Document Z4aGmNN1RwXKn1Nee7YzDEvpL

Heating Ventilating Air Conditioning Guide 1939 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- aSoIve problems by drawing lines as indicated by dotted line, entering chart at lower left band scale. 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 750 Chapter 39. Piping and Duct Insulation 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 Table 16. Heat Gains for Insulated Cold Pipes Rates of heat transmission given in Btu per hour per degree Fahrenheit temperature difference between fluid in pipe and surrounding still air Based on materials having conductivity, k = 0.30 Nominal Pipe Sizes (Inches) H 1 m 2 2M 3 3J4 4' S 6 8 10 12 Ice Water Thickness Thickness of . Insolation (Indies) Bto Per Linear Foot Btu Per Sq Ft Pipe Surface 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 0.502 .0.119 0.431 0.139 -0.403 0.155 0.357 0.174 0.351 0.200 0.322 0.228 0.303 0.269 0.293 0.295 0.282 0.294 0.248 0.349 0.239 0.404 0.233 0.455 0.201 0.559 0.198 0.648 0.194 Brink Thickness Thickness of Insulation (Inches) 2.0 2.0 2.0 2.4 2.5 2.5 2.6 2.7 2.9 2.9 3.0 3.0 3.0 3.0 3.0 Btu Per Linear Foot 0.098 0.111 0.124 0.131 0.134 0.151 0.170 0.186 0.191 0.209 0.241 0.259 0.318 0.383 0.438 Btu Per Sq Ft Pipe Surface 0.446 0.405 0.352 0.300 0.270 0.244 0.226 0.202 0.183 0.176 0.165 0.150 0.140 0.135 0.131 Hravt Brine Thickness Thickness of Insulation (Inches) Btu Per linear Foot' Btu Per Sq Ft Pipe Surface 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 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 17, 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 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.