Document YjmbbL0q8QY2BKxqmYnJYGKjE

476 CHAPTER 32 1960 Guide 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 point for various humidities can be readily as certained from a psychrometric 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-mid 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 7. 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 low for as 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 time 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 8 may be used for making estimates of the thickness 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 8 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 . Table 7... .Heat Gains for Insulated Cold Pipes Safe! of hoof froRsmissoa given in Bto por hoot per liaoor foot with Ml air conditions of 90 F. Bated oa materials having a thermal conductivity, k " 0.30 of o mean.temperature of the room and the pipe surface Nominal Pipe Site [laches) Pipe Temperature 40 F Pipe Temperoture 15 F -15Pipe Temperature F Nombtol Thickness of bindoften (Indies) Actual Thidcness of (mutation (Enchei) Btu por Uneor Foot Nominal Thickness of Insulation (Inches) Actual Thickness of Insulation (Inches) Blu por Uneor Foot Nominal Thickness of btsuiatloa (Inches) Actual Thkkness of insulation (indies) Btu per Linear Foot K 1H IX 1X 22X 3 3H 4 5 6 8 10 12 14 16 18 20 24 30 H 0.76 H 0.66 H 0.77 H 0.91 l 1.04 6.6 IK 7.9 IK 8.4 IK 8.8 1X 9.0 IK 1,57 1.47 1.59 1.66 1.54 7.0 8.0 8.8 9.7 11 IK IK IiKx 2 1.57 1.47 1.59 1.66 1.82 10 12 13 14 15 1 1.05 11 1 1.05 12 1 1.01 14 1M 1.29 14 IK 1.54 13 IK 1.58 13 IK 1.33 16 IK 1.55 17 2 1.79 17 2 2.04 17 2 2.11 15 2 1.86 19 2 2.05 20 2K 2.79 18 2K 2.54 21 ix1K 1.50 16 1.47 19 IK 1.52 23 iK 1.57 27 IX 1.56 31 2 2.00 20 2 2.03 23 2 2.02 28 2 2.07 33 2X 2.56 32 2K 2.57 23 2K 2.53 28 2K 2.65 33 2K 2.57 39 3 3.06 40 iK 1.43 36 2K 2.43 37 3 2.93 44 IK 1.43 41 2K 2.43 41 3 2.93 50 IK 1.43 46 2K 2.43 45 3 2.93 55 ik 1.43 51 2K 2.43 50 3 2.93 60 iK 1.43 60 2K 2.43 59 3 2.93 71 IK 1.43 74 2K 2.43 73 3 2.93 87 Industrial Insulation 477 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 than 42 F, the time required to cool it to the freezing point will be equal to (t -- 32)/10 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 8. 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 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 freezing point will be 60/80 of the tim* 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 8, 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 glected. When these factors enter into the computations it is necessary to enlarge the factor of safety. Also as stated, the time shown in the table is that required to lower the water to the freezing point. A longer period would be required to freeze the water, but the-danger point is reached when freez ing starts. The flow of water will stop and the entire line will be in danger as soon as the water freezes across the section of the pipe at any point. If it is necessary to calculate the time required for the water to cool to 32 F, the following equation may be used ft log, - (4) toAere H -- hours for water to cool to 32 F. Tp = inside radius of pipe, inches. ti = initial water temperature. Cp may be obtained from manufacturers' catalogs and hand books. See Equation 1 for definitions of other symbols. When water must remain stationary longer than the times designated in Table 8, the only safe way to insure against Table 8------ Data for Estimating Requirements to Prevent Freezing of Water in Pipes with Surrounding Air at --18 F Number of Horn fo Cool 42 , Wafer fo Freezing Poinf Nominal Pipe Size {Inches) Wafer Flow Required ft 42 Flo Prevent Freez ing, Pounds per linear Foot of Pipe per Hour Hildcnes* of badation in Indies (Conductivity, k = 0.30) 2 3 4 234 K l ix 3 4 5 6 8 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 6.02 7.96 9.88 13.90 18.10 22.20 0.57 1.16 2.02 2.90 5.08 7.20 9.69 12.20 17.25 22.70 28.10 0.54 0.68 0.84 0.95 1.24 0.45 0.94 0.40 0.79 1.47 1.73 1.98 2.46 2.96 3.43 1.11 1.78 2.45 1.43 1.93 freezing is to install a steam or hot water pipe parallel to the cold water pipe or to place an electric resistance heater along the side of the exposed water line. The heating system and the water line are then insulated so that the heat losses from the heating system are not excessive, and the heating effect is concentrated against the water pipe where it is needed. For this form of protection 2 in. of an efficient insulation may be applied. APPLICATION OF INDUSTRIAL INSULATION Small pipes are normally insulated with half-sectional in sulation split horizontally and furnished with factory applied jackets forming hinge and lap. Large piping is insulated with segmental blocks, wired ot banded in place, or with blanket insulations, particularly where removal for frequent servicing of the pipe is' necessary. Indoors, sectional insulation is ap plied with the canvas lap pasted. Outdoors, it is necessary to provide a weatherproof finish, fittings and bends are insu lated with portions of standard preformed insulation, blanket insulations, or insulating cements.. Fitting insulation should be carefully specified to be compatible with the pipe insula tion. Insulation on lines carrying cold water, brine, or other cold fluids must be protected to prevent the infiltration of water vapor into the insulation. flat, curved, and irregular surfaces such as boilers, breech ings, tanks, and vessels, are normally insulated with blocks or lagging, or with blanket forms of insulation. The insulation is secured in a variety of ways depending upon the form of insulation and contour of the surface to be insulated. One specification includes wiring the blocks in place, stretching a hexagonal mesh netting tightly over the insulation, and fastening securely with wires. Over the netting a coat of min eral wool or asbestos cement is applied with a second coat of asbestos cement trowelled to a smooth even finfch For out door installations weather protection is then applied. For insulation over surfaces at temperatures below ambient, water-vapor protection must be applied for both indoor or outdoor applications. f