In the concept of spray chillers, as shown schematically in Fig. 6.62, rather than immersing the tubes in liquid as is done in the conventional shell-andtube evaporator, liquid refrigerant from a circulating pump is sprayed over the tubes. The operating level of the liquid refrigerant in the evaporator is maintained below the bottom tubes. The idea of sprayed-tube evaporators is not new, and even though the overall heat-transfer coefficient of the sprayed tube evaporator usually exceeds that of the flooded type, the additional complexity of the pump and spray assembly usually dissuaded designers from choosing them. In recent years, however, another strength of the spray-tube evaporator has emerged in importance, and that feature is the low refrigerant charge required.
The improvement in heat-transfer coefficient in comparison to a flooded evaporator occurs because the sprayed tube permits easy escape of the vapor bubbles, which once they form, insulate the heat-transfer surface. In fact, there is an optimum circulation rate in the sprayed-tube evaporator that yields the maximum boiling coefficient. When the flow rate is too high, the liquid film becomes thick and insulates the tubes. When the flow rate is too low some of the tube surfaces do not become wetted. A typical circulation rate is 5 times the rate evaporated. The pump adds a small amount to the power cost, and if the pump fails, the evaporator is essentially out of operation. Often a reserve pump becomes part of the unit.
Reducing the refrigerant charge is an objective either from the standpoint of cost in the case of halocarbon refrigerants or for purposes of safety in the case of ammonia. As a comparison30, a flooded chiller with a refrigerating capacity of 1407 kW (400 tons of refrigeration) would require an ammonia charge of 5900 kg (13,000 lb), while the sprayed-tube evaporator would require only 159 kg (350 lb). The liquid leg that provides an adequate static head to the pump inlet contains much of the refrigerant charge.