LTV Rising-Film Evaportator
In a Swenson single-effect, Long Tube Vertical rising-film evaporator (see Figure 1), evaporation occurs inside the evaporation tubes, so it is used primarily to concentrate non-salting liquors. To provide for good heat transfer, the temperature difference between the heating medium and the liquor should be greater than 15°F. Tubes are normally ¾” to 2” in diameter and from 10 to 30 feet long.
The operation of the rising-film evaporator is straightforward. Liquor is fed into the bottom liquor chamber and enters the tubes. There the liquor is heated with condensing steam or any other suitable heat-transfer fluid. If the vapor pressure of the feed equals or exceeds the system pressure at the bottom tubesheet, vaporization will occur immediately. For colder feed, the lower portion of the tubes is used to preheat the liquor to its boiling point. Vaporization then begins at the height within the tubes where the vapor pressure of the feed liquor equals the system pressure.
As the liquor climbs up the inside of the tubes, additional vapor is generated and the velocity of the liquid-vapor mixture increases to a maximum at the tube exit. The outlet mixture impinges upon a deflector, mounted above the top tubesheet of the heat exchanger, where gross, initial separation of the liquid from the vapor occurs.
Additional entrained liquor is separated from the vapor by gravity as the vapor rises in the vapor body. A mesh or centrifugal entrainment separator can be installed near the top of the vapor body to remove most of the remaining traces of liquid from the vapor. The exit vapor is routed to either the next effect of a multiple-effect evaporator system, to a compressor or to a condenser. A Swenson vertical-tube surface condenser is shown in the Figure 1. The concentrated liquor is discharged from a connection near the bottom of the vapor body.