overall heat transfer coefficient

[miltiadis]

Hi.I am working on a project concerning heating of a marine fuel tank (with heating coils and hot oil as heating medium).I am trying to calculate the area required for the heating of the tank and i have reached to a point where i have to calculate the overall heat transfer coefficient (U).I know that i have to calculate the following resistances
hi film coefficient on the inside of coil
ri fouling factor on the inside of coil
rw resistance due to conduction of heat through coil
ro fouling factor on the outside of coil
ho film coefficient on the outside of coil
Assuming that the product of the tank is steady how can i proceed with the calcs of ho?
Another question is that i have found in EXXON MOBILS Design practices a formula for calculating overall heat transfer coefficient for tank heaters (steam,finned)
Uo=(11.7(Tsteam - Tfluid)^0.14)/?f^0.4
?f:abs viscosity of process fluid
Do you know if i can proceed the calculations with the above mentioned formula?
Thanks in advance

 

[srfish]

Hi,

Your post says "hot oil as heating medium" but the proposed equation is for steam?

 

[gwolf2]

It´s a bit more complicated than that formula. The hardest link in the chain is estimating the natural convection of the fuel around the heating coil, closely followed by the surface degredation on the inside and outside of the heating coil.

You ought to be driving the oil through the coil at a speed well away from the laminar/turbulent transition so that´s OK to calculate.

Other than that it´s a 1D heat transfer problem assuming that the coil metal conductivity is 10-100 times the other conductivities (which metal usually is) to even out the transition between unsymmetric external natural convection and symmetric internal forced convection.

Try any good standard heat transfer text book. Not sure about getting numbers for the fouling factors w.r.t. conduction properties. There´s probably some approximate data out there. Just remember that any coating due to oxide will have a very small effect on conductivity because it is so thin. You may be able to ignore it on a first pass calc.

gwolf

 

[davefitz]

Not my particular field, but if the heating medium is condensing steam at the ID, the value of h,i is very high, so the governing reistance to heat transfer is outside fouling Ro followed by inside fouling Ri.

The outside fouling can be very high if the steam temp is above the temp needed to cause the oil to separate the carbon rich heavies from the volatiles in the film layer.The surface would then accumulate a thick coating of carbon- so you would firstneed to ensure the sat temp of the steam is well below the temp that this fractionation occurs.

 

[gwolf2]

davefitz.

>but if the heating medium is condensing steam at the ID,

It´s not steam

"...heating of a marine fuel tank (with heating coils and hot oil as heating medium)..."

 

[25362]

We shouldn't forget the overall heat transfer coefficient will be lower than the lowest coefficient on either side.