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KEY FACTORS IN DUCT DESIGN LAYOUT
1. Aspect Ratio of Duct
a. Relationship of the long side to the short side.
b. Keep the ratio as low as
possible, round is best - less friction -
loss for a given cross
sectional area.
c. Stack of oval is not the best.
d. Space limitation will limit the
size of round pipe.
e. 8" sided duct is the modern
trade-off from an equipment and clearance stand point.
2. Duct Heat Gain or Loss
a. Ducts with large aspect
ratio's have more heat gain or loss.
b. Ducts carrying small air quantities at
low velocity have the greatest heat gain or loss.
c. The addition of insulation to a
duct can significantly affect heat gain or loss.
d. Ducts that run through
unconditioned space should be insulated.
3. Duct Friction And Turbulence
a. Static pressure constantly
decreases as air flow through a duct system.
b. This decrease in pressure is
partly the result of friction (air rubbing against duct walls).
c. Static pressure losses may also
be caused by turbulence within the duct, especially near
fitting that
change
the direction or reduce the flow of the air (air-chums).
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| EFFECTIVE
LENGTHS OF COMMON FITTINGS
1. Duct Fittings
500
- Offset starting collar..............10'
501 -
Straight starting collar............35'
502 -
Round starting collar...............35'
2. Round Branch Fittings
(Takeoff)
502
- Round starting collar...............35'
502 -
Round starting collar w/elbow...35'-50'
581 - Square to round straight............15'
582
- Square to round elbow...............40'
585 - Square to round elbow...........40'-45'
586 - Round to round..................20'-25'
587 - Round to round..................20'-25'
651 - Round to round..................20'-25'
(586,587,651 Takeoff are adjustable and have a
wider mouth than
body.)
3. Round Elbows and
Angles
200-203
Elbows.........................5'-10'
207-210 Angles........................10'-20'
4. Register
Boots
608 - Universal, straight..................5'
610
- 90 degree, elbow....................30'
612 - End.................................50'
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Duct Elbows - Radius Throat
Up to 11".....10'
12" - 21".....15'
22" - 27".....20'
29" - 33".....25'
Square Throat
Up to 11".....40'
12" - 15".....55'
16" -
21".....75'
22" -
27"....100'
28" -
33"....125'
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