3.1.1 Temperature variation of different state monitors
Figure 4 presents temperature variations of different monitors of LTC. It can be seen that the refrigeration device is started since 14 min. Different monitors undergo initial temperature rise and then reduce gradually. About 254 min later, -80oC target temperature is achieved and freezer stops. Thereafter, different temperature monitors enter into periodic fluctuation variations. This is to say the pull-down process approximately lasts for 4 hours. For monitor T1, it experiences an initial slight temperature rise, and then drops to -3.7oC rapidly. During temperature drop period, some unusual phenomenon occur, such as the temperature fluctuation and sudden temperature reduction, as described in Fig. 4(b). Under the compression effect of compressor, the discharge temperature T2 achieves a high value. After initial temperature increase, T2 starts to reduce. As high-temperature discharge gas goes through long pipe from LTC compressor to pre-cooled condenser, obvious temperature drop occurs under the cooling of external environment. Thereafter, high-temperature discharge gas gets further cooled in pre-cooled condenser. Other monitors also experience initial rapid temperature increase. The maximum temperatures of T2, T2’, T3 and T3’ are achieved in 52-60s with values of 127.1oC, 101.2oC, 45.0oC and 43.8oC, respectively. Afterwards, four monitors experience temperature drop and finally reach 100.2oC, 70.3oC, 37.9oC and 35.9oC, once refrigeration system is shut down. The associated temperature reductions are 26.9oC, 30.9oC, 7.1oC and 7.9oC. The temperature drop of refrigeration gas in pre-cooled condenser reduces from 56.2oC to 32.4oC during pull-down process, while the temperature variation range of refrigerant passing through CHE is 1.2-2.0oC.
While − 80oC freezing temperature is achieved, refrigeration system will be shut down. Subjected to external heat invasion, the air temperature within freezer rises, when it increases to certain temperature limit, refrigeration system will restart. Thereafter, refrigeration unit comes into periodic start-stop variations, and different test points experience extensive temperature fluctuations. Generally, during stable operation of freezer, different monitors basically maintain in relative stable temperature ranges. The average temperature values of T1, T2 and T2’ are − 3.6oC, 94.2oC, and 67.4oC, as shown in Fig. 4(a), and the average temperature values of T3 and T3’ are 37.1oC and 36.0oC, as shown in Fig. 5(a). It is easy to obtain that during periodic operation of freezer, the temperature decline from the outlet of compressor to the inlet of pre-cooled condenser is 26.8oC, and the temperature drop of refrigerant gas in pre-cooled condenser is about 30.3oC. The cooling effect generated by long pipe is close to that generated by pre-cooled condenser. However, there is only 1.1oC temperature reduction generation while refrigerant fluid passes through CHE. That is to say the latent heat exchange is the main heat exchange mode in CHE. Moreover, with time continuing, the average temperatures of different monitors have slight temperature reductions, and it is obviously reflected by temperature profile of monitor T1. This phenomenon may be attributed to the increase of refrigeration efficiency after long term operation.
Figure 6 displays temperature distributions of different monitors of HTC. Similar to temperature variations of LTC, different temperature monitors of HTC also undergo initial temperature rise once freezer is activated. While the maximum temperatures are achieved, different monitors start to experience temperature decline with different reduction rates. If the air temperature within freezer reduces to the required freezing temperature, CRS is shut down and then comes into periodic start-stop fluctuations, as shown in Fig. 6(a). Different from T5, monitor T5’ is close to high-temperature compressor and its temperature experiences slight reduction during pull down phase. While monitor T5 undergoes obvious temperature rise and rapid temperature drop. Once high-temperature vapor flows out of compressor, it goes through a long way and comes into anti-condensation coil. The maximum temperature difference between T6 and T6’ is about 33.2oC. While high-temperature vapor flows into anti-condensation coil, it undergoes continuous temperature reduction cooled by environment air. Moreover, the inlet and outlet vapor temperature monitors also have same variation profile. As Fig. 6 shows, temperature variation profiles of T6’ and T6’’ are almost parallel except the initial stage. The temperature reduction is about 25.4oC after high-temperature vapor goes through anti-condensation coil. As HTC condenser is close to drying filter, monitors T7 and T7’almost have same temperature variation profile, with difference of 0.4oC. After 4 hours pull-down process, -80oC low temperature is achieved and final values of different monitors are − 10.3oC, 14.1oC, 107.9oC, 74.7oC, 49.3oC, 39.1oC and 38.7oC, respectively. 254min later, CRS enters periodic start-stop switching phase, and different monitors experiences periodic temperature fluctuations. After 48 operation cycles, temperatures of different monitors tend to be certain values. For instance, T5 is about − 9.2oC during periodic operation of CRS. T5’ and T6 tend to be 19.8oC and 107.5oC. As for T6’ and T6’’, two parameters maintain to 75.2oC ad 47.3oC. The associated temperature reduction through anti-condensation coil is about 27.9oC. This means that there is still an obvious thermal load at the sealing section of freezer door and great insulation and sealing measure should be adopted to reduce external heat leakage. While for monitors T7 and T7’, both almost have same fluctuation profile during periodic operation process, and the average temperatures of two monitors are 42.2oC and 42.1oC.
After detailed comparison and analysis, it is easy to see the start-stop ratio of CRS reduces with time. Once the target temperature is first achieved, CRS stops for 2.5min and then restarts and works for 19min, the start-stop ratio is 7.6. With time continuing, CRS works for short term and stops for long time. For the last operation cycle, the working time and resting time of CRS are 8.5min and 3.5min, and the corresponding start-stop ratio is about 2.43. The main reason is given as follows. As time continuous, the operation efficiency of CRS gets obviously improved and external heat invasion can be removed in short term. Correspondingly, the rest time of freezer increases and the work time of freezer decreases. Therefore, the start-stop ratio of CRS reduces with time. That is to say the long-term operation of CRS has positive effects on reducing power consumption and improving system operation efficiency.
