Document M42yy7RyR8n4nq9E2Bdr1gLOL

" AMERICAN SOCIETY OF HEATING & YENTILATIN6 ENGINEERS 6HDEI923] Dia. of Pipe In. TABLE 45. CAPACITIES OF ONE PIPE RISERS A llowable Velocity in Feet per Second 18 22 Cond. lb. hr. Sq. ft. Rad. B.t.u. loss hr. Based on 240 B.t.u. Cond. lb. hr. B.t.u. loss hr. Sq. ft. Rad. Based on 240 B.t.u. ZA i Wi " l Vi 2 8.9 14.5 25.2 34.2 56.5 8,640 14,070 24,460 33,190 54,830 36.0 58.6 101.9 138.1 228.4 10.9 17.8 30.8 41.8 69.0 10,580 17,270 29,890 40,560 66,960 44.1 72.0 124.5 169.0 279.0 The table gives the capacity of the pipe in pounds of steam-per hour. B.t.u.' transmitted per hour, and square feet of radiation based on 240 B.t.u. per sq. ft. Fio. 31. Curves Showing Effect of Constrictions in Various Sizes of Pipe ' The investigation has brought out the following outstanding points which seem of sufficient importance to the heating industry to be sum marized : 1. All pipe should be reamed--pipe cut by an ordinary wheel cutter and not reamed will decrease the capacity of a one-pipe system by as much as 30 per cent; 2. Pipe with any unusually large bump, blister, or other constriction on the inside should not be used--the capacity of a one-pipe system is no greater I than the capacity of the free area left by its greatest constriction; ' ; 3. Unions and all other fittings entering into a riser should be of full size opening; , .: 4. The .size of pipe for one-pipe risers carrying any quantity of steam should be determined in accordance with Column 1 of Table 45, or by Column 2 after applying the desired factor of safety. 64 HOT WATER HEATING HOT water heating may be divided into two classes, forced and gravity circulation. The first is used on large units and other installations where it would be difficult to obtain uniform heating by gravity circulation. In many cases, approaching the natural dividing line between the two systems, a combination of forced and gravity cir culation is installed, in which a pump is used as an auxiliary to circulate the water during periods of heavy firing. GRAVITY HOT WATER SYSTEMS Gravity hot-water heating may be divided into several classes, de pending on the method of running the flow and return mains. For residences, the two-pipe, up-feed system generally is used. This means that the flow and return mains are run on the ceiling of the basement, with branches and risers feeding the several radiators from below. Where attic space is available, and in shops and factories the down-feed system often is used. This may have either single or double drops, although the single drop is preferable. The chief advantage of the downfeed system is that it raises the mean effective circulating head, and obviates many of the fine adjustments that are required with the upfeed system. Sometimes, when there is plenty of circulating head available, a two-pipe up-feed system with a single-pipe basement circuit is installed. This system, while it cuts down the amount of piping in the basement, increases the size of the mains because it divides the avail able circulating head into two circuits. The basic principle of gravity circulation is shown in Fig. 32. As the water in column A is heated, and its density decreases it is over balanced by the cold water in B. The water in column B, being colder, always outweighs the water in column A, and thus circulation is estab lished. As the temperature difference between these two columns is increased, the difference in weight increases, with a consequent increase in the velocity of circulation. The velocity of circulation also varies with the difference in.elevation between the heater and the radiators.. A circuit similar to that shown in Fig. 33 will not circulate, because the hot water is not overbalanced by the cold. Adding pressure to the water will not increase the velocity of circulation in any way, except that it may allow greater temperature differences to exist between the flow and return columns. The velocity of circulation cannot be increased or decreased by changing the pitch of the majns. In actual practice, the circulation. of water 'in a hot-water heating system is much more complicated than that -shown in a simple circuit Material for this section was prepared especially for The Guide 1923 by William Hutton. Wlnsted. Conn., in collaboration with Philip Parker, Woburn. Mass.. W. P. Elder. Sherman Parker and A.' F. Karlson, Fitchburg, Mass. 65