Refrigerating System – Three-stage (ternary) cascade refrigeration system
Cascade refrigeration cycles are commonly used in the liquefaction of natural gas, which consists basically of hydrocarbons of the paraffin series, of which methane has the lowest boiling point at atmospheric pressure. Refrigeration down to that temperature can be provided by a ternary cascade refrigeration cycle using propane, ethane and methane, whose boiling points at standard atmospheric pressure are 231.1 K, 184.5 K and 111.7 K (Haywood, 1980). A simplified basic diagram for such as cascade cycle is shown in Figure 3.40. In the operation, the compressed methane vapor is first cooled by heat exchange with the propane in the propane evaporator before being condensed by heat exchange with the ethane in the ethane evaporator, so reducing the degree of irreversibility involved in the cooling and condensation of the methane. Also, because of the high temperature after compression, the gas leaving each compressor passes first through a water-cooled aftercooler. In large-scale plant of this type, the compressors become rotary turbo-machines instead of the reciprocating type ones.