Document 9JGEyNkOveo25xbBJy5m7X20R

528 CHAPTER 28 1946 Guide, Table 11. Thickness of Loose Insulation foe Use as Fill in Underground Conduit Systems 8tham Pressure ' Psio ` ob Condition Steam Temperature Fahrenheit Degrees Minimum Thickness op Insulation in Inches - Steam Lines Return Lines Pipes Less Pipes 4 In. Pipes Lareer Pipes I^ss Pipes 4 In. than A In. to 10 In; ' than 12 In. than 4 In. and Larger Minimum Distance Between and Return ; Hot Water, or 0 to 25 212 to 267 ik 2 2K IK ik i - 25 to 125 267 to 352 2 2K 3 IK IK IK Above 125, or superheat 352 to 500 3 3K IK IK IK, other experimental data which have been presented, the usual endeavor is to secure not less than 90 per.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 tem perature difference for use with the chart when applying it for under ground pipe line'estimates. REFERENCES *~Heat Loss from Copper Piping, by R. H. Heilman {Heating, Piping and Air Conditioning, September, 1933, p.458). 2--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 2)iitrict ^Jleatin9 Steam Distribution Piping, Selection of Pipe Sizes, Conduits for Piping, Pipe Tunnels, Overhead Distribution, Inside Piping,'' Fluid Meters and Metering, Steam Requirements, Rates, Utilization, Automatic Temperature Control 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 com- plete 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 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) (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. 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 electrical generating units, the pressure will be considerably lower- than if ' live steam, direct from the boilers, is used. The advantages of low pressure distribution (2 to 30 psi) are (1) smaller heat loss per square foot of pipe surface, (2) less trouble with traps and" valves, (3) simpler problems in pressure reduction at the buildings, and 529