Used Evaporators

Evaporators are used to concentrate a solution, separating the vapor from the solution. All evaporation equipment contains a heat exchanger for heating the process solution and a means to effectively separate the vapor from the residual liquid, called a vapor separator. Evaporator performance is rated by Lbs per hour or Kg per hour of evaporation. The greatest increase in steam economy is achieved by reusing the vaporized solvent. This is done in a Multiple Effect Evaporator by using the vapor from one effect as the heating medium for another effect in which boiling takes place at a lower temperature and pressure. Another method of increasing the utilization of energy is to use a Thermo Compression evaporator, in which the vapor is compressed so that it will condense at a temperature high enough to permit its use as the heating medium in the same evaporator. This type is also called a MVR or mechanical vapor recompression.

Types of Evaporators

Short tube vertical evaporators are natural circulation evaporators. These units consist of short tubes 4-6 feet long and 2” to 4” in diameter set between two horizontal tube sheets that span the evaporator body diameter. The tube bundle contains a large circular down comer that returns concentrated liquid above the top tube sheet to below the bottom tube sheet for product withdrawal. The driving force for flow of liquid through the tubes is the difference in density between the liquid in the down comer and the tubes.

Long tube vertical evaporators accomplish more evaporation than in all other types combined. They are also natural circulation evaporators. They are also categorized individually as rising film, falling film, and rising/falling film types. Basically these units consist of a vertical single pass shell and tube heat exchanger discharging into a relatively small vapor head. Tube length is typically12’ to 36’; and the tube diameters are from 0.75” to 2” OD. Units may be once through or recirculating, depending upon the application. Long tube evaporators are the most economical design.

• Rising Film – feed liquor enters at the bottom of the tube and boils part way up the tube with the mixture of liquid and vapor leaving at the top. As the liquid in the tubes boil the vapor generated from boiling fills the core of the tube forcing the liquid to the side wall. As the liquid moves up the tube more vapor is formed and the remaining liquid thins out and moves more rapidly up the tube increasing the heat transfer coefficients and reducing the residence time
• Falling Film – feed liquor enters the top of the tube and flows down the tube wall as a thin film. Since the film is moving in the direction of gravity, a finer and faster moving film results, yielding higher heat transfer coefficients and reduced contact times. The flow of vapor and liquid may be co-current in which the vapor-liquid separation takes place at the bottom, or counter-current where the liquid is drawn from the bottom and the vapor from the top

Forced circulation evaporators are made in a variety of arrangements for services where the feed and/or product liquor has a tendency to salt or scale and where the viscosities of the solutions are so high that natural circulation is not feasible. Force circulation is achieved by using pumps. Forced circulation leads to high tube side velocities and higher heat transfer coefficients and smaller heating surfaces. The pump withdraws liquor from the flash chamber and forces it through the heating element back to the flash chamber. Circulation is maintained regardless of the evaporation rate so this type of evaporator is well suited to crystallizing operations where solids must be maintained in suspension at all times.

Thin Film or Wiped Film are agitated evaporators - essentially a large diameter jacketed tube, in which the product is vigorously agitated and continuously removed from the tube wall by scraper blades or wipers mounted on a shaft rotating inside the tube. The material to be processed is continuously spread as a thin film on the tube walls by the mechanical agitator. This permits processing of extremely viscous and heat sensitive materials, as well as products that crystallize. The heating medium may be steam or hot oil on the jacket side.