We've all seen criminals or heroes crawling around inside ductwork on their way to rescue
or escape, but let's go them one better, and imagine ourselves driving a car through this
duct system. The duct will be the interstate, and our little car will have 3000 btu of cooling
in the back seat. Our car itself has no engine, it will be powered by the blower in the
air handler. There is no actual speed limit but we will try to maintain one because it is
the speed of our car which will determine how noisy the system will be. Let's shoot for
500 feet per minute it is a goog number for quietness. We do not want the occupants to have
to watch T.V. with the remote control in their hand, having to turn up the volume every
time the blower comes on. Our cars won't make much noise unless they are caught speeding
coming out of the registers.
If the duct is 8 inches tall, which is standard, we'll have to allow 2 inches of width
for our car. It would be nice if we had the road to ourselves, but we don't, it is a two-ton
highway; a highway delivering 24000 btu of cooling, so we have to make room for seven other
cars. We will need two inches of width for each car, plus an extra two inches duct width
for friction and spacing between cars, and end up with a duct that is 18 inches wide. (
8 cars X 2 inches per car plus an extra 2 inches for friction). So, our duct will be 8 inches
tall by 18 inches wide, to start with, and this main duct will be known as the supply trunkline.
When the blower comes on, the cars accelerate. The first room , on the right, needs 3000
btu to counter the heat gain in that room, so the car on the far right exits the trunk into
a "take-off". The take-off is an exit ramp that is slightly oversized so the car
will not have to decelerate to exit. This take-off is cut into the trunkline with a 7 inch
diameter, but then tapers to a 6 inch round pipe. Six inch round is the size the car needs
to maintain its speed, and it's load. If the car slows down, the 3000 btu will be reduced.
As the car approaches the actual point of release into the room (outlet) it is converted
back to a rectangular shape in what is known as a boot. In this case, the outlet is in the
floor, and the boot goes from 6 inch round to a 4 inch by 12 inch rectangle. This allows
room for a 4 by12 register to diffuse the air flow into the room, without changing its 3000
btu capacity or creating noise.
After the first car exits, there is no longer a need for the full 18 inch width, so the
trunk is be reduced by 2 inches. Two inches being the size of the lane we needed for each
car. With the trunk reduced to a sixteen inch width, the cars can continue in their lane
with a constant speed. This procedure will be repeated after every exit, assuring a constant
speed and load. The "return" system, is the set of ductwork that returns the air
to the furnace or airhandler. This system is designed in the same fashion, except the air
is entering the duct at each take-off instead of exiting. The trunk line then increases
in size some 2 inches in width for every 100 cfm we add to its capacity, until finally reaching
the 8 by 18 size at the furnace. Both of these highways, the supply and the return, should
be as flat and straight as possible. If turns must be made, they should be smooth and rounded,
any hills must be gentle; so that all lanes of traffic may proceed without having to slow
down. This is the basic concept of duct design, the flow of traffic within established lanes
and at a constant velocity.