**3. Experimental system and methodology**

This section describes the experimental system and method of the cascade MR refrigerator, which is one of several refrigeration systems commonly used to achieve the ultra-low temperature introduced in Section 2.

**Figure 8** shows the schematics of the experimental system, which is divided into low and high stage cycles. The high-stage cycle comprises of a compressor, condenser, expansion valve, and cascade heat exchanger, while the low-stage cycle comprises of a compressor, expansion tank, pre-cooling heat exchanger, cascade heat exchanger, intermediate heat exchanger, expansion valve, and evaporator. The cooling water side consists of an isothermal bath, process chilled water (PCW) pump, and inverter, and the flow rate of the cooling water is adjusted by controlling the number of revolutions. The brine side consists of a brine tank, brine pump, inverter, and heater. The inverter also controls the flow rate of the brine, and the load is controlled using the heater in the brine tank.

**Figure 9** explains the use of the pre-cooling heat exchanger, which was not mentioned in the aforementioned basic description of the cycle. The pre-cooling heat exchanger uses the cooling water going into the condenser of the high-stage cycle to primarily cool the high-temperature, high-pressure refrigerant discharged from the compressor of the low-stage cycle, and it handles a portion of the aftercooler capacity. A simulation analysis was performed to determine whether to use

**Figure 8.** *Schematics of experimental system of cascade MR refrigerator.*

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

**Figure 9.** *Comparison of cascade MR refrigerator with and without pre-cooling heat exchanger.*

the pre-cooling heat exchanger. It should be noted that its use is accountable for a portion of the capacity handled by the aftercooler, and

simultaneously has an adverse effect on the temperature of the cooling water entering the condenser of the high-stage cycle. The results of the simulation analysis show that the application of the pre-cooling heat exchanger increases the coefficient of performance (COP) by approximately 10% from 0.289 to 0.316. The actual device produced and experimented based on this is shown in **Figure 10**.

The experiments were carried out under the conditions shown in **Table 2** to determine the performance characteristics according to the MR composition of the cascade MR Joule-Thomson refrigerator that uses the MR as the working fluid. Cooling water of 18°C flows through the pre-cooling heat exchanger at 22 L/min and is supplied to the high-stage condenser, and the brine entering the evaporator is supplied at a constant flow rate of 40 L/min. The compressor selected was a standard commercial compressor. The discharge pressure should not exceed 3 MPa, and the suction pressure should be maintained above 0.1 MPa, according to the operational constraints of the compressor.

**Figure 10.** *Experimental device of cascade MR refrigerator.*

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


#### **Table 2.**

*Summary of experimental conditions.*

For reference, if the suction pressure is in a vacuum state, the oil supply in the compressor may not be smooth and moisture may enter. Therefore, the suction pressure of the compressor must be higher than vacuum pressure. To prevent the carbonization of the compressor oil, the discharge temperature is limited to be within 120°C.

The following method was used under the aforementioned experimental conditions. If the high-stage cycle reaches the target temperature, the compressor of the low-stage cycle is started. To prevent excessive pressure buildup, the valves installed before and after the expansion tank are opened during the process. The brine pump is operated as soon as the compressor starts, thereby exchanging the heat at the evaporator. Following this, the opening of the expansion valve is controlled in such a way that the suction pressure of the compressor does not drop to or below the atmospheric pressure, until the target temperature is reached. As the target temperature is approached, the temperature of the low-pressure side stream entering the intermediate heat exchanger decreases, and accordingly, the pressure of the high-pressure side stream decreases. To solve this problem, the valve of the pipe connected to the compressor suction tube and the expansion tank is opened to inject the refrigerant to maintain the pressure. If the target temperature

**Figure 11.** *P-h diagram of cascade MR refrigerator.*

is reached, the load of the heater in the brine tank is gradually increased, and if the steady state is reached, the cooling capacity, including the load put on the heater and the heat generation pump, is measured.

**Figure 11** shows the P-h diagram of the cascade MR refrigerator. The working fluid of the low-stage cycle consists of MR. When the flow rate of the cooling water entering the pre-cooling heat exchanger is sufficient, primary cooling proceeds up to the temperature of the cooling water, and the outlet temperature of the cascade heat exchanger is determined based on the evaporation temperature of the high-stage cycle. Here, when the low-stage cycle is viewed as a system, the sum of the cooling capacity at the evaporator and the work put into the compressor should be the same as the sum of the heat exhausted by the pre-cooling heat exchanger and the cascade heat exchanger. As the capacity of the pre-cooling heat exchanger and the aftercooler increases, the capacity of the evaporator increases. Because of its characteristics, the Joule-Thomson cycle using MR has a temperature gradient within the two-phase region, and the refrigerant temperature at the aftercooler outlet is limited to the temperature level of the cooling water. Therefore, in this study, the evaporator of the high-stage cycle, that is, the cascade heat exchanger, is used as the aftercooler of the low-stage cycle to increase the cooling capacity of the evaporator.
