Document 8R8RJkGX4zEkogNwa7pe8vkek

462 CHAPTER 32 1959 Guide - Table 7.... Thickness of Ptpe Insulation Ordinarily Used Indoors* Steam Pressor* POg or Condition Steam Temperature Fahrerftcit Degree* Thickness of lawifatioa Pipe* larger 4 In. Pipes 2 In. to 4 tn. Pipe* X* lo 1}4 tn. 0 to 25 25 to 100 100 to 200 . 212 to 267 267 to 338 338 to 388 1 io. 1X in. 2 in. 1 in. 1 in. 1M m. 1 in. 1 in. 1 in. Low Superheat High Superheat 388 to 500 500 to 600 600 to 700 2X in. 3 in. 3H in- 2 in. 2Kin3 in. 1X in- 2 in. 2 in. to veether is ardissrily ineolsted to s in. (role than shown in this table, and cohered with a waterproof cost of heat loss decreases, but the annual cost of insulation (first cost multiplied by percent fixed charges) increases. The sum of the two costs will first decrease with increasing thick ness of insulation, and then, depending on operating condi tions, will increase beyond a certain thickness. Therefore, the thiMrnasa at which the sum of the costs is a minimum, is the most economical. The method of ging the chart Fig. 6 is as follows: From the number of hours of operation per year at the bottom of the chart proceed upward to the line showing value of heat in dollars per million Btu using the appropriate scale A or B. Proceed right to temperature difference line using either scale A or B. Move upward to the line representing conductivity of the insulation being considered, then left to ' discount lias, upward to the percent fixed charge line, right to the pipe size or flat surface line and read the economic thickness at top on the scale corresponding to the scales se lected for value of* heat' and temperature difference. For ex- Toble 9 .... Data for Estimating Requirements to Prevent Freezing of Water in Pipes with Surrounding Air at --18 F Nttsber of Hoars to Cool 42 Wafer fo Freezing Point Nominal Pipe Sin (lodtet) I Wafer Flow Required at 42 F to Prevent Freex- I ing, Found* per linoar I Foot of Pipe par Hour Thtdmeu of Imdation in India (Cowdiidiolly, It " 0J0) 2 3 4 234 X 1 IX 2 3 4 5 6 8 10 12 0.42 0.83 1.40 1-94 3-25 4.55 5.92 7.35 .10.05 13.00 15.80 0.50 1.02 1.74 2.48 4.27 0.57 1.16 . 2.02 2.90 5.08 0.54 0.68 0.84 0.95 1.24 0.45 0.55 0.68 0.75 0.94 0.40 0.48 0.58 0.64 0.79 6.02 7.96 9.88 13.90 18.10 22.20 7.20 9.69 12.20 17.25 22.70 28.10 1.47 1.73 1.98 2.46 2.96 3.43 1.11 1.29 1.46 1.78 2.12 2.45 0.93 1.06 1.19 1.43 1.70 1.93 ample; with Scale B heat value and Scale A temperature difference or vice Versa, use Scale A-B economic thickness. If the intersection with the flat surface is on the fiat surface continued curve, the economical thickness is read on the prime scales A'-A', A'-B', or B'-B'. Cost of insulation refers to incremental cost in place; L.e., average incremental cost per unit thickness after the first unit. The first inch of insulation will usually cost more than each additional inch. This is due to cost of finish, contractor's setup, and operating costs, which do not increase in pro portion to thickness. The first inch of insulation, except in very unusual cases, provides enough saving in heat cost out of bare surface cost to more than cover these extra first costs. Table 8 ..., Heat Gains for Insulated Cold Pipes gate* of boat hmmnrioon given in Bfe per (hour! (Fahrenheit degree temperature difference between fluid to pipe and surrounding itiS air) Based on materials having conductivity, Ic = 0.30 Ice Water Thickness Brine Jhidraes* Heavy Brine Thiefavg Pipe Sin (indie*) Thickness of bwdatioa (Indies) Bfe Per linear Foot Bht Per Sq Ft Pipe Surface Thickness of Insdaiian (Indies) Bfe Per linear Foot Bfe Per Sq Ft Pipe Tbidcaeg of Inwfcrtioft (Inches) Btu Per linear Foot Bfe Per Sq FI Pipe Surface X H l IX IX 2 2H 3 3H 4 5 6 8 10 12 1.5 1.6 1.6 1 ;6 1.5 .1.5 1.5 1.5 1.5 1.7 1.7 1.7 1.9 1.9 1.9 0-110 0.119 0.139 0155 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 2.0 0.098 0.446 2.0 0.111 0.405 2.0 0.124 0.352 2.4 0.131 0.300 2.5 0.134 0.270 2.5 0.151 0.244 2.6 0.170 0.226 2.7 0.186 0.202 2.9 0.191 0.183 2.9 0.209 0.176 3.0 0.241 0.165 3.0 0.259 0.150 3.0 0.318 0.140 3.0 0.383 0.135 3.0 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 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 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 Pipe and Industrial Insulation 463 To obtain discount from, insulation list price, first obtain applied prices for the desired type of insulation in one-inch, two-inch, etc., thickness. Then determine the average incre mental cost per inch above one-inch thickness. Incremental cost in percent discount from list is -(-D where X = average incremental cost in place per inch above one inch. y a* list price for one inch. In figuring pipe insulation, use the list price for one-inch thickness for a pipe size consistent with the average incre mental cost per inch. For example, if the average incre mental cost per inch is based on applied prices for one, two, and three inches, the incremental cost of the second and third inches would be averaged and the pipe size for list price would be that nearest the average pipe size of the second and third inch. A rapid method for determining the economical thi<-Imres of insulation by use of tables has been published.1 LOW TEMPERATURE PIPE INSULATION Surfaces maintained at temperatures lower than the sur rounding air are insulated to reduce the flow of heat and to prevent condensation. The