Small-diameter heat exchangers for air conditioning can reduce cost and refrigerant charge, which is of great significance for realizing energy savings and emission reduction. In this study, the plate fins of a 5 mm single-row air-conditioning condenser are optimized to replace a 7 mm single-row air-conditioning condenser. The optimization variables are tube spacing, fin width, and fin spacing, and the objectives are to maximize heat transfer and minimize air-side pressure drop and cost. The Taguchi method was selected for the experimental design and single-objective optimization, and the grey relational analysis and comprehensive performance index method were used for multi-objective optimization. The final optimum tube spacing, fin width, and fin spacing were 16.0 mm, 16 mm, and 1.4 mm, respectively. The heat transfer was the same as that of the 7 mm tube condenser. The pressure drop was reduced by 35%, and the cost was reduced by 26%, validating the effective replacement of the 7 mm condenser.

After optimizing the compressor design, condenser tube diameter, and tube row arrangement, air supply volumetric flow rate, and refrigerant charge of the domestic dehumidifier. The optimized design schemes were obtained from the original combinations by non-dominated sorting, and the optimized design schemes were experimentally verified under three environmental conditions according to the test method of T/CAS 342-2020 to obtain a complete idea of dehumidifier multi-component improvement. The results show that the dehumidifying capacity of Scheme 5 is slightly increased by 2.5% at 27°C/60% RH, and its energy factor is significantly increased by 24.6%. When the air supply volumetric flow rate was increased from 210 to 268 m³·h⁻¹, the air supply temperature was reduced by 1.7°C, improving comfort. Under environmental conditions with higher dehumidification demand, i.e., at 35°C/90% RH, the energy factor of Scheme 4 can reach 4.3 L·kW⁻¹·h⁻¹, which is 34.4% higher than the original scheme, and the gap is further widened. The application of the 5 mm tube diameter condenser in dehumidifiers reduces the amount of refrigerant charge by 25.9%, reduces the cost by 7.5%, and strengthens the heat transfer effect and the homogeneity of the two-phase mixing, which shows the great application potential.