Convective heat transfer coefficient for air 6

[skuntz]

Where could I find some values for the heat transfer coefficient for air under natural convection? I have to believe that this has been worked out many times somewhere.

 

[SNORGY]

I typically use about 5 BTU/(hr-ft^2-F) for pipe to still air. Hasn't got me into trouble yet.

Regards,

SNORGY.

 

[corus]

Natural convection to air depends on the surface orientation as well as the temperature difference between the surfae and fluid.

I use formulae from McAdams Heat Transmission, which gives h=1.776(DT)^0.25 for air for a vertical surface.

corus

 

[IRstuff]

Seems a tad high. Most textbooks put it between 0.2 and 4 BTU/hr-ft^2-°F

http://www.tufts.edu/as/tampl/en43/lecture_notes/ch5.html

However, your mileage may vary, so you can, and should, calculate it for your specific scenario:

http://en.wikipedia.org/wiki/Natural_convection

TTFN

FAQ731-376

 

[25362]

J.P.Holman's Heat Transfer McGraw-Hill, Para. 7-9 (Table 7-2) gives simplified equations for free convection from various surfaces for laminar and turbulent flow.

 

[corus]

Sorry, I was using SI units.

corus

 

[TGS4]

The "Standard" way of calculating this can be found in the ASTM C680 Standard.

The formulae, calculations, etc, are from Fundamentals of Heat and Mass Transfer by Incropera and DeWitt.

 

[gwolf2]

I tend to use Holman too.

 

[QMax1]

Depending on your conservative slant, I would say 1 BTU/hr ft^2 F is a safe bet for free convection of air. If your conservative slant is at the high side, I think the 5 BTU/hr ft^2 F is a good number. Depending on what you are calculating, you may not need to use any formulas.

 

[SNORGY]

With a 7-month winter, with 2 of those months having frequent excursions into the -35 C range, I tend to predict a high heat transfer coefficient to make sure we insulate and trace the heck out of stuff.

There's nothing like starting up a sulphur plant in central Alberta in January, for example.

Regards,

SNORGY.

 

[ione]

Maybe this calculator can help

http://www.thermal-wizard.com/tmwiz/default.htm

 

[skuntz]

What a great resource. Thanks. By the way I found a copy of Holman's book at Albris for $1.99!