Duct Work & Sheet Metal

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Duct Work & Sheet Metal

If you've done the heat loss, and chosen a furnace or air handler. Now you have to design a system to distribute the conditioned air to each room. This system will be based on the cfm output of the blower, and the total cfm will have to be distributed proportionally to the rooms according to their needs. The btu and cfm output will seldom match exactly the house's requirements, so the extra will have to be rationed out. The furnace will have a specification sheet which will list the various blower speeds and outputs.

There are numerous methods of designing a ducted heating or cooling system. And if we sat around thinking hard enough, I'm sure we could come up with a couple more. We could engineer the heck out of the situation if we wanted to, but most of us don't get paid for creativity or unusual design techniques, so I'm going to review one proven method, and leave it at that.

In technical terms, the system will be a low velocity, reducing extended plenum perimeter system. It is more work saying it than installing it. In simple terms, it means that the trunk line tapers as it goes, and that the supply outlets will be near the exterior walls, in this case the floors, and the returns will be located on the inside walls. The ductwork size ,as always , is based on the friction component of the moving air versus the duct itself, and the blowers ability to counter this friction. Again, what this really means is that the air doesn't really want to move, but the blower will move it anyway's. It is always noted in units of inches of water, or In. Wg., and the velocity, or the speed of the air will be in FPM or feet per minute. These concepts and abbreviations are useful and helpful in their own right, but rapidly lose their value when you are crawling around on your belly measuring a trunkline through a crawl space, or dripping sweat in a two hundred degree attic. For residential applications with limited duct lengths, get one of those rotating duct calculators from a salesman, set it at point 1, and go; the chart below, approximates the cfm while the fpm remains under 700 for branches and 1000 for trunklines (Supply branches should be limited to output maximums of 8000 btu for heating, and 4000 btu of cooling unless construction methods dictate otherwise, and should always contain a manual damper for air flow adjustment).

DUCT DESIGN AND DYNAMICS

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 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.

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.