ENERGY MANAGEMENT HANDBOOKS phần 6

INDUSTRIAL INSULATION 451 surface temperature is directly related to the surface resistance R , which in turn depends on the emittance of the surface. As a result, an aluminum jacket will be hotter than a dull mastic coating over the same amount of insulation. This is demonstrated below. Calculation The objective is to calculate the amount of insulation required to attain a specifi c sur-face temperature. As noted earlier, th – ts = ts – ta 1 s Therefore, R1 = Rs ts – ta = tk flat = Eq tk pipe Fig. 15.3 Equivalent thickness chart. (From Ref. 16.) Therefore, Table 1 tk or Eq tk = kR th – ts s a 5.2 Equivalent thickness values for even insulation thicknesses. Nominal Pipe Size (in.) r1 Actual Thickness (in.) 1 1-1/2 2 2-1/2 3 3-1/2 4 1/2 0.420 1.730 3/4 0.525 1.626 1 0.658 1.532 1-1/4 0.830 1.447 1-1/2 0.950 1.403 2 1.188 1.337 2-1/2 1.438 1.287 3 1.750 1.242 3-1/2 2.000 1.217 4 2.250 1.194 4-1/2 2.500 1.178 5 2.781 1.163 6 3.313 1.138 7 3.813 1.120 8 4.313 1.108 9 4.813 1.097 10 5.375 1.088 11 5.875 1.079 12 6.375 1.076 14 7.000 1.069 16 8.000 1.059 18 9.000 1.053 201 0.000 1.048 24 12.000 1.040 30 15.000 1.032 Source: Ref. 1 6. 2.918 4.238 5.662 7.172 2.734 3.966 5.297 6.712 2.563 3.711 4.953 6.275 2.405 3.472 4.626 5.856 2.321 3.342 4.449 5.629 2.195 3.148 4.177 5.276 2.099 2.997 3.968 5.001 2.012 2.858 3.771 4.742 1.959 2.772 3.649 4.582 1.916 2.704 3.549 4.448 1.880 2.645 3.464 4.337 1.846 2.590 3.388 4.231 1.799 2.510 3.270 4.071 1.761 2.453 3.184 3.956 1.737 2.407 3.116 3.863 1.714 2.369 3.056 3.783 1.693 2.333 3.007 3.714 1.675 2.305 2.972 3.663 1.662 2.286 2.936 3.619 1.647 2.265 2.900 3.569 1.639 2.231 2.858 3.504 1.622 2.206 2.822 3.449 1.608 2.188 2.789 3.411 1.589 2.163 2.736 3.347 1.572 2.122 2.704 3.281 8.755 10.402 8.199 9.747 7.665 9.117 7.153 8.507 6.872 8.171 6.436 7.648 6.093 7.234 5.768 6.840 5.564 6.592 5.396 6.386 5.253 6.211 5.118 6.043 4.911 5.790 4.759 5.604 4.644 5.452 4.541 5.330 4.450 5.214 4.383 5.123 4.321 5.048 4.258 4.969 4.178 4.866 4.110 4.776 4.051 4.711 3.971 4.598 3.874 4.497 452 ENERGY MANAGEMENT HANDBOOK Table 15.3 Equivalent thickness values for simplified insulation thicknesses. Nominal Pipe Size (in.) r1 Actual Thickness (in.) 1 1-1/2 2 2-1/2 3 3-1/2 4 1/2 0.420 1.730 3/4 0.523 1.435 1 0.638 1.715 1-1/4 0.830 1.281 1-1/2 0.950 1.457 2 1.188 1.438 2-1/2 1.438 1.383 3 1.750 1.286 3-1/2 2.000 1.625 4 2.230 1.281 4-1/2 2.300 1.564 5 2.781 1.202 6 3.313 1.138 7 3.813 8 4.313 9 4.813 10 5.375 11 5.875 12 6.375 14 7.000 16 8.000 18 9.000 20 10.000 24 12.000 30 15.000 Source: Ref. 16. 3.053 4.406 6.787 8.253 2.660 3.885 5.996 7.447 2.770 4.013 5.358 6.702 2.727 3.333 4.552 5.777 2.382 4.025 5.253 6.476 2.367 3.398 4.446 5.561 2.765 3.657 4.737 5.815 2.114 2.968 3.889 4.868 2.459 3.258 4.166 5.251 2.010 2.806 3.659 4.059 2.351 3.152 4.905 4.962 1.893 2.639 3.489 4.339 1.799 2.555 3.317 4.122 1.804 2.495 3.230 4.153 1.776 2.445 3.391 4.010 1.752 2.579 3.232 3.971 1.810 2.457 3.108 3.850 1.793 2.428 3.140 3.793 1.777 2.405 3.103 3.745 1.647 2.265 2.900 3.569 1.639 2.231 