The Swenson calandria evaporator (see Figure 6) is applied less often today than it was years ago, though there are still a number of companies that prefer this evaporator for various applications. For most applications, however, the lower equipment cost for other designs has prompted the replacement of calandria evaporators with LTV falling-film, LTV rising-film, and forced circulation evaporators.
The calandria evaporator has a heat exchanger (with tubes usually less than six feet long) integral with the vapor body. The level is maintained in the upper portion of the tubes or above the top tubesheet and the circulation pattern is up through the tubes and down through a central pipe called a “downcomer”. Circulation is created by the difference in specific gravity between the body liquor and the headed liquor and vapor generated inside the tubes, plus a vapor lift effect.
The Swenson calandria evaporator can be used for salting-type applications; however, an agitator located inside or beneath the downcomer is recommended to suspend the salt crystals in the lower portion of the body. Although the agitator creates some flow through the tubes, most of the flow is still created by “thermo-syphon”. The calandria is also used for batch-type concentration of liquors.