Refrigeration cascade systems

Fig. 4. Single-pressure, mixed refrigerant cascade system for producing LNG...

Example. With the example cascaded system at an evaporator temperature of -100°F, the horsepower requirement is 6.2hp/ton refrigeration. A ton of refrigeration is equal to 12,000 BTU/hr. 

The Mollier diagrams for compound and cascade systems (Figures 2.9 and 2.10) indicate the enthalpy change per kilogram of circulated refrigerant, hut it should he home in mind that the mass flows are different for the low and high stages. 

Stream 2. At 200 K, either ethane or ethylene would be suitable refrigerants, with ethane being slightly better from the point of view of the power requirements. As for Stream 1, a cascade system would be required for heat rejection to ambient temperature. 

Rather than use the simple cycle shown in Figure 24.44 for the liquefaction of natural gas, much more complex arrangements using multiple cycles (with both pure and mixed refrigerants) and cascade systems can be used. 

The refrigeration necessary to run the plant is supplied by a cascade system consisting of liquid ammonia, a closed ethylene refrigeration cycle, and a high-pressure nitrogen cycle. 

CASCADE SYSTEMS - Arrangement in which two or more refrigerating systems are used in series uses evaporator of one machine to cool condenser of other machine. Produces ultra-low temperatures. 

However, such a fortunate situation does not often occur as in the case of feed consisting of refinery off-gases which are usually a mixture from several sources. This requires a more complex cycle. A typical cycle is essentially a cascade system employing components of the feed at different temperature levels. In order to provide the refrigeration required to maintain the... 

In a cascade system, economy of horsepower may be realized by use of interstage flashes and by the utilization of refrigerant at two or more temperature levels. 

The cascade system can be adapted to any cooling curve i.e., the quantity of refrigeration supplied at the various temperature levels can be chosen so that the temperature differences in the evaporators and heat exchangers approach a practical minimum (smaller temperature differences reduce the irreversibility, and therefore lower power consumption). 

Heat Pumps. Because of added capital and complexity, heat pumps are rarely economical, although they were formerly commonly used in ethylene/ethane and propylene/propane spHtters. Generally, the former spHtters are integrated into the refrigeration system the latter are driven by low level waste heat, cascading to cooling water. 

Figure 3-6. Scheme for liquifying methane using the cascade refrigeration system. 

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