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.
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.