Factors affecting Overall heat transfer coeff

[Nivrah]

To those experienced with HX's,
An external body did a heat and mass balance in our plant where they used a HX with the following specs for a given set on inlet conditions:
Area: 180 sq metres.
LMTD: 4.6
U: 3436 W/m2.⁰C
Duty: 2823 kW.

Assume there is no fouling,
My question is: Is U a constant for a given HX??
Or should U change according to different inlet conditions like flowrates??

 

[TD2K]

U is not a constant.

The overall heat transfer coefficient will vary depending on flow rates and changes in physical properties: heat capacity, density, viscosity, thermal conductivity and density. Likely a few that I missed, pull out your heat transfer textbook.

 

[Nivrah]

TD2K,
thanks for your input..it confirmed my thoughts.

So this means a vendor must have some sort of graph showing different values of U for different conditions right!
are they meant to provide me with varying values of U for differing inlet given a specific fluid, density etc..? or is it up to me to ask for this info?
thanks,

 

[Latexman]

These days vendors use software to estimate U. There is software on the market you or your company can buy to estimate U. Or, you can develop your own.

Good luck,
Latexman

 

[IRstuff]

Is this a liquid to liquid HX? Given that the only way heat can be removed is through flow, then it should be evident that flow rate is a significant factor in the htc.

Your state "given set on inlet conditions" so that "body" had specific conditions for the calculation, since they also came up with an LMTD.

TTFN
faq731-376

 

[rakuday]

Nivrah,

Usually, the vendors will calculate U for the stated design conditions only. U is a function of different parameters and it is really difficult to have a graph covering all these parameters.

If you have doubts over some specific conditions, you can have them crosschecked with the vendor and request for U and heat loads for those conditions of interest. Else you can even calculate U from some of the basic HT books. To start, you can refer Process HT by DQ Kern or Fundamentals of heat exchanger design by RK Shah.

 

[TD2K]

The GPSA data sheet also has a summary sheet showing how changes in physical properties on the shell side and tube side will affect the film heat transfer coefficient for that side.

 

[TD2K]

GPSA data book, sorry

 

[rmw]

Or....

You can do a search on this site for the relevant terms in your original question and start reading. Plenty has been said, and I can state that quite a few years ago, it was a question just like yours that brought me to find this site, helped me with my understanding of the issue and I have been a member since.

I think you will find plenty of information that will help you determine just what is going on with your Hx deep in the bowels of this site.

Good searching,

rmw

 

[Nivrah]

@ rakuday,
yeas, the vendor gave me the U value based on design conditions. i think I will as the vendor for values of U based on some different inlet consitions. Or if the flowarates change by 10 %, i can conservatively assume that U is a constant.