Heat Recovery Steam Generator (HRSG) Learning Center
< back to esteemprojects.com
Thermal properties of water and steam Thermal properties of flue gas Combustion, supplementary firing Heat transfer coefficients Intube pressure drop Gas side pressure drop across tubes Ducting pressure losses Acid dew point of flue gas NOx and other considerations

Intube Pressure Drop


The intube pressure drop may be calculated by any number of methods available today, but the following procedures should give sufficient results for HRSG design. The pressure loss in HRSG tubes and fittings is normally calculated by first converting the fittings to an equivalent length of pipe. Then the average properties for a segment of piping and fittings can be used to calculate a pressure drop per foot to apply to the overall equivalent length. This pressure drop per foot value can be improved by correcting it for inlet and outlet specific volumes.

Friction Loss:
Dp = 0.00517/di*G2*Vlm*F*Lequiv
Where,
Dp = Pressure drop, psi
di = Inside diameter of tube, in
G = Mass velocity of fluid, lb/sec-ft2
Vlm = Log mean specific volume correction
F = Fanning friction factor
Lequiv = Equivalent length of pipe run, ft
And,
Vlm = (V2-V1)/ln(V2/V1)
For single phase flow,
V1 = Specific volume at start of run, ft3/lb
V2 = Specific volume at end of run, ft3/lb
For mixed phase flow,
Vi = 10.73*(Tf/(Pv*MWv)*Vfrac+(1-Vfrac)/rl
Where,
Vi = Specific volume at point, ft3/lb
Tf = Fluid temperature, °R
Pv = Press. of fluid at point, psia
MWv = Molecular weight of vapor
Vfrac = Weight fraction of vapor %/100
rl = Density of liquid, lb/ft3
Fanning Friction Factor:
The Moody friction factor, for a non-laminar flow, may be calculated by using the Colebrook equation relating the friction factor to the Reynolds number and relative roughness. And the Fanning friction factor is 1/4 the Moody factor. For a clean pipe or tube, the relative roughness value for an inside diameter given in inches is normally 0.0018 inch.
With this, we can calculate the factor,
Reynolds number =
Inside Diameter, inches =
Friction factor, F:
Equivalent Length Of Return Bends:

The equivalent length of a return bend may be obtained from the following curves based on Maxwell table and can be corrected using the Reynolds number correction factor.

Lequiv = FactNre*Lrb
Where,
FactNre = Reynolds number correction
Lrb = Equivalent length of return bend, ft
Return Bend Equivalent Length:
Return Bend Equivalent Length
Reynolds Correction:
Reynolds Correction
Where,
G = Mass velocity, lb/sec-ft2
Di = Inside tube diameter, in
Visc = Viscosity, cp

Now that we have all the details described, we can calculate the pressure drop for some typical heater coils.

Coil Data
Tube inside dia., in: Pipe straight length, ft:
Bend radius, in: Number of returns:
Process Data
Mass vel., lb/sec-ft2: Viscosity, cp:
Spec. vol. at start, ft3/lb: Spec. vol. at end, ft3/lb:
Pressure Drop, psi:

Disclaimer:

The formulas and correlations presented herein are all in the public domain and are to be used only as a learning tool. Note that any product, process, or technology in this document may be the subject of other intellectual property rights reserved by sponsors or contributors to this site. This publication is provided as is, without any warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of fitness for a particular purpose, or non-infringement.

The formulas, correlations, and methods presented herein should not be considered as being recommended by or used by the sponsors of this site. The purpose of this site is educational and the methods may or may not be suitable for actual design of equipment. Only a fired heater design engineer is qualified to decide if a calculation or procedure is correct for an application.