Fig. 170.—Diagram Showing the Discharge and Spacing of Taylor Nozzles.
The nozzles should be selected to give the best distribution, to consume all of the head available, and to give the proper cycle of operation. The entire head available should be consumed in order that the fewest number of nozzles may be used. An excellent study of the characteristics of various types of nozzles has been published in Bulletin No. 3 of the Engineering Experiment Station at Purdue University, 1920. As a result of the tests on the nozzles shown in Fig. 169, it was determined for all nozzles, except No. 8, that
Q = Ca√(2gh);
in which Q = the rate of discharge in cubic feet per second; C = a coefficient shown in Table 88; a = the net cross-sectional opening of the nozzle in square feet; h = the pressure on the nozzle in feet of water.
| TABLE 88 | |||||||
|---|---|---|---|---|---|---|---|
| Coefficients of Discharge for Sprinkler Nozzles Shown in Fig. 169 | |||||||
| Nozzle Number | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Coefficient | .648 | .756 | .696 | .666 | .675 | .598 | .569 |
It is evident that if the head on the nozzles is constant and the nozzle throws a circular spray, the intensity of dosing at the circumference will be greater than nearer the center. This difficulty is overcome by so designing the dosing tank from which the sewage is fed that the head on the nozzle and the quantity thrown will vary in such a manner that the distribution over the bed is equalized. Intermittent action is obtained by an automatic siphon which commences to discharge when the tank is full and empties the tank in the period allowed for dosing. Under such conditions the tank should discharge for a longer time at the higher heads than at the lower heads as there is more territory to be covered at the higher heads. The design of the tank to do this with exactness is difficult, and the construction of the necessary curved surfaces is expensive. Where a dosing tank is used for such conditions it has been found satisfactory to construct the tank with plane sides sloping at approximately 45 degrees from the vertical (or horizontal). A tank with curved surfaces is shown in Fig. 171. The dosing siphon is usually placed in the tank as shown in the figure. The head and quantity of discharge through the nozzles can be varied also by maintaining a constant depth in a dosing tank by means of a float feed valve, and varying the head and quantity discharged to the nozzles by a butterfly valve in the main feed line, or by the use of a Taylor undulating valve designed for this purpose. The butterfly valve is opened and closed by a cam so designed and driven at such a rate that the required distribution is obtained. The Taylor undulating valve is opened and closed at a constant rate, the shape of the valve giving the required variations in head and discharge. Other methods of control have been attempted but have not been used extensively.
Fig. 171.—Section of 12–inch Siphon and Dosing Tank, for King’s Park, Long Island.
An example of the design of the nozzle layout and dosing tank for a sprinkling filter follows: