Methods of measuring circulation composition for zeotropic binary mixture refrigerants

• Novel, cost-effective ex-situ PTD gas method to measure binary mixture composition • PTD gas method measures composition within ± 0.02 accuracy in the presence of oil • Compared three in-situ methods against ex-situ PTD gas method in a chiller system • Calibration of in-situ methods is recommended to avoid 3% discrepancy in COP • Aids in low-GWP blend systems with testing, control, diagnostics, and maintenance Zeotropic blends allow for tailoring refrigerant properties to achieve desirable global warming potential (GWP) and flammability characteristics. However, composition shift between the circulation and the nominal composition in blends, necessitate accurate measurement of the circulation composition. This study reviews existing ex-situ and in-situ methods for measuring circulation composition, and introduces a novel, cost-effective ex-situ measurement technique based on the pressure-temperature-density (PTD) gas method. This method measures binary mixture composition in the presence of oil, with an uncertainty of ±0.02 in mass fraction. It is more portable and affordable than traditional gas chromatography (GC), though with some tradeoffs in accuracy. In a chiller system using R134a and R32 blends, the PTD gas method detects a circulation composition shift of 0.03-0.06 from the nominal value. Additionally, three in-situ estimation techniques – pressure-temperature-quality (PTx), pressure-temperature-enthalpy (PTh), and pressure-temperature-density (PTD) liquid – are evaluated for measuring circulation composition without sample withdrawal. The PTx and PTh methods underestimate composition by 0.02-0.08, while the PTD liquid method overestimates by 0.08-0.12. These discrepancies cause the peak COP to shift relative to the ex-situ PTD gas measurement. The peak COP is 2-3% lower using nominal composition, PTx and PTh methods, whereas the PTD liquid method overestimates it by nearly 3%. Thus, calibration of in-situ methods is recommended to enhance accuracy of in-situ methods using an ex-situ technique. These findings may aid experimental analysis, blend system control and field diagnosis, potentially enabling precise topping-off of leaking systems without full refrigerant replacement.