3 replies to this topic

[cat]

Hi,

 

I have an existing heat exchanger (no longer in use as this was replaced before) which i wish to re-utilize for a different application (different flow and temperatures), but I am not sure if I can really utilize the existing equipment.

 

My existing heat exchanger has the following design data:

 

Fluid service: Water on both sides

Fluid details

     Hot side:

     Flow: 242 m3/hr

     DT: 12.6 C (67.6 to 55)

     DP: 12 kpa

 

     Cold side:

     Flow: 300 m3/hr

     DT: 10 C (33 to 43)

     DP: 61 kpa

 

Heat exchanger details:

      Q: 3489 kw

      LMTD: 23.3

      U: 3256 W/m2K

      A: 46

     Countercurrent

     94 plates

     1 pass

 

Now, for my new fluid:

 

Fluid service: water on both sides

Fluid details

     Hot side:

     Flow: 200 m3/hr

     DT: 10 C (43 to 33)

     DP: 100 kpa (max)

 

 Based on the heat load on the hot side, Q is only 2322 kW.

 

The cold side fluid is not yet designed fully (pump not yet bought), but would like to target of 10C DT max which would translate to ~200 m3/hr (30C to 40C), although the preference is 8C DT (30-38C) which would translate to a flow of 250 m3/hr. This water is going to atmospheric conditions

 

My initial thought - based on nearly similar flowrates and the Q for the new conditions are lower than what was the original design, it may be usable.

 

Now as I am into detailing, when I calculated the UA based on the new conditions for the new service, the UAcalc =5x Udesign. This may be due to lower DT across the fluid (less driving force), and in such, I need to compensate with a larger area.

 

Is the exchanger usable based on Q only, or not? If it is usable, will I not be getting the target temperatures, ie,. instead of 33, it may higher than this, say around 41C (since 10C/5 = 2C)?

 

Thanks in advance for the help.

 

Regards,

cat

 

[IGC]

Probably not.

 

The LMTD has reduced from 23 to 3 based on your required conditions.  As you point out this causes the required surface are to rapidly rise.  A rough guess would be an additional 2 of this unit based on surface area alone.  

 

You may be better off sending the info to the original supplier and have them come up with possible outlet temperatures as they will have the correct correlations for their plates to calculate a possible U value for you.  

[cat]

@IGC, I tried to compute using charts (for h values) from an article in CEP (prelim sizing for P&F), and I got an area of 112m2 (which is ~3x), and your rough estimate seems to be close enough. I am attaching the excel file, if someone would be kind enough to check it out.

 

Note: i tried using the same method for the other P&F that we are using just to validate the thought process, and seems to be close in providing a quick estimate

 

 

 P&F prelim.xlsx   9.6KB   487 downloads

Edited by cat, 20 August 2019 - 08:28 AM.

[IGC]

Honestly I think it would be even higher for the area.  The U value you have is higher than the original scenario.  You can't have a higher pressure drop with reduced flow when considering the same unit and the relationship that is shown in that article between dP and h should give you a lot lower h value.  It wouldn't surprise me if the h value was around 4000 to 6000 W/m2K in this case.  

 

Regardless, it seems that you will need a different unit.  Sorry!