Number of Risers to be Installed.
—Much difference of opinion exists among the various manufacturers of vacuum cleaning systems as to the maximum length of hose that should be used with a cleaning system, and as this maximum length determines the number of risers to be installed, some fixed standard is necessary. As already stated in [Chapter VI], the author considers that this maximum should be fixed at 75 ft.; that is, the risers should be so spaced that all parts of the floor of the building can be reached with 75 ft. of hose. Where 50 ft. is used as a maximum, as is recommended by many manufacturers, the number of risers would be increased, incurring a greater cost of installation and requiring the operator to shift his hose from one inlet to another more often than would be the case where fewer inlets were used, and more time would be required in cleaning, with a slight reduction in the power. The author does not consider that this reduction in power would be sufficient to offset the additional time required to change the hose from one inlet to another.
The best and quickest way to determine the number of risers necessary is to cut a piece of string to the length representing 75 ft. on the scale of the plans, and by running this around the plan using corridor doors for access to all rooms, wherever possible, locate the riser so that every point can be reached with the string. In the case of the building illustrated in [Fig. 61] four risers located as shown will be necessary.
Size of Risers.
—Before we can determine the size of risers to be installed it is necessary to determine the probable number of sweepers that will be attached to any one riser simultaneously. In the case of the building ([Fig. 61]) it is possible that there may be four sweepers attached to one riser and it is also possible that there may be but one, and two sweepers to a riser is considered to be a safe assumption. The author uses the following rule in determining the size of risers to use:
Where the number of sweepers is double the number of risers, assume that all sweepers will be on one riser simultaneously.
Where the number of sweepers is equal to the number of risers, assume that half the sweepers will be on one riser simultaneously.
Where the number of sweepers is half the number of risers, assume that one-quarter of the sweepers will be on one riser simultaneously.
When no lifts occur a low velocity in the riser is not objectionable and the size of the riser should be made equal to the size of the horizontal branch thereto throughout its length, wherever this branch is not larger than 2¹⁄₂ in. diameter. When larger, reductions in the riser can be made until 2¹⁄₂ in. is reached when this size should be maintained throughout the remainder of its length. No riser should be made less than 2¹⁄₂ in. unless a lift is necessary.
Before finally fixing the size of riser to be used in any case the size of the branch in the horizontal lines serving the same must be approximately determined.