Effects of Fin Length on Frosting and Defrosting Characteristics of Small-Diameter Copper Tube-Fin Heat Exchangers

Frost buildup on copper tube-fin heat exchangers reduces their performance in cold, humid conditions. Fin length plays a key role in balancing heat transfer and frost resistance. This study experimentally examines how fin length affects thermal and frosting behavior. Four heat exchangers with fin lengths of 15.1 mm, 18.53 mm, 20.3 mm, and 23.5 mm were tested at 2 °C/1 °C dry-bulb/wet-bulb air temperature and −6 °C coolant temperature under constant static pressure. Results show that longer fins increase total heat transfer—peak capacity rose from 512 W to 566 W—but reduce heat transfer per unit area by about 30%. Operating time before defrosting increased by 30.6%, from 45.7 min to 59.6 min, due to lower frost density. Total frost mass grew, but unit-area frost decreased by 12.7%. During defrosting, longer fins achieved greater absolute airflow recovery (from 195 to 213 m3h−1), though defrosting efficiency per gram of frost declined. Short fins (15 mm) suit space-limited systems needing high surface efficiency. Long fins (23 mm) benefit large systems requiring long run times and strong post-defrost performance. Medium lengths (17 mm to 20 mm) offer a practical balance for general use. These findings support better heat exchanger design in frost-prone applications.