We've all seen criminals or heroes crawling around inside ductwork on their way to rescue or escape, but let's go 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 that will determine how noisy the system will be. Let's shoot for 500 feet per minute—it is a good 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.
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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 2-ton highway delivering 24000 Btu of cooling, so we have to make room for seven other cars. We will need 2 inches of width for each car, plus an extra 2 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 its 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 by 12 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 reduced by 2 inches. Two inches is the size of the lane we needed for each car. With the trunk reduced to a 16 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 air handler. 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 inch 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.