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The design of the ductworks in ventilation systems are often done by using: Velocity Method, Constant Pressure Loss Method
(or Equal Friction Method, Static Pressure Recovery Method.|
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|| Forced-air System,
DUCTWORK & DISTRIBUTION
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.
Here's a chart of the relationships you should end up with
(all measurements in inches):
2 1/4 x 10||
4 x 10|
2 1/4 x 12
12 x 4|
2 1/4 x 12|
3 1/4 x 10
|4 x 12|
4 x 10
6 x 12|
3 1/4 x 14||
4 x 14||
8 x 12|
4 x 14
||6 x 14||
8 x 14|
8 x 8||
8 x 14||
10 x 14|
8 x 10
|| ||14 x 14|
8 x 12|| ||
20 x 14|
8 x 14
|| || |
8 x 16|| ||
24 x 14|
8 x 18
|| || |
8 x 20|| ||
30 x 12|
8 x 22
|| ||30 x 16|
|| ||8 x 24|| || |
10 x 22
|| || |
10 x 24
|| || |
|| ||10 x 30|| || |
Again, what this really means is that the air doesn't really want to move, but the blower will move it anyways. 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 trunk line 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 trunk lines (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).
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observation panels to full framed and hinged access doors. Duct Protection Consumables and accessories to protect ductwork from
dirt, moisture and debris during transportation, storage and installation and weld pins ductwork; Duct connectors, Cable suspension
systems, Air control devices, Gaskets, sealants & adhesives Access panels, Weld pins. Warm air heating & air conditioning,
duct cleaning, preventative maintenance, maintaining a forced-air system, air filter and blower motor.
The most common method of home heating is the use of air warmed by a furnace and forced through ducts that
carry it to the rooms in the house. This method is known as forced air heat. Forced air is, by far, the most common form of home
heat, because it uses natural gas or liquid propane, which are generally more affordable than other fuels. Gas forced-air systems are
widely used in cool climates worldwide. A gas forced-air furnace - running on natural gas or liquid propane (LP) - draws in
surrounding air, channels it across a set of heated plates, known as a heat exchanger and then uses a blower to circulate the air
throughout the house.
A chamber on top of the furnace, known as a plenum, leads the warmed air from the furnace to a network of ducts that carry the warm air
to heat registers or vents mounted on walls or ceilings. To keep the cycle going, return ducts carry cooled air from each room back to
the furnace so it can be reheated and recirculated. Older systems use gravity to carry warm air throughout the house and cool air back
to the furnace. Advances in home design have required some changes in today's forced-air systems. Conventional forced-air heat
operates by recycling indoor air. In drafty older homes, this worked well since fresh air trickled in from outdoors. Problems arise in
newer, superinsulated homes, where air contaminants can be constantly recirculated, causing respiratory ailments and other health
problems. Many Building Codes now require a fresh air intake in new construction to reduce such hazards. Some homes use a heat
recovery ventilator, which improves air quality without significant heat loss by drawing prewarmed outdoor air into the system.
Builders have also begun installing high-velocity (HV) forced-air systems. These systems increase living space by using small-diameter
tubes that require far less space in ceilings and walls than sheet-metal ducts.
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A variety of Ventilation and Heating units for Bathrooms and Restrooms. 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
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
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Heating, Air Conditioning, Ventilation - HVAC Industry Glossary of Terms
|HVAC(R): Heating Ventilation and Air Conditioning an R may be added to
|Split system: Refrigeration system where the refrigerant is piped from one
part to another, such as residential heat pump or a/c system.|
|Packaged or self-contained: Refrigeration system where everything including
the air moving hardware is kept in one box, such as a window air conditioner or a roof-top unit.|
|Forced air: heating and or cooling system that connects to the conditioned
space with duct-work that uses air as the moving fluid. The heating or cooling can come from any number of sources.|
|Radiant: System that uses hot surfaces to radiate or convect heat into the
environment. Without the use of fans or blowers |
|Heat Pump: Refrigeration device that can be used to move heat to or from air
or water, or to or from air to water then the water is used to heat or cool air. In some parts of the world they are also known
as "Reverse Cycle". |
|Evaporator: The part of a refrigeration system that gets cold. It is called
the evaporator because it is the part of the system that evaporates the refrigerant from liquid to vapor.|
|Condenser: The part of a refrigeration system that gives up heat from the
refrigerant and changes the refrigerant from a vapor to a liquid. Condensers can be either water cooled or air cooled.|
|Compressor: The heart of any refrigeration system that pumps the refrigerant.
(absorption systems that use ammonia or lithium bromide do not have compressors). Compressors can be reciprocating, rotary,
scroll, disc, or screw.
|Receiver: Tank on the liquid side of a system that holds excess refrigerant
in the system that needs to be there for proper operation.|
|Accumulator: Tank on the suction side of a system that holds excess
refrigerant to prevent slugging the compressor with liquid.|
|Outdoor coil: The coil on a heat pump system that is located out side or in
the ground loop of a ground-sourced heat pump. This is so not to be confused with the
condenser on an air-conditioning system.|
|Indoor coil: The coil on a heat pump that is located inside. This is so not
to be confused with the evaporator on an air conditioning system.|
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