Reheater Design Parameters
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The purpose of a reheater is to heat up the massecuite which you just cooled down in the crystallisers. You cooled the massecuite in the crystallisers in order to grow the crystals by depositing sucrose in the mother liquor onto the crystals created in the pan.
The reason you now reheat the massecuite is to reduce the viscosity of the massecuite to make the separation of the crystal from the mother liquor in the centrifugal easier.
It is obviously important that when you reheat the massecuite you don't allow the crystals you have so carefully nurtured to redissolve.
The reheater is a finned tube heat exchanger. The massecuite flows on the outside of the tubes, over the fins. Hot water flows through the tubes, giving up heat along the way.
If the crystallisers have been doing their work properly the massecuite will be at about 40°C. In order to be free spinning the massecuite should be heated to about 55°C. The hot water water temperature should not exceed the massecuite outlet temperature by more than 2°C.
Design Data
C-massecuite specific heat capacity, in the above temperature range, is about 1.7 to 1.8 kJ/kg/K. The overall heat transfer coefficient (OHTC) for a reheater is about 0.011 kW/m2/K. The flow velocity of water in the finned tubes should be in the range 1.5 to 2 m/s, in order to achieve the stated OHTC. Care must be taken to ensure that the number of passes and length of each pass ensure the correct flow velocity without causing excessive pressure drops due to friction.
