The Effect of Tube Wetting Rate on Kestner Performance at Sezela Factory

Sezela sugar factory uses long tube climbing film evaporators of the Kestner type for first effect evaporation. The plant trial showed that the Kestner specific evaporation rate (kg/m/h) and heat transfer coefficient can be improved by increasing the tube wetting rate (kg/min/tube). Increasing the wetting rate by 62% increased the specific evaporation rate by 40%. Introduction The climbing film evaporator was patented in 1899 by Paul Kestner. These long tubed vessels, typically 7 m in height, are common in the South African sugar industry and are referred to as Kestners. Kestners are typically used as first and second effect vessels in multiple effect evaporator stations. The authors are not aware of Kestners being used as third, fourth or fifth effects in South Africa. Although common in the local sugar industry, the performance of Kestners has been inconsistent. However, the Umzimkulu Kestner is viewed as a benchmark by the authors. The Umzimkulu Kestner has good evaporation rates and low fouling rates, and can operate for up to eight weeks without any tube cleaning. The Sezela Kestners, on the other hand, produce lower evaporation rates and higher fouling rates. The Kestners are cleaned on a two-week cycle. The Kestners also have a history of blocked tubes due to juice drying out in the tubes. Kestner performance has been studied by a number of South African authors. The studies were mainly on pilot Kestners. James et al. (1978) found that the heat transfer coefficient (HTC) increased with feed rate. This paper also quotes a specific evaporation rate of 35 to 40 kg/m/h. Walthew and Whitelaw (1996) also studied Kestner performance in a pilot plant and showed that increasing the feedrate increased HTC and decreased the fouling rate. The Umzimkulu (UK) and Sezela (SZ) Kestner design data, together with the current operating data, are given in Table 1. The comparison of specific evaporation rates shows that the UK Kestner performs better by approximately 26%. The tube wetting rate is also higher in the UK Kestner. The authors attempted to simulate the UK Kestner wetting rate on one of the SZ Kestners. Proc S Afr Sug Technol Ass (2008) 81: 184 189