Document G5oYj86VaV2xBRZDJRnBdgn8n

I ^T^T^LATING Alt CONDITIONING-.-GUIDE IMP Fig. 6. Chart for Dethroning Economical Thickness of Insolation CHAPTER 40. PIPE AND DUCT INSULATION | j, ! | . i j j i i" i of economic thickness is desired this may be accomplished through the the use of the chart, Fig. 6. The dotted line on the chart illustrates its use in solving a typical example. In using the chart, start with the scale at the left bottom margin representing the given number of hours of operation per year; then proceed vertically to the line representing the given value of heat; thence horizontally to the right, to the line representing the given temperature difference; thence vertically to the line representing the con ductivity of the given material; thence horizontally, to the left, to the line representing the given discount on that material; thence vertically to the,curve representing the required per cent return on the investment; thence horizontally to the right, to the curve representing the given pipe size; thence vertically to the scale at the top right margin where the economical thickness may be read off directly. UNDERGROUND PIPE INSULATION Underground steam distribution lines are carried in protective struc tures of various types, sizes and shapes. (See Chapter 43). Detailed data on commonly used forms of tunnels and conduit systems have been published by the National District Heating Association*. 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. 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 mem branes 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 theories5 previously developed, together with Table 19. Thickness of Loose Insulation for Use as Fill in Underground Conduit Systems Steam Pressures (Lb Gage) or Conditions St&am Temperatubes 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 Minimum Distance Between AND Return Hot Water, or 0 to 25 212 to 267 ix 2 2X ik IX i 25 to 125 267 to 352 2 2H 3 IK IX IK Above 125, or superheat 352 to 500 2X 3 3X IX IX `Handbook of the National District Heating Association, Second Edition, 1932. `Theory of Heat Losses from Pipes Buried in the Ground, by I. R. Allen (A.S.H.V.E. Transactions, Vol. 26. 1920, p. 335). 703