- heating medium separated from evaporating liquid by tubular heating surfaces,
- heating medium confined by coils, jackets, double walls, flat plates, etc.,
- heating medium brought into direct contact with evaporating liquid, and
- heating with solar radiation.
Evaporators with tubular heating surfaces dominate the field. Circulation of the liquid past the surface may be induced by boiling (natural circulation) or by mechanical methods (forced circulation). In forced circulation, boiling may or may not occur on the heating surface
Solar evaporators require tremendous land areas and a relatively cheap raw material, since pond leakage may be appreciable. Solar evaporation generally is feasible only for the evaporation of natural brines, and then only when the water vapor is evaporated into the atmosphere and is not recovered.
Evaporators may be operated batchwise or continuously. Most evaporator systems are designed for continuous operation. Batch operation is sometimes employed when small amounts must be evaporated. Batch operation generally requires more energy than continuous operation.
Batch evaporators, strictly speaking, are operated such that filling, evaporating, and emptying are consecutive steps. This method of evaporation requires that the body be large enough to hold the entire charge of the feed and the heating element be placed low enough not to be uncovered when the volume is reduced to that of the product. Batch operation may be used for small systems, for products that require large residence times, or for products that are difficult to handle.
A more frequent method of operation is semibatch in which feed is continuously added to maintain a constant liquid level until the entire charge reaches the final concentration. Continuous-batch evaporators usually have a continuous feed, and over at least part of the cycle, a continuous discharge. One method of operation is to circulate from a storage tank to the evaporator and back until the entire tank is at a specified concentration and then finish the evaporation in batches.
Continuous evaporators have continuous feed and discharge. Concentrations of both feed and discharge remain constant during operation.
Evaporators may be operated either as once-through units or the liquid may be recirculated through the heating element. In once-through operation all the evaporation is accomplished in a single pass. The ratio of evaporation to feed is limited in single-pass operation; single-pass evaporators are well adapted to multiple-effect operation permitting the total concentration of the liquid to be achieved over several effects. Agitated-film evaporators are also frequently operated once through. Once-through evaporators are also frequently required when handling heat-sensitive materials.
Recirculated systems require that a pool of liquid be held within the equipment. Feed mixes with the pooled liquid and the mixture circulates across the heating element. Only part of the liquid is vaporized in each pass across the heating element; unevaporated liquid is returned to the pool. All the liquor in the pool is therefore at the maximum concentration. Recirculated systems are therefore not well suited for evaporating heat sensitive materials. Recirculated evaporators, however, can operate over a wide range of concentration and are well adapted to single-effect evaporation.
There is no single type of evaporator which is satisfactory for all conditions. It is for this reason that there are many varied types and designs. Several factors determine the application of a particular type for a specific evaporation result. The following sections will describe the various types of evaporators in use today and will discuss applications for which each design is best adapted.
Evaporators may be operated batchwise or continuously. Most evaporator systems are designed for continuous operation. Batch operation is sometimes employed when small amounts must be evaporated. Batch operation generally requires more energy than continuous operation.
Batch evaporators, strictly speaking, are operated such that filling, evaporating, and emptying are consecutive steps. This method of evaporation requires that the body be large enough to hold the entire charge of the feed and the heating element be placed low enough not to be uncovered when the volume is reduced to that of the product. Batch operation may be used for small systems, for products that require large residence times, or for products that are difficult to handle.
A more frequent method of operation is semibatch in which feed is continuously added to maintain a constant liquid level until the entire charge reaches the final concentration. Continuous-batch evaporators usually have a continuous feed, and over at least part of the cycle, a continuous discharge. One method of operation is to circulate from a storage tank to the evaporator and back until the entire tank is at a specified concentration and then finish the evaporation in batches.
Continuous evaporators have continuous feed and discharge. Concentrations of both feed and discharge remain constant during operation.
Evaporators may be operated either as once-through units or the liquid may be recirculated through the heating element. In once-through operation all the evaporation is accomplished in a single pass. The ratio of evaporation to feed is limited in single-pass operation; single-pass evaporators are well adapted to multiple-effect operation permitting the total concentration of the liquid to be achieved over several effects. Agitated-film evaporators are also frequently operated once through. Once-through evaporators are also frequently required when handling heat-sensitive materials.
Recirculated systems require that a pool of liquid be held within the equipment. Feed mixes with the pooled liquid and the mixture circulates across the heating element. Only part of the liquid is vaporized in each pass across the heating element; unevaporated liquid is returned to the pool. All the liquor in the pool is therefore at the maximum concentration. Recirculated systems are therefore not well suited for evaporating heat sensitive materials. Recirculated evaporators, however, can operate over a wide range of concentration and are well adapted to single-effect evaporation.
There is no single type of evaporator which is satisfactory for all conditions. It is for this reason that there are many varied types and designs. Several factors determine the application of a particular type for a specific evaporation result. The following sections will describe the various types of evaporators in use today and will discuss applications for which each design is best adapted.
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