2.858 3.504 1.622 2.206 2.822 3.449 1.608 2.188 2.789 3.411 1.589 2.163 2.736 3.347 1.572 2.122 2.704 3.281 9.972 12.712 8.965 10.642 8.112 9.581 7.070 8.420 7.759 9.179 6.733 8.027 7.015 8.195 5.965 7.046 6.266 7.256 5.577 6 543 5.907 7 080 5.230 6.461 5.237 6.015 4.969 5.821 4.842 5.768 4.786 5.583 4.591 5.361 4.519 5.271 4.456 5.241 4.258 4.969 4.178 4.866 4.110 4.776 4.051 4.711 3.971 4.598 3.874 4.497 Table 15.4 Rs Valuesa (hr • ft2 °F/Btu). t – t (°F) 10 25 50 75 100 With Wind Velocities Wind Velocity (mph) 5 10 20 Still Air Plain, Fabric, Dull Metal: ε = 0.95 0.53 0.52 0.50 0.48 0.46 0.35 0.30 0.24 Aluminum: ε = 0.2 0.90 0.88 0.86 0.84 0.80 0.41 0.35 0.28 Stainless Steel: ε = 0.4 0.81 0.79 0.76 0 75 0 72 0.40 0.34 0.27 Source: Courtesy of Johns-Manville, Ref. 16. aFor heat-loss calculations, the effect of R is small compared to R , so the accuracy of R is not critical. For surface temperature calculations, Rs is the controlling factor and is therefore quite critical. The values presented in Table 15.4 are commonly used values for piping and flat surfaces. More precise values based on surface emittance and wind velocity can be found in the references. INDUSTRIAL INSULATION Example. For a 4-in. pipe operating at 700°F in an 85°F ambient temperature with aluminum jacketing over the insulation, determine the thickness of calcium silicate that will keep the surface temperature below 140°F. Since this is a pipe, the equivalent thickness must fi rst be calculated and then converted to actual thickness. STEP 1. Determine k at t = (700 + 140)/2 = 420°F. k = 0.49 from Table 15.1 or appendix Figure 15.A1 for calcium silicate. 453 the insulation equal to the heat loss off the surface, fol-lowing the discussion in Section 15.4.2. Figure 15.4 will be used for several different cal-culations. The following example gives the four-step procedure for achieving the desired surface temperature for personnel protection. The accompanying diagram outlines this procedure. STEP 2. Determine R from Table 15.4 for aluminum. ts – ta = 140 – 85 = 55. So Rs = 0.85. STEP 3. Calculate Eq tk: Eq tk = (0.49)(0.85) = 4.24 in. 700 – 140 140 – 85 Example. We follow the procedure of the fi rst example, again using aluminum jacketing. STEP 4. Determine the actual thickness from Table 15.2. The effect of 4.24 in. on a 4-in. pipe can be accomplished by using 3 in. of insulation. Note: Thickness recommendations are always increased to the next 1-in. increment. If a surface tem-perature calculation happens to fall precisely on an even increment (such as 3 in.), it is advisable to be conserva-tive and increase to the next increment (such as 3-1/2 in.). This reduces the criticality of the Rs number used. In the preceding example, it would not be unreasonable to recommend 3-1/2 in. of insulation, since it was found to be so close to 3 in. To illustrate the effect of surface type, consider he same example with a mastic coating. Example. From Table 15.4, Rs = 0.50, so Eq tk = (0.49)(0.50) ————— = 2.49 in. This corresponds to an actual thickness require-ment on a 4-in. pipe