Introduction

The air-source heat pump has been approved to be one of the most economical ways to provide heat. So promoting the utilization of the air-source heat pump in cold regions as an environmental protective heating mode could effectively ease the air pollution, which was caused by coal heating in winter. The normal air-source heat pumps usually adopt the conventional single-stage vapor compression (CSVC) system. However, the thermodynamic performance of the CSVC system degrades drastically as the ambient temperature goes down, such as the lack of heat capacity, low coefficient of performance (COPh), and unreliability. This is the main barrier for the wide application of the CSVC system in cold regions. Aimed at the above problem, considerable research has been proposed to enhance the heating performance, such as refrigerant injection [1, 2], additional heat source [3], cascade system [4], applying electronic expansion valve [5], inverter technology [6], optimizing existing components [7], mixed-refrigerants [8], and nanofluids [9]. In the past decade, the applications of refrigerant injection technology attracted more and more attentions and became an effective way to improve the heat pump performance and keep the compressor working steadily.

The flash tank vapor injection (FTVIC) system was applied to air conditioners at 1979 [2, 10]. Moreover, due to that the compression and discharge processes of scroll compressor are 540° of rotation, so the scroll compressors are less prone to slug [11]. This also explains why the vapor injection cycles are mostly applied to scroll compressors. Therefore, comprehensive researches on the vapor injection technology of scroll compressor were conducted by experimental comparison and thermodynamic model of vapor injection process. However, due to the addition of the vapor injection pipe in the vapor injection system, the research method is different from that of the conventional vapor compression system [12,13,14]. Ma and Zhao [15] experimentally compared the heating performances of the FTVIC system and the sub-cooler vapor injection (SCVIC) system with the scroll compressor and obtained that refrigerant injection with flash tank could improve the heating capacity and COPh by 10.5 and 4.3%, respectively, compared with those of the SCVIC system. Wang et al. [16] thermodynamically modeled an FTVIC system with scroll compressor and a newly designed hybrid economizer and concluded that the hybrid economizer could enhance the cooling capacity by 1.5 ~ 3.5% and provided the condition for the application of thermostatic expansion valve (TXV) by generating the superheated vapor. The same author [17] also gave attention to the dynamic work process of the scroll compressor [18] and proposed that the injection process could be considered as a continuous “adiabatic throttling + isobaric mixing” process [19]. Xu et al. [20] tested the FTVIC system in the transient and steady-state process and found that the electronic expansion valve coupled with the PID controller is effective to adjust the injection ratio, and the optimum heating performance could be obtained when the superheat of the injected vapor is 4 ~ 6 K. In order to explore the time-varying characteristics of the vapor injection cycles in depth, Qiao et al. [10] analyzed the dynamic behavior of a R410A FTVIC system with scroll compressor by developing a transient, lumped-parameter model and concluded that the major transient flow characteristics and heat transfer can be captured by the proposed model, which was successfully verified by the experimental data [21]. Investigations on the scroll compressor from system level and component level are more in-depth and comprehensive. The refrigerant injection technology has been proved to be a good way to enhance the system performance. However, the scroll compressor is designed to have a fixed volume ratio. As the working condition varied, the over-compression and under-compression are prone to occur for the scroll compressor. Therefore, the poor adaptability is unfavorable for applying the vapor-injected scroll compressor in residential air conditioner.

