Document zD7XGV5O57VmxLVXzj9M1bMR

304 ' ' ' , Chapter 15 ____________ 1945 Guide tliatToTCircuit ^ shows tHat~tHe7S_foFSection 4-12 musFbe 19,700 milinches in order ' lhat the total fh may be 15.7. This is practically equal to the 20,000 fh, assumed for Circuit 8 and this shows how simple it is to secure well-balanced circuits in a reversed return system.. Example 8. The direct return, forced circulation systenTshown in Fig. 15 is similar to Fig. 14, except that the water passing through Unit 1 returns directly to the boiler and the total length of this circuit is about 130 ft, whereas, the total length of Circuit 8 is about 1990 ft, or about 15 times as long. -Solution. The design must begin with Circuit 8. The calculations for this circuit if tabulated as shown for Circuit 8 of Fig. 14 will show that the total fh is 318,200 milinches, or 26.5 ft, as compared with 15.7 ft for the reversed return system. In order that each of the eight units will receive its correct share of the water, the fh of each of the other seven circuits must also be 26.5 ft. For Circuit 1, for example, the total fh for Section 0-1 and 16-17 is 15,600 milinches; hence, the fh in Section 1-16 (through the air conditioning unit), must be 302,600 milinches, or 25.15 ft to prevent Unit 1 having an advantage over Unit 8. Comparing the reversed return system of Fig. 14 with the direct return system of Fig. 15, it appears that the head against which the pump must deliver the 720 gpm is 15.7 ft as compared with 26.5 ft for the direct return and that the installation of the reversed return would require 130 ft of 8 in. pipe not necessary for the direct return system. Thefh in the lines joining the pump to the pipe system shown in Figs. Hand 15 is not included in this calculation. CHAPTER 16 5- 'lAJelding. Pipe Material, Types of Pipe Used, Dimensions of Pipe Com mercially Available, Expansion and Flexibility of Pipe, Pipe Threads and Hangers, Types of Fittings, Welding as Applied to Erection of Piping, Valves, Corrosion of Piping IMPORTANT considerations in the selection, and installation of pipe and fittings for heating, ventilating, and air conditioning work are dealt with in this chapter. PIPE MATERIALS Use of corrosion-resistant materials for pipe, including special alloy steels and irons, wrought-iron, copper and brass, has increased con siderably during the past few years. The recent development of copper, brass, and bronze fittings which can be assembled by soldering or sweating permits the use of thin-wall pipe arid thereby has reduced the initial cost of such installation. The following brief discussion indicates the variety of pipe materials and the types, of pipe available. Wrought-Steel Pipe. Because of its low price, the great bulk of wrought pipe used for heating and ventilating work at the present time is of wrought steel. The material used for steel pipe is a mild steel made by the. acid-bessemer, the open-hearth, or the electric-furnace process. Ordinary wrought-steel pipe is made either by shaping Sheets of metal into cylindrical form and welding the edges together, or by forming or drawing from a solid billet. The former is known as welded pipe, the latter as seamless pipe. Many types of welded pipe are available, although the smaller sizes most frequently used in heating and ventilating work are made by the lap-weld, resistarice-weld, or butt-weld process. While the lap-weld and resistance-weld processes produce a better weld than the butt type, lap-weld and resistance-weld pipe are seldom manufactured in nominal pipe^ sizes less than 2 in. Seamless pipe can be obtained in the small sizes' at a somewhat higher cost. . Seamless steel pipe is frequently used for high pressure work or where pipe is desired for close coiling, cold bending, or other forming operation. Its advantages are its somewhat greater strength which permits use of a thinner wall and, in the small sizes, its freedom from the occasional tendency of welded pipe to split at the weld when bent. . Wrought-iron Pipe. Wrought-iron pipe is claimed to be more corro sion-resisting than ordinary steel pipe and therefore its somewhat higher first cost is said to be justified on the basis of longer life expectancy. Wrought-iron pipe may be identified by the spiral line marked into each length, either knurled into the metal or painted on it in red or other bnght color. Otherwise, there is little difference in the appearance of wrought iron and steel pipe, although microscopic examination of polished and etched specimens will readily disclose the difference. Cast-Ferrous Pipe. There are now available several types of castmetal pipe iriade of a good grade of cast-iron with or without additions of nickel, chromium, or other alloy. This pipe is available in sizes from 1in. to 6 in., and in standard lengths of 5 or 6 ft with external and internal diameters closely approximating those of extra strong