To obtain the friction loss to inlets on risers 3 and 4 the friction loss in the pipe “bc” must be added to the above figures. With 50 ft. of 3¹⁄₂-in. pipe carrying 280 cu. ft. free air the friction loss is 0.6 in. when the vacuum in the pipe is 12 in. and 0.4 when the vacuum in the pipe is 8 in.
The total loss of vacuum to inlets on risers 3 and 4 will be 2.91 in. if 1-in. hose is used and 1.95 in. if 1¹⁄₄-in. hose is used. In this case, the total loss from inlet to vacuum producer is approximately equal to the maximum variation of vacuum permitted at sweeper outlets when 1¹⁄₄-in. hose is used, but is greater than when 1-in. hose is used.
However, it is the variation in vacuum at the hose cock farthest from and that nearest to the vacuum producer that fixes the maximum variation allowable. In this case it will be the difference in vacuum between an inlet on riser 1 or 2 and a similar inlet on riser 3 or 4. The difference in vacuum at the bases of these risers will be the friction loss in the pipe “bc,” and the total difference in friction in the risers will occur when one sweeper is attached to the lowest inlet on one riser, and one sweeper on the eighth and one on the seventh floor on the other riser. The friction loss in the riser having the two sweepers attached to its upper inlets will be:
15 ft. of 2¹⁄₂-in. pipe from seventh to eighth floors, 70 cu. ft. of free air per minute, or 0.051 in. with a density equivalent to 6-in. vacuum, and 0.075 in. with a density equivalent to 10-in. vacuum.
85 ft. of 2¹⁄₂-in. pipe from first to seventh floors, 140 cu. ft. free air per minute, or 0.25 in. with a density equivalent to 6-in. vacuum, and 0.42 in. with a density equivalent to 10-in. vacuum.
The total difference in vacuum at the hose cocks will be:
0.051 + 0.25 + 0.4 = 0.7 in. with 6-in. vacuum at the hose cock.
0.075 + 0.42 + 0.6 = 1.15 in. with 10-in. vacuum at the hose cock.
Either of these values are well within the maximum variation. It is, therefore, evident that when the vacuum producer cannot be centrally located that a piping system which will give the most nearly equal length of pipe to each riser will yield the best results.
A vacuum cleaning system for serving a passenger car storage yard will best illustrate the effect of long lines of piping. A typical yard having 8 tracks, each of sufficient length to accommodate 10 cars, is shown in [Fig. 63]. The vacuum producer in this case is located at the side of the yard at one end, which is not an unusual condition.