Experimental investigation on oil-gas separator of air-conditioning systems

Abstract

The oil-return system plays an important role in the variable refrigerant flow (VRF) systems because it ensures the reliable operation of the VRF systems. The oil-gas separator is the most essential component of the oil-return system, and the separation efficiency of the separator directly influences the performance of the VRF systems. Therefore, in this paper, a test rig was built to measure the oil discharge ratio of the compressor and the separation efficiency of the oil-gas separator. A sound velocity transducer was used to measure the oil mass concentration instantaneously, because the sound velocity was changed with the mass ratio of oil to refrigerant. The separation efficiency of the separator could be obtained by comparing the mass fraction of oil to refrigerant before and after the separator was connected to the system.

Keywords

variable refrigerant flow system / oil-gas separator / separation efficiency

Introduction

The variable refrigerant flow (VRF) system plays an important role in the central air-conditioning field. However, the oil-return problem constitutes a major difficulty in design and development of the VRF system with long piping. The oil separator is an important component in this system, which could return the separated oil to the compressor. Oil droplets are separated by the centrifugal force caused by the tangential inlet fluid to overcome the lift force and gravity in the oil separator.

However, the oil separator could not separate all of the oil, and a certain amount of the oil still discharges from the separator with vapor refrigerant and circulates through the system. The oil circulates with the refrigerant and has a significant impact on refrigerant evaporation heat transfer performance. Previous investigations showed that the effect of oil circulation might deteriorate heat transfer at higher oil concentrations [¹]. Additionally, when the oil concentration was between 3% and 5%, the heat transfer coefficient decreased seriously by 9.3%-36.5% for the heat exchanger of 5 mm in diameter [²]. Related research results of flow pressure drop characteristic indicate that the presence of oil always increases flow frictional pressure drop [³,⁴]. Therefore, it is important to quantify oil concentration accurately in the system for designing and operating the VRF system.

There are many factors influencing the efficiency of oil separator. Wu and Wu [⁵] found that the diameter of the outlet pipe was the most significant factor influencing the drag coefficient, based on which he suggested the right diameter of the outlet pipe to get the best performance. Raoufi et al. [⁶] found that the efficiency of the separator increased with the decrease of the diameter of the cyclone vortex finder. Jiang [⁷] proposed that the larger ratio of the length to the diameter of the separator is, the better separation efficiency is to the oil-gas separator used in oil-injection screw compressor system.

The measurement method for oil concentration in the refrigerating systems has been investigated over the past years. Of various methods, the sampling method is the commonly one applied to measuring oil ratio in the practical application. Meanwhile, several other methods for monitoring the oil concentration have also been developed, including light absorption [⁸], refractive index [⁹], relative dielectric constant [¹⁰], acoustic velocity [¹¹], viscosity [¹²], thermal conductivity [¹³], and density [⁸,¹⁴], which can detect any change in oil concentration immediately by amplifying the signal of corresponding physical property with specialized equipment.

In this paper, the sound velocity transducer is proposed to be used to measure the oil concentration in a refrigerating system. Since the sound velocity of refrigerant/oil mixture is directly related to the oil concentration change, the mass ratio of the oil to the mixture could be obtained based on the correlated relationship between oil concentration and sound velocity of the mixture. The oil discharge ratio of the compressor is studied under different conditions, and the separation efficiency of the oil separator is also investigated based on the sound velocity method.

Experimental setup

The experiment setup was built to measure the oil concentration in the refrigeration system, as shown in Fig. 1. The system contains a complete refrigeration cycle composed of a compressor, a condenser, a sub-cooler, an expansion valve, and an evaporator. The evaporator is a calorimeter, which could measure the refrigerating capacity. The system could also obtain the performance of the compressor. On the base of the cycle, an online sound velocity transducer is installed after the sub-cooler in the system, which could obtain the oil fraction in the liquid refrigerant in real time.

As shown in Fig. 1, there are three channels between the compressor and the condenser. The first channel connects the compressor and condenser directly, where valve V00 is used to control it. There is a high efficient oil separator connected in the second channel, and the valve V11 is mounted before the high efficient oil separator, while valve V12 is used after it. Through this channel, the mixture of refrigerant and the oil discharged from the compressor enters the high efficient oil separator where nearly all the oil is separated from the mixture. The separated oil is first accumulated in the oil reservoir, and then returns to the compressor through valve V33. The oil level in the reservoir could be controlled and thus the oil in the system could be adjusted. The third channel contains valve V21, the objective oil separator, and valve V22. The separated oil from the objective oil separator can either enter the oil reservoir through valve V31, or return to the compressor directly through valve V32. Under the same condition, by comparing the oil concentrations between the first channel and the third channel obtained from the sound velocimeter, the separation efficiency of the objective oil separator can be calculated.

