Introduction
With the sustained and rapid development of the national economy, refrigeration and air conditioning facilities have been widely used in all aspects of national life. Refrigeration compressor is the main energy consuming equipment of refrigeration and air conditioning system. The energy consumption of the compressor accounts for approximately 70% of the whole air conditioning cooling system load [
1]. Thus, improvement in the energy saving performance of the refrigeration compressor will significantly improve the energy efficiency ratio of refrigeration and air conditioning.
In the operation of the compressor, the moving parts are always working in the bad conditions, such as heavy load, high speed, high temperature, and so on. The operation performance and service life of the compressor are influenced by the lubrication and its frictional characteristics directly. A non-normal use of a running part will destroy the whole equipment. Therefore, the effective reduction of the friction loss is of great importance to the protection of the normal operation of the compressor for inevitable friction dissipation. The main purpose for the use of refrigeration oil is to create a safe and reliable fluid lubrication environment in the refrigeration cycle. Hydrodynamic pressure in the surface of the moving parts forms a certain thickness of oil film with a certain degree of pressure-bearing capacity, and this oil film could reduce the wear and friction power consumption of the contact surface and ensure reliability and efficiency. Takuya et al. [
2] used the elastic fluid Reynolds equation to analyze the lubrication characteristics of sliding bearing. It was considered that the refrigerant oil viscosity in the gap could weaken the bearing capacity, increase the wear and tear, and the possibility of damage. Okaichi et al. [
3] used the hydrodynamic lubrication characteristics to analyze the influence of the bearing morphology on the friction loss and reliability. It was found that the oil film thickness reached the ideal state when the outer diameter of the bearing and the bearing width were 16 and 22 mm. In the refrigeration cycle phase, leakage refrigerant and refrigerant oil was partially miscible. Fukuta et al. [
4] found that the viscosity of refrigerant oil was affected by the molecular forces. When the concentration of refrigerant in the refrigerant oil increased, the molecular acting forces of the lubricating oil decreased and the viscosity of the lubricating oil decreases significantly. In addition, the circulation flow of the refrigerant oil took away the heat and wear debris generated by the friction of the internal moving parts of the compressor, which played a role in cooling and cleaning the surface of friction.
The application of nano particles in the refrigeration lubricants for solving the environmental protection and improving the energy efficiency of the refrigeration system is an effective method [
5,
6]. Bi et al. [
7] prepared Al
2O
3, TiO
2 nano refrigerant oil by the basis of mineral refrigerant oil, showing that the cooling rate of the refrigerator was faster, and the power consumption was reduced by more than 20% after using nano refrigerant oil. Xing et al. [
8] prepared fullerene nano refrigerant oil. It is found that the sample had good dispersion and stability which could be maintained for more than 3 years. Meanwhile, the COP values were increased by 5.6% and 5.3% respectively by adding them to the two home refrigeration compressors. Wang et al. [
9] pointed out that nano refrigerant oil can effectively improve the phase solubility and energy efficiency of mineral oil and environmental protection refrigerant after applying NiFe
2O
4 to the air conditioning system. However, there are few reports on the performance of domestic refrigerator compressor using C70 and NiFe
2O
4 nanocomposites based nano-oil. In this work, C70 and NiFe
2O
4 nanocomposites were added to refrigeration oil as lubricants. The four-ball friction tester was used to simulate and test the frictional property. Finally, the application of the nanocomposites in domestic refrigerator compressors was investigated by compressor calorimeter experiments.
Experiment
In this study, the nano refrigeration oil was prepared by combining physical and chemical methods. C70/NiFe2O4 was added to the refrigeration oil (56EP) when the nanocomposites were modified by dispersant of span 80. After that, the mixed oil was added to the NETZSCH machine which could make the material dispersed through mechanical agitation. The concentration was investigated in order to obtain the nano-oil performances at different concentrations. The basic parameters of pure oil are listed in Table 1, and the TEM image of prepared nano oil is shown in Fig. 1.
The friction coefficient is an important factor in the evaluation of the tribological properties of nano-oil, which is determined based on the SH/T0762-2005 issued by China’s National Development and Reform Commission. Figure 2 demonstrates the four-ball friction tester which can evaluate the friction and wear performance. The four balls were respectively fixed on the tee and oil box of the tester. The sample of oil was put into the oil box (oil surface covered with the bottom of the ball). The test steel ball is a level 2 standard G Cr ball, with the hardness between 59 and 61, and a diameter of 12.7 mm. The steel balls used in the experiment were washed with petroleum ether. The experiment condition was kept the same as that of the working condition of the compressor. That is, the temperature was kept at 50°C±2°C, the rotating speed was kept at 1200 r/min, the load was kept at 392 N, and the experiment data were collected in 60 min.
