Document LgwL9MwZmJ2O0pjp6X0D3Yd1Q

HEATING VENTILATING AIR CONDITIONING CUIDE 1942 surrounding air temperature conditions. Because of the damage and service interruptions which 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 tem perature is assumed to be 10 F above, and the surrounding air temperature 50 F below the freezing point of water (temperature difference, 60 F). The last column of Table 11 gives the minimum quantity of water at initial temperature of 42 F which 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 expressed in feet. As an additional factor of safety, and in order to provide against Nominal Pipe SlZB (Inches) 1 2 3 4 5 6 8 10 12 Table 11. Data for Estimating Requirements to Prevent Freezing of Water in Pipes with Surrounding Air at --18 F Number op Hours to Cool 42 F Water to Freezing Point Water Flow Required at 42 F to Prevent Freezing, Pounds per Linear Foot or Pipe pes Hour 2 0.42 0.83 1.40 1.94 3.25 4.55 5.92 7.35 10.05 13.00 15.80 Thickness of Insolation in Inches (Conductivity, k -- 0.30) 34 0.50 0.S7 1.02 1.16 1.74 2.02 2.48 2.90 4.27 5.08 6.02 7.20 7.96 - 9.69 9.88 . 12.20 13.90 17.25 18.10 22.70 22.20 28.10 2 0.54 0.68 0.84 0.95 1.24 1.47 1.73 1.98 2.46 2.96 3.43 3 0.45 0.55 0.68 0.75 0.94 1.11 1.29 1.46 1.78 2.12 2.45 4 0.40 0.48 0.58 0.64 0.79 0.93 1.06 1.19 1.43 1.70 1.93 temporary reductions in flow occasioned by reduced pressure, it is ad visable to double the rates of flow listed in the table.'~~It'must be empha sized 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 52 F instead of 42 F, the time required to cool it to the freezing point will be prolonged to twice that given in the table, or the rate of flow of water may be reduced so that the quantity required will be one-half that shown in the last column of Table 11. However, 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 freezing 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 11, the loss of heat stored in the insulation, the effect of a varying temperature 782 CHAPTER 43. PIPE AND DUCT HEAT LOSSES difference due to the cooling of pipe and water, and the resistance of the outer surface of the insulation to the transfer of heat to the air have all been neglected. 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 freezing 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. When water must remain stationary longer than the times designated in Table 11, the only safe way to insure against freezing is to install a steam or hot water line 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 Table 12. Thicknesses of Pipe Insulation Ordinarily used Indoors3 Steam Pressures (Lb Gage) or Conditions 0 to 25 25 to 100 100 to 200 Low Superheat Medium Superheat High Superheat Degrees Fahrenheit 212 to 267 267 to 338 338 to 388 388 to 500 500 to 600 600 to 700 . Thickness or Insulation Pipes Larger Than 4 In. 1 in. 1M in- 2 in. 2H in. 3 in. 3H in. Pipes 2 In. to 4 In. 1 in. 1 in. 1M in. 2 in. 2% in. 3 in. Pipes H In to 1H In. 1 in. 1 in. 1 in. 1 Vi in. 2 in. 2 in. All piping located outdoors or exposed to weather is ordinarily insulated to a thickness M in. greater than shown in this table, and covered with a waterproof jacket. 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 insu lation may be applied. ECONOMICAL THICKNESS OF PIPE INSULATION The thicknesses of insulation which ordinarily are used for various temperature conditions are given in Table 12. Where a thorough analysis of economic thickness is desired this may be accomplished through the use of the chart, Fig. 6. The dotted line on the chart illustrates its use in solving a typical example. In using the chart, start with the scale at the left bottom margin representing the given number of hours of operation per year; then proceed vertically to the line representing the given value of heat; thence horizontally to the right, to the line representing the given tem perature difference; thence vertically to the line representing the con ductivity of the given material; thence horizontally, to the left, to the line representing the given discount on that material; thence vertically to the curve representing the required per cent return on the investment; thence horizontally to the right, to the curve representing the given pipe size; thence vertically to the scale at the top right margin where the economical thickness may be read.off directly. 783