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Rating A Finned Tube Air Heater

air heater air cooled tubeside velocity finned tube cross flow fouling pressure drop

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#1 zavtranguyen

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Posted 04 April 2020 - 03:27 AM

Dear experts, i'm doing a rating design for a existing heat exchanger.
 
It's a Finned tube Air Heater, from the Original design:
Outside: Air flow with 123231 kg/h, inlet temp = 23.2 C; Op.pressure = 5.98 kPa(g)=> Outlet temp = 115 C
Tubeside: Saturated steam: 5710 kg/h, 140 C, 260 kPa(g), outlet temp = 140 C 
 
That is the design. Actually we use  Air flow only 88000 kg/h (inlet tem = 35 C, outlet  = 87C), the steam consumption is around 1900 kg/h and condensate outlet temp = 38-42C, the steam inlet nozzle 8", so the control valve at the steam line opens ~ 8%, these problems cause a big loss in condensate pressure that we can't recover the condensate to the header.
 
 
Since problems mentioned above, we want to use Steam condensate (145-147C) as the heating fluid for the first bundle (the heat exchanger has 2 bundle), the steam condensate flow rate is about 21000 kg/h with carbon steel pipe size 6".
The problem now is the condensate velocity (tubeside) ~ 0.1 m/s, any serious problem for this velocity (Any fouling occurs? The condensate temp inlet 145-146, oulet ~ 96C).
I need your help to resolve this issue.
Please find attached doc for more esstential information.
 I would appreciate your help.
Thank you so much.


Edited by zavtranguyen, 22 April 2020 - 02:23 AM.


#2 PingPong

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Posted 04 April 2020 - 10:59 AM

Without a drawing (preferably a P&ID) of the existing system, as well as a drawing of your proposed system, this is not clear.


You mention 2 bundles. Are those bundles in series on the air side? If so it could be that you need to feed steam to only one bundle for the current operation at 40 % of design duty. Has that been tried already?



#3 zavtranguyen

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Posted 04 April 2020 - 07:51 PM

Without a drawing (preferably a P&ID) of the existing system, as well as a drawing of your proposed system, this is not clear.

You mention 2 bundles. Are those bundles in series on the air side? If so it could be that you need to feed steam to only one bundle for the current operation at 40 % of design duty. Has that been tried already?

Dear Mr.PingPong, i have attached more information above, so you can check the configuration.

- These two bundle is in series: 2 nozzles for steam inlet (for each header) and 2 nozzles for condensate outlet.

- "If so it could be that you need to feed steam to only one bundle for the current operation at 40 % of design duty" ==> for that point: you're right, if we use one bundle, condensate could be recovered. But in mentioned modify (using avaialbe hot condensate 147C) we also intend to save energy- this is steam, that is why we choose that solution and we hope that the steam bundle in new change will not to be used (if the first bundle duty is enough)

 

 



#4 Bobby Strain

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Posted 04 April 2020 - 10:36 PM

So, proceed with your plan.

 

Bobby



#5 zavtranguyen

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Posted 05 April 2020 - 03:50 AM

So, proceed with your plan.

 

Bobby

Dear Sir. Bobby

In this modify, we use 21000kg/h of hotCondensate for the first bundle which has 206 finned tubes (ID = 20 mm), so the problem we 're facing is low tubeside velocity which is 0.1 m/s, as attached excel workbook.

So my question: is there any issue with this velocity with your expriences?

 

Thank you so much

Nguyen



#6 PingPong

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Posted 05 April 2020 - 07:38 AM

You mention 206 tubes per bundle, however in the spreadsheet it says 361 tubes/bundle in the first two sheets. That would reduce tubeside velocity to only 0.06 m/s.

 

Assuming the condensate is clean low tubeside velocity does not cause fouling.

 

Low tubeside velocity will however give a low tubeside film coefficient. Note also that you would make it worse by routing the condensate through the bundle from bottom to top, as natural convection would then counteract and further reduce the film velocity. In this case it is better to feed the condensate at the top and withdraw at the bottom of the bundle.



#7 zavtranguyen

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Posted 05 April 2020 - 10:04 AM

You mention 206 tubes per bundle, however in the spreadsheet it says 361 tubes/bundle in the first two sheets. That would reduce tubeside velocity to only 0.06 m/s.

 

Assuming the condensate is clean low tubeside velocity does not cause fouling.

 

Low tubeside velocity will however give a low tubeside film coefficient. Note also that you would make it worse by routing the condensate through the bundle from bottom to top, as natural convection would then counteract and further reduce the film velocity. In this case it is better to feed the condensate at the top and withdraw at the bottom of the bundle.

Dear Mr.PingPong,

- The attached workbook has 3 sheets: "HX_DesignDataSheet"- 361 tubes you see is when 2 bundles (206 tubes for first bundle, 155- second)are taken into account  for the heat exhanger.

- "Assuming the condensate is clean low tubeside velocity does not cause fouling" ---> i worried about fouling problem after referring to the Design standards, there is min. velocity for tubeside water not lower 1 m/s. and by the way, the min. velocity mentioned in Design standards is implied for fouling issues or tubeside film coefficient?

 

- "Low tubeside velocity will however give a low tubeside film coefficient. Note also that you would make it worse by routing the condensate through the bundle from bottom to top, as natural convection would then counteract and further reduce the film velocity. In this case it is better to feed the condensate at the top and withdraw at the bottom of the bundle."  ---> Yes,  i ll change the direction to from the top.

 

Thank you so much.



#8 PingPong

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Posted 05 April 2020 - 11:41 AM

I don't know what the exact text is in your Design Standard.

 

Minimum velocity is normally specified for fouling fluids. In the case of water it applies to cooling water, sea water, river water, brackish water, raw water, .....

 

Steam condensate is very clean and therefor not fouling.



#9 zavtranguyen

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Posted 06 April 2020 - 12:59 AM

You mention 206 tubes per bundle, however in the spreadsheet it says 361 tubes/bundle in the first two sheets. That would reduce tubeside velocity to only 0.06 m/s.

 

Assuming the condensate is clean low tubeside velocity does not cause fouling.

 

Low tubeside velocity will however give a low tubeside film coefficient. Note also that you would make it worse by routing the condensate through the bundle from bottom to top, as natural convection would then counteract and further reduce the film velocity. In this case it is better to feed the condensate at the top and withdraw at the bottom of the bundle.

Dear Mr.PingPong,

"Note also that you would make it worse by routing the condensate through the bundle from bottom to top, as natural convection would then counteract and further reduce the film velocity. In this case it is better to feed the condensate at the top and withdraw at the bottom of the bundle."

 

Dear Mr.PingPong,

Do you any document mentioning about "heat transfer film coefficient for vertical tubeside upward flow" and  how does natural convection affect to forced convection? or any rule to select right direction for fluid flow? I just want to get more detail about this,

 

Thanks for your help.

 

 

 

 



#10 PingPong

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Posted 06 April 2020 - 02:36 AM

When a fluid is cooled, natural convection is downward. That's common knowledge.

So If a slow streaming fluid is cooled in a vertical pipe the optimal flow direction is from top to bottom so that natural convection adds to the forced convection. That's common sense.

Having the fluid flowing in opposite direction of natural convection would lower the heat transfer coefficient. That's obvious.

 

I am sure that there are textbooks (e.g. VDI Heat Atlas), websites and software that allow one to estimate/calculate the effect of natural convection in combination with forced convection.


Edited by PingPong, 06 April 2020 - 02:41 AM.





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