Document Dv8d6G9eJwndEkMXJoJBrbZm4

Heating Ventilating Air Conditioning Guide 1938 Table 17. Data for Estimating Requirements to Prevent Freezing of Water in Pipes Nominal Pips Size (Inches) 1 2 k 0.42 i 0.83 ik 1.40 2 1.94 3 3.25 4 4.55 5 5.92 6 7.35 8 10.05 10 13.00 12 15.80 Number op Hours to Cool Water to Freezing Point Water Required to Flow to Prevent Freezing. Pounds per Linear Foot op Pipe per Hour 3. 0.50 1.02 1.74 2.48 4.27 6.02 7.96 9.88 13.90 18.10 22.20 Thickness of Insulation in Inches 42 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 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 Table 18. Thicknesses of Insulation Ordinarily used Indoors3 Steam Pressures (la Gage) or Conditions 0 to 25 25 to 100 100 to 200 Low Superheat Medium Superheat High Superheat Steam Temperatures 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 loan 4 In. 1 in. IK in. 2 in. 2*^ in. 3 in. ZH in. Pipes 2 In. to 4 In. 1 in. 1 in. IK in. 2 in. .2^ in. 3 in. Pipes HIn. to 1H In. 1 in. 1 in. 1 in. 1 K 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. Table 19. Thickness of Loose Insulation for Use as Fill in Underground Conduit Systems Steam Pressures (Lb Gage) ob Conditions Steam Temperatures Degrees Fahrenheit Minimum Thickness op Insulation in Inches Steam Lines Return Lines Pipes Less Pipes 4 In: Pipes Larger Pipes Less Pipes 4 In. than 4 In. to 10 In. than 12 In. than 4 In. and larger Hot Water, mor 0 to 25 212 to 267 2 2K ik IK m25 to 125 267 to 352 2 3 IK IK Above 125, or superheat 352 to 500 2K 3 3K IK IK Minimum Between AND Return i IK IK 738 Chapter 39. Piping and Duct Insulation with other experimental data which have been presented, the usual endeavor is to secure not less than 90 per cent efficiency for underground piping. Table 19 can be used as a guide in arriving at the minimum thickness of loose insulation fills to use for laying out conduit systems. Other factors such as the number of pipes and their combination of sizes, as well as the standard conduit sizes, are primary controlling factors in the amount and thickness of insulation for use. When sectional insulation is applied to lines in tunnels or conduits, usual practice is to apply the most efficient materials in. less in thick ness than that determined by the use of Fig. 3. The data in Fig. 3 are based on conditions of insulation exposed to the air, whereas normal ground temperature is substituted for air temperature in determining the tem perature difference for use with the chart when applying it for under ground pipe line estimates. PROBLEMS IN PRACTICE 1 What precautions must be taken in selecting insulation used for covering pipe lines carrying materials at temperatures lower than the dew point? Materials intended for this service should be as moisture proof as possible and in addition an outer covering should be applied which is proof against diffusion of air and water vapor. If the material permits the diffusion of air, the air will reach a point in the covering where the temperature is below the dew point. The condensed water will gradually accumulate until the covering becomes saturated, which will increase the conductivity and perhaps lower the mechanical strength of the covering until it becomes worthless. 2 Compute the total annual heat loss from 165 ft of 2-in. bare pipe in service 4000 hours per year. The pipe is carrying steam at 10 lb pressure and is exposed to an average air temperature of 70 F. The pipe temperature is taken as the steam temperature, which is 239.4 F, obtained from Table 8, Chapter 1. The temperature difference between the pipe and air = 239.4 -- 70 ** 169.4 deg. By interpolation of Table 1 between temperature differences of 157.1 F and 227.7 F, the heat loss from a 2-in. pipe at a temperature difference of 169.4 deg is found to be K677 Btu per hour per linear foot per degree temperature difference. The total annual heat loss from the entire = 1.677 X 169.4 X 165 (linear feet) X 4000 (hours) = 188,000,000 Btu. 3 t Coal costing $11.50 per ton and having a calorific value of 13.000 Btu per pound is being burned in the furnace supplying steam to the pipe line given in Question 2. If the system is operating at an over-all efficiency of 55 per cent determine the monetary value of the annual'heat loss from the line. The cost of heat per 1 million Btu supplied to the system -- 1,000,000 X 11.5 (dollars) -5- 13,000 (Btu) X 2000 (lb) X 0.55 (efficiency) = $0,804. The total cost of heat lost per year = 0.804 X 188 (million Btu) = $151.15.4 4 If the steam line given in Question 2 is covered with 1-in. thick 85 per cent magnesia, determine the resulting total annual heat loss through the insula-' tion. Also compute the monetary value of the annual saving and the per centage of saving over the heat loss from the bare pipe. By interpolation of Table 9 between temperature differences of 157.1 F and 227.7 F, the coefficient of transmission for 1-in. magnesia on a 2-in. pipe is found to be 0.525 Btu per hour per square foot of pipe surface per degree temperature difference at a temperature difference of 169.4 deg. The total hourly loss per square foot of insulated pipe will then be 0.525 X 169.4 -- 89.04 Btu. From Table 5 the area per linear foot of 2-in. pipe is 4A closely approximate solution of this problem may be quickly made by use of the estimating chart given in Fig. 1.