Convection

Heat Transfer CONVECTION HEAT TRANSFER The basic relationship for heat transfer by convection has the same form as that for heat transfer by conduction: (2-9) Q h A DT where: = rate of heat transfer (Btu/hr) Q h = convective heat transfer coefficient (Btu/hr-ft²-°F) A = surface area for heat transfer (ft²) DT = temperature difference (°F) The convective heat transfer coefficient (h) is dependent upon the physical properties of the fluid and the physical situation. Typically, the convective heat transfer coefficient for laminar flow is relatively low compared to the convective heat transfer coefficient for turbulent flow. This is due 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 flow situations occurring during heat transfer by convection. Example: A 22 foot uninsulated steam line crosses a room. The outer diameter of the steam line is 18 in. and the outer surface temperature is 280^oF. The convective heat transfer coefficient for the air is 18 Btu/hr-ft²-^oF. Calculate the heat transfer rate from the pipe into the room if the room temperature is 72^oF. Solution: Q h A DT h (2 p r L) DT � � � � � � 18 Btu hr ft² °F 2 (3.14) (0.75 ft) (22 ft) (280°F 72°F) 3.88 x 10⁵^Btu hr Many applications involving convective heat transfer take place within pipes, tubes, or some similar cylindrical device. In such circumstances, the surface area of heat transfer normally given in the convection equation ( ) varies as heat passes through the cylinder. In addition, Q h A DT the temperature difference existing between the inside and the outside of the pipe, as well as the temperature differences along the pipe, necessitates the use of some average temperature value in order to analyze the problem. This average temperature difference is called the log mean temperature difference (LMTD), described earlier. Rev. 0 Page 19 HT-02