Nevertheless, the high cost-effective character of rotary compressor leads to its wide application. At present, there are two different vapor-injected rotary compressors: twin-cylinder rotary compressor and single-cylinder rotary compressor. For the twin-cylinder rotary compressor, the isentropic process is two-stage compression. Some previous researches show that the application of refrigerant injection into the twin-cylinder rotary compressor could also enhance the heating capacity and COPh effectively. Ko et al. [22] studied the impact of cylinder volume ratio and found that the heating capacity and COPh of the air-to-water heat pump with vapor injection were improved by 48 and 36%, respectively. Further, they proposed that the system performance can be greatly affected by the cylinder volume ratio. Under different frequencies and ambient temperatures, Heo et al. [23, 24] researched the variation of the heating performance with a FTVIC system by experiments and numerical simulation. The results showed that the refrigerant injection technology could enhance the system performance significantly, and the optimum cylinder volume ratio Rv is different under various design conditions. Heo et al. [25] also experimentally explored the control strategy of the SCVIC system and concluded that the optimum sub-cooler pressure ratio Rp and injection ratio Rm were varied in the range between 0.4 and 0.8 and between 0.1 and 0.3, respectively. Ma et al. [26] compared the heating performance difference between the twin-cylinder rotary compressor and the scroll compressor at ambient temperatures of 7, − 7, and − 15 °C. Under the similar heating capacity, the COPh of the twin-cylinder rotary compressor system was slightly higher than that of the scroll compressor system.

Although the above research shows that the twin-rotary vapor-injected compressor has some significant merits, it has two separate cylinders and an intermediate mixing chamber. The compressions and injection processes of conventional twin-rotary compressor are independent due to the existence of the intermediate mixture chamber. Compared with the single-cylinder rotary compressor, the twin-rotary compressor is more expensive and has more easily worn parts, which are not conductive to improve the cost-effective for the minitype residential air conditioners.

Therefore, in order to further reduce the cost of the compressor for low ambient temperature heating, applying the refrigerant injection technology in single-cylinder rotary compressor attracts the interest of researchers recently. The cylinder injection and blade injection are the two main types of vapor injection technology used in single-cylinder rotary compressor. For the cylinder injection, Jia et al. [27] investigated the heating performance of a R410A FTVIC system by experiments. The injection pressure was fixed at the optimum value for different systems. The test results indicated that the heating capacity Qh and COPh were improved by 2.29 and 1.94%, respectively, compared with those of the twin-cylinder vapor-injected rotary compressor system when the ambient temperature was above − 15 °C. Their further research found that the cylinder injection could enhance the heating capacity by 12% averagely over that of the CSVC system when the ambient temperature was higher than − 15 °C [28]. For the blade injection, Wang et al. [29], Liu et al. [30], and Wang et al. [31] investigated the performance of the air-source heat pump by simulation based on a verified numerical model. The results under different ambient temperatures showed that the blade injection could overcome the back-flow of the cylinder injection and enhance heating capacity and COPh of the air-source heat pump by 23.1–28.2 and 4.5–8.1%, respectively, compared with the CSVC system with a regular single-stage rotary compressor.

The aforementioned studies indicated the single-cylinder rotary compressor with different injection structure could enhance the heating performance significantly like the vapor-injected scroll compressor/two-cylinder rotary compressor under low ambient temperature. The existing research mainly focused on the variations of heating capacity and COPh with ambient temperature. There are few researches on the variations of heating performance with injection pressure and compressor frequency for a single-cylinder vapor-injected rotary compressor system at present.

To evaluate the effects of injection pressure and compressor frequency on the system performance, an R410A FTVIC system with single-cylinder vapor-injected rotary compressor was built in this study. The heating performance of the R410A SCRCVI system and the CSVC system were measured and analyzed by comparison. The experimental results will be evaluated for use in the future optimizing of the injection structure and the control strategy.

The single-cylinder vapor-injected rotary compressor and flash tank

For the single-cylinder vapor-injected rotary compressor, the injection hole and reed valve were installed on the cylinder wall. The schematic diagram of the rotary compressor is stated in Fig. 1. It includes four working processes in each rotation period: the suction, the refrigerant injection, the compression, and the discharge. The refrigerant is injected into the working chamber as the meshing line of the piston rotating over the suction port. When the pressure of working chamber is larger than the injection pressure, the reed valve, settled on the injection hole, will be closed with the proposed of stopping the vapor reflux. Then the refrigerant vapor would be further compressed to the discharge pressure, and the rotary completes the compression and discharge processes. Due to the cylinder injection structure, it should be noted that as the piston rotates from the injection hole to the suction port, the injected gas will flow into the suction tube, which will decrease the volume efficiency of the compressor at the suction process.

