Velocity head , impeller

The velocity head JT in a pipe flow is related to Hquid velocity hy H = I Qc The Hquid velocity in a mixing tank is proportional to impeller tip speed 7zND. Therefore, JTin a mixing tank is proportional to The power consumed by a mixer can be obtained by multiplying and H and is given... 

Power consuiTmtion has also been measured and correlated with impeller Reynolds number. The velocity head for a mixing impeller can be calculated, then, from flow and power data, by Eq. (18-3) or Eq. (18-5). 

An impeller with a high fluid head is one with high peripheral velocity and discharge velocity. Such impellers are useful for (I) rapid reduction of concentration differences in the impeller discharge stream (rapid mixing), (2) production of large interfacial area and small droplets in gas-hquid and immiscible-liquid systems, (3) sohds deagglomeration, and (4) promotion of mass transfer between phases. 

The impeller discharge rate can be increased at the same power consumption by increasing impeller diameter and decreasing rotational speed and peripheral velocity so that N D is a constant (Eq. 18-4)]. Flow goes up, velocity head and peripheral velocity go down, but impeller torque Tq goes up. At the same torque, N D is constant, P and Q <=< Dl. Therefore, increasing impeller diameter at... 

Howe er, most conventional pump impellers receive the fluid into the impeller eye, at the center or inside diameter of the impeller. There are single suction impellers, and dual or double suction impellers with two eyes, one on each side. Dual suction impellers are mostly specified for low NPSH applications because the eye area is doubled (it ean reeeive twice as much fluid at a lower velocity head). Dual suction impellers arc mostly found on split case pumps where the shaft passes completely through the impeller. But they can afso be found mounted onto the end of the shaft in some special pump designs. 

Sometimes you hear people say that the pump is operating off its cw. t velocity, the impeller diameter and design are correct, if the pump installed and functioning correctly, including the mechanical seal and impossible to operate off the curve. The pump will be somewhere on between shut-off head and maximum flow a zero elevation. 

The impeller (wheel) of the centrifugal compressor imparts kinetic energy to the gas by increasing the gas velocity through the rotation of the impeller. A static pressure rise in the impeller comes from part of this energy, and the balance is converted to velocity head, which in turn converts to additional pressure rise in the compressor wheel assembly. (See Figures 12-43, 12-44B, 12-461, and 12-47.)... 

Centrifugal pump consists of a rotor (impeller) in a casing in which a liquid is given a high velocity head that is largely converted to pressure head by the time the liquid reaches the outlet. 

The power put into a fluid mixer produces pumping Q and a velocity head H. In fact all the power P which is proportional to QH appears as heat in the fluid and must be dissipated through the mechanism of viscous shear. The pumping capacity of the impeller has been measured for a wide variety of impellers. Correlations are available to predict, in a general way, the pumping capacity of the many impeller types in many types of configurations. The impeller pumping capacity is proportional to the impeller speed N and the cube of the impeller diameter D,... 

If these three relations are combined it is seen that at constant power, one can vary the ratio of flow to impeller velocity head by a choice of D given by Eq. (2)... 

By way of review, all the power that we apply to the mixing impeller produces a pumping capacity, Q, and a velocity head, H, within the discharge from the impeller, such that power P, is proportional as shown below. 

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