Interstage Matching And Coupling Characteristics In A Two-Stage Compression Heat Pump System With Variable Capacity

Two-stage compression technology with intermediate injection characteristics has been justified to improve the vapor compression device’s flexibility of running under lower temperatures. When this technology is used in air source heat pump (ASHP) system, in order to satisfy various working conditions, volume capacity ratio of the two compressors and their coupled characters will change. However, there are few researches on the characters of these changes. As a result, the control of a two-stage ASHP compression system in practice is always found to be unstable and compressor failure of motor burnout can be found sometimes. With the aim of improving the two-stage compression technology and enhancing its practical applicability, this dissertation focuses on the volume capacity ratio, dynamic coupling relationship of the two compressors, and injection factors that influences the system performance, in a two-stage compression system with varying conditions.Firstly, based on theoretical cycle analysis, it is found that the two-stage compression cycle with one-stage throttling and intermediate incomplete cooling which uses R410A as working substance is more suitable for practical use for its good compromise among system heating performance, controllability and stability, etc. Then based on the above theoretical analysis, a measuring plant with variable capacity and adjustable injection parameters is built up. The plant uses inverter compressor and electric expansion valves, and macroscopic parameters can be measured under variable working conditions.Secondly, based on the structure of the rotary compressors, classification method of the running mode for the studied system is given. Then a dynamic coupling model of the low-and high-stage compressor that can reveal the forming process of the intermediate saturation pressure, and a model of the two-stage compression heat pump system with intermediate injection characteristics are established and experimentally verified.Thirdly, based on the research of heating performance of varying conditions, quantitative impact of adjustable injection parameters on system performance are analysised. Results show that, when the evaporation temperature is between-30-0℃, the condensation temperature is between35-40℃, and theoretical displacement ratio of the low-and high-stage compressor tε is between1.13-5.08, the system heating capacity grows with the increase of ε, and the maximum improvement can reaches0.4of COPhot. And with injection process, the system heating capacity can be increased by8%-15%, and COPhot can be increased by5%～12%, compared with the no-injection process.Fourthly, based on the above established model, combined with thermodynamic state diagram, the thermodynamic processes of the studied system, and the influences of theoretical displacement ratio of the low-and high-stage compressor ε and injection factors on system performance are analysed. It is pointed that the studied system system has the characters of two compressors’asynchronism, exhaust intermission of the discharge process, discontinuousness of the compression process, and the fluctuation feature of thermodynamic parameters in the intermediate control volume. The injection quantity and specific enthalpy is a pair of coupled injection parameters with heat exchange in the intermediate heat exchanger as the constraint, which has a big influence on the formation of the thermodynamic parameters of the intermediate control volume which are formed by the dynamic coupling compress process. The injection specific enthalpy is qualitative factor that influences the quality of the injection process, while injection quantity is the quantitative factor. The intermediate pressure increases accordingly when the injection mass increases. And the intermediate mixing process is firstly a stable "constant pressure" process, then changes to a new stable "constant pressure" process via a mass varied "constant specific volume" transition process.Finally, based on the simulation and experimental results, the variations of the system performance parameters under variable conditions are quantitatively analyzed. It is advanced that the heating capacity of the system is very sensitive to the variation of the intermediate pressure and the COP is insensitive to that. And then the controllable running parameters are optimized based on the principle that the heating capacity, heating coefficient of performance, or the exergetic efficiency is maximum. The results show that the intermediate pressure ratio characteristic parameter between1.1and1.3, theoretical displacement ratio of the low-and high-stage compressor between1.2-3.2, the relative injection mass value between0.1and0.3, and the superheat of the high-stage compressor between10℃-20℃, are suitable for the studied system when the evaporation temperature is between-30-0℃and the condensation temperature is40℃.