3 replies to this topic

[Tawanda]

Hie, i have two heat exchangers with each with a face area of 2.49cubic meters, 120 tubes each, heating agent which is steam at 1034KPa, steam consumption 2000kg/hr. My question is if i change them to stainless steel, (currently there are copper ) how will this affect the time i heat my product. heat transfer will be to air for process use. Taking into consideration that thermal conductivity of stainless steel at 225degrees is 19 while that for copper is 398  k - W/(m.K)

[PingPong]

The effect of the thermal conductivity (or thickness) of the metal tube wall has a negligible effect on the overall heat transfer coefficient U (W/m².K).

 

You can replace the copper tubes by stainless steel tubes with the same inside diameter without any detectable effect on the performance of the exchanger.

 

It is a common mistake by laymen to think that the heat transfer rate is determined by the thermal conductivity of the tubewall, but that is not so, it is determined by the film coefficients (partial heat transfer coefficients) of both mediums. The impact of the tubewall is negligible as long as it is not an insulator (e.g. glass or polymer).

 

In your case the U value will be almost completely determined by the air side film coefficient, which depends on the air velocity (and air physical properties).

Edited by Art Montemayor, 11 March 2014 - 03:33 PM.

[Art Montemayor]

Well stated and documented PingPong.  My field experience backs up what you write.

 

I have exchanged copper and carbon steel with stainless steel tubes in many exchangers while monitoring their effectiveness.  I was never able to detect any percievable difference between the different tube alloys.

[chemks2012]

Tawanda,

 

This is reason why there is no effect of k on overall heat transfer coefficient, U.

 

 

Also, you will have the same surface area and LMTD. My rough calcs states, there is hardly any difference [say about 0.5 kW] in heat transferred by two types of heat exchangers you mentioned.

 

PingPong said it all.

 

KS