11 replies to this topic

[Achal Gupta]

hello,

i am a student in second year and i am learning heat exchanger design in aspen plus. I have been facing problem of cross over in temperatures. My hot inlet is at 210 F and is leaving at 105 F. Cold inlet is at 95 F and is leaving at 115 F. I have tried increasing the outlet temperature of cold fluid....lowering the outlet cold temperature, but its not helping. Also i am confused in how to change the exchanger geometry. I am using 2-4 shell and tube type heat exchanger. Please help me.

Edited by Achal Gupta, 08 July 2015 - 08:57 AM.

[shan]

It seems that you have the freedom to vary the outlet temperature of cold fluid.  If you specify the outlet temperature at 100 F, I am sure that you will resolve your heat exchanger temperature cross over problem.

[Achal Gupta]

hi shan

i am afraid to say it didnt solved the problem.....and i by mistake wrote something wrong i am rewritting the corrected version

I have tried increasing the outlet temperature of hot fluid....lowering the outlet cold temperature, but its not helping. 

Edited by Achal Gupta, 08 July 2015 - 08:57 AM.

[shan]

How come?  If the hot fluid outlet temperature is from 210 F to 105 F and cold fluid temperature is from 95 F to 100 F, there is no a common temperature to cross over. 

You may need re-rewriting your question with clearly indication of specified process parameters and calculated values.

[srfish]

With those temperatures and a 2 shell pass-4 tube pass shell-and-tube heat exchanger, there shouldn't be a problem. Using R = 0.19 and P=0.913, the LMTD chart gives a value of 0.93.

[shan]

My best guess is the exchanged duties from the hot fluid to the cold fluid duty are not balanced at the specified temperature range.  However, there is not enough information to prove or disprove my guess.

[Chirag Prajapati]

To deal with temperature cross over in shell and tube heat exchanger designing two options are available

 

1)  Change the terminal temperatures of hot and cold side fluid if possible to avoid temperature cross over.  

 

2)  Consider two shell in series for designing

[Achal Gupta]

hi shan
the question given to me is as follows:
 
33,114 lb/hr of n-butyl alcohol at 210 F is to be cooled to 105 F using water from 95 F to 115 F. Available for the purpose is a 19.25 inch ID, two pass shell exchanger with 204 tubes 0.75 inch OD, 16 BWG, 16'0" long on 1-in. square pitch arranged for four passes. vertically cut baffles are spaced 5 in. apart. Pressure drops of 10 psi are allowable. validate this heat exchanger using aspen plus.

 

when i put all of this information in aspen plus and excecute the error and warning that i get are as follows:

 

 

Calculations begin ...

 

 

    Block: B1       Model: HEATX

  **  ERROR

      TEMPERATURE CROSSOVER DETECTED

      RE-CALCULATING WITH MINIMUM APPROACH TEMP. SPEC

 

  *   WARNING

      CALCULATED SHELLSIDE DELTA-P > THAN MAXIMUM ALLOWABLE DELTA-P

      MAXIMUM DELTA-P USED IN CALCULATIONS

 

 

->Simulation calculations completed ...

[shan]

Do you specify the heat exchanger as "countercurrent" or "concurrent"?

[Achal Gupta]

its countercurrent shan

[srfish]

I am not familiar with the error messages in Aspen but maybe I can throw a little light on the situation.

 

The error message: "Temperature crossover detected" is correct. There is a temperature cross. The other message: "recalculating with minimum approach temp. spec" is opaque. Are the outlet temperatures on your output the same as the input? If not that would explain the warning. You are correct in using a two pass shell for those temperatures. A one pass shell would give an LMTD correction factor lower that the required minimum of 0.750. As mentioned earlier, the two pass shell gives a factor of 0.93.

 

It appears that the maximum allowable of 10 Psi was exceeded, thus the warning. But with a liquid flow rate of 33,114 lb/hr and 5 inch baffle spacing, the shell side pressure drop would't be over 10 Psi.

[Achal Gupta]

thanks srfish...i got it.

and i got the right result

a

thank you shan and srfish for helping me