Document Znqz2dp0D81LzdGrevbjz2eK7

American Society of Heating and Ventilating Engineers Guide, 1929 with the square of the tube diameter, and inversely with the 0.63 power of the absolute viscosity in centipoises. In this formula, c and 2 are taken at the temperature of the.body of the liquid rather than at the temperature of the film. A formula given by McAdams and Frost from experiments on water, using film temperature is: , 15.4 K /, , 60\ jD V\-8 / - -- i1 + t) hv t = ratio of length of tube to diameter. As an approximation, the above formula may be used for liquid flowing outside tubes by using four times the hydraulic radius for D. The Chapter XVII--Heat Exchangers for Water and Oil A-COUNTER CURRENT FLOW or rujios Fig. 2. C-heating fujid by CONOCNSING PURE VAPOR 0* COOLING FLUID BY VAPORIZING PURE FLUID Temperature Differences for Various Conditions of Heat Transfer turbulence is usually much greater than is indicated by this means, especially when cross flow occurs. The value of h found from the equations is, of course, only the heat transfer rate from the tube through one liquid film. If the problem to be considered is the transfer of heat frbm oil to oil, or from water to water, the reciprocals of the three heat transfers through heating film, tube, cooling film, and any scale or dirt, must be added to obtain the overall resistance. In the absence of non-condensable gases, the coefficient'of heat transfer from condensing steam to a tube may be from 1500 to 3000, or rs = 0.0007 to 0.0003. Oil in the steam increases the resistance greatly and. the resistance for condensing oil vapors, benzol, or other materials is much higher than for steam. The resistance of the tube wall to heat flow can be easily calculated, knowing the conductivity of the metal. It is based upon: the.; logarithmic mean diameter of the tube, which can be taken as the arithmetic average diameter, with very little error in all ordinary '280 UX*- mean temp ctrr dmdcd by orbatcst temr an: Fig. 3. Chart for Calculation of Logarithmic Mean Temperature Difference cases. If a = thickness in inches, Dz = average diameter of tube, and K = heat conductivity in B.t.u. per hour per square foot per degree per foot thickness, rt = i2~K ~ res`stance of tube wall to heat flow. 12 K hi =---a--- = conductance of tube wall. Values of K are given in the following table for different metals. There is a slight variation in K with temperature, but this change can be dis regarded in almost all cases. Silver............. Copper........... Aluminum..... Bronze........... Wrought Iron Cast Iron.;__ Steel.... ...:.... ;. Lead............... .K 240 220 120 64 26 36 26 20 From the above table, the resistance of a copper tube Ys in. thick would . 0.125 be --- Z2\j XI/- = 0.000047, while that of a steel tube of .the same 281