In shell-and-tube evaporators the boiling refrigerant chills a liquid such as water or a brine/antifreeze. Some of the major forms of construction include: (1) refrigerant boiling in the tubes chilling in the shell the liquid flowing across the tube bundle with its direction repeatedly reversed by baffles, and (2) refrigerant boiling in the shell surrounding the tubes through which the liquid passes.
Refrigerant in tubes. Most water chillers using halocarbon refrigerants vaporize the refrigerant in the tubes using a superheat-control expansion valve and are thus direct expansion. An exception in the air-conditioning industry are the water chilling packages using centrifugal compressor. The refrigerant boils in the shell in these evaporators because excessive drops in pressure would result were the low-density refrigerant vapor to be sent through the tubes. In industrial refrigeration systems the circuits could be arranged for forced liquid overfeed or could be operated flooded using a surge drum.
Refrigerant in shell. Probably the most popular type of liquid-chilling evaporator in industrial refrigeration practice directs the liquid to be chilled through the tubes which are surrounded by the boiling refrigerant in the shell. These heat exchangers are usually called flooded chillers. Whenever the refrigerant boils in the shell there must be some way of separating the liquid from the vapor before the vapor leaves the evaporator and passes on to the compressor. Two approaches to achieving the liquid-vapor separation are shown in Fig. 6.59 and Fig. 6.60. In the liquid chiller of Fig. 6.59 the tubes do not fill the shell volume, but there is separation space left at the top of the shell. The level-control valve regulates the flow of liquid refrigerant to the bottom of the vessel. When ammonia is the refrigerant an oil pot is provided which is periodically drained of the oil that collects there. Another approach to separation is shown in Fig. 6.60 where a small vessel is mounted above the shell of the main evaporator, which now has all of its internal volume occupied by tubes. This design is used for brine chillers serving ice rinks as well as for many other brine-chilling applications.
The design of liquid chillers is a technical specialty not addressed here. Instead, this section emphasizes the concerns of one who selects and operates liquid chillers. As the required temperature of the liquid being chilled drops, a number of changes occur—all of them bad. The regrettable result is often the inability to chill the liquid to the required temperature. Such a failure is caused by the degradation of the heat-transfer coefficients, both of the refrigerant and the fluid being chilled, especially in the case of brines and antifreezes.
The total resistance to heat transfer, Rtotal, of the chiller is the sum of the individual resistances,
An estimate of the boiling coefficient of the refrigerant on the outside of the tubes is provided by the graph29 in Fig. 6.61. Both for the liquid heat transfer coefficient inside the tubes and for the boiling heat-transfer coefficient of the refrigerant outside the tubes the heat-transfer coefficients drop as the temperature drops.
Equations 6.14 and 6.15 along with Fig. 6.61 can be used by the designer or purchaser to estimate the size of a liquid chiller, although it is ultimately the manufacturer of the heat exchanger who is responsible, since the manufacturer is versed in the specialties of heat-exchanger design.