Heat loss from a pipe outside - how to account for wind?

[Emanon]

Dear members,

I wish to send a warm liquid through a pipe and into a kettle/boiler. For this I want to calculate the heat loss
and end up finding out exactly how much heat I need to put into the system in order to have the liquid end up at a preset temperature in the kettle. But in this case everything is outside.

I've found many formula's (with different assumptions) that allow me to calculate the heat loss through a pipe, such as this one:
http://www.engineersedge.com/heat_tr...ator_12921.htm

But my question is: is there any way I can account for the wind causing even more heat loss?
Note: the pipe is not insulated.


Kind regards,

Erik

[JAlberts]

Heat transfer analyses under the catagory of "forced convection heat transfer" may be applicable to your problem and I found a large number of potentially heplful references by doing a simple online search under that title.

Maybe Kelly has something specific she can refer to you from her incredible knowledge base.

[Emanon]

Thank you.
I thought wind wouldn't fall under forced convection, because you don't force it...
But that means I could just "estimate" a mean value for the wind and work with that.

I should have enough information about forced heat convection to work with.
Thanks again!

[Kelly_Bramble]

See the attachment...

Heat_Transfer_-_A_Practical_Approach_-_Yunus_A._Cengel_p405_-_406.pdf

[Emanon]

Thank you very much!

[Emanon]

Sorry to prolong this post, but I've got some new questions.

The first assumption from Example 7-5 in the pdf states that steady operating conditions exist. That should be true once the system is up and running. But of course there's a certain time before this steady state has been reached from cold start-up. For as far as I know cold-startup information in general is rare. Should I simply find out how much heat I need by experimenting?

Also, I don't think I quite understand the concept of the film temperature, T_f. What exactly does the film temperature mean/represent? (English is not my primary language, so I'm not sure whether "film" means anything or refers to anything special other than a thin strip of material, sorry).
I know you simply take the average between the surface temperature and the temperature of the surrounding air, but is it nothing more? And is calculating the average film temperature applicable to any temperature?

[JAlberts]

For the heat input required during the startup period I suggest the following. Since the maximum cooling effect of the heat transfer will be at your maximum fluid temperature the key issue then becomes the desired maximum time for the start up process. During this period you must supply an adequate heat input rate to raise the fluid in the pipe to the required operating temperature. The calories/kg/degree centigrade required by the fluid properties and the start up time required, ignoring heat loss, will determine the calories/hour input rate required for fluid heating alone. The second factor during this start up period is the heat loss through the pipe. This can best be represented by the average heat loss value between the heat transfer rate at the beginning of the start up and that at the maximum operating temperature. Assuming that your pipe temperature at start up is the same as the surrounding air temperature then using 1/2 of your calculated operating heat loss rate is a reasonable approximation for this value. Then, this average heat loss rate must be added to the fluid heating input rate to determine the final desired heat input rate during the start up period. Of course, at the end of that period the initial heat input should be reduced to the steady state heat input requirement; and, I would assume that this would be achieved by the same thermostatic control system used to maintain the correct fluid temperature during standard operating conditions.

Of course, this above calculation assumes that your worst case forced convection wind heat loss is going to exist during the start up period; otherwise, the heat loss rate will be less; and, at a minimum, the standard natural air convection flow heat loss rate or something between that and your maximum forced convection wind flow loss rate.

With regard to your question about "film temperature" as you state, this is a transition layer that can exist at the boundry layer between a heated surface and its adjacent surrounding fluid and is used to address such issues as turbulent flow in this area; or, in the case of heat exchangers, boiling or vapor formation in that region which cam effect the heat transfer between the adjacent fluid and pipe surface. In your application with standard wind flow around the pipe this should not be a significant factor; and, using the average of the pipe surface and air temperatures should be sufficient.

[Emanon]

Thank you both very much for the explanations!