Evaporators vertical-type natural circulation

Vertical-type natural circulation evaporator. In this type of evaporator, vertical rather than horizontal tubes are used, and the liquid is inside the tubes and the steam condenses outside the tubes. Because of boiling and decreases in density, the liquid rises in the tubes by natural circulation as shown in Fig. 8.2-lb and flows downward through a large central open space or downcomer. This natural circulation increases the heat-transfer coefficient. It is not used with viscous liquids. This type is often called the short-tube evaporator. A variation of this is the basket type, where vertical tubes are used, but the heating element is held suspended in the body so there is an annular open space as the downcomer. The basket type differs from the vertical natural circulation evaporator, which has a central instead of annular open space as the downcomer. This type is widely used in the sugar, salt, and caustic soda industries. 

Several types of evaporators exist.34 The older, more traditional, evaporators are the Roberts and the Kestner, both rising film, tubular evaporators. The Roberts, first introduced in the 1800s, is known as a short tube, natural circulation, vertical tube evaporator. The tubes, inside which the evaporation takes place, are in the range of 1.5 to 3 meters in length. The Kestner evaporator consists of numerous long vertical tubes, 6 to 7.5 meters long, inside a cylindrical shell. In both, the juice to be concentrated is fed to the bottom of the tubes and heated, causing the juice to... 

The natural circulation evaporator is the classic evaporator type. In order to achieve larger heating surfaces, the agitator evaporator was soon replaced by the tube evaporator. The vertical arrangement with relatively short tubes results in a natural circulation of the product in the system separator/heater. 

Evaporators employ heat to concentrate solutions or to recover dissolved solids by precipitating them from saturated solutions. They are reboilers with special provisions for separating liquid and vapor phases and for removal of solids when they are precipitated or crystallized out. Simple kettle-type reboilers [Fig. 8.4(d)] may be adequate in some applications, especially if enough freeboard is provided. Some of the many specialized types of evaporators that are in use are represented on Figure 8.16. The tubes may be horizontal or vertical, long or short the liquid may be outside or inside the tubes, circulation may be natural or forced with pumps or propellers. 

Mechanical cleaning is fairly easy with such units, and the capital investment is relatively low. Circulation stops, however, if the heat input is interrupted, creating the danger of the settling of any solids suspended in the liquor. This type of unit is not well suited to viscous liquids because of the low heat transfer coefficients associated with the low velocities of natural convec-tion.f Short-tube vertical evaporators have largely been surpassed by other types, particularly for applications involving liquors that foam, deposit excessive scale, are excessively viscous, or are heat sensitive. 

Heat transfer is improved by an increase of the velocity of the liquid. According to natural convection and the buoyancy force of rising bubbles a circulation of the Uq-uid is induced in a short-tube vertical evaporator, see Fig. 7.1-2. In the narrow tubes, liquid is evaporating to a certain degree and recirculated in the central return passage after separation of the vapor. This type is less recommended when a fouling of the heat transfer area by product components takes place. Then it is reasonable to separate the tube bundle and the head space in which the entrainment of hquid is reduced, see Fig. 7.1-3. The cleaning of the tubes can be easily carried out. 

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