Abstract:
Refrigeration, air conditioning, and heat pumps have contributed significantly to the growth of various industrial sectors including food, thermal control of manufacturing process, building and medicine. The vapour compression refrigeration (VCR) system is still the most commonly used system due to its simplicity. However, a significant energy loss occurs during the expansion process in the throttling device. Hence, there have been proposals to modify the expansion process to one that is isentropic to recover the expansion work. This can be achieved by using alternative technologies: an ejector or an expander. In general, ejectors are simpler to build than expanders however they require a more extensive modification to the cycle. By contrast, expanders are easier to install and are especially suitable for retrofitting. This research aims to improve the energy efficiency and performance of a subcritical VCR system by using an expander. In order to achieve this objective, a unique rotary vane mechanism having four vanes made up of two intersecting bars was adopted and developed as the expander in this study and it is the first reported example of such an expander in the literature. The geometry of the stator is not circular but is instead designed so that there is always contact between the vanes and the inner wall of the stator. This differs from most traditional rotary vane machines which use centrifugal forces and/or springs to maintain contact between the vanes and the stator. This configuration reduces the vane tip forces, although the internal leakages might increase because of the additional leakage paths.
The first set of experiments was carried out on an open system using compressed air as the working fluid. The analysis of the experimental data allowed a better understanding of the expander’s performance in static and dynamic conditions. The main parameters studied were volumetric efficiency, isentropic efficiency and output power. Dynamic tests were carried out to study the effects of different grades of lubrication on the performance of the expander at various operating conditions. The maximum volumetric efficiency was 31.2% for grade 46 oil. The highest isentropic efficiency was 31.2% for grade 46 oil and it was 45.6% for grade 22 oil. Static leakage tests were carried out to investigate the leakage characteristics. There was no leakage from the prototype’s housing. The average contributions of internal leakages through the radial clearance, vane tips, rotor slot and end face gaps were 37.2%, 33.3%, 16.2% and 14.1%, respectively. It was observed that the internal leakage paths were generally independent of each other. Mathematical and numerical models of the expander were developed and validated with the experimental data. The analytical model associates a geometrical description of the prototype, thermodynamic processes, heat transfer, forces and frictions, and radial clearance leakage. The theoretical model showed the effects of internal leakages and frictions on the performance of the expander. Three-dimensional numerical simulations were carried out using computational fluid dynamics (CFD). The effects of vane tip clearance gap sizes and expander rotational speeds on the performance of the expander were studied. Finally, a static CFD model was developed to simulate the individual leakage paths in the expander, the interaction between various operating parameters and the internal leakages, and the effects on the total efficiency.
A refurbished heat pump system with R22 as the working fluid was investigated. The relationships between the expander’s rotational speed, condensing load and degree of subcooling with the output work and expander efficiency were studied. Additionally, the thermodynamic behaviours of the system fitted with the expander or with a throttle valve were compared and discussed. The maximum expander isentropic efficiency of the prototype was 41.9% at the optimum rotational speed of 750 rpm and the Coefficient of Performance (COP) was improved by 4.16% compared to that of the system with an expansion valve. This heat pump test rig had significant pressure losses which were due to the components in the circuit such as mass flow meter, sub cooler and bends in the pipeline. This resulted in the low performance of the prototype and the system. Also, the test rig did does not have the flexibility to test how different expander cycle configurations affect the performance.
To overcome the limitation on the previous R22 heat pump system, a customised R134a vapour compression test rig was constructed to further investigate the expander, its effects on a VCR cycle and how different expander arrangements affect its performance. Locating the expander immediately after the condenser gave the best performance. The expander prototype improved the COP by 6.4% compared to the system with a throttling valve. The maximum expansion isentropic efficiency was 34.9% at 500 rpm.
In summary, this work aimed to improve the energy efficiency and performance of a subcritical VCR system using a four-intersecting-vane expander. The results show that the concept can effectively improve the COP of a system without affecting the thermodynamic behaviour significantly, suggesting its suitability for retrofitting to existing systems. Various factors may affect the general performance of the expander, including the operating conditions, lubrication system and expander arrangement in the cycle. Further optimisation of the expander’s performance is possible and this should be investigated in the future.