4.  PERFORMANCE OF EVAPORATORS (CAPACITY AND ECONOMY)

The performance of a steam-heated evaporator is measured in terms of its capacity and economy. Capacity is defined as the number of kilogram of water vaporized per hour. Economy (or steam economy) is the number kilogram of water vaporized from all the effects per kilogram of steam used. For single effect evaporator, the steam economy is about 0.8 (<1). The capacity is about n -times that of a single effect evaporator and the economy is about 0.8 n for a n -effect evaporators. However, pumps, interconnecting pipes and valves are required for transfer of liquid from one effect to another effect that increases both equipment and operating costs.

5. THERMAL/ POCESS DESIGN CONSIDERATIONS

Many factors must be carefully considered when designing evaporators. The type of evaporator or heat exchangers, forced or natural circulation, feeding arrangement, boiling point elevation, heat transfer coefficient, fouling, tube size and arrangement are all very important.

Types of evaporators have already been discussed and the guidelines for selection of most suitable evaporator are addressed in the next section.

5.1. Tube size, arrangement and materials

The selection of suitable tube diameter, tube length and tube –layout is determined by trial and error calculations. The details are discussed in design of shell and tube heat exchangers ( module # 1). If the pressure drop is more than the allowable pressure drop further adjustments in tube diameter, tube length and tube-layout is required.

A variety of materials including low carbon steel, stainless steel, brass, copper, cupronickel etc. are used. However the selection of tube materials depends on the corrosiveness of the solution and working conditions.

5.2. Heat transfer coefficients

The heat transfer coefficient of condensing steam in shell side is normally very high compared to the liquid side. Therefore tube side (liquid side) heat transfer coefficient practically controls the rate of heat transfer.

The overall heat transfer coefficient should be either known/ calculated from the performance data of an operating evaporator of the same type and processing the same solution. Typical values of overall heat transfer coefficient are given in Table 3.1 .

 

Table 3.1. Typical overall heat transfer coefficients in evaporators.