3 replies to this topic

[sheiko]

Hello,

I need to calculate external heat transfer coefficient outside a horizontal pancake spiral coil placed in a vertical vesssel (a glycol dehydrator).

In addition to the correlation Nu = f(Re, Pr), i need to know:
1/ How to determine equivalent diameter outside the coil
2/ How to calculate the velocity through the coil

Could you please help me?

Any reference (book, article) will be more than welcomed.

Regards

[Art Montemayor]

Sheiko:

I have done this applidation as well the one with hairpin loops in the bottom of a storage tank in order to keep the contents warm or hot, I did this with Phenol and about 3 or 4 other stored liquids. The design critieria I took was taught to me by my mentor and he always advised me that any calculations based on theoretical, induced, natural convection can't be relied upon as representing real life. They are merely good, theoretical exercises and can never be applied, per se, in real life applications.

I followed his advice and never had an application that did not produce or was not successful. Dependiing on the fluid and other physical conditions, I applied an overall coefficient from experience - 50 - 100 btu/ft2-oF - when dealing with conventional liquids and 50 psig steam inside the coils.

I honestly believe it is a waste of time to apply theoretical equations to an application that is almost 75-90% dependent on empirical information and phenomena.

I know how to calculate the equivalent length of the coil, but why would you (or anyone) want to know the equivalent diameter outside the coil? Furthermore, if you know the diameter of the pipe forming the spiral coil, you can easily find the superficial velocity. I don't see any problem here since you must first assume a pipe (or tube) outer diameter. I am currently in Madrid, Spain so I am isolated from my personal engineering files and resources in my home library. Therefore, I'm sorry I can't give you the equations and relationships for now.

[breizh]

Bonjour ,
let you try Kern "process heat transfer" page 721

Regards

Breizh

[sheiko]

Salut Breizh,

I have indeed checked in Kern but there is no guidelines for determining what i am looking for (equivalent diameter and average velocity through the coil)...

Art,

Thanks. Looking forward your files.