Experimental Study on the Equilibrium Vapor Pressures of Mixed Weak Acid–Salt Solutions for Frost-Free Air-Source Heat Pumps

Weak acid–salt solutions have become an alternative to antifreeze in frost-free air-source heat pumps (FFASHPs) owing to their low corrosiveness and cost. Although mixed weak acid–salts are effective for further reducing the freezing points and vapor pressures of the solutions, corresponding data are currently lacking, especially for temperatures ≤273 K. In this study, the vapor pressures of mixed potassium acetate (CH₃COOK)–sodium formate (HCOONa)–water (H₂O) and sodium acetate (CH₃COONa)–potassium formate (HCOOK)–H₂O were measured and correlated. Considering the circulation of antifreeze solutions in FFASHPs, 1:1 and 1:5 mass ratios of CH₃COOK to HCOONa and CH₃COONa to HCOOK were selected, respectively. The tested solute concentrations ranged from 0 to 50 wt %, and the tested temperatures ranged from 258 to 333 K. In total, 145 vapor pressure data points, ranging from 0.1121 to 12.9726 kPa, were obtained and fitted with modified Antoine equations. The average absolute deviations between the measured and calculated values were within 2.26%. The vapor pressures of the 44 wt % CH₃COOK–HCOONa (CH₃COOK:HCOONa = 1:1) and 42 wt % CH₃COONa–HCOOK (CH₃COONa:HCOOK = 1:5) solutions were approximate to those of the 30 wt % CaCl₂ solution below 278 K, and the mixed weak acid–salt solutions could be regenerated at lower temperature conditions.