Some special industrial and laboratory applications require very low temperatures. It is very difficult and not economically advantageous to achieve this temperature with a single vapor compression mechanical cycle. In two-stage cooling systems, the energy consumed in the compressors is less and the compressor outlet temperature is lower. EER value is higher than single-stage systems, but initial setup costs are high. Since the difference between the condensing pressure in the condenser and the evaporation pressure in the evaporator is large, the compression ratios of the compressors used in such systems are also very high. As the compression ratio increases, the efficiency of the compressors decreases and the energy expenditures increase excessively. In addition, compressor costs are rising.
The following shows the two-stage refrigeration cycle using R-134A and R-404A as refrigerants. The two-stage cooling system consists of two separate vapor compression mechanical cooling systems. The evaporator of the first stage cooling system (R-134A) becomes the condenser of the second stage system (R-404A) (heat exchanger). Therefore, the condenser condensing temperature of the second stage system is lowered, as a result, the temperature of -50°C is more easily reached in the evaporator of the second stage cooling system.
The energy consumed in the two-stage cascade cooling system (R-134a/R-404a) given above; It is equal to the sum of the energy consumed in the first stage compressor, second stage compressor, first stage condenser fan and second stage evaporator fan. Since it is known that the cooling capacity is equal to the cooling energy obtained in the second stage evaporator, the EER value is calculated with the equation given below.
In addition, it becomes inevitable to use a two-stage cooling system according to the physical and thermodynamic properties of the refrigerants. Especially, the widespread use of carbon dioxide systems, which have recently started to be used again due to environmental concerns, brings cascade cooling systems to the fore. As seen below, in a cascade cooling system using carbon dioxide; in the first stage ammonia, R-404A, R-134a etc. fluid can be used. The condenser condensing temperature of the first stage cooling system is 40°C and the evaporator evaporation temperature is -10°C. The second stage represents the CO2 side, the condenser condensing temperature is -5°C and the evaporator evaporation temperature is -25°C.
The temperature difference in the heat exchanger is 5°C. EEV stands for electronic expansion valve.
Showing the Cascade Cooling System in P-h and T-s Diagrams
The cascade cooling system using the NH3/CO2 couple is given below. Here, ammonia is used as refrigerant in the first stage (auxiliary cooling circuit). In the second stage (main cooling circuit), carbon dioxide was used as the refrigerant. The cycle shown in blue in the pH diagram represents the system with carbon dioxide (condensation temperature: -5°C, evaporation temperature -45°C) and the red color represents the cycle using ammonia refrigerant (condensation temperature: 40°C, evaporation temperature -10°C).
Cascade refrigeration cycle consists of two different refrigeration cycles. Ammonia (NH3) is used in the first stage, while carbon dioxide (CO2) is used as the refrigerant in the second stage. For the 1st stage in this system; (5-6) shows compression in the compressor, (6-7) condensation in the condenser, (7-8) expansion in the pressure reducer, and (8-5) evaporation in the evaporator. Likewise, in the second stage, (1-2) compression in the compressor, (2-3) condensation in the condenser, (3-4) expansion in the pressure reducer and (4-1) evaporation in the evaporator.
Optimum Intermediate Temperature
- The intermediate temperature between two cascade circuits is a design parameter that plays an important role in determining the coefficient of performance (COP) of the entire system.
- Assuming that there is no temperature difference between the two liquids in the evaporator-condenser, in reversible cycles, the optimum stage temperatures are the geometric mean of the condensation and evaporation temperatures of the cascade system. For example, it is assumed that the circuits each have the same temperature rating, since the actual values do not differ by more than 10% from the ideal values.
- The optimal temperature difference between the two fluids will depend not only on the heat transfer characteristics of the refrigerants in the two circuits, but also on the economics of design (operation vs. capital cost).
- The larger the temperature difference, the lower the COP of the system.
Evaporator / Condenser
- The temperature difference between the condensation of the low-temperature refrigerant and the evaporation of the high-temperature refrigerant from 2.5°C to 5°C, called the approximation, to keep the compressor cost-effective and energy consumption low.
- Seems like a reasonable balance.
- Since the surface tube type evaporator-condenser is bulky, innovative designs of condensers must be developed. It is reported that the use of an expensive but compact surface and plate type condenser minimizes the leakage of carbon dioxide into the ammonia system.
- When carbon dioxide and ammonia are used as refrigerants, it is a big problem that the mixture of carbon dioxide and ammonia in the evaporator-condenser forms a solid precipitate as a result of chemical reaction.
- For the safe operation of evaporator-condensers, double-surface welded pipes made of quality materials, rigorous testing and continuous leak monitoring are recommended for surface and tube type condensers.
Automatic Cascade Cooling Systems
The automatic cascade cooling system using only one compressor was proposed by Ruhemann in 1946. These auto-cascade systems or systems known as automatic cooling cascades are used to make the temperature -180°C in single compression cascade systems with multicomponent zeotropic refrigerant mixtures. Compared to other dual or multi-cascade cooling types, autocascade systems have many advantages. These advantages are the compact design of the system elements; its reliability, safety and flexibility in use; The many limitations of the auto cascade system, such as the relative ease and affordability of maintenance and the lubrication fluidity of the auto cascade system, need to be avoided. As an example, consider the two-loop cascade system operating with R-23 and R-22. Let us show the vapor pressure of these substances on a Dühring drawing below.
- In the R-23 circuit the pressure is 2.47 bar for the evaporation temperature of -65°C and again in the R-23 circuit the pressure is 16.27 bar for the condensation temperature -15°C.
- To condense R-23, the R-22 refrigerant must be boiled at less than -15°C.
- If the evaporation temperature is selected as -22°C, the suction pressure in the R-22 circuit is the same as in the R-23 circuit.
- Also, if the condensation temperature in the R-22 circuit is adjusted to 42°C, the discharge pressure in the two circuits will be the same.
- The steam can then be mixed with suction and then compressed together, provided it separates.
- Separation can be accomplished by partial condensation using a circuit as shown in Figure 2.17.
When the mixed steam passes into the condenser, the temperature will be too high for R-23 to condense at a pressure of 16.27. But R22 can condense, provided the partial pressure in the mixture is high enough. The condensed R-22 is subsequently separated from the R-23 vapor and expanded to the low side pressure where it boils at -22°C. Then the cooling effect -15°C is used to condense the R-23 vapor and this liquid goes to the evaporator where it boils at -65°C.
The main disadvantage of the auto-cascade system is that it requires the use of a special mixture of refrigerant. This feature causes three maintenance-related problems.
First, since the refrigerant mixture consists of different types of refrigerant with different boiling points, a leak in the system causes an imbalance in the ratio of the remaining refrigerants.
- To restore the system to proper working condition, all of the remainder of the refrigerant must be replenished with a new and potentially costly charge to ensure the correct mixing ratio.
- Second, because the mixture is proprietary, it is not readily available in conventional refrigerant supply sources and can also be difficult and costly to obtain.
- Third, because such systems are not widely used, it is sometimes difficult to find skilled maintenance personnel who have mastered the repair and maintenance procedure.
Source: friterm.com; Hüseyin BULGURCU