Evaporative recirculated systems

The system shown in Fig. 11-75 is direct expansion where diy or slightly superheated vapor leaves the evaporator. Such systems are predominantly used in small applications because of their simplicity and light weight. For the systems where efficiency is crucial (large industrial systems), recirculating systems (Fig. 11-77) are more appropriate. 

Direct-expansions are more appropriate for smaller systems which should be compact, and where there are just one or few evaporators. Overfeed (recirculation) systems should be considered for aU applications where first cost for additional equipment (surge drums, low-pressure receivers, refrigerant pumps, and accessories) is lower than the savings for the evaporator surface. 

Open recirculating systems These are more amenable to inhibition since it is possible to maintain a closer control on water composition. Corrosion inhibition in these systems is closely allied to a number of other problems that have to be considered in the application of water treatment. Most of these arise from the use of cooling towers, ponds, etc. in which the water is subject to constant evaporation and contamination leading to accumulation of dirt, insoluble matter, aggressive ions and bacterial growths, and to variations in pH. A successful water treatment must therefore take all these factors into account and inhibition will often be accompanied by scale prevention and bactericidal treatments. 

The presence of water, or water vapour, affects the chemistry of thermal modification and heat transfer within the wood (Burmester, 1981). Under dry treatment conditions, the wood is dried prior to thermal modification, or water is removed by the use of an open system, or a recirculating system equipped with a condenser. In closed systems, water evaporated from the wood remains as high-pressure steam during the process. Steam can also be injected into the reactor to act as a heat-transfer medium, and can additionally act as an inert blanket to limit oxidative processes. Such steam treatment processes are referred to as hygrothermal treatments. Where the wood is heated in water, this is known as a hydrothermal process. Hydrothermal treatments have been extensively studied as a... 

Continuous water recirculation system, whereby cooling water picks up heat by being pumped through a process heat source or heat exchanger and then over a cooling tower, open to the air, whereupon the sensible heat gained is lost by an evaporation process. 

The principal problem with the falling-film LTV evaporator is that of feed distribution to the tubes. It is essential that all tube surfaces be wetted continually. This usually requires recirculation of the liquid unless the ratio of feed to evaporation is quite high. An alternative to the simple recirculation system of Fig. ll-122/i is sometimes used when the feed undergoes an appreciable concentration change and the product is viscous and/or has a high boiling point rise. The feed chamber and vapor head are divided into a number of liqu... 

Hydrogen excess and recirculation is costly, the gas carries away evaporated liquid, which has to be condensed in condensers with low heat transfer coefficients due to the presence of gas after that the gas must be recompressed and recycled to the reactor. Compression is an expensive unite operation and, moreover, in recirculation systems a purge is required to prevent accumulation of inert components this inevitably leads to hydrogen losses. Short switch over times must be feasible to reduce losses to off-spec products to a minimum. For the same reason also rapid startup and shut down methods must be developed. This also leads to long on-stream-times and a good productive use of the plant. 

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. Circulatory systems are therefore not well suited for evaporating heat sensitive materials. Circulatory evaporators, however, can operate over a wide range of concentrations and are well adapted to single-effect evaporation. 

For most evaporative units, the steam flow is directly proportional to the production rate and to the temperature rise in the recirculation system and the AT" in the heat exchanger. Both of these values have an important influence on the scaling rate in cases where materials of inverted solubility are being handled or where there are scale-forming components within the system. In systems where this value is critical, the AT can be monitored and used to reset the steam rate. An interlock should be provided that will cut off the steam flow to the system if the circulating pump or propeller circulator in a draft tube baffle unit is off due to motor overload or power failure. 

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