Document qmRx44pq2j8kQ4vvKxEk93VQn

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 HORIZONTAL SUPPLY MAINS The horizontal mains supplying the risers at the top of a down-feed system must be liberally sized unless the house tank is set at a much higher elevation than usual. To provide a gravity head on the highest fixtures of 15 lb-per square inch it is necessary for the water line in the house tank to be nearly 40 ft higher, and with the line loss considered this becomes about 45 ft. Such heights are not often practical and as a result the pressure on the highest fixtures either is reduced to 7 lb (which is sufficient to operate a flush valve), or flush tank water-closets are sub stituted, or a separate cold and hot water supply is installed with a small pneumatic tank to give the increase in pressure necessary. The chief objection to the use of a pneumatic tank is that a separate hot water heater is required and this heater must be located either sufficiently below the highest fixtures to obtain a gravity circulation, or it must be provided with a circulating pump in order to force the hot water to-the top floor level. The most common solution is to place the house`tank as high as the structural and architectural conditions will permit and then, to use liberally-sized lines between the house tank arid the upper fixtures, say for the two top stories, below which the riser sizes may be reduced to .those indicated in Fig. 2 and Table 2. Where the house tank is only one story above the top fixtures, flush tank water-closets must be used and the drop in the entire run from the house tank down to the farthest fixture should not exceed 1 lb; the less, the better. This means that if the total equivalent run to the farthest top fixtures supplied is 300 ft, the drop per 100 ft should not exceed ^ or 0.33 lb per 100 ft. The friction curves shown in Fig. 3 may be used for quickly determining the proper size of pipe to give any desired drop in pounds per 100 ft of equivalent run. OVERHEAD' DISTRIBUTION MAIN Example S. 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 and 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 maximum flow rate of 400 gpm may be expected, of the horizontal main where a maximum flow rate7of 200 gpm may be expected, and of the riser down to the fixture level where the maximum'flow rate is'approximately 100 gpm? 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 1 lb X 100 ----- 600--------- 0,16671b. Referring to Fig; 3 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 to .the top floor, then reduce to 4 in. for the branch to the floor below the top, and belowThis the sizes in Table 2 could be followed. In such a case, flush tank closets Should doubtless be substituted. ' Had the tank been 'set 10 ft higher, the head available to be used up .in .friction, but 776' RATE OF FLOW IN.GALLONS PER MINUTE CHAPTER 45. WATER SUPPLY PIPINC AND WATER HEATING still giving the same pressure at the top fixtures, would have been 0.43 lb X 10 ft or 4.3 lb lit? the.1115 drop used previously, would give-a total allowable drop of ,T rv , b --' .3 "> which, divided by the 600 ft equivalent run gives a drop per 100 ft 777