Brochure LTV Evaporator

Swenson Technology, Inc.

LTV RISING-FILM EVAPORATOR

In a Swenson single-effect, LTV rising-film evaporator (see Fig. 5), evaporation occurs inside the evaporator tubes, so it is used primarily to concentrate non-salting liquors. To provide for good heat-transfer rates, the Delta T between the heat-transfer medium and the liquor should be greater than 15^oF, and preferably greater than 20⁰F. Tubes are normally 3/4" to 2" in diameter and from 10 to 30 feet long.
Operation of the rising-film evaporator is straightforward. Liquor is fed into the bottom liquor chamber and then into the tubes. It is heated with condensing steam or any other suitable heat-transfer medium. 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 that height within the tubes where the vapor pressure of the feed liquor equals the system pressure.

Evaporator fig3

Fig. 3. Typical Fluid-Temperature Profile of Rising-Film Evaporator

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 liquor is separated from the vapor by gravity as the vapor rises in the vapor body. A mesh-type 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 conducted to the next effect of a multiple-effect evaporator, to a compressor or to a condenser. A Swenson vertical-tube surface condenser is shown in Fig. 5. The concentrated liquor is discharged from a connection near the bottom of the vapor body.

     Heat-transfer rates are enhanced in the non-boiling section by surface or local boiling and in the boiling section by nucleate boiling. As expected, the heat-transfer rates in the boiling zone are several times greater than those in the non-boiling zone, so it is important to reduce the non-boiling zone to a minimum.
     A typical fluid-temperature profile taken with a traveling thermocouple for a rising-film evaporator is shown in Fig. 3. The maximum temperature occurred at a point eight feet above the bottom tubesheet where the boiling began.
     Different two-phase flow schemes are created in the boiling zone, including slug flow, where a slug of liquor is followed by a slug of vapor, similar to the perking in a coffee percolator; annular flow, where a ring of liquor encases a center core of vapor and liquid mist; and mist flow, where vapor blankets the tube surface. Mist flow should be avoided because poor heat transfer results when there is not enough liquid present to wet the tube walls. To avoid mist flow, it is sometimes necessary to recycle concentrated product from the vapor body to the bottom liquor chamber so as to supplement the feed liquor. (This recirculation line is shown in Fig. 5.)
     The LTV rising-film evaporator can also be adapted to concentrate moderate scaling liquors when a pump is used to recycle concentrated product from the vapor body to the bottom liquor chamber. This is known as forced recirculation.
     The vapor body shown in Fig. 5 is integral with the heat exchanger. When the heat exchanger is too large to use the integral configuration, when quicker access to the tubes is desired for maintenance, or when surge volume in the vapor body is required for level control, the vapor body is separated from the heat exchanger, generally as shown in Fig. 7. A skirt-type baffle replaces the deflector as the initial separator.

Evaporator Fig.4.

Fig. 4. Swenson Double-Effect, LTV Rising-Film Evaporator