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Temperature Cross- Please Read, Maybe Worth Your Time


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#1 Student5642

Student5642

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Posted 10 February 2019 - 07:21 PM

After spending the weekend trying to understand my problem, I feel I am back to square one with this.

 

So I am trying to design a heat exchanger that is part of a liquid air energy storage plant, the heat exchanger is on the turbine expansion side (not relevant but a little background). 

 

A preliminary study has calculated for the HX as 

Thot in ; 616K Liquid

Thot out; 308 Liquid

 

Tcold in; 298K Gas
Tcold out; 602.3 K Gas

 

The flow rate of Hot is 75.75kg/s and the flow rate of the cold stream is 173 kg/s.

Now I understand I have a temperature cross, so I believe I have two options. Either design multiple shells in series heat exchangers or design a double pipe heat exchanger. Is this correct to assume?

 

Secondly, if I go with the option of designing multiple shells in series I must first determine a suitable Ft factor (correction factor) to account for the non pure counter correct flow. (Is that correct?) 

 

If that is correct, then I have found through a few correlations, an F factor of 0.8. I don't know where to go from here to continue designing my heat exchanger because i don't know how to determine the correct number of shells to avoid a temperature cross.

 

With further research, I did indeed find a rudimentary graphical method of drawing triangles between the plotted inlet and outlet temperatures of the hot and cold streams to find the number of shells. This was found as 18 shells. 

I understand that this is probably an impossible situation as 18 shells in series would result in a ridiculously large pressure drop (is this correct) 

 

Coming back to the point of possibly designing a double pipe heat exchanger, my initial calculations find that either the pressure drop is so significant it is not worth it, or that the length of the double pipe heat exchanger exceeds around 30 meters. 

 

Please can someone advise regarding this? 






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