Refrigerant and lubricant charge in air condition heat exchangers: Experimentally validated model

• Refrigerant and oil mass distribution of an air conditioning system is measured.
• Measurements are used to develop and validate a semi-empirical model.
• Model predicts the refrigerant and oil mass well in plate-and-fin evaporator.
• Model indicates oil separation in the inlet header of microchannel condenser.

The paper presents a semi-empirical model to predict refrigerant and lubricant inventory in both microchannel condenser and plate-and-fin evaporator of an air conditioning system. Validated against experiments, where mass of refrigerant and lubricant are isolated under steady state and afterwards measured within ±1% and ±2.5% uncertainty, respectively, the model predicts refrigerant mass in both heat exchangers within 20%. As for lubricant, initial application of the model predicts mass well for evaporator but consistently under-predicts for condenser. Analysis shows that the lubricant might be separated in the condenser inlet header. Accumulated in the bottom, the lubricant-rich liquid may start to fill in the microchannel tubes. The temperature profile in the infrared image supports this hypothesis, as the temperature of the bottom channels is much lower. After modifying the model by counting in these liquid filled channels, both refrigerant and lubricant mass in the condenser can be modeled within 15% error.