**4. Experimental results and discussions**

This section provides a brief overview of the data obtained using the experimental device of the cascade MR refrigerator introduced in Section 3. The specific composition proportions of the MRs below are not specified because they are protected by patent rights. In terms of changing the composition proportions, the total charging mass of MR was kept constant, and the mass proportions of the high boiling point refrigerants.

**Figure 12.** *Transient temperature of each measuring point (including R290).*

#### *Perspective Chapter: Ultra-Low Temperature Chillers for Semiconductor Manufacturing Process DOI: http://dx.doi.org/10.5772/intechopen.98547*

R290 and R600a were adjusted. In other words, the mass of three different types of refrigerants was increased by the same proportion as the reduced charging amount of the high boiling point refrigerant, and the total charging amount of the MR remained constant [15].

#### **4.1 Experiments with MR compositions of R290, R116, R23, and R14**

**Figure 12** shows the compressor discharge temperature and suction temperature, evaporator inlet temperature, and expansion valve inlet temperature according to the operating time in the refrigerant's initial composition proportion (R290, R116, R23, and R14) state. The device was operated by applying the MR composed of the above four refrigerants, and the opening of the expansion valve was controlled according to the rapid pressure change. Here, the temperature was decreased while leaving the suction valve of the expansion tank open, and as the target temperature was reached, the valve was closed, and the brine heater was operated.

The experiment was conducted while adjusting R290 in a mass fraction range of 5–50%, as shown by MR1–MR8, to examine the performance of the device based on the proportion change of the high boiling point refrigerant (R290) in the refrigerant charging amount of the same mass. The charging amount of R290 was the highest in MR1 and the lowest in MR8. **Figure 13** shows the compressor discharge and suction temperature, evaporator inlet temperature, and expansion valve inlet temperature based on the composition ratio of R290. When the mass ratio of R290 decreased, the compressor suction temperature increased from −67.8°C to −48.2°C. In both cases of MR7 and MR8, the expansion valve inlet temperature was clearly high when the mass fraction of the high boiling point refrigerant dropped below a certain level. However, the evaporator inlet temperature after going through the expansion valve was between −109.2 and − 106.7°C, showing no significant effect.

**Figure 14** shows a graph by comparing the changes in the cooling capacity and time when the composition of the high boiling refrigerant is changed. As the proportion of the high boiling point refrigerant increases, the cooling capacity shows a decreasing tendency. MR8 shows the highest cooling capacity at the 2.36 kW level, whereas MR1 shows the lowest value with 1.69 kW. However, there is a disadvantage in that the average pressure of the device increases because of the mid/low boiling point refrigerant that has a relatively higher pressure at room temperature as the proportion of the high boiling point refrigerant decreases. Furthermore, the cooling time was defined as the time when the brine supply temperature reaches −100°C, as measured through experiments. As the mass fraction of R290, a high boiling point refrigerant, increased, faster cooling time was achieved.
