Mixer Drive Systems

Invariably a new mixer supplied to the tyre industry, and most mixers supplied to the rest of the rubber industry today would have a variable speed drive system. The days of single speed and two-speed mixers are numbered due to the signifieant inerease in versatility given by a variable speed drive. 

There are three options for a variable speed (VS) drive variable voltage direct current (DC) electrieal drives, variable frequency alternating current (AC) electrical drives, and hydraulic drives. 

Of these, the first VS drives were invariably of the DC type. As drive sizes increase, the cost of motors escalates rapidly, but this type of drive still has advantages at low speeds over other types of electrical drives, as full torque (or full load current) is available down to a stall condition. This has particular advantages in feeding second pass materials at a low rotor speed. 

Variable frequency AC drives suffer from torque limitations at low speeds, and it is not uncommon to have to feed second pass materials at a higher rotor speed than is required for mixing, simply to prevent 

Layout of tangential mixer operated using two hydraulic drives 

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Rotor speed control is an integral part of the process of rate of temperature rise. The modem mixer is generally supplied with variable speed, constant torque electric motors driving via gear reducers or by a hydraulic drive system which has a motor shaft mounted tachometer to control rotor speed. Systems that can monitor, adjust and record the mixer rotor speed provide improvement in resultant compound uniformity. Changes in rotor speed within a cycle can be programmed. 

Key a, mixer b, condenser c, syringe d, syringe drive system e, reactor f, oil bath g, vapor trap and h, nitrogen flow meter. 

Bio-Logic Instrument and Laboratories (Meylan, France) manufactures an SFM-3 stopped-flow instrument (Fig. 4.17) that consists of three independent drive syringes driven by stepping motors, two mixers and a delay line, three observation windows, replaceable cuvettes, no stop-syringes, and efficient temperature regulation. At maximum flow rate, the minimum dead times range from 1.0 to 4.9 ms for fluorescence detection and 1.3 ms for transmittance. Currently, the Bio-Logic MOS-IOOO optical system employs fluorescence or absorbance detection, which is not suitable... 

Fig. 1. Set-up of the PTB laser system. The Nd YAG laser is frequency stabilized onto a selected iodine absorption line using the phase modulation method. The probe beam is modulated at 2.05 MHz by an electro-optic modulator (EOM), the pumb beam is frequency shifted by an acousto-optical modulator (AOM). The driving AOM rf power is chopped in order to cancel frequency offsets introduced by the Doppler background using a lock-in detection scheme. The transmitted probe beam signal is detected by a photodiode (PD) and mixed with the EOM rf in a double balanced mixer (DBM)...

Three types of impellers are commonly used in the low viscosity region, propellers, Fig. 1 turbines. Fig. 2 and axial flow turbines. Fig. 3. Impellers used on small portable mixers shown in Fig. 4, are oflen inclined at an angle as well as being off-center to give a good top-to-bottom flow pattern in the system. Fig. 5. Large top-entering drives usually use either the axial flow turbine or the radial flow flat blade turbine. For aerobic fermentation, the radial flow disc turbine is most common and is illustrated in Fig. 6. 

To test the response time of any detector, one normally challenges the detector with a step function. For example, consider a flow system (Balko et al., 1981) with a mixer and two driving syringes, an observation tube, and a thermistor as the detector as shown in Figure 9. If acid and base are mixed, a step increase in heat occurs because the reaction is so fast. Thus, 0.02M NaOH and 0.0 M HC1 mixed in the ratio of one to one will give a temperature rise of about 69 mK since Mi for the heat of formation of water at 25°C and in 0.1M KC1 is about 13.75 x 103 cal/mol (Hale et al., 1963). The time it takes the temperature of the sensor to rise to Me of its total change is called the sensor s time constant, t. The time to rise half way is ti/2. Since most physical devices respond exponentially, at least to a first approximation, the output E may be expressed as... 

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