2.2. Basket-type Vertical Evaporators

The construction and operational features of basket-type evaporators are very similar to those of the standard evaporator except that the downtake is annular. The tube bundle with fixed tube sheets forms a basket hung in the centre of the evaporator from internal brackets. The diameter of the tube bundle is smaller than the diameter of evaporator vessel, thus forming an annular space for circulation of liquid. The tube bundle can be removed for the purpose of cleaning and maintenance and thus basket evaporators are more suitable than standard evaporators for scale forming solutions. The vapor generated strikes a deflector plate fixed close to the steam pipe that reduces entrained liquid droplets from the vapor.

2.3. Long-Tube Vertical Evaporators

This is another most widely employed natural circulation evaporator because it is often the cheapest per unit of capacity. The long vertical tube bundle is fixed with a shell that extends into a larger diameter vapor chamber at the top ( Figure 3.2 ). The long-tube vertical (LTV) evaporator consists of one pass shell and tube heat exchanger. In this type of evaporator, the liquid flows as a thin film on the walls of long (from 12 to 30 feet in length) and vertical heated tube. Both rising film and falling types are used. Tube length usually varies from 20 to 65 ft. The main advantage of this type of evaporators is higher heat transfer rate. The feed enters at the bottom and the liquid starts boiling at lower part of the tube. The LTV evaporators are commonly used in concentrating black liquors in the paper and pulp industries.

 

Figure 2.2 . Long-Tube Vertical Evaporators.