Fig. 1
figure 1

Schematic diagram of rotary compressor

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Figure 2 shows schematic diagram of the newly designed flash tank used in the test-bed. There were three spiral flow channels in the novel flash tank. When two-phase refrigerant flowed through these channels, the centrifugal force was added to enhance the gas–liquid separation efficiency. The refrigerant from the upper-stage electronic expansion valve enters the flash tank from the two-phase inlet. Then the two-phase refrigerant was separated the two-phase refrigerant into vapor and liquid phases by centrifugal force and gravity. The separated vapor refrigerant was injected into the compressor from the injection hole, while the saturated liquid refrigerant flowed out of the flash tank from the port at the bottom.

Fig. 2
figure 2

Schematic diagram of flash tank

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Experimental setup and test procedure

Experimental setup

A test-bed of the air-source heat pump with flash tank was built. The refrigerant injection technique coupled with the inverter rotary compressor was applied in the system under the testing condition. Figure 3 illustrated the schematic diagram of the R410A SCRCVI test-bed. The thermodynamic P–h diagram of the R410A SCRCVI system is represented in Fig. 4. The experimental setup was initially equipped with a four-way reversing valve, a single-cylinder rotary compressor, a flash tank, a ball valve, a upper-stage electronic expansion valve (EEV1), the indoor–outdoor heat exchanger, and a lower-stage electronic expansion valve (EEV2). The indoor heat exchanger and outdoor heat exchanger were mounted separately in two closed environment chambers. The single-cylinder rotary compressor was driven by a DC inverter. The upper-stage EEV1 was equipped at the inlet of the flash tank to control the injection pressure and the injection mass flow rate. The function of lower-stage EEV2 was to control the superheated degree of the suction vapor in the range between 3 and 5 K. The working condition of the system could be changed by shutting off the ball valve installed on the injection pipe. It meant that the SCRCVI system was turned into the CSVC system when the ball valve was shut off. As the compressor frequency was fixed at certain value, the injection pressure could be changed by adjusting the opening of the EEV1. In the meantime, the EEV2 was automatically adjusted depending on the superheated degree of the suction vapor. The specifications of the main components are illustrated in Table 1.

Fig. 3
figure 3

Schematic diagram of the experimental setup

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Fig. 4
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Pressure–enthalpy diagram

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Table 1 Specification of system components
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Test condition

In the present study, only the heating model without frosting condition was considered. The experimental condition is given in Table 2. This setup used R410A as the working fluid. The heating performance of the SCRCVI system was comprehensively investigated by varying the ambient temperature Tod, compressor frequency f, and injection pressure Pinj. The corresponding comparison heating performance test of CSVC system was conducted under different compressor frequency at the same experiment condition mentioned above.

Table 2 Experiment conditions
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Data analysis method

The system-heating COPh, air-side capacity, refrigerant-side capacity, and refrigerant-side mass flow rate are calculated from Eqs. (1)–(5). The power consumption W was acquired by a power meter.

Heating capacity

$${Q_{\text{h}}} = {\dot m_{\text{air}}} \times ({h_{\text{out}}} - {h_{\text{in}}})$$
(1)

Coefficient of performance

$${\text{CO}}{{\text{P}}_{\text{h}}} = \frac{{{Q_{\text{h}}}}}{W}$$
(2)

Injection mass flow rate

$${\dot m_{\text{inj}}} = {\dot m_{\text{to}}} - {\dot m_{\text{eva}}}$$
(3)

The injection mass flow ratio

$${R_{\text{m}}} = \frac{{{{\dot m}_{\text{inj}}}}}{{{{\dot m}_{\text{to}}}}}$$
(4)

The injection pressure ratio

$${R_{\text{p}}} = \frac{{{P_{\text{inj}}} - {P_{\text{s}}}}}{{{P_{\text{dis}}} - {P_{\text{s}}}}}$$
(5)

The calibration of all the measuring instruments was conducted before testing. The measurement uncertainties for the key system performance parameters are given in Table 3. The theory of propagation of errors including systematic and random errors was applied to analyze the accuracy of the test data. With the purpose of reducing random error, five measurement values under setting condition would be collected in the interval of 3 min, as the system state parameters of air-side and refrigerant-side kept steady for about 10 min.

