considerably large temperature and pressure difference), single vapor compression refrigeration cycles become impractical. One of the solutions for such cases is to perform the refrigeration in two or more stages (i.e. two or more cycles) which operate in series. These refrigeration cycles are called cascade refrigeration cycles. Therefore, cascade systems are employed to obtain high temperature differentials between the heat source and heat sink and are applied for temperatures ranging from -70°C to 100°C. Application of a three-stage compression system for evaporating temperatures below -70°C is limited, due to difficulties with refrigerants reaching their freezing temperatures. Impropriety of threestage vapor compression systems can be avoided by applying a cascade vapor-compression refrigeration system.
Cascade refrigeration systems are commonly used in the liquefaction of natural gas and some other gases. A large-capacity industrial cascade refrigeration system is shown in Figure 3.37.
The most important advantage of these cascade systems is that refrigerants can be chosen with the appropriate properties, avoiding large dimensions for the system components. In these systems multiple evaporators can be utilized in any one stage of compression. Refrigerants used in each stage may be different and are selected for optimum performance at the given evaporator and condenser temperatures.
Conventional single compressor, mechanical refrigeration system condensing units are capable of achieving temperatures of about –40°C. If lower temperatures are required then cascade refrigeration systems must be used. A two-stage cascade system uses two refrigeration systems connected in series to achieve temperatures of around -85° C. There are single compressor systems that can achieve temperatures colder than –100°C but they are not widely used. These systems are sometimes referred to as ‘auto cascading’ systems. The main disadvantage of such systems is that it requires the use of a proprietary blend of refrigerant. This characteristic results in three service-related problems:
• A leak in the system can easily cause the loss of only some of the refrigerant making up the blend (due to the refrigerant blend which is made up of different types of refrigerant with different boiling points), resulting an imbalance in the ratio of the remaining refrigerants. To return the system to proper operation, all of the remaining refrigerant must be replaced with a new and potentially costly charge to insure a proper blend ratio.
• The blend is proprietary and may not be readily available from the traditional refrigerant supply sources and therefore may be hard to obtain and costly.
• These types of cascade systems are not widely used; it is hard to find well-qualified field service staff that are familiar with repair and maintenance procedures.
Of course, these and other issues can cause undesirable expense and downtime.