Experiences with pre-evaporators at Racecourse Mill

In the 2008 season, a new pre-evaporator vessel, called PE1A, was installed at the Racecourse Mill as a precursor to the introduction of cogeneration at the factory. PE1A and the existing pre-evaporator, called PE-north, were designed as steam-efficiency measures to provide vapour to the evaporating crystallizers, juice heaters, co-located sugar refinery and, potentially, to other value-adding operations. Both pre-evaporator vessels are Robert type evaporators, with the PE-north vessel an older standard design and the PE1A vessel incorporating new design features. PE1A is configured to take letdown steam from the cogeneration boiler (Racecourse no.4 boiler) and to return the condensate to the boiler. PE-north takes exhaust steam from the milling turbines and turbo-alternators supplied by Racecourse no.3 boiler. The specifications for condensate supplied as boiler feedwater to the cogeneration boiler, which operates at 8,000 kPa, are more stringent than the industry typical boilers, operating at 1,700 kPa. PE1A is operated as a juice-evaporator vessel during the crushing season and as a steam transformer during the non-crush season. The operation of both pre-evaporator vessels is described. Severe corrosion issues are noted for the PE1A vessel, which is not prevalent for the PE-north vessel despite the latter being considerably older and used in a similar role. The causes of the high corrosion levels for PE1A are discussed, and some measures to alleviate the operational conditions at Racecourse are described.