Table 3 Instrumentation and propagated uncertainties
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Results and discussion

The researches on the heating performances of the SCRCVI system were carried out by varying the compressor frequency f and injection pressure Pinj. The variations on the heating capacity Qh of the SCRCVI system and the CSVC system are shown in Fig. 5. The experimental results demonstrated that the Qh enhanced by refrigerant injection increased first and then decreased slightly according to the Pinj. In comparison with the CSVC system, the refrigerant injection improved the heating capacity significantly. An optimum injection pressure corresponding to the maximum Qh could be obtained from the variation regularity in the testing condition. The trend in heating capacity Qh could be explained by the variations of the system parameters. As the Pinj increased, the increase in refrigerant mass flow rate injected into the working chamber led to the increase in the total mass flow rate and also enhanced the inter-cooling effect in the cylinder. As a result, the discharge temperature/pressure of the SCRCVI system decreased, which caused the enthalpy in the condenser to decrease slowly. However, the growth rate of total mass flow rate was higher than the reducing rate of enthalpy drop in the condenser as the injection pressure increased, which gave great contribution to the increase in the Qh in the low-pressure range. When the Pinj was above the optimal pressure, the Qh of the SCRCVI system decreased slightly due to the reduction in enthalpy drop in the condenser playing the dominate role on the fluctuation of the SCRCVI system-heating capacity. The increasing trend of heating capacity according to the compressor frequency can be explained by the increase in the total mass flow rate and rotational speed. Under the testing condition, the Qh of the SCRCVI system improved about 9.1 ~ 29.5% over that of the CSVC system, and the average improvement was more than 21%. The optimum injection pressure corresponding to the maximum heating capacity was about 800 ± 30 kPa when the compressor frequency was changed in the range between 50 and 70 Hz. The maximum Qh of the SCRCVI system at f = 90 Hz was 75.8% higher than the maximum value at f = 50 Hz. Therefore, for the cold regions, using inverter technology to drive the single-cylinder rotary compressor with refrigerant injection was an effective way to improve the heating capacity.

Fig. 5
figure 5

Variation of heating capacity with injection pressure

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Figure 6 shows the variation of power consumption with injection pressure at the ambient temperature Tod = − 10 °C. From experimental results, it could be observed that the power consumption W of the SCRCVI system was higher than that of the CSVC system. In addition, the power consumption W of the SCRCVI system obviously rise according to the compressor frequency and slightly increased according to the injection pressure. For the compressor frequency from 50 to 90 Hz, the power consumption W of the SCRCVI system was 6.6 ~ 24.7% higher than that of the CSVC system. The power consumption W of the SCRCVI system at f = 90 Hz increased about 94.6% (686 W) over that at f = 50 Hz. And yet, when the injection pressure increased from 750 to 1050 kPa, the power consumption W at the compressor frequency f = 80 Hz was just improved by 13.6% (160.9 W). The main reasons for this tendency were that: (1) as the injection pressure increased, the discharge temperature and pressure decreased, the total and injection mass flow rate both increased, and the specific enthalpy of suction vapor kept constant. As a result, the increase in total mass flow rate had an important influence on the variation of the power consumption, so the shaft power of the system compressor increased slowly according to the injection pressure; (2) as the compressor frequency increasing, the rotational speed rise. Under the same injection pressure and suction pressure, the displacement volume was significantly improved with the rise of rotational speed, which finally led to the shaft power increasing obviously.

