Evaporation parallel-feed operation

Fig. 7.2-1 Three effect evaporation unit with parallel-feed operation... 

Parallel-feed multiple-effect evaporators. Parallel feed in multiple-effect evaporators involves the adding of fresh feed and the withdrawal of concentrated product from each effect. The vapor from each effect is still used to heat the next effect. This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt. 

The feed to a multiple-effect evaporator is usually transferred from one effect to another in series so that the final product concentration is achieved in only one effect. In forward feed operation, raw feed is introduced in the first effect and passed from effect to effect parallel to the steam flow. The concentration of the liquid increases from the first effect to the last and product is withdrawn from the last effect. Forward feed operation is advantageous when... 

Appmach Dugway Proving Ground proposes to use two evaporative vapor generators operated in parallel to generate agent vapors. Agent vapor concentrations will be controlled by adjustments of dilution air feed rates. 

The principle of operation of the diffusion still is first described as a single-stage device. This is illustrated in Figure 1, in which the system is shown comprised of two parallel heat transfer surfaces—one supplied with heated feed water, the other cooled. The surfaces are closely spaced. The heated water is supplied to surface a for evaporation. 

Because of the unpredictable performance of thermocompressors, control of evaporators using them is more difficult than for a conventional system where it is necessary to set only steam and feed rates to maintain a constant evaporation rate. One way to provide flexibility with better operating stability is to use two or more thermocompressors in parallel. This permits capacity control without loss In energy economy. 

---