1. MECH 8250 – Building Systems Winter 2015
MSYS AC Systems
MECH 8250 – Building Systems Winter 2015
DUCT DESIGN By
Satwinder Singh
10/11/2018
By Satwinder Singh

2. Duct design Air flow in ducts Major and Minor Losses in Ducts
Loss coefficient for some fittings
Equivalent length for a fittings
10/11/2018
Satwinder Singh

3. Air Flow in Ducts

4. Velocity air pressure, Pv
Pv in in water and V in ft/min
Pv in Pa and V in m/s
Mass Density ρ
62.4 lbm/ft3 and 999 kg/
Pressure changes during flow in ducts.
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Satwinder Singh

5. Friction Loss Tedious task to solve by equations
Pressure Loss Charts have been prepared.
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Satwinder Singh

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8. Equivalent of a circular duct
The pressure loss due to friction is greater for a rectangular duct than for a circular
duct of the same cross-sectional area and capacity.
For most practical purposes ducts of aspect ratio not exceeding 8 : 1 will have the same lost head for equal length
and mean velocity of flow as a circular duct of the same hydraulic diameter.
When the duct sizes are expressed in terms of hydraulic diameter Dh and when the equations for
friction loss in round and rectangular ducts are equated for equal length and capacity,
an equation for the circular equivalent of a rectangular duct is obtained:
Dh = Hydraulic diameter
a and b are the dimension of a rectangular duct
10/11/2018
Satwinder Singh

9. Equivalent of a circular duct
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Satwinder Singh

10. Elbows are generally efficient fittings in that their losses are small when the turn
is gradual. When an abrupt turn is used without turning vanes, the lost pressure will
be four or five times larger. When considering the lost pressure in divided flow fittings,
the loss in the straight-through as well as the loss through the branch outlet
must be considered.
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Satwinder Singh

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Satwinder Singh

12. Example: Pressure Loss
Compute the loss in total pressure for a round 90-degree branch and straight-through section, a tee. The common section is 12 in. in diameter, and the straight-through section has a 10 in. diameter with a flow rate of 1100 cfm. The branch flow rate is 250 cfm through a 6 in. duct.
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Satwinder Singh

13. Equivalent lengths
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Satwinder Singh

14. Example:
Compute the equivalent lengths for the fittings in the duct system of Fig The fittings are an entrance, a 45-degree wye, the straight-through section of the wye fitting, a 45-degree elbow, and a 90-degree elbow.
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Satwinder Singh

15. What is the total pressure loss on the critical path?
Example:
What is the total pressure loss on the critical path?
Critical Path: 1-a-3 with Equivalent length of ft
We pick average pressure (friction) loss for duct and calculate the total pressure loss for the system.
Friction loss to be designed for = 0.08”/100ft
P = ft x 0.08”/100ft = 0.102”
Path 1: 1-a-2
Le = Li Lwye,S + 50 =
= ft
Path 2: 1-a-3
Le = Li Lwye,Br + Lelbow =
= ft
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Satwinder Singh

16. Duct Accessories Turning vanes Dampers Fire dampers Linear
Airfoil (More efficient)
Dampers
Parallel blades
Opposed blades)
Fire dampers
Turning vanes prevent turbulence and high loss in total pressure where turns are
necessary in rectangular ducts where we don’t have space for large-radius turns.
When turning vanes are used, an abrupt 90-degree
turn is made by the duct, but the air is turned smoothly by the vanes.
Turning vanes
are of two basic designs. The airfoil type is more efficient than the single-piece flat
vane.
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Satwinder Singh

17. Turning Vanes
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Satwinder Singh

18. Dampers
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Satwinder Singh

19. Duct Design Volume (Q) is a function of cross sectional
area (A) and velocity (V)
Q=AV
however, momentum, friction and turbulence must also be accounted for in the sizing method
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Satwinder Singh

20. Air Flow in Ducts

21. Static Pressure
Force required to overcome friction and loss of momentum due to turbulence
As air encounters friction or turbulence, static pressure is reduced
Fans add static pressure
Static pressure is measured in Inches-water gauge
Positive pressure pushes air
Negative pressure draws air
Straight ducts have a pressure loss of “w.g./100’
based on diameter and velocity
10/11/2018
Satwinder Singh

22. Equivalent Length
Describes the amount of static pressure lost in a fitting that would be comparable to a length of straight duct
10/11/2018
Satwinder Singh

23. Duct Construction
Round ductwork is the most efficient but requires greater depth
Rectangular ductwork is the least efficient but can be reduced in depth to accommodate smaller clearances
Avoid aspect ratios greater than 5:1
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Satwinder Singh

24. Equal Friction Method
Presumes that friction in ductwork can be balanced to allow uniform friction loss through all branches
1. Find effective length (EL) of longest run
2. Establish allowed static pressure loss/100’
ΔP=100(SP)/EL
3. Size ducts
4. Repeat for each branch
Note: velocity must be higher in each upstream section
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Satwinder Singh

25. Equal Friction Method - Example
The system shown is supplied air by a rooftop unit that develops 0.25 in. wg total pressure external to the unit. The return air system requires 0.10 in. wg. The ducts are to be of round cross section, and the maximum velocity in the main run is 850 ft/min, whereas the branch velocities must not exceed 650 ft/min.
Size the ducts using the equal-friction method. Show the location of any required dampers. Compute the total pressure loss for the system.
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Satwinder Singh

26. Equal Friction Method - Example
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Satwinder Singh

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