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Design Margin

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#1 Chemitofreak


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Posted 11 June 2018 - 04:55 AM



Can any one explain the meaning of the following, I have came across the same in one of the Air Cooler Datasheet:


Exchanger to be designed 125% of flow and 120% of surface Area


Does it mean that the exchanger shall have 25% margin on the flowrate and 20% excess area on top of that.


For me 20% excess area for a Air Cooler is over the top requirement.


Thanks in advance. 



#2 Chemitofreak


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Posted 15 June 2018 - 02:20 AM

# Response Awaited.

#3 Saml


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Posted 15 June 2018 - 06:33 AM

It means what the client wanted to mean. The only way to know is to ask for clarification. If you are reviewing an existing exchanger, you would have to re-calculate it and make an educated guess as to what was designed.


- May be that it should work at 125% flow or have 20% excess area whatever gives the bigger design.

- Or may the client wanted a design for 125% flow and then to add 20% area.

#4 Technical Bard

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Posted 20 June 2018 - 11:05 PM

Depending on the service, this might be a BAD idea.  If the exchanger is fouling, adding extra surface and designing for extra flow will oversize the exchanger, reduce velocities and increase the propensity for fouling.   It the air cooler doesn't have very good control (VFDs / speed control on fans, louvers), excess air will lead to overcooling, which might not be a good thing - it depends on the process.


This criteria will oversize the air cooler by 50%.  Add traditional fouling factors to that and you might have twice the air cooler you need (and pay twice as much for it). 


I would ask the Client if that was their intention.

#5 Process Engineer_DES

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Posted 28 June 2018 - 11:08 PM

As pointed out earlier, this will oversize the air cooler. A 25% margin over the operating flow will give a higher area, and an additional 20% on the already high area will lead to an even larger air cooler. This will lead to the following issues: 

Over cooling and resulting condensation (depending on service)

Higher cost due to bigger structure

Larger plot area (in case plot plan is predetermined)

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