Document pYwmGVj5OQKYmg8mzJOmB4Bj

American Society of Heating and Ventilating Engineers Guide, 1937 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, in part, upon whether or not it has been passed through electrical generating units. If it has, the pressure will be considerably lower than if live steam direct from the boilers, is used. The advantages of low pressure distribu tion (2 to 30 lb per square inch) are (1) smaller heat loss from the pipes (2) less trouble with traps and valves, and (3) simpler problems in pressure reduction at the buildings. With distribution pressures not exceeding 40 lb per square inch there is little danger even if the full distribution pressure should build up in the radiators through the faulty operation of a reducing valve; but with pressures higher than this a second reducing valve or some form of emergency relief is usually desirable to prevent excessive pressures in the radiators. The advantages of high pressure distribution are (1) smaller pipe sizes and (2) greater adaptability of the steam to various operations other than building heating. The different kinds of apparatus which frequently must be served require various minimum pressures. Kitchen equipment requires from 5 to 15 lb per square inch, the higher pressures being necessary for apparatus in which water is boiled, such as stock kettles and coffee urns. An increased amount of heating surface, which is easily obtained in some kinds of apparatus, results in quicker and more satisfactory operation at low pressures. For laundry equipment, particularly the mangle, a pres sure of 75 lb per square inch is usually demanded although 30 lb per square inch is sufficient if the mangle is equipped with a large number of rolls and if a slow rate of operation is permissible. Pressing machines and hospital Sterilizers require about 50 lb per square inch. PIPE SIZES The lengths of pipe, steam quantities, and initial and terminal pressures having been chosen, the pipe sizes can readily be calculated by means of the Unwin pressure drop formula. This formula, which gives pressure drops slightly larger than actual test results, is as follows: 0.0001306 W*L (l + ^-) where P= 1D> P = pressure drop, pounds per square inch; W = weight of steam flowing, pounds per minute. L = length of pipe, feet. D = inside diameter of pipe, inches. d = average density of steam, pounds per cubic foot. (1) This formula is similar to the Babcock formula given in Chapter 32. Information on provision for expansion will be found in Chapter 34.. In general, return lines when installed follow the contour of the land, and Table 1 gives sizes of return pipes for various grades. It is evident that at points where the grade is great, smaller pipes can be installed. 688 Chapter 37--District Heating CONDUITS FOR PIPING Conduits for steam pipes buried underground should be reasonably waterproof, able to withstand earth loads and to take care of the expan sion and contraction of the piping without strain or stress on the couplings, or without affecting the insulation or conduit. Expansion of the piping mUst be carefully controlled by means of anchors and expansion joints or bends so that the pipes can never come in contact with the conduit. Anchors can be anchor fittings or U-shaped steel straps which partially encircle the pipes and are firmly bolted to a short length of structural steel set in concrete. Table 1. Capacity of Returns for Underground Distribution Systems in Pounds of Condensate per Hour Sira* Pitch or Pipb feb 100 Ft. 6' 1' V r 5' i 448 998 1890 2240 3490 IH 7m 1740 2700 4980 2490 4190 7380 3990 5740 10700 4880 7480 13900 6480 9480 16900 3 13900 22500 30900 37400 50400 4 30900 44800 64800 79700 105000 5 54800 79800 120000 144800 195000 6 90000 138000 187000 237000 312000 8 190000 277000 404000 508000 660000 10 344000 498000 724000 900000 1190000 12 555000 798000 1148000 1499000 1990000 10' 5490 9480 14500 24900 74800 154000 294000 449000 938000 -- 20' 749013500 20900 36900 105000 229000 418000 ___________ -- Size of pipe should be increased if it carries any steam. In laying out conduits of this type the following points should be borne in mind: 1. An expansion joint, offset, or bend should be placed between each two anchors. 2. If the distance between buildings is 150 ft or less and the steam line contains highpressure steam, the line may be anchored in the basement of one building and allowed to expand into the basement of the second building. If the steam line contains low-pressure steam (up to 4-lb pressure), this method may be used if buildings are 250 ft or less apart. 3. If the distance between buildings is between 150 ft and 300 ft and the steam line contains high-pressure steam, the lines should be anchored midway between the buildings and allowed to expand into the basements of both buildings. If the steam line contains low-pressure steam this method may be used if buildings are between 250 ft and 600 ft apart. No manhole is required at the anchor, and a blind pit is all that is necessary. 4. For longer lines, manholes must be located according to judgment and depending upon the expansion value of the type of expansion joint or bend that is used. The minimum number of manholes will be required when an expansion bend or an anchor with double expansion joint is placed in each manhole and the pipes are anchored mid way between manholes. 5. A proper hydrostatic test should be made on the assembled line before the insula tion and the top of the conduit are applied. The hydrostatic pressure should be oneand-one-half times the maximum allowable pressure and it should be held for a period of at least two hours without evidence of leakage. In any case the pressure should be np. less than 100 lb per square inch. The styles and construction of conduits commonly used may be classi fied as follows. Some of the more common forms are illustrated in Fig. 1. Wood Casing: The pipe is enclosed in a cylindrical casing usually having a wall 4 in. thick and built of segments which are bound together by a wire wrapped spirally around. 689