Document 3NJZaVdDQRQbp7wKBMzaEYDO

542 CHAPTER 28 1948 Guide Table 11. Thickness of Loose Insulation for Use as Fill in Underground Conduit Systems Steam . Pressure Pbig or Condition Steam Temperature Fahrenheit Hot Water, or 0 to 25 212 to 267 25 to 125 267 to 352 Above 125, or superheat 352 to 500 orMinimum Thickness 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. t.nnn 12 In. than 4 In. and larger m 2 2J4 m V/s 2 2K 3 i a l 'A 3 m iH m Minimum Distance Between Steam Return i IK m on commonly used forms of tunnels and conduit systems have been published by the National District Heating Association 2. Pipes in tunnels are covered with sectional insulation to provide maximum thermal efficiency and are also finished with good mechanical protection in the form of metal or waterproofing membrane outer jackets. In some instances, where actual submersion of hot lines may occur it has been found good practice to firmly secure the covering with corrosion resistant wire, then sew on a wire-inserted asbestos fabric jacket with wire. This jacket is porous. The principle of withstanding submersion is that water may enter as water, then actually boil at the pipe surfaces and escape as steam without rupturing the insulation or jacket. Conduit systems are in more general use than tunnels. Pipes carried in conduits may be insulated with sectional insulation; however, the more usual practice is to fill the entire section of the conduit around 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 difficult to determine accurately due to the many variables which have to be. considered. As a result of theories 3 previously developed, together with other experimental data which have been presented, the usual endeavor is to secure not less than 90 pier cent efficiency for underground piping. Table 11 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. 6. The data in Fig. 6 are based on conditions of insulation exposed to the air, whereas normal ground temperature is substituted for air temperature in determining the temperature difference for use with the chart when applying it for under ground pipe line estimates. REFERENCES 1 ""Heat Loss from Copper Piping, by R. H. Heilman {Heating, Piping and Air Conditioning, September, 1933, p. 458). * Handbook of the National District Heating Association, Second Edition, 1932. * Theory of Heat Losses from Pipes Buried in the Ground, by J. R. Allen (A.S.H.V.E. Transactions Vol. 26, 1920, p. 335). Chapter 29 DISTRICT HEATING Steam Distribution Piping, Selection of Pipe Sizes, Conduits for Piping, Pipe Tunnels, Overhead Distribution, Inside Piping, . Metering, Steam Requirements, Rates, Utilization, Automatic Temperature Control THE term district heating refers to the heating of several buildings from a central plant as in the heating of portions of cities, and is also used in connection with the heating of groups of buildings as in institu tions and factories.' It is usually preferable, in a group of industrial or institutional buildings, that they be heated from a central plant rather than by individual plants. Fuel can generally be burned more efficiently, less labor is required, and often a central plant is cheaper to install. Those phases of district heating which frequently fall within the province of the heating engineer are outlined here with data and. information for solving incidental problems in connection with institutions and factories. Some data are included to cover the piping peculiar to 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. STEAM DISTRIBUTION 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 since the local conditions control the layout 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) (square foot of equiva lent direct radiation) is a safe figure. This allows for line condensation which, however, is a small part of the total at times of maximum load. Miscellaneous steam requirements such as laundry, cooking, or process 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 with the heating load and will, seldom occur at the time of the heating peak. Unusual features such as large heaters for swimming pools should not be overlooked. The pressure at which the steam is to be distributed will depend upon (1) boiler pressure, (2) whether exhaust or live steam, (3) pressure require ments of apparatus to be served. If steam has been passed through 543