ASTM C680 1

[trashcanman]

If someone has the ASTM C680 standard, please confirm an item.
I have and old magazine (HPAC, Nov. 94) article for calculations for insulated piping systems. In it is stated that the standard has for convective coefficient the following (for pipes):

Hcv=C*((1/d)^.2)*((1/Tavg)^.181)*((1+1.277*wind)^.5)=BTUH/Ft^2*Deg.F

d=pipe od, inches
Tavg = average temp of air film
wind=air speed, mph
C=1.016 for horizontal pipes and C=1.235 for longer vertical cylinders.

The wind part is what I want to confirm. Is that what the standard really says?

Also, if you examine the equation, it appears that the higher the average air film temp, the lower the Hcv becomes, approaching 0. The same holds for increasing pipe OD.

Something does not look right. Thanks for replies in advance.

 

[pmover]

trashcanman,

in reviewing ASTM C680, there is nothing that resembles the Hcv equation. furthermore, the following paragraph is an excerpt regarding wind speed.

"X1.3.5.7 Wind Speed—Wind speed is defined as wind speed
measured in the main airstream near the subject surface. Air blowing across real objects often follows flow directions and
velocities much different from the direction and velocity of the
main free stream. The equations used in Practice C 680
analysis yield “averaged” results for the entire surface in
question. Because of this averaging, portions of the surface will
have different surface temperatures and heat flux rates from the
average. For this reason, the convective surface coefficient
calculation cannot be expected to be accurate at each location
on the surface unless the wind velocity measurements are made
close to the surface and a separate set of equations are applied
that calculate the local surface coefficients.

in review of ASTM C680 (June 2004 edition), most eqns and references are most likely provided in heat transfer textbooks - at least that i recognize.

while this is not much help, it is good that you are investigating the HPAC article.

fyi, ASTM C680 contains VB code for determining all needed heat transfer values and results. it may be worth your $ to purchase it and abandon the HPAC article.

good luck!
-pmover