The conventional method of measuring the oil concentration in the refrigerating system is the sampling method, which is commonly considered as the standard method. The sampling method is accurate; however, it requires stricter operation of the sampling and measurement. Furthermore, it is time consuming and reduces the amount of oil in the system, which influences the reliability of the system. In addition, it is not an online measurement and, therefore, unable to provide real time measurement of the oil concentration.

Therefore, in this paper, the sound velocity method is introduced to provide real time measurement of the mass fraction of oil to refrigerant, for the sound velocity is changed with the mass ratio of oil to refrigerant.

Since the sound velocity of refrigerant/oil mixture is directly related to the change in oil concentration, the mass ratio of the oil to the mixture could be obtained based on the correlated relationship between oil concentration and sound velocity of the mixture. The mixture of refrigerant and oil flows into the tube of the sound velocimeter, and the sound signal is launched from the emitter, flows through the mixture, and then is received by the receiver. The sound velocity transducer measures the velocity of the sound in the mixture. The measured values are transmitted via a twin core cable to the evaluation unit. The evaluation unit determines the concentration for display or output and control purposes.

The sound velocity method has many features and benefits, such as high resolution, excellent repeatability, short response time, high resolution temperature measurement and low thermal inertia. Most important of all, it virtually has no influence on pressure, flow rate and viscosity.

The sound velocimeter can collect and save the data in real time. Since there are no other redundant operations, it can be used to measure the oil concentration under variable conditions, while increasing the efficiency of the experiment. The sound velocimeter used in the paper is SPRn 4214 LS, with a sound velocity range from 200 to 1600 m/s and accuracy of 0.01 m/s.

In addition, all the measurement sensors are shown in the schematic diagram of the experimental setup. The main sensors contain the pressure transducers which measure the suction and discharge pressures of the compressor, and the temperature transducers which measure the suction and discharge temperatures of the compressor, as while as the temperature of the refrigerant liquid before the expansion valve. The accuracy values of measurements and the control ranges of the sensors are listed in Table 1.

Experimental results and discussion

Validation of sound velocity method

To validate the accuracy of the sound velocity method, the oil concentration measured by the sound velocimeter was compared with that obtained by the sampling method. The comparison of the results under the same condition is illustrated in Fig. 2.

As shown in Fig. 2, the relative deviation of the measurement results between the sound velocimeter and the sampling method is from -7.72% to 10.27% with an average relative deviation of 2.63%. According to Yang et al. [⁸], the relative deviation of measurement data between the density method and the sampling method is from -7% to 10% with an average relative deviation of 4.28%, while the relative deviation of measurement data between the light absorption method and the sampling method is from -8% to 23%. Therefore, it is practicable to use the sound velocimeter to measure the oil concentration in the refrigerating systems.

Oil discharge ratio of compressor

The oil discharge ratio of the compressor was measured by the sound velocimeter to investigate the influencing factors. The oil discharge ratio of the compressor means the oil concentration when the oil separator was not connected to the system. When the valve V00 was open in Fig. 1, while all other valves V11–V33 were closed, the mixture of the refrigerant and oil discharged from the compressor directly entered the condenser, instead of going through the oil separator. Under this condition, the oil circulated through the whole system with the refrigerant, and the oil concentrations were constant in the system. Therefore, the oil concentration measured by the sound velocimeter behind the sub-cooler was also the oil discharge ratio of the compressor.

During the testing, the suction and discharge pressures of the compressor, the suction superheat degree, and the sub-cooling degree before the expansion valve were maintained at a special operating condition. The testing compressor is the scroll compressor with a displacement volume of 65 cm³ and an oil amount of 500 mL. The frequency or the rotary speed of the compressor varied from 20 r/s to 100 r/s.

The oil discharge ratio of the compressor was investigated at the common suction and discharge pressure when the VRF was at standard refrigerating. At this time, the suction pressure was 0.89 MPa, the discharge pressure was 2.79 MPa, the suction superheat degree was 11.2°C, and the sub-cooling degree before the expansion valve was 8.3°C.

The oil discharge ratios of the compressor at different rotary speeds were depicted in Fig. 3.

It could be seen from Fig. 3 that the oil discharge ratio increased with rotary speed rising, showing a linear relation. The reason for this is that when the speed of the compressor increases, more oil is pumped to the compression chamber through the oil pump. On the other hand, the oil discharged from the compression chamber with the refrigerant stays in the compressor shell for a shorter period of time. As a result, less oil can be separated into the compressor, and the efficiency of the first separation in the compressor is insufficient. Therefore, much more oil enters the system with the discharged refrigerant, leading to a high oil discharge ratio.

