Document J3NyNe112Oz97aRN3nJjYRwJ2

-H HEATING VENTILATINC AIR CONDITIONING GUIDE 1940 D = diameter of round ducts, feet. V = velocity of air in the duct, feet per minute, at specified temperature. d = density of air, pounds per cubic foot, at the specified temperature at which V is measured. e = naperian base of logarithms = 2.718. In using formulae 8, 9, and 10, one of the duct air temperatures will be unknown and will be solved for by substitution of the other known or assumed values. The assumed values dependent upon the mean duct air temperature can be determined exactly by the.cut-and-try method. Heat Transmission Through Duct Walls Insulated with Materials Table 15. of Varying Conductivities* Values are expressed in Biu per hour per square foot of flat surface per degree Fahrenheit difference in temperature between air inside and still air outside at 90 F for mlA nir and 50 F for warm air in ducts Cold Am ! AlE CoNDUCTivrrT Insulation at 86 F Thickness Insulation (Inches) 40 F 60F I 80 F I 0F | 120 F Temperature Difference 50 F 1 30 F 10 F | 40 F 70 F 150 F 100 F 180 F 130 F 0.200 0.250 0.300 0.350 0.4S0 0.550 , A 2 A 2 A 1 y2 2 A lA 2 72 2 72 2 0.319 0 175 0.121 0.092 0.382 (L214 0.149 0.114 0.440 0 252 0.176 0.135 0.494 0 286 0.202 0.156 0.323 0 177 0.122 0.093 0.328T 0.324 0.180 0.178 0.124 | 0.095.j 0.387 0.217 0.151 0.115 0.392 0.220 0.153 0.117J 0.390 0.218 0.445 0.255 0.178 0.137 0.450 0.258 0.180 0.139 0.448 0.256 ... . . . 0.499 0.289 0.204 0.158 0.505 0.292 0.207 0.160 0.502 0.290 n 506 0 356 o!254 0.198 Q 6R2 n 417 0.302 0.236 0.602 0.599 0.360 0.358 0.257 ........ 0.200 1 ........ 0.688 1 0.685 0.422 1 0.418 0.305 1 ........ 0.239 1 ........ 0.330 0.181 0.125 0.337 0.184 0.127 0.397 0.221 0.154 0.404 0.225 0.156 0.457 0.260 0.181 0.466 0.264 0.184 0.511 0.295 0.208 0.521 0.300 0.211 0.610 0.364 0.259 0.621 0.370 0.263 0.699 0.714 0.425 0.432 0.307 0.312 1 0.344 0.188 0.129 0.412 0.229 0.159 0.475 0.268 0.187 0.530 0.306 0.215 0.633 0.376 0.267 0.730 0.440 0.31/ .For round ducts less than 30 in. diameter, increase heat transmission values by the following percent- ages: Thickness of Insulation (Inches) 1 1H 2 111% 11 n21 to 30 in. Duct Diameter--------------- --------------------------- 12 to 21 in- Duct Diameter.------------------------------------------ 3% 696 CHAPTER 40. PIPE AND DUCT INSULATION - Heat losses for insulated ducts are given in the warm air column of Table 15: The losses are based on a uniform series of material con ductivities at 86 F mean temperature and an air temperature of 50 F Outside of the duct. The losses may be interpolated for odd material conductivities and temperatures. The conductivities of various materials will be found in Table 2 of Chapter 5. For cases where the surrounding air temperature is other than 50 F, the losses may be selected on the basis of temperature difference. i. Recently, a new prefabricated insulated duct built entirely of asbestos i has been placed on the market. Example 4 Determine the entering air temperature and heat loss lor a duct 24 X 36 in. cross-section and 70 ft in length, insulated with Yi in. of a material having a con ductivity of 0.35 Btu at-86 F mean temperature, carrying air at a velocity of 1200 fpm, measured at 70 F, .to deliver air at 120 F with air surrounding the duct at 40 F. Solution. Assume the entering air temperature to be 130 F. Thus, the mean tem perature difference will be 85 F. Referring to the warm air column of Table 15 and interpolating for .85 F temperature difference, the overall heat transmission coefficient is found to be 0.516 Btu. From Table 4 Chapter 1 the density of air at 70 F and,29.92 in. Hg. is found to be 0.07423 lb per cubic foot. Substituting these and the other given values in Formula 8: 28.8 X 6 X 0.07423-X 1200 0.516 X 10 X 70 = '42.61 . .. 120 (42.61 -1-1) - 80 ll ~ ------ 42l;r-T--_ ' 123 6 Based on 123.8 F entering air temperature, the new mean temperature difference will' be81.9 F arid the new transmission coefficient will be 0.515. Resubstitutirig in Formula* 8, ti becomes 123.9 F. Substituting in formula 6: Q = 0.515 X 10 X 70 ^lg3-9 2+ 129j _ 40 J Q = 29.543 Btu LOW TEMPERATURE INSULATION Surfaces maintained at temperatures lower than, the surrounding air are insulated to reduce the flow of heat and to prevent condensation and frost. ;The insulating material should absorb a minimum amiount. of moisture, for. one. reason'that the absorption of moisture substantially increases the ^conductivity of the material. ; This property is particularly important in the case of surfaces to be: insulated that are below the dew pointof the; surrounding air. In such cases, due to! vapor pressure difference,' it; is necessary to seal the .surface of the insulating material against the penetration of water vapor which would condensef within the material, causing a serious increase in heat flow, possible breakdown of the material'and corrosion of metal surfaces. An insulating material with a high degree of moisture absorption might pick up moisture before application and then, when the seal is in place and the temperature of the insulated surface reduced, release that moisture to the cold' surface. The thickness. :of insulation required to prevent sweating. is that thickness which will raise the temperature of the-outer-surface-of the 697 I