Document evbym3YpKqEZxJJVqvpvko0V4

HEATING VENTILATING AIR CONDITIONING' GUIDE 1940 -temperatures. For cases where the surrounding air temperature is. other than 90 F the gain's may be selected on the basis of temperature difference. INSULATION OF PIPES TO PREVENT FREEZING If the surrounding air temperature remains sufficiently low for an ample period' of time, insulation' cannot prevent the freezing of still water, or of water flowing at such a velocity that the quantity of heat carried in the water is not sufficient to take care of the heat losses which will result and 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, and if the velocity of the water flowing in the pipe is main tained at a sufficiently high rate, freezing may be prevented. Table 17 may be used for making estimates of the thickness of insu lation necessary to take care of still water in pipes at various water and 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 temperature is assumed to be 10 F above, and the surrounding air tem perature 50 F below the freezing point of water (temperature difference, 60 F). The last column of Table 17 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 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 17: 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 wate'r to be supplied will be 80/60 of that given. In making calculations to arrive at the values given in Table 17, the loss of heat stored in the insulation, the effect of a varying temperature difference due to the codling 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 700 ML CHAPTER 40. PIPE AND DUCT INSULATION The thicknesses of. insulation which ordinarily are used for various temperature conditions are given in Table 18. Where a thorough analysis, Table 18. Thicknesses of Insulation Ordinarily used Indoors1 Steam Pressure (Lb Gags) or Conditions 0 to 25 25 to 100 " 100 to 200 Low Superheat Medium Superheat High Superheat .han*1LP4S^ Steam Temperatures Degrees / Fahrenheit 212 to 267 267 to 338 338 to 388 388 to 500 500 to 600 BOO to 700 Thickness op Insulation Pipes larger loan 4 Id. 1 in. 1J4 in. . 2 in. in. 3 in. 3J<j in. Pipes 2 In. to 4 In. 1 in. 1 in. 1in. 2 in. 2^ in. 3 in; Pipes HIo. to IX la. 1 in. 1 in. 1 in. IJ-f in. 2 in. 2 in. * in. EreatCT 701