Refrigeration cycle cascade

The mixed refrigerant cycle is a variation of the cascade cycle and involves the circulation of a single multicoiTmonent refrigerant stream. The simplification of the compression and heat exchange services in such a cycle can offer potential for reduced capital expenditure over the conventional cascade cycle. 

Figure 11-56. Cascade type low-temperature refinery refrigeration cycle. (Used by permission Charls, J., Jr. Tech Paper RP-468, 1950. Dresser-Rand Company.

Complex refrigeration cycles can be built up in different ways. Figure 24.29 shows a cascade cycle with multistage compression and expansion of the high-temperature cycle. 

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. 

Carnot Refrigerator and Heat Pump Basic Vapor Refrigeration Cycle Actual Vapor Refrigeration Cycle Basic Vapor Heat Pump Cycle Actual Vapor Heat Pump Cycle Working Fluids for Vapor Refrigeration and Heat Pump Systems Cascade and Multistaged Vapor Refrigerators... 

There are several variations of the basic vapor refrigeration cycle. A cascade cycle is used when the temperature difference between the evaporator and the condenser is quite large. The multistaged cycle is used to reduce the required compressor power input. 

Neglecting kinetic and potential energy changes, a steady-state and steady-flow mass and energy balance on the components of the cascade vapor refrigeration cycle have the general forms ... 

The cooling load of the cascade vapor refrigeration cycle is the rate of heat added in the evaporator of the lowest temperature cycle. The power added to the cycle is the sum of the power added to the individual compressors ... 

The following example illustrates the analysis of the cascade vapor refrigeration cycle. 

A cascade vapor refrigeration cycle made of two separate vapor refrigeration cycles has the following information ... 

Determine the mass rate flow of the top cycle, power required by compressors 1 and 2, total power required by the compressors, rate of heat added to the evaporator, cooling load, and COP of the cascade vapor refrigeration cycle. 

Natural gas expander cycle 2 Refrigerant expander cycle 3 Cascade cycle 4 Mixed refrigerant cycle... 

With three or more stages for each refrigerant, the power consumption for the cascade cycle is found to compete quite favourable with other liquefaction processes, especially in arctic conditions. This is mainly because of the low flow rate of refrigerant. The cascade cycle is also more flexible in operation, since each circuit of refrigerant can be separately controlled. For the classical cascade, however, an overlap may occur of refrigerants like methane and ethylene. Thus, as methane condenses at an elevated pressure, this will inherently cause some throttling losses. 

The mixed refrigerant cycle (MRC) uses a single mixed refrigerant instead of multiple pure refrigerants as the cascade cycle. The mixed refrigerant consists normally of nitrogen, ethane, propane, butane and... 

Figure 14 Cascade refrigerant cycle, only one stage per refrigerant.

Another cycle that has been used exclusively for large natural gas liquefaction plants is the mixed refrigerant cycle. Since this cycle resembles the classic cascade cycle in principle, it can best be understood by reference to a simplified flow sheet of that cycle presented in Fig. 6. 

Soave modification of, 491-492 Reduced coordinates, 85, 477 Refrigeration, 274-290 Refrigeration cycles absorption, 288-289 Carnot, 275-276 cascade, 286-287... 

In the past the liquefaction of natural gas used a classic cascade cycle. The process required 120,000 hp for liquefaction of over 150 million standard cubic feet (mmscf) per day. Provisions are made for some of these cycles to use seawater for cooling. Later, baseload LNG plants utilized mixed refrigerant cycles, such as Air Products and Chemicals, Inc. s propane precooled mixed refrigerant system. Baseload plant capacities range from about 70 mmscf/day to about 350 mmscf/day of LNG. Baseload plants move LNG from remote sites by ship to populated areas. For... 

The mixed refrigerant cycle was developed to meet the need for hq-uefying large quantities of natnral gas to minimize transportation costs of this fuel. This cycle resembles the classic cascade cycle in principle and may best be understood by referring to that cycle. In the latter, the natural gas stream after purification is cooled successively by vaporization of propane, ethylene, and methane. Each refrigerant may be vaporized at two or three pressure levels to increase the natnral gas coohng efficiency, but at a cost of considerable increased process complexity. 

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