Circulation of massecuite within vacuum pans is critical to maximizing throughput and controlling quality during sugar production. In recent years, there has been a great deal of debate regarding the optimal length of tubes for both low head batch pans and continuous vacuum pans (CVPs). Here is what you need to know.
Batch Vacuum Pans
In batch pans, in order for circulation to occur, it is necessary before boiling to fill the pan to cover the top of the tubes. During boiling, the massecuite level reaches about 1500 to 2100 mm above the top tube plate, creating a high hydrostatic head and causing elevation of boiling point. This in turn lowers the differential temperature between the massecuite and the calandria steam and increases the likelihood of color formation.
The final head above the top tube plate is a function of the tube length. In general, wide short-tube calandrias require less additional head than long-tube calandrias. Although circulation is promoted by vapor bubbles within the tubes, their effect is dispersed in the open volume above the tubes. Indeed, with high hydrostatic heads towards the end of boiling, the vapor bubbles may only form in a zone above the top tube plate. As a result, short-tube calandrias circulate better than long-tube calandrias during the latter stages of boiling, particularly in the absence of stirring. For this reason, tube length is critical in the design of batch pans, with good designs featuring tube lengths between 800 – 1200 mm.
Continuous Vacuum pans
The impact of tube length in continuous vacuum pans is very different. The massecuite level is constant from the first stages to the last (typically about 300 mm above the top tube plate). There is minimal hydrostatic head at the top of the tubes and the delta-T between the calandria steam and the massecuite is not suppressed, which allows for the use of lower pressure / temperature steam. Continuous pans generally exhibit good circulation with a relatively high proportion of vapor in the tubes, ultimately lowering effective pressure, even at the bottom of the tubes. Because of this, tube length has very little impact on heat transfer and color formation in CVPs.