For a thin-walled tube, the inner surface area ( A 1), outer surface area ( A 2), and log mean surface area ( A lm), are all very close to being equal. The equation can be simplified without losing much accuracy if the tube that is being analyzed is thin-walled, that is the tube wall thickness is small compared to the tube diameter. &=& \left( 18 ~ $$Įquation 2-10 for the overall heat transfer coefficient in cylindrical geometry is relatively difficult to work with. Calculate the heat transfer rate from the pipe into the room if the room temperature is 72☏. The convective heat transfer coefficient for the air is 18 Btu/hr-ft 2-☏. and the outer surface temperature is 280☏. The outer diameter of the steam line is 18 in. Values of h have been measured and tabulated for the commonly encountered fluids and flow situations occurring during heat transfer by convection.Ī 22 foot uninsulated steam line crosses a room. This is due to turbulent flow having a thinner stagnant fluid film layer on the heat transfer surface. Typically, the convective heat transfer coefficient for laminar flow is relatively low compared to the convective heat transfer coefficient for turbulent flow. The convective heat transfer coefficient ( h) is dependent upon the physical properties of the fluid and the physical situation. The basic relationship for heat transfer by convection has the same form as that for heat transfer by conduction:Ĭonvective heat transfer coefficient (Btu/hr-ft 2-☏) For flow in a pipe, T b is the average temperature measured at a particular crosssection of the pipe. For boiling or condensation, T b is the saturation temperature of the fluid. For flow adjacent to a hot or cold surface, T b is the temperature of the fluid "far" from the surface. The exact definition of the bulk temperature ( T b) varies depending on the details of the situation. In practice, analysis of heat transfer by convection is treated empirically (by direct observation).Ĭonvection heat transfer is treated empirically because of the factors that affect the stagnant film thickness:Ĭonvection involves the transfer of heat between a surface at a given temperature ( T s) and fluid at a bulk temperature ( T b). Heat transfer by convection varies from situation to situation (upon the fluid flow conditions), and it is frequently coupled with the mode of fluid flow. Heat transfer by convection is more difficult to analyze than heat transfer by conduction because no single property of the heat transfer medium, such as thermal conductivity, can be defined to describe the mechanism. The transfer of heat from the surface of a heat exchanger to the bulk of a fluid being pumped through the heat exchanger is an example of forced convection. The transfer of heat from a hot water radiator to a room is an example of heat transfer by natural convection. The term forced convection is used if this motion and mixing is caused by an outside force, such as a pump. The term natural convection is used if this motion and mixing is caused by density variations resulting from temperature differences within the fluid. As an example of heat transfer, white objects absorb very little heat – check that they will be slower to heat up during hot summers.Heat transfer by the motion and mixing of the molecules of a liquid or gas is called convection.Ĭonvection involves the transfer of heat by the motion and mixing of "macroscopic" portions of a fluid (that is, the flow of a fluid past a solid boundary). The more radiation a body absorbs, the higher the heat transfer. Radiative heat transfer – electromagnetic waves can also transfer heat when they come into contact with matter. See how a fireplace works: heated air expands and rises throughout the room, while cooler air descends towards the fire where it is heated. Conduction is a very effective method of heat transfer in metals, but gases such as air conduct heat poorly.Ĭonvective heat transfer – usually refers to fluids or gases that exchange heat with other objects during their free movement. There are three different types of heat transfer:Ĭonductive heat transfer – the transfer of heat from one molecule to another through direct contact between objects. We explained more about heat flow in our thermal equilibrium calculator. When the temperature increases, the kinetic energy of the molecules also increases. The energy in the form of heat is transferred from the molecules in the first system to the second system. Heat transfer occurs when one system comes into contact with another low-temperature system.
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