Document jmN6KJb6JJojyd4k31BNZ6g62

American Society of Heating and Ventilating Engineers Guide, 1934 Table 12. Thicknesses of Insulation Ordinarily used Indoors^ (Lb Gaoe) ob Condition 0 to 25 25 to 100 100 to 200 Low Superheat Medium Superheat ' High Superheat Steam Temperatures Deobees Fahrenheit 212 to 267 267 to 338 338 to 388 388 to 500 500 to 600 600 to 700 Thickness op Insulation Pipes Larger loan 4 In. 1 in. 1)4 in. 2 in. 2y2 in. 3 in. X3 in- Pipes 2 In. to 4 In. 1 in. 1 in. IH in- 2 in. 1)4 in- 3 in. Pipes M In to 1H In- 1 in. 1 in. 1 in. 1)4 in. 2 in. 2 in. All piping located outdoors or exposed to weather is ordinarily insulated to a thickness than shown in this table, and covered with a waterproof jacket. in. greater the pipes with high quality, loose insulating material. The insulation must be kept dry at all times, and for this purpose effective waterproofing membranes enclose the insulation. A drainage system is also provided to divert water which may tend to enter the conduit. The economical thickness of insulation for underground work is dif ficult of accurate determination due to the many variables which have to be considered. As a result of theories developed by J, R. Allen3, together with experimental data presented by others, the usual endeavor is to secure not less than 90 per cent efficiency for underground piping. Table 13 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. . Use of Fig. 3 involves conditions of insulation exposed to the air, whereas normal ground tem perature is substituted for air temperature in determining the tempera ture difference for use with the chart when applying it for underground pipe line estimates. Table 13. Thickness of Loose Insulation for Use as Fill in Underground Conduit Systems Steam Pressure (Lb Gage) or Condition Minimum Thickness op Insulation in Inches Temperature Degrees Fahrenheit Steam Lrnss 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 Minimum Distance Between Steam and Return Hot Water, or 0 to 25 212 to 267 25 to 125 267 to 352 l)4 2 2 2 )4 1)4 3 IX IX IX ix i ix. Above 125, or superheat 352 to 500 . ix 3 3 X IX ix IX Theory of Heat Losses from Pipes Buried in the Ground, by J. R. Alien (A.S.H.V.E. Transactions, Vol. 26, 1920). 518 Chapter 36 DISTRICT HEATING Underground Steam Piping, Selection of Pipe Sizes, Provision for Expansion, Capacity of Returns with Various Grades, Pipe Con duits, Pipe Tunnels, Service Connections, Steam per Square Foot of Heating. Surface, Fluid Meters and Metering THIS chapter deals with those phases of district heating which frequently fall within the province of the heating engineer. Data and information are included for solving incidental problems in connection with institutions and factories and for the design of building heating systems which are to be supplied with purchased steam, A complete district heating installation should not be attempted without a thorough study of the entire problem by men competent and experienced in that industry. The Handbook and other publications of the National District Heating Association and the references at the end of this chapter should be consulted. UNDERGROUND STEAM PIPING The methods used in district heating work for the distribution of steam are applicable to any problem involving the supply of steam to a group of buildings. The first step is to establish the route of the pipes, and in this matter the local conditions so fully control the layout that little can be said regarding it. Having established the route of the pipes, the next step is to calculate the pipe sizes-. In district heating work it is common practice to design the piping system on the basis of pressure drop. The initial pressure and the minimum permissible terminal pressure are specified and the pipe sizes are so chosen that the required amount of steam, with suitable allowances for future increases, will be transmitted without exceeding this pressure drop. The steam velocity is therefore almost disregarded and may reach a very high figure. Velocities of 35,000 fpm are not con sidered high.' By the use of this method the pipe sizes are kept to a minimum with consequent savings in investment. The steam flowing through any section of the piping can be computed from a study of the requirements of the several buildings served. In general a condensation rate of 0.25 lb per hour per square foot of equiva lent heating surface is a safe figure. This allows for line condensation which, however, is a small part of the total at times of maximum load. Any unusual requirements such as those for process steam should be individually calculated. * The steam requirements for water heating should be taken into account, but in most types of buildings this load will be relatively small compared 519