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HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 Example 2. What pressure drop should be used for the steam piping of a system if the measured length of the longest run is 500 ft and the initial pressure is not to be over 2-lb gage? Solution. It will be assumed, if the measured length of the longest run is 500 ft, that when the allowance for fittings is added the equivalent length of run will not exceed 1,000 ft. Then, with the pressure drop not over one half of the, initial pressure, the drop could be 1 lb or less. With a pressure drop of 1 lb and a length of run of 1,000 ft, the drop per 100 ft would be Jfj lb, while if the total drop were Vi lb, the drop per 100 ft would be Ho lb. In the first instance the pipe could be sized according to Column D for Ho lb Pot 100 ft, and in the second case, the pipe could be sized according to Column C for. H4 lb. On completion of the sizing, the drop could be checked by taking the longest line and actually calculating the equivalent length of run from the pipe sizes determined. If the calculated drop is less than that assumed, the pipe size is all right; if it is more, it is probable that there are an unusual number of fittings involved, and either the lines must be straightened or the column for the next lower drop must be used and the lines resized. Ordinarily resizing will be unnecessary. ONE-PIPE GRAVITY AIR-VENT SYSTEMS One-pipe gravity air-vent systems in which the equivalent length of run does not exceed 200 ft should be sized as follows: 1. For the steam main and dripped runouts to risers where the steam and condensate flow in the same direction, use Hs-lb drop (Column D). 2. Where the riser runouts are not dripped and the steam and condensation flow in opposite directions, and also in the radiator runouts where the same condition Occurs, use Column L. 3. For up-feed steam risers carrying condensation back from the radiators, use Column J. 4. For doom-feed systems the main risers of which do not carry any radiator con densation, use Column H. 5. For the radiator valve site and the stub connection, use Column K. 6. For the dry return main, use Column U. 7. For the wet return main use Column T. On systems exceeding an equivalent length of 200 ft, it is suggested that . the total drop be not over lb. The return piping sizes should correspond with the drop used on the steam side of the system. Thus, where HU-lh drop is being used, the steam main and dripped runouts would be 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 H4 lb); the dry return from Column R;, and the wet return from Column Q. With a ^2-lb drop the sizing would be the same as for fit lb except that the steam main and dripped runouts would be sized from Column B, the main riser on a down-feed system from Column B, the dry return from' Column O, and the wet return from Column N. Notes on Gravity One-Pipe Air-Vent Systems 1. Pitch of mains should not be less than H in. >n 10 ft. 2. Pitch of horizontal runouts to risers and radiators should not be less than H 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 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. 294 CHAPTER 15. PIPING FOR STEAM HEATING SYSTEMS 4. Supply mains, branches to risers, or risers, should be dripped where necessary. 5. Where supply mains 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. 2 assuming that this is all there is to the system or that the riser and run shown involve the longest run 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 J4 lb the drop per 100 ft will be slightly less than H6 lb. It would be well in this case to use U4 lb, and this would result in the theoretical sizes indicated in Table 8. These theo 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. Table 8. Pipe Sizes for One-Pipe Up-feed System Shown in Fig. 2 Part or System Section or Pipe Branches to radiators.. Branches to radiators.. Riser............................... Riser............................... Riser............................... Branch to riser...... ....... Supply main................. Branch to supply main Dry return main.......... Wet return main.......... Wet return main.......... Wet return main......... 0 to b b to c c to d d toe e to/ / tog gtoh h to j ftok k to m m to n n to p Radiation Supplied (Sq Ft) Theoretical Pipe hub (Inches) Practical Pipe size (Inches) 100 2 2 50 1H 200 2 2 300 2H 2H 400 2H m 500 3 3 600 3 3 600 3 H 3 H 600 3 3 600 zH 3 600 2 600 1 2 600 1 2 600 1 2 Fig. 2. 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 He 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 lb is desired, the drop per-100 ft would be % lb 295