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Paper ID: 134, Page 5 𝜂 𝑄⁄𝑊 𝑄 (4) where 𝑊 is the compression work. CO2 Compressor The mass flow rate of a single semi-hermetic reciprocating compressor can be defined as (Wang et al. 2021): 𝑚 (5) where 𝜂 is the volumetric efficiency of the compressor, 𝑣 is the specific volume of CO2 at the compressor suction state, which depends on the suction gas superheat Δ𝑇 and the evaporation temperature 𝑇 , and 𝑉 is the displacement rate of the compressor. In terms of compressor performance, the compression ratio is defined as the discharge pressure (𝑃) over the suction pressure (𝑃, that is: In this study, the volumetric efficiency for a semi-hermetic compressor manufactured from Bitzer 𝑟 (6) can be estimated with the function of compression ratio (Bitzer 2019), of which correlation is: 𝜂 1.17850.1755𝑟0.0152𝑟 (7) 𝑅 0.9999 The isentropic efficiency of a Bitzer compressor can be estimated with real performance data. Hence, when the compression ratio is below 3, the correlation is: 𝜂 161.48 413.74𝑟 438.47𝑟 247.09𝑟 78.081𝑟 13.117𝑟 0.915𝑟 (8) 𝑅 0.9952 When the compression ratio is over 3, then the correlation is: 𝜂 0.192 0.6443𝑟 0.2813𝑟 0.0517𝑟 0.0035𝑟 (9) 𝑅 0.9944 Therefore, the energy required for compression work can be calculated from: 𝑊 , (10) where h, is a specific enthalpy at the discharge state during an isentropic compression process and h is the specific enthalpy at the compressor suction state. Then, the enthalpy at the discharge state post-compression can be calculated from: h h (11) For a single compressor, the discharge temperature 𝑇 can be worked out once the enthalpy and pressure at the discharge state have been known. The equations with constants for calculating the CO2 property has been derived by Huang et al. and simplified by Span and Wagner (Huang et al. 1985), (Span & Wagner 1996). 3 PRELIMINARY VALIDATION The mathematical model of a semi-hermetic reciprocating compressor has been validated against the experimental data recorded from an existing installation, as shown in Appendix. The system operated with the arrangement of parallel compression, in which compressor 1 worked as a main compressor and compressor 2 operated as an auxiliary compressor. The technical data for the compressor (Type: 4HTE- 20K) used in the system at 50Hz is given as the displacement rate is 12 m3/h. Compressor 1 operated with the VSD ranging from 30 to 60 Hz, while compressor 2 operated with an FSD of 50 Hz. The power consumption for a compressor changed with conditions at the suction and discharge states. The predicted and actual power consumptions for compressor 1 with VSD and compressor 2 with FSD are displayed in Fig.8. In terms of the compressor with VSD, the results show an acceptable agreement with most errors within ±10%. But the simulation has the highest errors when the system is warming up, i.e. the actual power consumption is below 6 kW. However, for the FSD compressor, the difference between the predicted power and the actual power is within ±5%, which is more accurate 6th International Seminar on ORC Power Systems, October 11 - 13, 2021, Munich, GermanyPDF Image | NOVEL TRANSCRITICAL CO2 HIGH- TEMPERATURE HEAT PUMP
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