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HE heat transfer coefficient inquiry!

HE heat transfer coefficient inquiry!

HE heat transfer coefficient inquiry!

(OP)
Hi everybody,
This is my first thread in this mechanical engineering division so my question is simple.Often we are asked to size shell-u tube heat exchangers and as far I know the most critical parameter is defining the overall heat transfer coefficient. Different approaches-values, result to alternative designs, so my inquiry has to do with a reliable source from which these values can be obtained with a safety margin. For example in natural gas-water shell(water) tube (natural gas) HE we use U=120 Btu/hr/ft2/deg.F. Is this value conservative?. Can we go with a higher value.?
Thanks in advance.  

RE: HE heat transfer coefficient inquiry!

This one is a reliable source. http://www.processassociates.com/process/heat/uvalues1.htm

Better option is to calculate U values from individual heat transfer coefficients of shell side and tube side and then deriving overall heat transfer coefficient. You can refer either Holman or Incropera & Dewitt.

RE: HE heat transfer coefficient inquiry!


Kindly consider the OHTC is always lower than the lowest individual HTC, and fouling factors should be selected on basis of accumulated experience.

For ballpark work your U coefficient doesn't seem to be conservative, it may even be too optimistic. Anyway, I suggest you follow quark's advice.

OHTC = U = overall heat transfer coefficient

RE: HE heat transfer coefficient inquiry!

In addition to determining the OTC, you also need to make a corection due to the HX configuration- ie, number of passes, counterflow vs parrellel flow, etc. The standard estimate of using LMTD is not accurate unles you have at least 5 passes or one of the fluids has an effectively infinite Cp.

Se compact heat exchanger theory , e- NTU method, etc.

RE: HE heat transfer coefficient inquiry!

What is the operating pressure on the natural gas ? The pressure is an indication of the value of the natural gas heat transfer coefficient. This coefficient will vary approximately with the square root of the operating pressure.

RE: HE heat transfer coefficient inquiry!

start with Donald Q. Kern, Process Heat Transfer

saludos.
a.

RE: HE heat transfer coefficient inquiry!

Another good resource: www.engineeringpage.com
Provides both experience values as calculation routines.

Regards

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