Internal calandrias

Many special types of equipment have been developed for particular industries, possibly extreme examples being the simple open ponds for solar evaporation of brines and recovery of salt, and the specialized vacuum pans of the sugar industry that operate with syrup on the tubeside of calandrias and elaborate internals to eliminate entrainment. Some modifications of basic types of crystallizers often carry the inventor s or manufacturer s name. For their identification, the book of Bamforth (1965) may be consulted. 

Two general types of evaporators are used, and their names refer to the type of circulation used to transfer heat to the liquor for evaporating the water. Natural circulation evaporators rely on a thermosiphon to circulate liquors while forced circulation units use a pump to achieve the required circulation. The heating tubes may be inside or outside the evaporator body, but most designs, especially the older calandria style evaporators, use internal tubes for heating (Figure 2). 

Internal calandria, vertical short tube preferred for fouling and crystal forming systems. OK for foaming, fouling, crystal formation, and some suspended solids. Viscosities <1000 mPa s. Relative to agitated film retention time of 168 1 and volume 3 1. 

Internal calandria, horizontal short tube including all auxiliaries vapor piping, barometric condenser, vacuum equipment, integral piping, condensate receivers. 

Internal calandria, vertical short tube including vapor piping, barometric condenser, vacuum equipment, integral piping, condensate receivers. FOB cost 220000 at 45 m heat transfer area, n = 0.55 for the range 10-600. L+M = 1.6-1.9. L/M =... 

Internal calandrias are not without problems, however. Hydrodynamics of internal calandrias are difficult to predict. Downcomers must be adequately provided in most systems apparent liquid levels are actually controlled in the downcomers. A froth exists above the top tubesheet this froth can build to appreciable heights. Liquid circulation must migrate across the tubesheet to the nearest downcomer. In doing so, it is agitated by the two-phase mixture leaving the tube ends. Froth can be reduced somewhat by extending the tube ends beyond the tubesheet to provide an undisturbed liquid flow path. However, the two-phase mixture must be separated before liquid can accumulate in this path this is often not accomplished satisfactorily. 

In order to control internal calandrias, it is recommended that the bottom liquid level tap be a distance below the tubesheet equal to approximately 25% of the shell diameter. The top liquid level tap should be located a distance above the top tubesheet equal to approximately 250% of the shell diameter. 

Internal calandrias must be carefully evaluated. External calandrias are much more predictable consequently less heat transfer surface may be required. This should be considered in any cost comparison. 

Internal calandrias present some unusual problems. Understanding the effects of the froth created above the top tubesheet will permit adequate design and operation. Downcomers must be properly sized. However, external calandrias generally are preferred. 

Under certain circumstances, such as a large LOCA combined with a loss of emergency core coolant injection, the pressure tube will overheat and (depending on the internal pressure) sag or strain into contact with the calandria tube. This requires models of the pressure tube thermomechanical transient behaviour, to predict the extent of deformation and the pressure tube temperature and internal pressure when/if it contacts the calandria tube. Separate channel thermomechanical models have been used to-date, using somewhat artificial boundary conditions (fixed steam flow rate) the system thermohydraulic codes now incorporate this capability, allowing more realistic predictions of the distribution of flow to each channel. 

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