Document 7OJRaR8y8ay2GbM3MGnm5K3OV

CHAPTER 28 1946 Guide greater than those given. Therefore, it is essential that insulation be ' applied in such a manner that air circulation within it or between it aiid the pipe is avoided. Fig. 4 shows the loss of heat from canvas-covered, cylindrical surfaces of various outside diameters when the surface to air temperature difference is low. The data are from tests made at Mellon Institute. The frequent practice of omitting insulation.on that portion of a pipe which passes through a masonry wall or which may be in contact with other metals should be avoided. Physical contact between the pipe surface and other structural materials of high thermal conductivity will ; , Table 8. Heat Gains for Insulated Cold Pipes Li - Rates of healjtransmission given in Btu per (hour) (Fahrenheit degree temperature iU ' ' . -difference betweenfluid in pipe and surrounding still air) I | Based on materials having conductivity, k - 0.30 jU ;[ i } _' Nominal Pips SlZB ' (Inches) LI [i n` ' i {l \ f. - ;. ' : Vt. H 1 Ws 2 2H 3 3H 4 5 6. 8 TO .12 Ice Water Thickness Brine Thickness Heatt Brine Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Btu Per Sq Ft Pipe Surface 1.5 1.6 1.6 1.6 1.5 1.5 1.5 1.5 1.51.7 1.7 1.7 ' 1.9 1.9 1.9 0.110 0.119 0.139 0.155 0.174 0.200 0.228 0.269 0.295 0.294 0.349 0.404 0.455 0.559 0.648 0.502 0.431 0.403 0.357 0.351 0.322 0.303 0.293 0.282 0.248 0.239 0.233 0.201 0.198 0.194 Thickness of Insulation (Inches) 2.0 2.0 2.0 2.4 2.5 2.5 2.6 2.7 2.9 2.9 3.0 3.0 3.0 3.0 3.0 Btu Per .Linear Foot Btu Per. Sq Ft Pipe Surface Thickness of Insulation (Inches) 0.098 0.446 0.111 0.405 0.124 0.352 0.131 0.300 0.134 ; 0.270 0.151 0.244 0.170 0.226 0.186 0.202 0.191 0.183 0.209 0.176 0.241 0.165 0.259 0.150 0.318 0.140 0.383 0.135 0.438 0.131 2.8 2.9 3.0 . 3.1 * 3.2 . 3.3 ' 3.3 3.4 3.5 3.7 3.9 4.0 4.0 4.0 4.0 Btu Per linear Foot 0.087 0.094 0.104 0.113 0.118 0.134 0.147 0.162 0.176 0.182 0.202 0.228 0.263 0.309 0.364 Btu Per Sq Ft .Pipe Surface 0.394 0.340 0.294 0.260 0.238 0,214 0.197 0.176 0.167 0.154 0.138 0.130 0.116 0.110 0.108 I ' , result in heat transfer much greater than that shown in'Tables 1 and 2 ri ' s for transfer from bare pipe to air. ' 1 ' The saving due to use of insulation on piping is illustrated in Example 4-' . . Example 4- If the steam line given in Examples 1 and 8 is covered with 1 in. thick : 85 per cent.magnesia, determine the resulting total annual loss through the insulation.' ' Also compute the monetary value of the annual saying and' the percentage of saying " ' over the heat loss from the bare pipe. ,' i ' Solution. . By referring to Fig. 1, the coefficient for 1 in. magnesia on a'.2-in. pipe is jj- , found to be 0.300 Btu per (hour) (linear foot of pipe) (degree temperature difference) , at a temperature difference of 169.4 F. The total hourly loss per linear foot of pipe will " then be 0.300 X' 169.4 = 50.8 Btu. The total annual loss- through the insulation = 50.8 X 165 (linear feet) X 4000 (hours) = 33,500 Mb. The annual bare pipe-loss as determined in the solution of Example 1 was found to be 181,600 Mb. The saving due ' . ( to insulation is then 181,600 -- 33,500. = 148,100 Mb per year. . From the solution of Example 8, it was found that the heat.supplied to the system cost S0.S04' per thousand Mb;- Therefore, the monetary value of the saving = 0,804 . (dollars) X 148.1 (thousand Mb). = $119.07, or 81,5 per cent of the cost when using uninsulated' pipe.' - ' ' ;* Pipe Insulation 523 LOW TEMPERATURE PIPE INSULATION Surfaces maintained at temperatures lower than the surrounding a}r are insulated to reduce the flow of heat and to prevent condensation. The insulating material should absorb a minimum amount of mois ture, because the absorption of moisture substantially increases the conductivity of the material. This property is particularly important in- Fig. 5. Thickness of Pipe Insulation to Prevent Condensation on Outer Surface1 .. the insulation of surfaces that are below the dew-point of 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 ofwater vapor which would condense 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. . There are a number of methods. ' of, producing vapor seals, some of which have been worked out by insula tion manufacturers to suit their products and others by applicators and