Module 3 : Design of Evaporator

Lecture 1 : Introduction and Evaporator Classifications

    

2.5. Rising or Climbing Film Evaporators

The LTV evaporator is frequently called a rising or climbing film evaporator. The liquid starts boiling at the lower part of the tube and the liquid and vapor flow upward through the tube . If the heat transfer rate is significantly higher, the ascending flows generated due to higher specific volume of the vapor-liquid mixture, causes liquid and vapor to flow upwards in parallel flow. T he liquid flows as a thin film along the tube wall. This cocurrent upward movement against gravity has the advantageous effect of creating a high degree of turbulence in the liquid. This is useful during evaporation of highly viscous and fouling solutions.

2.6. Forced Circulation Evaporators

Forced circulation evaporators are usually more costly than natural circulation evaporators. However the natural circulation evaporators are not suitable under some situations such as:

All these problems may be overcome when the liquid is circulated at high velocity through the heat exchanger tubes to enhance the heat transfer rate and inhibit particle deposition. Any evaporator that uses pump to ensure higher circulation velocity is called a forced circulation evaporator. The main components of a forced circulation evaporator are a tubular shell and tube heat exchanger (either horizontal or vertical), a flash chamber (separator) mounted above the heat exchanger and a circulating pump (Figure 3.4). The solution is heated in the heat exchanger without boiling and the superheated solution flashes off (partially evaporated) at a lower pressure are reduced in the flash chamber. The pump pumps feed and liquor from the flash chamber and forces it through the heat exchanger tubes back to the flash chamber.

Forced circulation evaporator is commonly used for concentration of caustic and brine solutions and also in evaporation of corrosive solution.

 

Figure 3.4 . Vertical tube forced-circulation evaporator.