Ducting Pressure Losses

Fired heater designers utilize ducting for many purposes in a fired heater design. They are used for connecting flue gas plenums to stacks, distributing combustion air to burners, transfering flue gas to and from air preheat systems, etc. The pressure losses through ducting pieces may be individually analyzed or the may be analyzed as a system.

We will first explore ducting losses by looking at the individual pieces. The following formulas and coefficients are from the American Petroleum Institute Practice API RP533.

Straight duct run friction loss:
Dp = (0.002989 * Fr * rg * Vg2)*Le/De
Where,
 Dp = Pressure drop, inH2O Fr = Moody friction factor rg = Average gas density, lb/ft3 Vg = Velocity of gas, ft/sec Le = Equivalent length of piece, ft De = Equivalent diameter of piece, ft
And for round duct,
De = Diameter
And for rectangular duct,
De = (2 * Width * Height)/(Width + Height)
Moody friction factor, Fr

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

 Duct surface Roughness Very rough 0.01 Medium rough 0.003 Smooth 0.0005

 Reynolds number: Roughness: Friction factor:

90° Round section elbow loss:
 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

90° Rectangular section elbow loss:
 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

Elbow of any degree turn loss:

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

Sudden contraction loss:
 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

No contraction change of axis loss:
 Dp = Vh * Cl Where, Vh = Velocity head of gas, inH2O Cl = Loss Coefficient From Table Included Angle(N°) Coefficient(Cl) <=14 0.15

Sudden enlargement loss:
 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

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

90° Round miter elbow loss:
 Dp = Vh * Cl Where, Vh = Velocity head of gas, inH2O Cl = Loss Coefficient From Curve

90° Rectangular miter elbow loss:
 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

Pressure loss across stack damper:

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.

Dp = 0.25 * Vh
Where,
 Dp = Pressure drop, inH2O Vh = Average velocity head of stack, inH2O

Draft gain or loss:

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.

Dp = (ra - rg)/5.2 * A
Where,
 Dp = Draft gain or loss, inH2O rg = Density of flue gas, lb/ft3 ra = Density of ambient air, lb/ft3 A = Height of gas path, ft