As introduced above, three thermocouples are attached to chamber walls of two compressors and oil separator. The related temperature variation profiles are depicted in Fig. 7. It can be seen that once CRS is activated, wall temperature of three components experience sudden temperature rise firstly, then THTC undergoes slight temperature decrease, and TLTC and TOS drop rapidly along with obvious fluctuations during pull-down phase. When it comes into periodic operation stage, both THTC and TLTC fluctuate with time and have gradual reduction trend, while TOS almost keeps relative stable variation. During whole process, THTC is < 70oC, and the maximum value of TLTC is obtained in the initial stage with a value of 66.9oC. While CRS stops, high-temperature vapor is still filled in compressor chamber. Heated by high-temperature vapor, both THTC and TLTC increase with time, as shown in Fig. 7. Moreover, TOS is always less than TLTC during whole process. The cooling effect of lubricant oil on low-temperature compressor gets greatly reflected.
3.1.2 Temperature variation of different monitors in freezer
Figure 8 displays temperature variations of 4 core monitors and 3 side monitors in compartment No.1. As Fig. 8(a) shows, 4 core test points experience rapid temperature reduction during pull-down period. This is mainly because refrigerant flows from top to bottom, the air in compartment No.1 gets cooled firstly, and air in compartment No.4 is cooled lastly. Therefore, monitor Tcore1 has the largest temperature reduction rate and monitor Tcore4 has the lowest temperature reduction rate during the first 60min. However, as time continuous, cold air sinks and hot air moves up, which causes temperature reduction rate of bottom monitor becomes larger than top monitor. Influenced by combination of direct cooling of evaporation coil and air circulation flow caused by natural convection within freezer, the air in bottom section obtains lower temperature than that in top section. Once CRS stops, temperatures of four core monitors are − 77.3oC, -78.7oC, -80.9oC and − 82.1oC, respectively. When CRS comes into periodic start-stop switching phase, temperature fluctuation amplitudes of four monitors are so small that their temperature profiles almost linearly reduce with time. After 48 cycle operation, final values of Tcore1-Tcore4 are − 80.2oC, -81.6oC, -82.8oC and − 83.1oC, respectively. During > 10 hours operation, temperature reductions of four monitors are 2.9oC, 2.9oC, 1.9oC and 1.0oC, respectively. 660min later, temperatures of four core monitors all reduce to -80.0oC, which means that all freezer space is cooled below − 80.0oC. Figure 8(b) presents temperature variation profiles of the top, left and right monitors in compartment No.1. Meanwhile, temperature profile of Tcore1 is also added for detailed comparison. It is evident to see three side monitors experience rapid temperature drop firstly and then gradually reduce during pull-down stage. Their temperature reduction rates are higher than that of Tcore1. When it comes into periodic start-stop operation, temperatures of three side monitors experience fluctuating variations. Largely different from core monitors, temperature of side monitors experience extensive fluctuations. The main difference between core monitor and side monitors is attributed to their locations. As monitors Ttop, Tleft and Tright are set on the top, left and right side of compartment No.1, these three monitors are directly influenced by evaporation cooling coil, so three monitors have rapid temperature reduction during pull-down phase. While CRS operate periodically, refrigerant R170 experiences alternating cooling and heating, so temperatures of Ttop, Tleft and Tright undergo continuous fluctuations. However, as the core monitor is arranged in the core of compartment, the air in the core is not directly cooled by evaporation coil and only cooled by conduction and convection. Due to low thermal conductivity of air and some conduction delay, the periodic operation of CRS has not caused obvious effects on temperature variation of core monitors. During periodic operation of CRS, three side monitors all vary from − 83.8oC to -87.2oC.
Figure 9 displays temperature variations of four back monitors. It is easy to see four monitors, located on the back side of freezer, experience initially rapid temperature drop, then their temperatures reduce gradually and come into fluctuation variations finally, as shown in Fig. 9(a). As the evaporation coil is wound around the side of freezer from top to bottom, so monitor Tback1 has the largest temperature reduction rate, and the temperature reduction rate of Tback4 is the lowest among four monitors during pull-down phase, as shown in Fig. 9(b). In general, as back monitors are directly attached to the back side of freezer, their temperatures are largely influenced by evaporation cooling of refrigerant. When compared to core monitors, four back monitors have rapid temperature reduction rates. For instance, it only takes 134 min for monitor Tback1 reaching − 80oC during pull-down period, and the time consumptions for monitors Tback2- Tback4 reducing to -80oC, are 141min, 147min and 211min, respectively. While for different core monitors, it costs > 240min their temperatures drop to -80oC. During periodic operation of CRS, four back monitors have fluctuating temperature variation profiles. Moreover, among four back monitors, the bottom monitor Tback4 has a slower temperature reduction and a higher temperature variation profile. This is mainly because low-temperature refrigerant flows from top to bottom, when it arrives at compartment No.4, most of refrigerant liquid evaporates. There is less phase change heat capacity used for cooling. That is to say the cooling effect becomes weaken when two-phase refrigerant flows through compartment No.4. Hence, monitor Tback4 has a low temperature reduction rate during pull-down phase and high temperature values during periodic operation phase. The lowest temperature of four back monitors can reach − 86.8oC.
Based on above introduction, it is easy to find that 660 min later, all space of freezer can reach below − 80oC, including core sections of compartments No.1 and No.2. Moreover, the lowest temperature could reduce to -87.2oC. To sum up, -80oC target temperature has been experimentally demonstrated and confirmed.