of 2 in. This compares with 3 in. required for an aluminum-jacketed system. It is of inter-est to note that even though the aluminum system has a higher surface temperature, the actual heat loss is less because of the higher surface resistance value. Graphical Method The calculations illustrated above can also be car-ried out using graphs which set the heat loss through STEP 1. Determine ts – ta, 140 – 85 = 55°F. STEP 2. In the diagram, proceed vertically from (a) of ∆t = 55 to the curve for aluminum jacketing (b). STEP 2a. Although not required, read the heat loss Q = 65 Btu/hr ft2) (c). STEP 3. Proceed to the right to (d), the appropriate curve for t – t = 700 – 140 = 560°F. Interpolate between lines as necessary. STEP 4. Proceed down to read the required insulation resistance Rt = 8.6 at (e). Since R = tk/k or Eq tk/k, tk or Eq tk = RIk tm = ——————— = 420°F k = 0.49 from appendix Figure 15.A1 and tk or Eq tk = (8.6) (0.49) = 4.21 in. which compares well with the 4.24 in. from the earlier calculation. The conversion of Eq tk to actual thickness re-quired for pipe insulation is done in the same manner, using Figure 15.3. A better understanding of the procedure involved in utilizing this quick graphical method will be obtained 454 after working through the remainder of the calculations in this section. 15.4.4 Condensation Control On cold systems, either piping or equipment, in-sulation must be employed to prevent moisture in the warmer surrounding air from condensing on the colder surfaces. The insulation must be of suffi cient thickness ENERGY MANAGEMENT HANDBOOK to keep the insulation surface temperature above the dew point of the surrounding air. Essentially, the cal-culation procedures are identical to those for personnel protection except that the dew-point temperature is substituted for the desired surface temperature. (Note: The surface temperature should be kept 1 or 2° above the dew point to prevent condensation at that tempera-ture.) Dew-Point Determination The condensation (saturation) temperature, or dew point, is dependent on the ambient dry-bulb and wet-bulb temperatures. With these two values and the use of a psychrometric chart, the dew point can be determined. However, for most applications, the relative humidity is more readily attainable, so the dew point is determined using dry-bulb temperature and relative humidity rather than wet-bulb. Table 15.5 is used to find the proper dew-point temperature. Calculation This equation is identical to the previous surface-temperature problem except that the surface tempera-ture ts now takes on the value of the dew point of the ambient air. Also, t now represents the cold operating temperature. tk or Eq tk = kRs ts – ta Fig. 15.4 Heat loss and surface temperature graphical method. (From Ref. 16.) INDUSTRIAL INSULATION Example. For a 6-in.