Document oMpqmROB59rvQN1rdnJbBmJVE

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 5. Where supply mains are decreased in size they should be dripped, or be provided with eccentric couplings, flush on bottom. 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 34 lb the drop per 100 ft would be or lb; therefore, Column D would be used for all O steam mains where the condensation and steam flow in the same direc tion. If a total drop of 34 lb is desired, the drop per 100 ft would be lb and Column B would be used. If the total drop were to be 1 lb, the drop per 100 ft would be x/% lb and Column E 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 H*, lb of 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 II, but the down-feed steam risers`sup plying the radiators should be sized by the appropriate Columns B through G, 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 9 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, especially when it is desirable to carry a moderate or low fire, and (3) because with large variation in pressure the value of gradu ated valves on radiators is destroyed. 298 CHAPTER 16. PIPING FOR SJEAM HEATING SYSTEMS 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 I. 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./? 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 34 lb to 34 lb, if possible. Thus, for a 400 ft equivalent run the drop per 100 ft should be not over 34 lb divided by 4, or lb. In this case the steam mains would be sized from Column jB; the radiator and undripped riser runouts from Column /; the risers from Column B, because Column H gives a drop in excess of % lb. On a down-feed system, Column B would have to be used for both the main riser and the smaller risers feeding the radiators in order not to increase the drop over lb. The return risers would be sized from the lower portion of Column 0 and the dry return main from the upper portion of the same column, while any wet returns would be sized from Column N. The same pressure drop is applied on both the steam and the return sides of the system. Notes on Vapor Systems 1. Pitch of mains should not be less than in. in 10 ft. 2. Pitch of horizontal runouts to risers and radiators should not be less than l/2 in. in 10 ft. Where this pitch cannot be obtained runouts over 8 ft in length should be one size larger than called for in the table. 3. In general it is not desirable to have a supply main smaller than 2 in. 4. When necessary, supply main, supply risers, or branches to supply risers should be dripped separately into a wet return, or may be connected into the dry return through a thermostatic drip trap. VACUUM, ORIFICE, SUB-ATMOSPHERIC SYSTEMS Vacuum, atmospheric, sub-atmospheric and orifice systems are usually employed in large installations and have total drops varying from )4 to 34 lb. Systems where the maximum equivalent length does not exceed 200 ft preferably employ the smaller pressure drop while systems over, 200 ft equivalent length of run more frequently go to the higher drop, owing to the relatively greater saving in pipe sizes. For example, a sys tem with 1200 ft longest equivalent length of run would employ a drop per 100 ft of 34 lb divided by 12, or lb. In this case the steam main would be sized from Column C, and the risers also from Column C (Column H could be used as far as critical velocity is concerned but the drop would exceed the limit of lb). Riser runouts, if dripped, would use Column C but if undripped would use Column I; radiator runouts, Column I; return risers, lower part of Column S; return runouts to radiators, one pipe size larger than the radiator trap connections. 299