Heat Transfer CONVECTION HEAT TRANSFERThe basic relationship for heat transfer by convection has the same form as that for heat transferby conduction:(2-9)Qh A DTwhere:= rate of heat transfer (Btu/hr)Qh = convective heat transfer coefficient (Btu/hr-ft^{2}-°F)A = surface area for heat transfer (ft^{2})DT = temperature difference (°F)The convective heat transfer coefficient (h) is dependent upon the physical properties of the fluidand the physical situation. Typically, the convective heat transfer coefficient for laminar flowis relatively low compared to the convective heat transfer coefficient for turbulent flow. This isdue to turbulent flow having a thinner stagnant fluid film layer on the heat transfer surface.Values of h have been measured and tabulated for the commonly encountered fluids and flowsituations occurring during heat transfer by convection.Example:A 22 foot uninsulated steam line crosses a room. The outer diameter of the steam lineis 18 in. and the outer surface temperature is 280^{o}F. The convective heat transfercoefficient for the air is 18 Btu/hr-ft^{2}-^{o}F. Calculate the heat transfer rate from the pipeinto the room if the room temperature is 72^{o}F.Solution:Qh A DTh (2 p r L) DT18Btuhr ft^{2} °F2 (3.14) (0.75 ft) (22 ft) (280°F72°F)3.88 x 10^{5}^{Btu}hrMany applications involving convective heat transfer take place within pipes, tubes, or somesimilar cylindrical device. In such circumstances, the surface area of heat transfer normally givenin the convection equation ( ) varies as heat passes through the cylinder. In addition,Qh A DTthe temperature difference existing between the inside and the outside of the pipe, as well as thetemperature differences along the pipe, necessitates the use of some average temperature valuein order to analyze the problem. This average temperature difference is called the log meantemperature difference (LMTD), described earlier.Rev. 0 Page 19 HT-02

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