Mechanical circulators are installed in vacuum pans in order to reduce the boiling time, reduction in sugar colour with an increase in pan yield. With increase in heat transfer coefficient, the strike time reduces up to 30%, which in turn increases the production capacity of the unit.
The mechanical construction of a circulator consists of motor driving the gearbox which offers the required speed reduction to the connected shaft and impellers. A sealing arrangement is provided between the circulator and the pan so as to maintain the vacuum. This seal may be of the packed gland type. A variable speed direct torque control drive (VFD) is used to drive the motor.
The drive is programmed to run at the fastest speed possible while keeping within the motor's allowable power and torque capabilities. If the massecuite viscosity increases, the drive senses the increase in required torque and slows down the motor to prevent any component from being overloaded. If the massecuite viscosity increases to a point where the drive is running at the lowest speed allowable, for motor cooling, then the drive trips.