Power Draw Or Torque Measurement

small scale static mixers do not fit correctly into a circular pipe. Consideration mnst be made as to how significant this is for the mixing process being investigated. 

It is wise to use a size of static mixer for which manufacturer s friction factor data are available and to ensure confidence in any data of this type before using them in any further correlation. Whenever possible, the friction factor should be measured during the experiment. 

In a stirred tank mixing system, perhaps the most fundamental measurement one can make is that of the power draw of the system, as many scale-up rules depend heavily on the specific power input. The power draw, P, of an impeller is characterized by its power number, Po (also denoted Np). The following expressions can be used to calculate the power draw of an impeller  

All the energy supplied to the fluid by the agitation system must eventually be dissipated as heat, so one possible method of measuring the power draw is to insulate the system and measure the temperature rise over time. In practice, this is extremely difficult to do accurately, due to problems with the effectiveness of the insulation and with the fluid physical properties being a function of temperature. Consequently, the method is rarely used, and methods based on measuring the reaction torque of the system are preferred. 

(4-2) shows, the power input, P, into the system can be calculated if the impeller speed, N, and torque, Tq, are known. If the impeller diameter, D, and fluid density, p, are also known, the power number, Po, for an impeller can be calculated. 

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