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Mechanical circulators are installed in vacuum pans in order to reduce the boiling time. But published data from Huletts Refinery in Durban has shown a reduction in sugar colour and an increase in pan yield as a result of the use of a pan stirrer.
Hugot quotes Webre in stating advantages of pan stirrers as follows
The stirrer shall be vertical top suspended type. The stirrer shaft shall be supported by bearings mounted inside the gearbox casing. The stirrer gearbox shall be supported on the pan itself. The pan shall be designed to carry the weight and live loads imposed on it by the stirrer.
If a bearing is required at the bottom of the shaft, it shall be lubricated by massecuite. The bearing housing shall be so designed that the bearing can be removed and replaced from the outside of the pan.
The material of construction of shaft and impeller shall be the same as that of the pan in which it is mounted, except that above the pan strike level the shaft may be of carbon steel. A rigid flanged coupling shall be used to join the carbon steel part to the stainless steel part of the shaft.
The shaft shall be coupled to the gearbox by means of a hollow shaft coupling. The gearbox shall have an American Gearbox Manufacturers Association (AGMA) service factor of no less than 1.5.
A gland or seal shall be provided to prevent air from passing between the shaft and the pan shell in to the pan body. This seal may be of the packed gland type.
Depending on the stirrer design baffles may be provided to reduce the circumferential velocity of massecuite in the pan downtake.
The critical speed of the shaft and impeller shall be no less than 120% of the maximum shaft speed. The impeller blade tip clearance shall be no less than 5% of the impeller diameter. The blade tip speed shall not exceed 10 m/s.
The stirrer shall be supplied with either
If a two speed motor is supplied the motor shall be wound such that the full load torque at the low speed is equal to full load torque at the high speed. Belts and pulleys shall be used to transmit power from the motor to the input of the gearbox.
The motor shall run at the high speed for as long as possible. When the motor current drawn reaches the rated full load current the switch gear must change the motor from the high speed windings to the low speed windings. The motor must continue to run at the low speed until the motor current drawn reaches the rated full load current for the low speed windings, at which point the motor must trip, an audible and visible alarm shall be given in the pan floor control room.
See an extract from the ABB two-speed motor catalog
If a variable speed direct torque control drive is supplied the motor shall be flange mounted on to the gearbox.
The drive shall be 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 must sense the increase in required torque and slow 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 must trip, an audible and visible alarm shall be given in the pan floor control room.
As can be seen from the specification above the power absorbed by the stirrer increases as the boil progresses. this is because the the viscosity of the massecuite increases as the brix increases. The graph below shows the variation of motor power with boil time for a stirrer fitted with a two speed motor.