HRSG designers utilize ducting for many purposes in a system design. They are used for connecting flue gas plenums to stacks, distributing combustion air to burners, transfering flue gas to the HRSG or for bypassing the HRSG. The pressure losses through ducting pieces may be individually analyzed or the may be analyzed as a system.

We can use the Colebrook equation to solve for the friction factor, with the roughness factor selected from the following:

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Radius/Diameter(R/D)	Coefficient(Cl)
0.5	0.90
1.0	0.33
1.5	0.24
2.0	0.19

		Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Height/Width(H/W)	Radius/Width(R/W)	Coefficient(Cl)
0.25	0.5	1.25
	1.0	0.37
	1.5	0.19
0.50	0.5	1.10
	1.0	0.28
	1.5	0.13
1.00	0.5	1.00
	1.0	0.22
	1.5	0.09
4.0	0.5	0.96
	1.0	0.19
	1.5	0.07

This may be used for a rectangular or round duct elbow of N ° turn.

	Dp = Vh * C90 * N/90
Where,
Vh = Velocity head of gas, inH2O
C90 = Loss coefficient from above for 90° turn
N = Number of degrees of turn

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Area2/Area1(A2/A1)	Coefficient(Cl)
< 0.2	0.34
0.2	0.32
0.4	0.25
0.6	0.16
0.8	0.06

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Included Angle(N°)	Coefficient(Cl)
30	0.02
45	0.04
60	0.07

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Included Angle(N°)	Coefficient(Cl)
<=14	0.15

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Area1/Area2(A1/A2)	Coefficient(Cl)
0.1	0.81
0.3	0.49
0.6	0.16
0.9	0.01

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Included Angle(N°)	Coefficient(Cl)
5	0.17
10	0.28
20	0.45
30	0.59
40	0.73

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Area1/Area2(A1/A2)	Coefficient(Cl)
0	1.0

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Curve

	Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Height/Width(H/W)	Coefficient(Cl)
0.25	1.25
0.5	1.47
1.0	1.50
4.0	1.35

This pressure loss is normally accounted for by rule of thumb. This may be 0.5 or 0.25 velocity head. We will use 0.25.

The draft gain or loss will be taken based on the height of the upward or downward flow of the flue gas. If the flow is upward, the pressure loss is negative.

Now we can try some of these piece calculations,

Now that we have some procedures for calculating the pressure loss for the various components that we might find in a duct system, how do we use them? The easiest way to analyze a ducting system and keep the pressure points straight, is to organize the system starting from the outlet and proceeding to the inlet. This may seem backwards at first, but when you examine the pressure at a given point in the system, you find that the pressure is always dependent on the downstream pressure. So, it makes sense to always work from outlet to inlet, then you always know the pressure of the outlet of the point you are at. To try this out, we will run the calculations for the simple example shown below.

For this example, we will assume we are picking up the flow at some point in a system, so we will calculate from the outlet(assumed to be to atmosphere) to the inlet without considering any condition at the inlet. We will assume the process conditions are those in above table.