Document oDakXbpK31qRVQdknEkJ4kRQ3

458 CHAPTER 32 1959 Guide Pipe and Industrial Insulation 459 fig. 1 .... Heat Loss through 1 In. Thick Pipe Insulation (Ua trrfk Tafah 6 for Various bnMtoi tivities of these two materials at mean temperatures of 8S5 and 352.5 F, interpolated from Table 1, are 0.677 and 0.466 Btu, re spectively. These values are substituted in Equation 2 and a trial calcula tion made. The actual thickness of diatomaceous earth covering is 3695 in. and that of the 85 percent magnesia is 2.125 in. For a nominal 6-in. steel pipe: n = 3312, r -- 6.407, and r* -- 8332. Then, 6.407 ,, ^, 8532 833 + 534 8532 lmogg., -- . 8o*5>3u2* Iuoan* -f,--A--m-- Hie temperature chop from the outer surface of the insulation to the surrounding air for a heat los of 785 Btu is found from Fig. 5 to be 495 deg for a 17-in. OD cylindrical surface, or 495 + 80 F room temperature ~ 1295 F surface temperature. Since a surface temperature of 135 F was assumed, it is evident that a temperature closer to 1295 F, or, for instance, 129 F should be used for recalculation: 1200 - 129 9* 833 +5.24 783 Btu. Since the temperature drop through each material is egual to the heat flow times the actual resistance of each material, the temperature drop through the diatomaceous earth is 789 X Rg. 2 .... Heat Loss through In. Thick Pipe Insulation ((/ wilfi Table 6 for Various laadaHota] 833 = 657 F, or the temperature between the two insulating materials is (1200 -- 657) -- 543 F. Since a temperature of 570 F between the two materials was assumed, it is obvious that a temperature closer to 543, or for instance 545 F may be selected. The mean temperatures of the two insulations corresponding to the new assumptions are (1200 + 545) ? 2 = 8725, and (545 + 129) f2 = 337, and the interpolated conductivities correspond ing to the new mean temperatures are 0575 and 0.461 for the diatomaceous earth and 85 percent magnesia, respectively. By substituting in Equation 2 1200 - 1291071 9* 5.63 78.5 Btu. 2.44 " 834 + 5.30 0.675 + 0.461 Again referring to Fig. 5, it is seen that the temperature drop from the outer surface of the insulation to the surrounding air for a heat loss of 785 Btu -- 49 deg, which corresponds to the surface temperature of 129 F last assumed. The heat loss is therefore 785 X 8532 4- 3312 = 202 Btu per aq ft of pipe surface. Since the surface area per linear foot of 6-in. pipe is 1.734 sq ft (Table 2), the heat loss per linear foot of pipe will be 202 X 1.734 = 351 Btu per hr. The rate of heat loss from a surface maintained at constant temperature is greatly increased by air circulation over the surface, in the case of well-insulated surfaces, the increases in losses due to air velocity are very small as compared with increases from bare surfaces, because of the fact that air fig. A .... Heat loss through Insulation on Rat Vertical Surface lUe with Table 6 for Voriow fauutetioRs) Rg. 3-------Heat Loss through 2 In. Thick Pipe Insulation (U with Table 6 for Voriow fns&fafions) flowing over the surface of the insulation can increase onlythe conductance of heat from surface to air, and cannot change the internal conductance of the insulation itself. The maximum increase in heat loss due to air velocity ranges from about 15 percent in the case of 1-in. thick insulation, to about 5 percent in the case of 3-in. thick insulation, provided that the insulation is thoroughly sealed so that air can flow only over the surface. 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. Therefore, it is essential that insulation be applied in such a manner that air circulation within it, or between it and the pipe, is avoided. Piping which is covered with a water-mixed product or insulation which will absorb moisture, particularly in damp locations, should be coated with a corrosion-resistant paint. TTiis precaution may prevent early failure due to external pipe corrosion. The frequent practice of omitting insulation on that por tion 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 struc tural materials of high thermal conductivity will result in beat transfer much greater than that shown in Tables 2 and 3 for transfer from bare pipe to air. The saving due to use of insulation on piping is illustrated in Example 4.' Example 4: If the steam pipe given in Example* 1 and t is covered with nominal 1 in. thick 85 percent magnesia, determine the readting total annual loss through the insulation. Also com pute the monetary value of the annual saving ami the percentage of saving over the heat los from the bare pipe. Solution: By referring to Fig. 1, the coefficient for 1-in. insula tion on a 2-in. pipe is found to be 0376 Btu per (hr) (linear ft of pipe) (deg temperature difference) at a temperature difference of 159.4 F. The heat insulation factor for 85 percent nupaa'a insulation, interpolated from Table 6, for a temperature differ ence of 159.4 F, is 1085. The total hourly loss per lm*r foot of pipe will then be 0376 X 159.4 X 1585 = 47.7 Btu. The total annual loss through the insulation = 47J x 165 (linear ft) X (4000 hr) = 31,480 Mb. The annual bare-pipe loss as determined in the solution of Example 1 was found to be 171,100 Mb The saving due to insulation is then 171,100 -- 31,480 = 139520 Mb per year. From the solution of Example t, it was found that the heat supplied to the system cost SO804 per 1000 Mb. Therefore, the monetary value of the saving -- 0804 (dollare) X 1396 (thousand Mb) -- $11233 or 8J6 percent of the cost when using nninmilntaH pipe. THICKNESS OF INSULATIONS TO BE APPLIED Thermal insulation is used in a variety of thicknesses for many varied reasons. Refrigerating equipment is normally designed for minimum heat gain, and heating equipment for minimum heat loss, within economical limits. This assures minimum-sized refrigeration or heating equipment, both from the standpoint of original as well as operational cost. Present refrigeration design practice limits the heat gain to 4 to 8 Btu per (hr) (sq ft) of exposed surface. Sometimes the principal object of the insulation is to limit the heat gain 0r loss to a specified value to control the temperature of a manu facturing or chemical process. Minor refrigeration and air-conditioning equipment and cold-water lines usually utilize only sufficient insulation thick ness to prevent condensation on the warm surface so as to prevent dripping. Occasionally the criterion for heated equip ment and pipes is the reduction of the outer surface to a tem perature (usually a maximum of 175 F to 200 F) low enough to prevent injury to personnel. ECONOMICAL THICKNESS OF industrial insulation The thicknesses of insulation which ordinarily are used for various temperature conditions are given in Table 7. Where a thorough analysis of economic thickness is desired, the thickness of insulation that will give the lowest sum of onnnal cost of heat loss and insulation may be found by 0f Fig. 6* As the thickness of insulation increases the annual