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Outside Film Coefficients Of Air-Cooled Heat Exchangers

film coefficients correlation air-cooled

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#1 Nikolai T

Nikolai T

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Posted 01 August 2019 - 01:03 AM



I would like to ask professionals if someone has correlations for outside film coefficients of Air-Cooled Heat Exchangers or can take a look on order of magnitude of values described below.


I faced with discrepancy between software results and plant data.


I found old paper with outside film coefficients diagrams in relation with surface ratio (Total/Bare), tube material, temperature of outside air and velocity of air in narrow cross section. After comparison it seems that calculated value  (972 kcal/h-m2-C) is almost twice of diagram value (542 kcal/h-m2-C). Both values based on bare outside tube area.


The Air-Cooled heat exchanger has horizontal tubes and forced draught.

Surface ratio (Total/Bare)=20

Velocity of air in narrow cross section=4,7 m/s

Face velocity=3,2 m/s


Velocity of air in narrow cross section is defined as (coefficient which depends on type of unit * air volume flowrate of fan * coefficient accounting presence of louvers)/air density at average temperature.


Could you please share your opinion?


Thank you,


Edited by Nikolai T, 01 August 2019 - 08:27 PM.

#2 breizh


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Posted 01 August 2019 - 01:49 AM

Hi Nikolai ,

a few documents to support your work (sorry for the quality).


other references:


Wolverine engineering data book II 






Good luck .


Edited by breizh, 01 August 2019 - 03:13 AM.

#3 PingPong


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Posted 01 August 2019 - 08:20 AM

I faced with discrepancy between software results with plant data.

So you are checking an existing aircooler?

From operating data you can calculate the actual U value.

However to estimate from that the air film coefficient you would first need to calculate the inside film coefficient of the fluid in the tubes. The air film coefficient obtained in this way will not be very accuarate.


Through the years the aircooler fins can become fouled and/or corroded which has a negative impact on the heat transfer. Software will not take that into account.


To calculate the theoretical air film coefficient you first of all need to know the tube diameter, tube pitch, fin material, fin height, fin thickness, fin spacing.


Various textbooks on heat transfer give simple formulas like the well known (but old) Briggs & Young equation.


The German VDI W√§rmeatlas (VDI Heat Atlas) gives other formulas for the air film coefficient, probably more accurate (deutsche Gr√ľndlichkeit) than B&Y:


Attached File  M1 Heat Transfer to Finned Tubes (VDI Heat Atlas).pdf   142.56KB   358 downloads

Edited by PingPong, 01 August 2019 - 08:33 AM.

#4 Nikolai T

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Posted 01 August 2019 - 07:26 PM

Dear Sirs,


Thank you for valuable help.


Yes, I'm checking an existing aircooler. I use EDR for modeling. Recently I modeled the aircooler in HTRI. The results are near the same. Probably the aircooler fouled despite cleaning while turnarounds. It was commissioned in 2006.

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