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.
|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|
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 = 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,
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.
|FactNre = Reynolds number correction|
|Lrb = Equivalent length of return bend, ft|
Return Bend Equivalent Length:
|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.