The plate-type of evaporator is the version of liquid chiller gaining most rapidly in popularity. It is an outgrowth of the plate-and-frame heat exchanger which has been applied in the food industry for many decades. The plate-and-frame heat exchanger consists of numerous plates that are gasketed in such a way that when these plates are bolted together one of the fluids flows between two of the plates and the other fluid between the pairs of adjacent plates. The plates are corrugated with a herringbone pattern that physically strengthens the plates and also promotes turbulence of the fluids, providing excellent convection heat transfer coefficients. This type of heat exchanger is appealing to such food industries as dairies because at the end of a work shift the bolts holding the plates in position can be loosened, permitting access to all surfaces for cleaning.
The conversion of the plate-and-frame heat exchanger transferring heat between two liquids poses the challenge of how to seal the refrigerant passages. This goal is achieved by a construction shown in Fig. 6.63 where instead of each plate capable of being separated, pairs of plates forming the refrigerant passages are brazed or welded. For halocarbon refrigerants, normal brazing of the edges suffices, but for ammonia either nickel brazing or welding is necessary. The liquid flows downward between its two boundary plates, while the refrigerant flows upward, thus, counterflow to the liquid. The refrigerant enters the evaporator at the lower right, with a portion of the refrigerant flowing upward through the first pair of refrigerant plates and the remainder passing on to succeeding pairs. At the end refrigerant pair on the left of the diagram, the near plate is shown in an exploded view to illustrate the refrigerant flowing upward. Each stream of refrigerant leaves its pair at the upper right and joins the streams from the other pairs, finally leaving the evaporator in the upper right corner. The construction illustrated in Fig. 6.63 has a confined gasketed passage between one refrigerant pair and another, so while the edges of the plates are welded, there is a small gasketed joint.
The outside boundary of the liquid passages are the outside of the refrigerant pair. Some small plate-type evaporators for halocarbon refrigerants are manufacturered without gaskets by brazing all the connections. Industrial evaporators, which are usually larger, are made by bolting together the refrigerant pairs. It is possible, then, to dismantle the evaporator to clean the liquid-side surfaces should they become fouled. Such dismantling means that the refrigerant passages are also opened to air, so the refrigerant must first be evacuated.
The major strengths of the plate-type evaporator are: (1) high heat-transfer coefficients, (2) low refrigerant charge, and (3) small size. These advantages are related, because whenever the heat-transfer coefficient can be improved, the heat exchanger can be smaller for a given refrigerating capacity. A small exchanger automatically results in lower refrigerant charge. Ranges of overall heat-transfer coefficients are reported by one source31 as 2500 to 4500 W/m2·K (440 to 790 Btu/hr·ft2·°F) for water/ammonia and 1500 to 3000 W/m2·K (265 to 530 Btu/hr·ft2·°F) for water/R-22. Another source32 suggests values for a flooded or recirculated water/ammonia evaporator of 2840 to 3975 W/2·K (500 to 700 Btu/hr·ft2·°F). For direct-expansion ammonia, an overall heat-transfer coefficient can be expected in the range of 2275 to 3400 W/m2·K (400 to 600 Btu/hr·ft2·°F).
The three major types of refrigerant feed—direct expansion, flooded with surge drum, and forced liquid overfeed—are all used with this type of evaporator. Flooded and liquid overfeed usually function more effectively, but the direct expansion arrangement is simplest. A difficulty with direct expansion is achieving uniform flow distribution to each of the refrigerant passages.