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Average Convective Coefficient (air) flow pipes

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Old   July 27, 2001, 21:13
Default Average Convective Coefficient (air) flow pipes
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chetansardesai
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process and space heat steam flows in a pipe, 14 " dia. there is a 3" thickness Calcium Silicate Insulation, we are adding an inch of insulation. steam temp is @ 383F. the insulated pipe is exposed to the atmosphere, and has some supports at its base (separated by say 15 feet). Ambient temp varies between 95F (@35C) to -20F(@-26C). Wind speed is about 15 miles/hr.

Q1. Will the insulated pipe surface temperature be in any way related to the ambient temperature? Q2. Will the average convection heat transfer coefficient (include the radiation part aswell) depend only on the wind speed? Q3. Will wind direction matter significantly? Q4. What are the average values of the convective heat transfer coefficient for air (ho)at the pipe surface? What assumptions are made while stating the average values of heat transfer coefficients?

Several sources (textbooks)have mentioned values between 4.0 btu/ft^2/F (for summer) and 6.0 btu/ft^2/F (for winter).

Wanted to get a nod from GURUs on this forum.

thanks.
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Old   July 28, 2001, 15:03
Default Re: Average Convective Coefficient (air) flow pipe
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John C. Chien
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(1). The heat conduction through the insulator is a function of the temperature gradient (and the material used). So, thicker insulator material will raduce that temperature gradient. (2). To maintain this conduction heat loss, it must be carried away from the air side. In other words, there is a temperature difference between the air temperature and the insulator surface temperature (which will be a function of ambient air flow movement). (3). So, high wind will lower the insulator surface temperature, which will in turn increase the temperature gradient in the insulator and heat loss. (if we assume that the metal pipe wall thickness is relatively thin, then it will remain the same as the steam temperature, which can be assumed constant locally because of huge steam mass flow rate) (4). If you assume the steam temperature and the insulator surface temperature, then you can calculate the heat conduction problem, which will give you the heat loss. In this case, the heat transfer on the air side will be the key to your solution.
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Old   July 28, 2001, 16:49
Default Re: Average Convective Coefficient (air) flow pipe
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chetansardesai
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thanks for your comments. I found some emperical equations in ASHRAE that mention both convective and radiative heat transfer coefficients.

In this case, we have a white surface with emmisivity =0.9 (assumed, the range mentioned in ASHRAE was 0.85 to 0.95)

For calculating heat lost (AIR SIDE) by ho*(2*pi*radius including insulation thickness*Length of the pipe)*(Surface temp of the pipe - Bin Temp)*hrs in the bin /year. If for the entire pipe length, we have estimated relative percentages of lengths that have a particular temp; eg. 1000 feet has 110F (measured on a particular day with low cloud cover), 3000 ft has 105F etc. Can we take a weighted average surface temperature for the entire year? i.e average surface temp = (1000*110+3000*105)/(1000+3000) ?

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Old   July 28, 2001, 17:09
Default Re: Average Convective Coefficient (air) flow pipe
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John C. Chien
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(1). If the variation is not large, as in your case between 100 and 105F, then I guess a linear model is good enough. That is, a simple weighted average can be used.
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