Page 276 - Water and wastewater engineering

COAGULATION AND FLOCCULATION 6-53

at 175 rpm has been shipped with the four impellers described below. The
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baffled rapid mix tank into which it is to be installed is 3.0 m in volume (1.56 m
diameter by 1.56 m deep). Which impeller should be used? Assume a water tem-
1
3

perature of 10 C, a G 1,000 s , a water density of 1,000 kg/m , and that the
motor power to water power efficiency is 80%. Show the calculations to justify
your decision.
Impellers

A. Propeller, pitch of 2, 3 blades, diameter 0.48 m, N p 1.0
B. Propeller, pitch of 2, 3 blades, diameter 0.61 m, N p 1.0
C. Turbine, 6 flat blades, vaned disc, diameter 0.48 m, N p 6.3

D. Turbine, 6 flat blades, vaned disc, diameter 0.61 m, N p 6.3
6-14. The town of Eau Gaullie has requested proposals for a new coagulation water treat-
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ment plant. The design flow for the plant is 0.1065 m /s. The average annual water
temperature is 19 C. The following design assumptions for a rapid-mix tank have
been made:
1. Number of tanks 1 (with 1 backup spare)
2. Tank configuration: circular with liquid depth 2 diameter

3. Detention time 10 s
1
4. Velocity gradient 800 s
5. Impeller type: turbine, 6 flat blades, N p 5.7
6. Available impeller diameters: 0.45, 0.60, and 1.2 m
7. Assume B (1/3) H

Design the rapid-mix system by providing the following:

1. Water power input in kW
2. Tank dimensions in m
3. Diameter of the impeller in m
4. Rotational speed of impeller in rpm

6-15. Your boss has assigned you the job of designing a rapid-mix tank for the new water
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treatment plant for the town of Waffle. The design flow rate is 0.050 m /s. The aver-
age water temperature is 8 C. The following design assumptions for a rapid-mix tank
have been made:
1. Number of tanks 1 (with 1 backup)
2. Tank configuration: circular with liquid depth 1.0 m
3. Detention time 5 s