Document aB8q6xnV487xzKbBV54KLOgX

American Society of Heating and Ventilating Engineers Guide, 1937 retical sizes, however, should be modified by not using a wet return less than 2 in. while the main supply, g-h, if from the uptake of a boiler, should be made the full size of the main, or 3 in. Also the portion of the main k-m should be made 2 in. if the wet return is made 2 in. Notes on Gravity One-Pipe Air-Vent Systems X. Pitch of mains should be not less them 34 in. in 10 ft. 2. Pitch of horizontal runouts to risers and radiators should not be less than ;n in 10 fl,,t. W...h.^e.rve this pHitkcvhii cannot boe otbtained runouts over 8 it in length should bp sizze larger than called for inn thLpe tmabWle*. 3. In general, it is not desirable to have a main less than 2 in. The diameter of the far end of the supply main should be not less than half its diameter at its largest part 4. Supply mains, branches to risers, or risers, should be dripped where necessary Table 9. Pipe Sizes for One-Pipe Up-feed System Shown in Fig. 1 Past or Ststeu . Section or Pipe Branches to radiators.. Branches to radiators.. Riser ___ Riser_____ Riser__________ ______ Riser......... Riser_____ a to b b to c c to d d toe Branch to riser Supply main........... Branch to supply main Dry return main.......... Wet return main Wet return main Wet return main ftog g to h htoj ftak ktom tn ton n to p Radiation Supplied (Sq Ft) ThboreticaiPipe size (Inches) Practical Pipe size (Inches) 100 50 200 400 500 600 600 600 600 600 600 600 2 134 2 236 234 3 3 m 3 2^ 134 1 1 1 2 134 .2 234 2)4 3 3 334 3 3 2 2 2 2 Fig. 1. Riser, Supply Main and Return Main of One-Pipe System TWO-PIPE GRAVITY AIR-VENT SYSTEMS The method employed in determining pipe sizes for two-pipe gravity air-vent systems is similar to that described for one-pipe systems except that the steam mains never carry radiator condensation.. The drop allowable per 100 ft of equivalent run is obtained by taking the equiva lent length to the farthest radiator as double the actual distance, and then dividing the allowable or desired total drop by the number of hundreds of feet in the equivalent length. Thus in a system measuring 400 ft from the boiler to the farthest radiator, the approximate equivalent length of run would be 800 ft. With a total drop of lb the drop per 100 ft would be ~ or Me. lb; therefore, Column D would be used for all steam mains where the condensation and steam flow in the same direc tion. If a total drop of % lb is desired, the drop per 100 ft would be 3^Ib 680 m Chapter 32--Piping for Steam Heating Systems. *and Column B would be used. If the total drop were to be 1 lb, the drop per 100 ft would be J4 lb and Column would be used. For mains and riser runouts that are not dripped, and for radiator runouts where in all three cases the condensation and steam flow in opposite directions, Column I should be used, while for the steam risers Column H should be used unless the drop per 100 ft is 36< lb or 3$2 lb, when Columns B or C should be substituted so as not to exceed the drop permitted. On an overhead down-feed system the main steam riser should be sized by reference to Column H, but the down-feed steam risers sup plying the radiators should be sized by the appropriate Columns B through C since the condensation flows downward with the steam through them. The riser runouts, if pitched down toward the riser as they should be, are sized the same as the steam mains, and the radiator runouts are made the same as in an up-feed system. In either up-feed or down-feed systems the returns are sized in the same manner and on the same pressure drop basis as the steam main; the return mains are taken from Columns 0, R, U, X, or AA according to the drop used for the steam main; and the risers are sized by reading the lower part of Table 8 under the column used for the mains. The hori zontal runouts from the riser to the radiator are not usually increased on the return lines although there is nothing incorrect in this practice. The same notes apply that are given for one-pipe gravity systems. TWO-PIPE VAPOR SYSTEMS While many manufacturers of patented vapor heating accessories have their own schedules for pipe sizing, an inspection of these sizing tables indicates that in general as small a drop as possible is recommended. The reasons for this are: (1) to have the condensation return to the boiler by gravity, (2) to obtain a more uniform distribution of steam throughout the system, (3) because with large variation in pressure the value of graduated valves on radiators is destroyed. For small vapor systems where the equivalent length of run does not exceed 200 ft, it is recommended that the main and any runouts to risers that may be dripped should be sized from Column D, while riser runouts not dripped and radiator runouts should employ Column /. The up-feed steam risers should be taken from Column H. On the returns, the risers should be sized from Column U (lower portion) and the mains from Column U (upper portion). It should again be noted that the pressure drop in the steam side of the system is kept the same as on the return side except where the flow in the riser is concerned. On a down-feed system the main vertical riser, should be sized from Column H, but the down-feed risers can be taken from Column D al though it so happens that the values in Columns D and H correspond. This will not hold true in larger systems. For vapor systems over 200 ft of equivalent length, the drop should not exceed lb to }?& lb, if possible. Thus, for a 400 ft equivalent run the drop per 100 ft should be not over }/% lb divided by 4, or lb. In this case the steam mains would be sized from Column B; the radiator , and 581