IMPROVED DESIGN OF A FALLING-FILM TUBULAR EVAPORATOR WITH A MAINTENANCE-FRIENDLY NOVEL JUICE DISTRIBUTOR

Abstract

Several variants of falling-film tubular evaporators (FFTE) are in use in the sugarcane industry. Most of these suffer from three main draw backs: (i) occasional tube chokes due to uneven distribution of juice, (ii) absence of headroom between the top tube sheet and juice distributor necessitating its dismantling for mechanical de-scaling of tubes, and (iii) occasional cracking of austenitic SS tubes fitted in carbon steel calandria caused by thermal stresses due to widely dissimilar expansion at elevated temperature. A new design of juice distributor has been developed. It comprises an inlet weir box and a five-stage cascading system that forms a uniform shower of juice across the entire cross-section. It is installed at 2 m above the top tube sheet to facilitate easy access to various tubes for inspection or mechanical de-scaling without its dismantling. A segmented tray plate bolted to the top tube sheet and having an individual tripodtype umbrella structure located over each tube ensures equal wetting of each and every tube, making the system failsafe. One 3300 m2 FFTE unit with this design of distributor is installed at a sugar factory in India. It has successfully completed three harvesting seasons without any tube choke. This plant produces plantation-white sugar by the double sulphitation process and has to carry out mechanical de-scaling during the crop to maintain heat transfer efficiency. Thermal stress analysis of another FFTE unit has been carried out using advanced software to simulate stress, buckling and deflection behaviour at elevated temperature for combination of carbon steel calandria and different grades of SS tubes to develop a structurally strong design with enhanced operational reliability. FFTEs with SS439 material tubes are installed as 2nd-4th effect evaporators at a 24,000 tcd sugar plant of the White Nile Sugar Company, Sudan. In the first crop, there was a problem of tube chocking due to intermittent supply of juice arising out of frequent stoppage of milling process. In subsequent crops, the supply of juice to the evaporator station has been largely consistent and, hence, the problem of tube chocking was not experienced. This plant has completed three more crushing seasons without any tube failure or structural deformity.