-diameter chilled-water line op-erating at 35°F in an ambient of 90°F and 85% RH, determine the thickness of fiberglass pipe insulation with a composite kraft paper jacket required to prevent condensation. STEP 1. Determine the dew point (DP) using either a psychrometric chart or Table 15.5. DP at 90°F and 85% RH = 85°F. (In Step 5, the thickness is rounded up, which yields a higher temp.) STEP 2. Determine k at t = (35 + 85)/2 = 60°F. k at 60°F = 0.23, from Table 15.1 or appendix Figure 15.A2. STEP 3. Determine Rs from Table 15.4. ∆t here is (ta,4– ts) rater than (ts – ta), ta – ts = 90 – 85 = 5°F, Rs = STEP 4. Calculate Eq tk. Eq tk = 0.23 0.54 85 – 90 = 1.24 in. 455 STEP 5. Determine the actual thickness from Fig-ure 15.2 for 6-in. pipe, 1.24 in. Eq tk. The actual thickness is 1.5 in. Graphical Method The graphical procedures are as described in Section 15.4.3. As the applications become colder, it is apparent that the required insulation thicknesses will become larger, with R values toward the right side of Figure 15.4. It is suggested that the graphical procedure not be used when the resulting R values must be de-termined from a very flat portion of the (t – t ) curve (anytime the numbers are to the far right of Figure 15.4). It is difficult to read the graph with sufficient accuracy, particularly in light of the simplicity of the mathematical calculation. Thickness Chart for Fiberglass Pipe Insulation Table 15.6 gives the thickness requirements for fi -berglass pipe insulation with a white, all-purpose jacket in still air. The calculations are based on the lowest Table 15.5 Dew-point temperature. Dry-Bulb Temp. (°F) 10 15 Percent Relative Humidity 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 5 – 35 – 30 10 – 31 – 25 15 – 28 – 21 20 – 24 – 16 25 – 20 – 15 30 – 15 – 9 35 – 12 – 5 40 – 7 0 45 – 4 3 50 – 1 7 55 3 11 60 6 14 65 10 18 70 13 21 75 17 25 80 20 29 85 23 32 90 27 36 95 30 40 100 34 44 105 38 48 110 41 52 115 45 56 120 48 60 125 52 63 – 25 – 21 – 17 – 14 – 12 – 10 – 8 – 6 – 5 – 4 – 2 – 1 1 – 20 – 16 – 13 – 10 – 7 – 5 – 3 – 2 0 2 3 4 5 – 16 – 12 – 8 – 5 – 3 – 1 1 3 5 6 8 9 10 – 8 – 4 – 2 2 4 6 8 10 11 13 14 15 16 – 8 – 4 0 3 6 8 10 12 15 16 18 19 20 – 3 2 5 8 11 13 15 17 20 22 23 24 25 1 5 9 12 15 18 20 22 24 26 27 28 30 5 9 14 16 19 22 24 26 28 29 31 33 35 9 13 17 20 23 25 28 30 32 34 36 38 39 13 17 21 24 27 30 32 34 37 39 41 42 44 16 21 25 28 32 34 37 39 41 43 45 47 49 20 25 29 32 35 39 42 44 46 48 50 52 54 24 28 33 38 40 43 46 49 51 53 55 57 59 28 33 37 41 45 48 50 53 55 57 60 62 64 32 37 42 46 49 52 55 57 60 62 64 66 69 35 41 46 50 54 57 60 62 65 67 69 72 74 40 45 50 54 58 61 64 67 69 72 74 76 78 44 49 54 58 62 66 69 72 74 77 79 81 83 48 54 59 63 67 70 73 76 79 82 84 86 88 52 58 63 68 71 75 78 81 84 86 88 91 92 56 62 67 72 76 79 82 85 88 90 93 95 97 60 66 71 77 80 84 87 90 92 95 98 100 102 64 70 75 80 84 88 91 94 97 100 102 105 107 68 74 79 85 88 92 96 99 102 105 107 109 112 72 78 84 89 93 97 100 104 107 109 111 114 117 2 3 4 5 7 8 9 10 12 13 14 15 18 19 20 21 23 24 25 27 28 29 30 32 33 34 35 36 38 39 40 41 43 44 45 45 47 49 50 50 52 53 55 55 57 59 60 60 62 63 65 65 67 68 70 70 72 74 75 75 77 78 80 80 82 83 85 85 87 89 90 90 91 93 95 94 96 98 100 99 101 103 105 104 106 108 110 109 111 113 115 114 116 118 120 119 121 123 125 ... - tailieumienphi.vn