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 heater design. The pressure loss in heater 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: 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: