Abstract: The low ambient temperature air source heat pump air heaters with vapor-injection technique can still provide thermal comfort even though under less than -25℃ ambient temperature condition. However, it is found that the control strategy of refrigerant mass flow rate at injection path significantly affects the system stability. Consequently, to prevent the compressor liquid hummer and improve system stability, refrigerant flow characteristic of the electronic expansion valve (EEV) was improved with an approximate linear relationship, meanwhile, the corresponding EEV control strategy was optimized. The experimental results shows that for the systems with related heating capacities of 3 kW and 4 kW, the compressor liquid hummer was prevented by optimizing the EEV flow characteristic and control strategy. Additionally, the COP deviation for various systems was within ±2% under both -12℃ and -20℃ ambient temperature conditions. Generally, applying the proposed optimization method is a promising way for enhancing the system stability.

Key words: Vapor injection, System's stability, Electronic expansion valve (EEV), Control strategy