Figure 4 showed the oil flow rate changed with the rotary speed of the compressor. Because the oil discharge ratio was proportional to the rotary speed, and the mass flow rate of the refrigerant also had a linear relation with the speed, the oil flow rate assumed a quadratic connection with the rotary speed of the compressor.

By changing the suction and discharge pressure, while maintaining the suction superheat degree at 11.2°C with the sub-cooling degree before the expansion valve at 8.3°C, the oil discharge ratios at different pressure ratios could be obtained as given in Fig. 5.

It could be observed from Fig. 5 that when the speed of the compressor was higher, the oil discharge ratio decreased more sharply as the pressure ratio increased at first, and when the pressure ratio reached a certain value, the oil discharge ratio gradually decreased to a constant plateau. When the speed was 90 r/s and the pressure ratio increased from 2 to 4, the oil discharge ratio decreased by 24.2%, but when the pressure ratio continued to increase, from 4 to 9, the oil discharge ratio decreased only by 9.5%. The same situation occurred with the speed at 60 r/s. When the pressure ratio increased from 2 to 4, the oil discharge ratio decreased by 37.9%, but when the pressure ratio increased from 4 to 9, the oil discharge ratio decreased only by 1.7%.

When the compressor speed becoming lower, the oil discharge ratio was even decreasing and its value was almost independent of the pressure ratio. As shown in Fig. 5, when the speed was at 30 r/s, as the pressure ratio increased from 2 to 9, the oil discharge ratio only changed from 0.41% to 0.37%. The reason for this was that when the speed of the compressor was lower, less oil was pumped from the oil tank to the compression chamber by the oil pump, and the discharged mixture could stay longer in the compressor shell, and could be separated more sufficiently in the compressor. Conclusively, when the oil discharge ratio became as low as 30 r/s, the oil concentration stayed almost constant regardless of the pressure ratio increase.

Separation efficiency of oil separator

The objective oil separator was connected to the system, indicating that valves V21, V22, V23 were open. The mixture of refrigerant and oil discharged from the compressor entered the separator, and was separated. Then the refrigerant entered the condenser, while the separated oil came into the oil reservoir through valves V23 and V31. The oil level was controlled in the oil reservoir, and then the oil turned back to the compressor through valve V33. Meanwhile, the oil concentration measured by the sound velocimeter was the one after the oil separator. By comparing this oil concentration with the oil discharge ratio of the compressor, the oil separation efficiency could be obtained.

Figure 6 shows the oil concentration after the oil separator in the system under the same conditions as in Fig. 5. As shown in Fig. 6, it could be seen that at different rotary speeds and different pressure ratios, the oil concentrations after the oil separator were almost the same under different conditions, which were from 0.20% to 0.25%. As the pressure ratio increased, the oil concentration increased slightly.

The explanation for this phenomenon may be interpreted as the fact that the diameters of the oil drop in the separator are not at the same pace. Since it becomes more and more difficult to be separated when the diameter is too small, every oil separator has its limited separation efficiency. Under the above conditions, the large size oil drops could be separated completely. However, when the oil drop was small enough to reach a certain value, it could not be separated anymore. Consequently, it would enter the system with the discharged refrigerant. When the pressure ratio increased, the discharge temperature ascended. As a result, the proportion of the small diameter oil drops increased, and more oil entered the system, leading to a higher oil concentration.

Comparing Fig. 5 with Fig. 6, the separation efficiencies under different conditions could be obtained, as shown in Fig. 7. Because the oil discharge ratios of the compressor vary under different conditions, and the oil concentrations after the separator are almost constant, the oil separation efficiencies are different from each other. At the same rotary speed, the separation efficiency decreased slightly as the pressure ratio increased, while as the rotary speed increased, the oil separation efficiency increased sharply.

Conclusions

In this paper, the sound velocity method to measure the oil concentration in the refrigerating system is experimentally investigated, which can measure the oil discharge ratio of the compressor and the oil separation efficiency of the oil separator in real time. This systematic study and analysis could be concluded as follows.

1) It is practicable to use the sound velocity transducer to measure the oil concentration in refrigerating systems, and the relative deviation of the measurement results between the sound velocity method and the sampling method is located within -7.72% and 10.27%, and the average relative deviation is 2.63%.

2) The oil discharge ratio increases with rotary speed rising with a nearly linear relation, while the oil flow rate assumes a quadratic connection with the rotary speed of the compressor.

3) When the speed of the compressor is higher, the oil discharge ratio decreases more quickly as the pressure ratio increases. Yet, when the pressure ratio reaches a limit value, the oil discharge ratio gradually stops changing. When the compressor speed becomes lower, the oil discharge ratio is even decreasing, which is almost independent of the pressure ratio.

4) Every oil separator has its limited separation efficiency, so the oil concentrations after the oil separator are almost the same under different conditions. As the rotary speed increases, the oil separation efficiency increases sharply.

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