Cascade refrigeration systems
significantly large temperature and pressure difference), one vapor compression refrigeration cycles become impractical. One of the solutions for such cases is to perform cooling in two or more stages (i.e., two or more cycles), who work in the series. These refrigeration cycles are called cascade refrigeration cycles. Thus, cascade systems used in order to obtain high temperature differences between the heat source and heat sink and are applied at temperatures from -70C -100C. Application of the three-stage compression system for boiling temperatures below -70C is limited, due to difficulties with the refrigerant temperature of freezing. The irrelevance of a threestage vapor compression systems can be avoided by applying a cascading steam-compression refrigeration machines.
Cascade refrigeration systems commonly used in the liquefaction of natural gas and some other gases. In large industrial cascade refrigerating system is shown in Fig. 3.37.
The most important advantage of these cascading system is that refrigerants can be selected with the appropriate properties, avoiding the larger components of the system.
In these systems with several evaporators can be used in any stage of compression. Refrigerants used at each stage may be different and selected for optimum performance at a given temperature of the evaporator and condenser.
Ordinary one pump, mechanical cooling system condensing units are capable of reaching temperatures of the order. At lower temperatures required then cascade refrigeration systems have to be used. A two-stage cascade system uses two refrigeration systems, connected in series to achieve a temperature of about -85C. There are single compressor system, which can reach temperatures lower than, but they are not widely used. These systems are sometimes called auto cascade systems. The main disadvantage of these systems is that it requires the use of a proprietary blend refrigerant. This characteristic results in three of problems connected with:
A leak in the system can easily lead to the loss of only some of the refrigerant components of the mixture of the refrigerant mixture, which consists of different types of refrigerants with different boiling points), as a result of an imbalance in the ratio of the remaining refrigerants. To return the system to function, all the other refrigerant should be replaced with a new and potentially expensive cost, to ensure the correct combination of attitude.
The mixture is proprietary and may not be readily available from traditional refrigerant power sources and therefore may be difficult and expensive.
These types of cascade systems are not widely used, it is difficult to find highly skilled field service staff who are familiar with repair and maintenance procedures.
Of course, these and other issues may cause unwanted costs and downtime...