In testing the efficiency of an air-cooled condenser, the refrigerant condensing temperature should be no more than approximately how many degrees higher than the leaving air temperature?

• A. 10°F
• B. 20°F
• C. 40°F
• D. 30°F

Answer: (D)

In testing an air-cooled condenser, the key idea is how close the refrigerant’s condensing temperature is to the temperature of the air leaving the condenser. This difference, called the approach temperature, reflects how effectively heat is being transferred from the refrigerant to the air. The smaller the gap, the more efficient the heat transfer tends to be, because the air is removing heat with a sufficient temperature driving force.

Practically, a typical target for this approach in air-cooled condensers is about 30°F. So the refrigerant condensing temperature should be no more than roughly 30°F higher than the leaving air temperature. If the gap is much larger, heat transfer is less efficient and airflow or fouling issues may be present. If the gap is smaller, it’s a tighter, more stringent condition that's harder to achieve in practice, but 30°F is the commonly used benchmark.