.5 and 2.0
Below 1.5, you have "too much pressure" in the tube,
since there is simply not enough hole area, and the
fan must work too hard, often overheating in the
process.
Above 2.0, you get "instability", since there is
too much opening for the fan, and "waves" of air
will result in the tube.
In the range of 1.5 to 2.0, you can adjust fan
speed and still retain "even" airflow.
Anything from 20% of maximum fan velocity to 100%
of maximum velocity should be "even". Below
20%, things may get screwy.
You may want to also consider the size of each hole
along the tube. They should all be the same size,
but they should all be 2 inches in diameter or less
assuming a tube of at least 6 inches in diameter.
Holes 2 inches or less work well on any size duct.
Larger holes can resonate, and "make music" like a
giant annoying flute. With your luck, they would
resonate at about 418 - 440 Hz, and sound to the
bees like queen piping, keeping all the bees in a
"piping freeze response state" all winter.
Not a good thing.
Now, what about ventilation for "each hive"?
Well, not to worry - the air exits the holes
and fans out from each hole at a roughly 22 degree
to 24 degree angle from each edge of the hole. If
the tube is above the hives, the fan-out should cover
enough area to spread the air around. A good test
would be to have an assistant pump a pair of smokers
at the fan intake, and watch the smoke fill the room.
This would be a 1:1 scale "wind tunnel test", if you
placed empty woodenware or cardboard boxes to stand
in for hives in appropriate positions.
If you are way, way off, and have concerns about there
being far too few holes that are open, you need to cover
ALL the holes, rotate the tube, and cut smaller holes
to get closer spacing. Again, I would start with even
spacing, and use duct tape to cover every Nth hole,
just to avoid all the math that would be required to
"design" the correct hole spacing up front.
If you want to get REALLY fancy, there are even devices
that you can install in the tube just downstream of the
fan to "straighten out" the airstream, and stop it from
swirling around in a corkscrew as it travels along the pipe.
This will increase the volume of air that exits the holes
above that of a system without such a gizmo installed.
If the tubes are above the hives, you want the holes to
be at "4:00" or "8:00", where "12:00" would be a hole facing
straight up at the ceiling. Straight down ("6:00") is NOT
as good, do the smoke test with the holes in various positions
and see.
...and you don't want any tube to be longer than 100 feet.
Tubes longer than 100 feet start to slow down the airflow
via friction at typical fan speeds, and make life much more
difficult. Long buildings need a pair of tubes, each fed
from each side.
I have not included the math behind most of what I have said,
as it is both complex and tedious. Duct tape is easier than
doing the math in a case like this, since you already have a
fait accompli "system" in place.
If things get really confusing, send a detailed dimensioned
sketch and an envelope stuffed with unmarked, non-consecutively
serial numbered $20 bills, and I'll churn through the math for you.
jim (Who has experience in reducing "hot air",
and is a charter member of the Society
For Not Only The Obedience Of, But The
Enjoyment Of The Laws Of Physics)
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