In addition, the role of the lower impeller in both mass transfer and mixing must be considered and the desirability of having multiple impellers in the tank can be considered.
If it is desired to consider axial flow impellers in a gas-liquid system for any reason, it should be remembered that the upward flow of gas tends to negate the downward action of the pumping capacity ofthe axial flow turbine. A radial flow turbine must have three times more power than the power in the gas stream for the mixer power level to be fully effective. On the other hand, the axial flow impeller must have eight to ten times more power than in the gas stream for it to establish the axial flow pattern.
In gas-liquid systems, one of the reasons that the power of the impeller is lower with the gas on than with the gas off is that the gas bubbles collect behind the impeller blade. This streamlines the blade, reducing power. Looking at Fig. 45, speculation can be made on what would happen if one were to fill up and streamline the back of the impeller with solid material. What happens is that with no gas rate, Fig. 46, the impeller draws less horsepower with the back of the blade streamlined than with the back of the blade flat.
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