insulating material should absorb a minimum amount of moisture, because the absorption of moisture substantially increases the conductivity of the ma terial. This property is particularly important in the insula tion of surfaces that are below the dew point of the surround ing 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 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 absorp tion might pick up moisture before application and then, when the vapor 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 insulation manufacturers to suit their products, and others by appiiers and users. Unless time-proven.methods are known, specifica tions of insulation manufacturers should be obtained and followed carefully. Equipment must be carefully protected against corrosion caused by condensation of water vapor. All metallic surfaces should be coated with a vapor-impervious barrier (some types of which have an asphaltic or tar base) without any breaks or openings in order to prevent corrosion. The thickness of insulation required to prevent condensa tion on the outer surface is that thickness which will raise the temperature of the outer surface of the insulation to a point slightly higher than the dew point of the surrounding vapor. The dew paint for various humidities can be readily as certained from a peychrometric chart. The external vapor barrier must be made as nearly perfect as possible in order to prevent migration of vapor into the insulation. The approximate required thickness of insulation to pre vent condensation on pipes and flat metallic surfaces may be obtained from fig. 7 in which a surface resistance of 0.606, corresponding to a film conductance of 1.65, was used in cal- culating the curves. This value provides a slight factor of safety and its use is known to give satisfactory field results. In using the chart it is advisable to specify the next thicker, rather than the next thinner, commercial insulation in cases where an intermediate thickness is indicated. Heat gains for pipes insulated with a material having an installed conductivity of 0.30 Btu per (sq ft) (hr) (F deg per in.) are given in Table 8. This table may be used for any of the commercial insulations offered for this purpose since they have conductivities very near the 0.3 value used. INSULATION OF PIPES TO PREVENT FREEZING If the surrounding air temperature remains sufficiently Iow'for an ample period of time, insulation cannot prevent the freezing of still water, or of water flowing at such a veloc ity that the quantity of heat carried in the water is not sufficient to take care of the resulting heat losses that will cause the temperature of the water to be lowered to the freezing point. Insulation can materially prolong the fan required for the water to give up its heat so that, if the velocity of the water flowing in the pipe is maintained at a sufficiently high rate, freezing may be prevented. Table 9 may be used for making estimates of the thirJmftga 'of insulation necessary to take care of still water in pipes at various water and surrounding air temperature conditions. Because damage and service interruptions may result from frozen water in pipes, it is essential that an efficient insulation be utilized. This table is based on the use of a material having a conductivity of 0.30. The initial water temperature is as sumed to be 10 deg above, and the surrounding air tempera ture 50 deg below, the freezing point of water (temperature difference, 60 F). The last column of Table 9 gives the minimum quantity of water at initial temperature of 42 F that should be supplied every hour for each linear foot of pipe, in order to prevent the temperature of the water from being lowered to the freezing point. The weights given in this column should be multiplied by the total length of the exposed pipe line ex pressed in feet. As an additional factor of safety, and in order to provide against temporary reductions in flow occasioned by reduced pressure, it is advisable to double the rates of flow listed in the table. It must be emphasized that the flow rates and periods of time designated apply only for the conditions stated. To estimate for other service conditions, the following method of procedure may be used. If water enters the pipe at any temperature other thou 42 F, the time required to cool it to the freezing point will be equal to (t -- 32)/I0 times that given in the table, or the rate of flow of water may be changed to 10/(t -- 32) times the indicated flow rate in the last columns of Table 9. How ever, if the water enters the pipe at 34 F, it will be cooled to 32 F in one-fifth of the time given in the table. It will then be necessary to increase the rate of flow so that five times the specified quantity of water will have to be supplied in order to prevent freezing. If the minimum air temperature is --38 F (temperature difference 80 F) instead of --18 F, the time required to cool the water to the freeing point will be 60/80 of the time given in the table, or the necessary quantity of water to be supplied will be 80/60 of that given. In making calculations to arrive at the values given in Table 9, the loss of heat stored in the insulation, the effect of a varying temperature difference due to the cooling of pipe and water, and the resistance of the outer surface of the in sulation to the transfer of heat to the air, have all been ne-