Fig. 6
figure 6

Variation of power consumption with injection pressure

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Figure 7 shows the variation of COPh with the change of the injection pressure at the compressor frequencies f = 50 ~ 90 Hz. As the injection pressure increasing, the COPh of the SCRCVI system increased first and then decreased. Corresponding to the variation tendency of the Qh and W for the SCRCVI system, the COPh peaked at the maximum Qh. In addition, the maximum COPh of the SCRCVI system decreased with increase in f, because the compressor frequency f had an important influence on the variation of the power consumption W. Compared with the CSVC systems, the maximum improvement in COPh under the compressor frequency of 50~90 Hz was in the range between 5.35 and 7.89%. When the SCRCVI system worked in optimum injection pressure, the system COPh was higher than 2.42. Moreover, it should be noted that at the high compressor frequencies, the increase in the SCRCVI system shaft power gave great contribution to improving the system Qh, and the effect of refrigerant injection becomes weaker. Therefore, the maximum COPh at the optimal Pinj decreased according to the f.

Fig. 7
figure 7

Variation of COPh with injection pressure

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The performances of the SCRCVI system were investigated in depth at different ambient temperatures, when the injection pressure was close to the optimum value, as well as the compressor frequency f was fixed at 80 Hz. All the tests were measured in the same psychrometer testing room. The variation of heating capacity with the change of ambient temperature is shown in Fig. 8. Both the heating capacities of the SCRCVI system and the CSVC system increased linearly with the ambient temperature at the compressor frequency f = 80 Hz. As the ambient temperature increases, the heating capacity difference between the SCRCVI system and the CSVC system remains nearly constant. In other words, the absolute increment of the system-heating capacity by applying vapor injection had little relationship with the ambient temperature, when the indoor dry-bulb temperature was set at 20 °C. Compared with the CSVC system, the vapor injection technology could enhance the heating capacity by 671 W averagely. However, the relative growth of the heating capacity increased with the decline in the ambient temperature due to the significant reduction in the heating capacity at low temperature. The maximum improvement was more than 42%, which could be obtained at Tod = − 15 °C. This also reaffirmed that the vapor injection technology was an effective way to ease the dramatic degradation of the system-heating performance in cold regions. The trend in variation of the heating capacity with the ambient temperature could be explained by the increase in suction vapor-specific volume and enthalpy difference in the condenser. As the ambient temperature decreases, the evaporation pressure decreased, the condensing pressure increased, the injection pressure decreased, and the discharge temperature of rotary compressor increased. As a result, the decline in the total mass flow rate played the dominant role on the system performance, which led to the decrease in the heating capacity eventually.

Fig. 8
figure 8

Variation of heating capacity with ambient temperature

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Figure 9 depicts the variation of power consumption with ambient temperature at the compressor frequency f = 80 Hz. Compared with the CSVC system, the power consumption W of SCRCVI system was larger when working in the same operating condition. As the ambient temperature increased from − 15 to 7 °C, the system power consumption W increased gradually. For the SCRCVI system, the average absolute improvement of the system W was about 190 W, and the relative growth rate increased from 13.4% at Tod = − 10 °C to 20.3% at Tod = 7 °C. The system W could be enhanced 16.8% averagely by applying the vapor injection technology to the single-cylinder rotary compressor. The changing trend of the W could be explained by the variation of the total mass flow rate. Since the suction vapor-specific volume decreased with the increase in the ambient temperature while the ∆(Pinj−Ps) stayed the same, the effect of vapor injection on the total mass rate became weaker with the increase in the ambient temperature.

Fig. 9
figure 9

Variation of power consumption with ambient temperature

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Figure 10 shows the variation of COPh with the change of the ambient temperature at the compressor frequency f = 80 Hz. As can be seen from the comparison of these figures, the SCRCVI system could exhibit higher COPh than the CSVC system at lower ambient temperature. However, the COPh improvement of the SCRCVI system over the CSVC system decreased with the ambient temperature. Under the operating condition n, the vapor injection technology could enhance the heating COPh about − 2.85 ~ 19.82%. As the ambient temperature increased, the system COPh declined gradually because the increase in the SCRCVI system shaft power gave great contribution to improving the system Qh, and the CSVC system was more sensitive to the ambient temperature than the SCRCVI system. Additionally, the effect of vapor injection became weaker and even harmful to the safe operation at higher ambient temperature, while the CSVC system showed superior heating performance (COPh) under this condition. Therefore, it was useful to improve energy efficiency at higher ambient temperature by switching the SCRCVI system to the CSVC system, if the CSVC system-heating capacity was sufficient for the heat load of a building.

