Heat transfer coefficient needed 3

[op9]

What would be an approx. value for the Overall Heat transfer coefficient between a mineral oil thermal fluid at 270deg C flowing in either 1.5" or 2" standard wall
steel pipe within a tank of liquid bitumen at say 150deg C? From this
I will calculate the approx flow rate of oil required. I know the length of pipe coil
within the tank ~ 75mtrs.

Rod Nissen
nissenr@one.net.au

 

[svanels]

Op9 your problem lacks some Data. To calculate the overall heat transfer coefficient you will need:

specific heat c_p {kJ/(kg*Celsius)}

viscosity u {kg/(m*s)}

thermal conductivity k { W/(m*Celsius) }

which are temperature dependent properties of the hot-oil.

Also you need an initial flow estimation, or estimated end temperature for the hot-oil.
Can you find the specs of the oil?

Regards Steven van Els
SAvanEls@cq-link.sr

 

[svanels]

I just found a book in our library:
Applied Heat Transfer V. Ganapathy
PennWell Publishing Company
ISBN 0-87814-182-0

There is an appendix "Estimating Forced Convective Heat-Transfer Coefficients Inside Tubes with various charts.

If you have an estimate for the mass flow, I can give you the coefficient h_i

I have the curves for SAE 10 and DOWTHERMA


Send me an email if you are interested.

Steven van Els
SAvanEls@cq-link.sr

 

[svanels]

To calculate the heat transfer coeficient in a coil (liquids):
known:
Temperature range K {kelvin}
density rho {kg/m³}
flow Q {m³/hr}
viscosity kinematic vc {stokes}
Prandtl Number Pr {adimentional}
specific heat c_p {kJ/kg*K}
diameter of coil d {m}
Thermal conductivity k {kJ/m*hr*K}

First calculate Reynolds Number
Depending on the regime, laminar or turbulent use the appropriate formula for calculating Nusselt's Number

For turbulent conditions

Nu_d = 0.023*Re_d^0.8*Prⁿ
where
n = 0.4 for heating and n = 0.3 for cooling
and 0.6 < Prandtl < 100

Related calculations

cross sectional area of pipe
A = pi*d²/ 4

Fluid Velocity
Vm = Q/A

Dynamic viscosity
u = vc * rho

Reynolds Number : Re_d = rho * Vm * d / u


Heat transfer coeficient:

h = k/d * Nu_d { Watt / m²*K }

Regards

Steven van Els
SAvanEls@cq-link.sr

 

Using the Dittus-Boelter correlation for turbulent internal flow will only give you the inside convective film coefficient, hi. If you're looking for the overall heat transfer coefficient U, you'll also need to determine the conductive resistance across the pipewall, and the external convective film coefficient, ho. The external film coefficient will vary widely, depending if the fluid in the tank is agitated, or only naturally convecting. This will most likely be your controlling heat transfer resistance if the tank isn't stirred, so choose your external heat transfer correlation carefully.

If the viscosity of the oil flowing inside the pipe doesn't vary excessively with temperature, then Dittus-Boelter is O.K. for calcualtion of an internal convective film coefficient. If the viscosity of the internal fluid does vary with temperature, you may want to utilize Seider-Tate. The Seider-Tate correlation has a viscosity correction factor to account for the temperature gradient between the bulk flowing fluid, and the fluid at the tube wall.

Hope this helps! Any decent heat transfer text should have several expamples of how to calculate the overall heat transfer coefficient for the system you are describing.
Matt