Document MGJd4Q473X0rqO6B6eBVjQw7y

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 PIPE SIZES FOR DOWN-FEED COLD WATER SYSTEM The risers for down-feed systems may be reduced considerably in size compared with those for up-feed systems because of the 0.43 lb per foot gain in pressure due to increasing hydrostatic head as the lowest story is approached. It has proved practicable to select down-feed riser sizes Fig. 4. Typical Layout for Down-Feed System House Tank- ------------------------ ------------------------------------------:------------------------------------------ 197 4' |t 8th. 197 4W.C.-F.V. 2U.-F.V. 3 Lav. 4" 255 215 4- 6W.C.-F.V. 4 Lav. 5* ^ 166 ? 7 th. 4W.C.-F.V. 2U.-F.V. 3 Lav. 211 * ILc-rv- . 145 2 6th. f y 4 W.C.-F. V. 2U.-F.V. 3 Lav. 196 2f 6W.C.-F.V. 117, 2 5th. t r 4W. C.-F; V. 2U.-F.V. 3 Lav. 180 2` fLc-FV' 25 T 4th. 10 Lav. . 160 2 4W. C.-F. V. 2 U.-.F. V. 3Lm- 11 J 3rd. i s. a ,, 130 2 |a 4W.C.-F.V. 2 U.-F. V. 3 Lav. 8 lias. 2nd. 98 .. lj 2W.C.-F.V. 1U.-F.V. ^ ^ 1 Lav. 4 t 1SS 1st *- ' <J) . 45 .f IT IW.C-tF.V. (2) ---- ' 282 122 ri iaa 4- 2 is. a 120 2" 1 a a 120 2' 1 a s. . 3W.C.-F.V. 119 2 1 Lav. ^ las. . 90 2\ 2 Lav. 89 if 2 3W.C.-F.V. 1 Lav. 4 1 1aa , (3) on the basis of a pressure drop of 30 lb per 100 ft. The 13 lb difference between 43 lb per 100 ft and 30 lb per 100 ft will usually take care of the friction in the fittings. The overhead mains, however, must be selected conservatively, as the pressure at the top will be low and the pressure drop available for friction will necessarily be small. In nearly all tall buildings the pressure is limited to that due to the hydrostatic head between the house tank and the main, though sometimes this is increased by the use of a pneumatic house tank. 822 CHAPTER 46. WATER SUPPLY PIPING AND WATER HEATING Where flush valves are used on top story closets the minimum practic able difference in elevation between the overhead mains and the bottom of an open type house tank is 20 ft, and flush valves specially adapted to operate on about 7 lb pressure must be used. In many installations gravity tank closets are used on the top story. Example 4. Suppose an installation has a house tank in which the water line is 20 ft above the level of the top fixtures to be supplied arid that the length of run to the farthest fixtures on this level is 400 ft with the pipe fittings adding another 200 ft, making an equivalent length of 600 ft. What would be the size of main coming out of the tank where a probable flow rate of 400 gpm may be expected, of the horizontal main where a probable flow rate of 200 gpm may be expected, and of the riser down to the fixture level where the probable flow rate is approximately 100 gpm? Table 5. Typical Calculation of Pipe Sizes on Down-Feed Riser with Flush Valve Water-Closets and Urinals (Riser No. 1. Fig. 4) Floor op Blog. 1st 2nd 3rd 4th 5th 6th 7th 8th Fixtures on Floor 1 S. s. I s. s. 1 s. s. 10 Lav. 4 W. C. 2 U. 3 Lav. 4 W. C. 2 U. 3 Lav. 4 W. C. 2 U. 3 Lav. 4 W. C. 2 U. 3 Lav. Gpm PER Fixture 4 4 4 3 Maximum Gpm on Floor 4 4 4 30 Maximum Gpm on Riser Usage (per cent) Probable Flow in Riser Gpm 4 100 4 8 100 8 12 92 11 42 58 25 Allowable Drop Lb per 100 Ft 30 30 30 30 45 180 30 60 39 249 291 40 117 30 45 180 30 66 39 249 540 27 145 30 45 180 30 60 39 249 789 21 166 30 45 180 30 60 39 249 1038 19 197 2 Pipe Size In. H H H 1 2 2 2 4 Here the level of the water in the house tank is 20 ft above the faucet of the highest fixture and the gravity pressure will be 0.43 lb X 20 ft = 8,6 lb and, if a total pressure drop of 1 lb is assumed, the pressure on the farthest fixture under times of peak load will be 8.6 lb -- 1 lb = 7.6 lb while the drop per 100 ft of equivalent run will have to be ouu Referring to Fig. 2 it will be noted that where the flow through the main is 400 gpm, an 8-in. pipe would be required; that where the flow is reduced to 200 gpm, a 6-in. pipe would be sufficient; and that where the flow is 100 gpm in the riser branch and riser, a 5-in. size would be correct. Of course these are somewhat excessive flows and the head from the tank is small so that large sizes are to be expected. It would be necessary to carry a 5-in. riser down to the branch of the top floor, then reduce to 4 in. for the branch to the floor below the top, and below this the pipes could be sized for a 30 lb drop per 100 ft. In such a case, tank closets should doubtless be used on the top floor. 823