Fig.10
figure 10

Variation of COPh with ambient temperature

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Figure 11 displays the variation of total mass flow rate for the SCRCVI system and CSVC system according to ambient temperature at the compressor frequency f = 80 Hz. Both the total mass flow rates of the SCRCVI system and CSVC system steadily increased with ambient temperature Tod. Compared with the CSVC system, the total mass flow rate improvement of the SCRCVI system was gradually reduced from 39.64% at Tod = − 15 °C to 16.23% at Tod = 7 °C as the ambient temperature increased, and the average total mass flow rate improved about 23.88% under the given condition. The average growth rates of the total mass flow rate for the SCRCVI system and CSVC system was 0.45 and 0.49 g s−1 K−1, relative to the ambient temperature, respectively. The average total mass flow rate of the SCRCVI system was about 2.9 g·s−1 higher than that of the CSVC system under various ambient temperatures. The reason for the trend in variation of the total mass flow rate was the different change rate between the two systems with the ambient temperature. Therefore, the decline of suction vapor-specific volume led to the increase in injection mass flow ratio Rm and the improvement of heating performance at lower ambient temperature eventually.

Fig. 11
figure 11

Variation of total mass flow rate with ambient temperature

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Figure 12 shows the variation of injection mass flow ratio Rm and injection pressure ratio Rp of the SCRCVI system according to the ambient temperature at the optimum injection pressure. The trend in variation of Rm was almost synchronized with that of the injection pressure ratio Rp. For the ambient temperature Tod from − 15 to 7 °C, the Rm and Rp were varied in the range between 0.195 and 0.219 and between 0.2 and 0.22, and the average value of the Rm and Rp was about 0.207 and 0.206, respectively. This trend could be explained as follows: The injection mass flow rate was mainly determined by the injection pressure, while the total mass flow rate was mainly determined by the ambient temperature. With increase in the ambient temperature, the optimum injection pressure and the suction pressure increased gradually, but the specific volume of saturated vapor decreased. As a result, the improvement of the injection mass flow rate synchronized with the improvement of the total mass flow rate. It meant that the Rm and Rp remained approximately constant under the given testing condition. Therefore, the Rp could be adopted as the signal to control the opening of the upper-stage electronic expansion valve EEV1 of the SCRCVI system in order to achieve the high heating performance and safe operation.

Fig. 12
figure 12

Variation of Rm and Rp with ambient temperature

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Conclusions

The heating performances of the R410A flash tank refrigerant injection system with a single-cylinder rotary compressor were studied by changing the compressor frequency f and the injection pressure Pinj under various ambient temperatures. A comprehensive performance comparison between the SCRCVI system and CSVC system was conducted to explore the optimum control strategy. The Qh and COPh of the SCRCVI system at the ambient temperature Tod = − 10 °C could be improved by 9.1 ~ 29.5% and 5.35 ~ 7.89% over that of the CSVC system, respectively. The peak value of Qh and COPh could be obtained at the optimum injection pressure. When the SCRCVI system worked at the optimum injection pressure, the trend in variation of heating capacity Qh, power consumption W, heating COPh, total mass flow rate, injection mass flow ratio Rm, and injection pressure ratio Rp with the ambient temperature were investigated in depth. For the ambient temperature from − 15 to 7 °C, the heating COPh improvement of the SCRCVI system over the CSVC system was 19.82 ~ − 2.85% because the Qh grow rate was lower than the W grow rate. The test results indicated that the SCRCVI could enhance the heating performance effectively at lower ambient temperature, and the CSVC system would exhibit higher COPh while the ambient temperature was beyond the critical value. The Rm and Rp under the given testing condition were varied in the range between 0.195 and 0.219 and between 0.2 and 0.22, respectively. The Rm and Rp fluctuated in the narrow range and remained approximately constant under different operating conditions. Therefore, the Rp could be adopted as the control parameter to adjust the Rm in order to achieve the high heating performance and safe operation.