Document 93Jz96nGn1gw2RG1BGwo2vwo3

American Society of Heating and Ventilating Engineers Guide, 1929 EFFECT OF AIR VELOCITY ON SURFACE LOSSES The rate of heat loss from a surface maintained at constant temperature is greatly increased by air circulation over the surface. Fig. 8 is based on Langmuir's equations {Trans. Am. Electro-Chem. Soc., Vol. 23). Other investigators have shown even greater increases in rates of heat loss from bare surfaces due to air velocity. Chapter XV--Heat Insulation for Pipes and Surfaces If the conditions are such that the air may circulate through cracks and crevices in the insulation, the increases may be far greater than those given above. Therefore, it is essential that insulation be sealed as tightly as possible. Pipe insulation out-of-doors should be provided with a weather-proof jacket, and other.outdoor insulation should be thoroughly weather-proofed. Fig. 5. Heat Loss through 1 in. thick Covering In the case of well-insulated surfaces the increases in losses due to air velocity are very small as compared with increases shown above for bare surfaces, because of the fact that air -flowing over the surface of- the insulation can increase only the rate of heat transfer from surface to air ' and cannot change the internal resistance to heat flow inherent in the insulation itself. The maximum increase in loss due to air velocity ranges from about 30 per cent in the case of Tin. thick insulation, to about.,. 10 per cent in the case of 3 in. thick insulation, provided that the insula tion is thoroughly sealed so that air can flow only over the surface. 254 Fig. 6.. Heat Loss through 11^ in. thick Covering ECONOMIC THICKNESS OF INSULATION Table 6 shows the thickness of insulation which are ordinarily used for' various temperature conditions. Where a thorough analysis of economic thickness is desired, this may be accomplished through the use of the chart, Fig. 9i The dotted line on the chart illustrates its use in solving a typical example. In order to use the chart, start at the lower left hand corner and proceed to the right to a point representing the given number of hours of operation per year; then proceed vertically to the line representing the given value of heat; thence horizontally, to the right, to the line repre- 255