To obtain the performance of the compressor when using C70/NiFe2O4 as additive, the compressor QX-c202E030 gA made by Zhuhai Lingda Compressor Co., Ltd., was randomly selected. Two compressors were selected to test the pure oil, and two compressors were prepared for the nano C70/NiFe2O4 oil. The result was tested according to GB/T5773-2004, the national standard for evaluation of the performance of compressors in China. The refrigerating capacity of the refrigerator compressor was tested by using the second refrigerant heat method. Two refrigeration loops were used to control the subcooling temperature and testing temperature as illustrated in Fig. 3. The second refrigerant was R114. The test condition of the compressor was tabulated in Table 2. The suction pressure of compressor refrigerants in the experiments were adjusted by the expansion valve, the suction temperature was controlled by the electricity and heat which were input to the second refrigerant, and the exhaust pressure was adjusted by changing the cooling water from the condenser. The refrigerating capacity was measured by calorimeter electricity and heat power meter while the motor power and the power frequency were measured by the power meter and the frequency meter. The experiment platform (Fig. 4) was provided by Zhuhai Lingda Compressor Co., Ltd., with a precision of 0.4% under the operating condition of the refrigeration compressor. The precision was determined by calculating the ratio of absolute error to the measure range.
Results and discussion
Lubricating ability of nano-oil
Figure 5 depicts the friction coefficient reduction percentage of the C70/NiFe2O4 nano-oil at different concentrations using the four-ball friction tester. It is clearly seen that the friction coefficients at different concentrations were lower than those of pure oil. The friction coefficient percentage increases when the concentration of C70/NiFe2O4 is 0.5 g/L and 2 g/L, with an increase of 13.2% and 18.5% compared to pure oil. When concentration increases from 0.5g/L to 1.8g/L, the friction coefficient percentage decreases from 13.2% to 9%. The wear reduction percentage increases sharply at lower concentrations but becomes steady at higher concentrations.
Figure 6 demonstrates the repeatability experiments to ensure the reliability of data and eliminate the error. Repetition measurement experiment was selected to demonstrate the reliability at a concentration of 0.25 g/L. The average friction coefficients for this experiment data are 0.084 and 0.082 respectively, with a maximum standard deviation of only 0.002.
According to the results, nano-oil can reduce the friction coefficient and the concentration can have a great influence on the result. The reason for this is probably that the C70/NiFe2O4 which is dispersed uniformly in lubricating oil plays a role in filling with the concave convex friction surface, which reduces the direct contact of the friction pair. This indicates that the C70/NiFe2O4 added can reduce the friction coefficient of lubricant oil in a certain range and can shorten the time needed.
Based on the above results, the nano-oil with the lowest friction coefficient is chosen which is C70/NiFe2O4 with a concentration of 2 g per pound and is labeled as new 1# and new 2#. Figure 7 shows the result of the temperature test. For comparison, the temperature was reduced in the nano-oil tested. The digital display instruments could measure the change of temperature when the oil frictional property test was started and the measurement lasted for 1 min. The results show that the new 1# and new 2# nano-oil in the four-ball friction conditions have a strong cooling effect.
Application in rolling piston type rotary compressor
The compressor performance test was conducted by Zhuhai Lingda Compressor Co., Ltd. The refrigerating capacity of a prototype compressor was measured, when secondary refrigerant flowed through the calorimeter. The compressor was measured before and after the performance test to ensure the weight changed within 10 g. First the base oil was added into the compressor to test. Then the nano-oil was tested in the compressor to estimate the effect of the introduction of nanoparticles on the experimental environment.
Table 3 presents the results of the refrigerating capacity test of the compressor. From the results of pure oil and nano-oil, it can be seen that the testing performance parameters fluctuate in the same type of compressor. The input power is reduced by 0.61% for the new 1# oil when using R22 as the working fluid. However, the input power is reduced by 0.31% for the new 2# oil. The operation temperature of the compressor is very high, which indicates that the concentration of C70/NiFe
2O
4 has a little influence on the viscosity [
10]. However, nanoparticles, as additive, are dispersed into the friction surface, which can increase the bearing capacity of oil film and reduce the direct contact with the metal surface. Besides, the refrigerating capacity is reduced by 0.29% for the new 1# oil, while the refrigerating capacity is increased by 0.94% for the new 2# oil. The refrigerating capacity increases due to the reduction of wear of the compressor and the increase of volumetric efficiency. However, the refrigerating capacity of the new 1# nano-oil is reduced. The reason for this is probably that the concentration of nano materials is too low. In the process of circulation, the nano-oil penetrates through the oil separator, and mixes with the refrigerant and is finally diluted, which results in the decrease in the lubricating performance of the nano-oil. The coefficients of performance were calculated by refrigeration capacity and input power. Compared to the original compressors, the COP are increased by 0.32% and 1.23% for the new 1# and the new 2# oil. This indicates that the refrigeration oil with C70/NiFe
2O
4 as additive can improve the performance of the domestic refrigerator compressor.
Conclusions
In this work, Fullerenes (C70) and NiFe2O4 nanocomposites as additive were dispersed in refrigeration oil. The tribological properties of the nano-oil and the performance of rolling piston type rotary compressor were tested. It is concluded that compared with pure oil, the friction coefficients of nano-oils significantly decrease. The concentration has an influence on the friction performance. Less frictional heating is produced during the compressor working process when using the nano-oil with C70/NiFe2O4 as additive. The rolling piston type rotary compressor when using R22 and C70/NiFe2O4 nano-oil has better performances. The COP is increased by 0.32% and 1.23% for the new 1# and the new 2# oil, respectively.
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