The portion of the main in the basement that serves the two risers on either side of the building (portion “ab,” [Fig. 61]) must be of such size as will produce the same loss in vacuum with 280 cu. ft. of air passing as the 2¹⁄₂-in. pipe gives with 140 cu. ft. of air passing. This may be determined from any table of equalization of pipes or may be obtained from the chart, [Fig. 48], in the following manner:
Find the intersection of the horizontal line “140” with the diagonal representing a 2¹⁄₂-in. pipe and pass on the nearest vertical to its intersection with the horizontal line “280.” The diagonal inclined toward the left passing nearest this intersection will be the pipe size required. In this case a 3-in. pipe will give a slightly greater friction and will be sufficient.
Unfortunately, it is rarely possible to locate the vacuum producer in as favorable a point as that given in the illustration, but an effort should always be made to select a location as nearly central to all risers as possible. The basements of modern office buildings are generally crowded and the space assigned to the mechanical equipment is limited and owing to the necessity of ventilation, the vacuum producer is generally located near the outside of the building.
Probably the best location that could be obtained in this case would be at “d” ([Fig. 62]). The length of piping to risers 1 and 2 would now be the same as that to all risers in case of [Fig. 61], but the distance to risers 3 and 4 will be increased 50 ft. It will be possible to increase the size of the pipe line “bd” to the maximum size to serve four sweepers, or 3¹⁄₂ in., the risers and their branches to remain 2¹⁄₂ in.
The total friction loss to risers 1 and 2 will now be:
| Entrance to tee in risers, 10 ft. plus 50 ft. | 60 ft. |
| Turn at base of riser, 10 ft., branch from “c” to riser 32 ft. | 42 ft. |
| Entrance to tee in main | 20 ft. |
| Total equivalent length of 2¹⁄₂-in. pipe | 122 ft. |
When 1-in. hose is used the density of the air entering the 2¹⁄₂-in. pipe is equivalent to a vacuum of 10 in. mercury and the friction loss in the 2¹⁄₂-in. pipe will be 1.9 in. mercury. When 1¹⁄₄-in. hose is used, the density of the air entering the pipe will be equivalent to a vacuum of 6-in. mercury and the friction loss in the 2¹⁄₂-in. pipe will be 1.32 in. mercury.
FIG. 62. ELEVATION OF LAYOUT FOR OFFICE BUILDING, SHOWING BEST LOCATION (AT D) FOR VACUUM PRODUCER.
The density of the air entering the 3¹⁄₂-in. pipe, “bd,” will be equivalent to a vacuum of 11.9 in. mercury when 1-in. hose is used, and to 7.32 in. mercury when 1¹⁄₄-in. hose is used. The friction loss in the 3¹⁄₂-in. pipe will be 0.31 and 0.23 in. mercury, respectively. Total friction loss to inlets on risers 1 and 2 will be 2.21 in. with 1-in. hose in use, and 1.55 in. with 1¹⁄₄-in. hose.