Document zQkMzwe0EjM7KNr1bKz6ZQmLg

536 CHAPTER 20 1058 Guide sized from Column C; radiator runouts and undripped riser runouts from Column L; up-feed risers from Column J; the main riser on a down-feed system from Column C. (it will be noted that if Column H is used the drop would exceed the limit of psi); the dry return from Column ft; and the wet-return from Column Q. With a 3^2-psi drop the sizing would be the same as for J44 psi, except that the steam main and dripped runouts would be sized from Column ft, the main riser on a down-feed system from Column B, the dry-return from Column 0, and the wet-return from Column N. Notes on Gravity One-Pipe Air-Vent 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 }{ in. . per foot. 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 main less than 2 in. The diameter of the far end of the supply main should not be less than half its diameter at its largest part. 4. Supply mains, runouts to risers, or risers, should be dripped where necessary. 5. Where supply mams are decreased in size they should be dripped, or be provided with eccentric couplings, flush on bottom. Example S: Size the one-pipe gravity steam system shown in Fig. 22 assuming that this is all there is to the system, or that the riser and main shown involve the longest rim on the system. Solution: The total length of run actually shown is 215 ft. If the equivalent length of run is taken at double this, it will amount to 430 ft, and with a total drop of fi psi the drop per 100 ft will be slightly less than psi; It would be well in this case to use J$4 psi, and this would result in the theoretical sizes indicated in Table 12. These theoretical 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. SIZING PIPING FOR ONE-PIPE VAPOR SYSTEMS Piping for one-pipe vapor systems is sized so as to permit only a few ounces pressure drop in the system. Otherwise, the method follows that outlined for sizing one-pipe gravity systems. SIZING PIPING FOR TWO-PIPE HIGH PRESSURE SYSTEMS Steam supply piping for two-pipe high pressure can be sized for greater pressure drops than that of the return piping. For a system using steam at 30 psig, the total pressure drop can be 5 to 10 psi, and for 150 psig systems, 25 to 30 psi. It has been observed that the maximum total pressure in the returns of a 30-psig system is about 5 psig, and that of a 150-psig system is about 20 psig, The pressure in the return mains is, of course, caused by the dis charge of traps and flashing of condensate into steam because the return line pressure is below that corresponding to the saturation temperature ol the condensate. The usual practice in the sizing of high pressure returns has been to size on the basis of lA psi per 100 ft of pipe for 30-psig sys tems, and 1 psi per 100 ft for 150-psig systems. This is an average figdre which corresponds generally to several of the previously published tables for the design of high pressure return piping. Notes on Two-Pipe High Pressure Systems Pitch of mains should not be less than 14 in. in 10 ft. Steam Heating Systems 537. Pitch of horizontal runouts to risers and heating units should not be less than in. per ft. SIZING PIPING FOR TWO-PIPE IX)W PRESSURE SYSTEMS Piping for two-pipe low pressure systems is sized in the same manner as for two-pipe vapor systems, except that the pressure , drop throughout the system can be based on H psi to 1 psi drop. SIZING PIPING FOR 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 Table 12. Pipe Sizes for One-Pipe Up-Feed System Shown in Fig. 22 Part op System Section of Pipe Radiation Supplied EDR Sq Ft Theoretical Pipe size (Inches) Practical Pipe size (Inches) Branches to radiators.......... 100 2 2 Branches to radiators.......... 50 1M 1H Riser......................................... o to b Riser................................... b to c 200 300 22 m 2H Riser................. c to d 400 2H 2M Riser................... Riser................. d to 6 500 3 3 e to / 600 3 3 Runout to riser............... f tog 600 3H 3K Supply main........................ Branch to supply main.... Dry return main... g to h A to/ /to * 600 600 600 33 w3 m2 Wet return main Wet return maip Wet return main k to m m to n n to p .600 000 600 . 1 1 2 2 2 Fig. 22. Riser, Supply Main and Return Main of One-Pipe System indicates that in general as small a drop as possible is recommended. The reasons for this are: (1) to have the condensate return to the boiler by gravity j (2) to obtain a more uniform distribution of steam throughout fi 6,Sys^> specially when it is desirable to carry a moderate or low a, (3) to prevent large variations in pressure which would nullify e value of graduated valves on radiators. except onnt vap?r systems where the equivalent length of run does not fbat l I* 's recommended that the main and any runouts to risers dripped, should be sized from Column D, Table 5, while riser un-f n n,*' dipped and radiator runouts should employ Column J. The tbo ,am risers should be taken from Column H. On the returns, ami s"ould be sized from Tables 6 and 7, Column U flower portion), noted tb *?aiLna *rom Column U (upper portion). It should again be same as n 6 pressure drop in the steam side of the system is kept the Oti n re*'urn de, except where the flow in the riser is concerned. Column <^wn"^eed system, the main vertical riser should be sized from mn " > but the down-feed risers can be taken from Column D, al-