Power propeller mixer

Baffles can be omitted when propeller mixers are top mounted at an angular off-center position (see Figure 5-23D) and vortex swirling is prevented. This is not recommended for large power systems on large tanks, due to shaft fatigue. 

A solution of sodium hydroxide of density 1650 kg/m3 and viscosity 50 mN s/m2 is agitated by a propeller mixer of 0.5 m diameter in a tank of 2.28 m diameter, and the liquid depth is 2.28 m. The propeller is situated 0,5 m above the bottom of the tank. What is the power which the propeller must impart to the liquid for a rotational speed of 2 Hz ... 

For producing an oil-water emulsion, two portable three-bladed propeller mixers are available a 0.5 m diameter impeller rotating at 1 Hz and a 0.35 m impeller rotating at 2 Hz. Assuming turbulent conditions prevail, which unit will have the lower power consumption ... 

Figure 7.6. Power number as a function of Reynolds number for a propeller mixer...

We find then that the coefficient P is between 2 and 4 for propellers and pitched blade impellers, and between 1 and 2 for flat disc turbines. That would mean that the latter are the most power-effective mixers. 

Turbines, propellers, and paddles power = 0.2 to 1.5 kW/m for mixing liquids with impeller discharge rate >20 x liquid flow rate into tank. Heat transfer, 0.4 to 2 kW/m but don t neglect heat input from the mixer mass transfer 2 to 4 kW/mT Turbines, propellers, and paddles power = 1 to 4 kW/m for mass transfer. Air agitation diffused air 0.3 to 0.5 NdmVs-m 1.5 to 6 dmVs m... 

POWER CORRELATIONS FOR SPECIFIC IMPELLERS. The various shape factors in Eq. (9.16) depend on the type and arrangement of the equipment. The necessary measurements for a typical turbine-agitated vessel are shown in Fig. 9.7 the corresponding shape factors for this mixer are Sj = DalD S2 = EID S = L/D , S4 == W /Od. S5 = J/D, and Sg = HfD,. In addition, the number of baffles and the number of impeller blades must be specified. If a propeller is used, the pitch and number of blades are important. 

Power Consumption of Mixers. Valentine and Macl ean in Perry s Chemical Engineers IJandbook, 3d ed, sec. 17, give theoretical horsepower requirements for propellers. They state that the power consump-... 

The action of a turbine or propeller in a Newtonian liquid of low viscosity can be likened to that of the impeller in a radial or axial flow pump respectively. That is, we can regard the mixer as being a caseless pump. The use of the flow rate (or discharge), Q, from a pump and the fluid head, //, is well known. Thus for a pump of unit efficiency these variables can be related to the power input... 

Note that the above power values are for power dissipated by the jet or propeller and do not include motor efficiency and bearing losses for the agitator, or pump efficiency and external pipe friction losses for the jet. The head loss by the recirculating fluid in a jet mixer can be significant proportion of the total fluid head required for mixing. 

For a wide range of applications, a portable mixer which can be clamped on the top or side of the vessel is often used. This is commonly fitted with two propeller blades so that the bottom rotor forces the liquid upwards and the top rotor forces the liquid downwards. The power supplied is up to about 2 kW, though the size of the motor becomes too great at higher powers. To avoid excessive strain on the armature, some form of flexible coupling should be fitted between the motor and the main propeller shaft. Units of this kind are... 

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