Solving for the head loss across the screen Ah,
As shown in Equation (5.5), the value of the coefficient can be easily determined experimentally from an existing screen. In the absence of experimentally determined data, however, a value of 0.84 may be assumed for Cd. As the screen is clogging, the value of A2 will progressively decrease. As gleaned from the equation, the head loss Ah will theoretically rise to infinity. At this point, the screen is, of course, no longer functioning.
The previous equations apply when an approach velocity exists. In some situations, however, this velocity does not exist. In these situations, the previous equations do not apply and another method must be developed. This method is derived in the next section on microstrainers.
Referring to Figure 5.3, the flow turns a right angle as it enters the openings of the microstrainer cloth. Thus, the velocity at point 1, V1, (refer to Figure 5.2) would be approximately zero. Therefore, for microstrainers: applying the Bernoulli equation, using the equation of continuity, and prefixing the coefficient of discharge as was done for the bar screen, produce
2 gC2dA 2
As in the bar screen, the value of the coefficient can be easily determined experimentally from an existing microstrainer. In the absence of experimentally determined data, a value of 0.60 may be assumed for Cd. Also, from the equation, as the microstrainer clogs, the value of A2 will progressively decrease; thus the head loss rises to infinity, whereupon, the strainer ceases to function. Although the previous equation has been derived for microstrainers, it equally applies to ordinary screens where the approach velocity is negligible.
Example 5.1 A bar screen measuring 2 m by 5 m of surficial flow area is used to protect the pump in a shoreline intake of a water treatment plant. The plant is drawing raw water from the river at a rate of 8 m3/s. The bar width is 20 mm and the bar spacing is 70 mm. If the screen is 30% clogged, calculate the head loss through the screen. Assume Cd = 0.60.
For screens used in shoreline intakes, the velocity of approach is practically zero. Thus,
2 gCdA 2
From the previous figure, the number of spacings is equal to one more than the number of bars. Let x = number of bars,
x = 54.77, say 55 Area of clear opening = 70 (55 + 1)(2000)
Example 5.2 In the previous example, assume that there was an approach
velocity and that the approach area is 7.48 m . Calculate the head loss. Solution:
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