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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 thefittings 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 forinlet and outlet specific volumes.

Friction Loss: p = 0.00517/di*G2*Vlm*F*LequivWhere,

p = 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)/lWhere,

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

l = 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 relativeroughness. 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 = 290925

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Inside Diameter, inches = 4.026

Friction factor, F:

Equivalent Length Of Return Bends:

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

Lequiv = FactNre*LrbWhere,

FactNre = Reynolds number correction

Lrb = Equivalent length of return bend, ft

Return Bend Equivalent Length:

Reynolds Correction:

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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:4.026

Pipe straight length, ft:400

Bend radius, in:4

Number of returns:9

Process Data

Mass vel., lb/sec-ft2:251.3695

Viscosity, cp:0.4293

Spec. vol. at start, ft3/lb:0.01887

Spec. vol. at end, ft3/lb:0.01929

Pressure Drop, psi: