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How To Calculate Lmtd With 2 Different Inlet Temperature (Mixer)

#mixer #heattransfer #lmtd

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#1 dhimas.ad


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Posted 21 December 2020 - 10:33 AM

So i want to mix 2 different liquid with different temperature on my mixer.


Here is the case :

I have 2 inlet liquid with inlet 1 is 73,3oC and 1 atm and inlet 2 is 116,3oC and 1 atm. I want the mixture of these liquid is 90oC and 1 atm. The mixture temperature when mixer condition adiabatic is 79,6oC.

My question is : how to calculate the LMTD when u have 2 different inlet temperature? Should i use average temperature (inlet1+inlet2)/2 then doing it with the usual formula or there is other formulas to calculate LMTD in this case? Or should i use Tinlet when the condition of the mixer is adiabatic which is 79,6oC?

(I want to find the heat transfer area for jacket heater of the mixer so the temperature of my mixture will be 90oC).


Thanks for your consideration for helping me. 

Edited by dhimas.ad, 21 December 2020 - 11:13 AM.

#2 Nikolai T

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Posted 22 December 2020 - 10:29 PM



If you have 79.6 C after mixing use the value to calculate temperature difference between mixture and heat medium, for example saturated steam.


You don't need to use inlet components temperatures.


Values of K for jacketed vessels there are on other threads.


heat medium - saturated steam:

250...950 kcal/m2*h*C - for water with forced mixing.

50...150 kcal/m2*h*C - for mineral oil with foced mixing.

Edited by Nikolai T, 23 December 2020 - 07:10 PM.

#3 Pilesar


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Posted 23 December 2020 - 10:40 AM

In real life, it would be more efficient to heat only the inlet 1 stream to reach the proper mix temperature. You normally don't want to cool the hot stream of inlet 2 and then heat it back up again after mixing. Calculate the LMTD using the inlet 1 temperature compared to the mixed feed temperature. Also notice the difference in the size of the cold-side exchanger flows for the two cases (heating inlet 1 vs heating the mixture.) It is a straightforward exercise to estimate the heat transfer area difference between the two cases. 

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