NATE Gas Heating Practice Exam

How does a heated heat exchanger transfer heat to the air?

1. Conduction

2. Natural convection

3. Radiation

4. Forced convection

The correct answer is: Forced convection

A heated heat exchanger transfers heat to the air primarily through forced convection. In a heat exchanger system, fans or blowers are used to move air across the heated surfaces of the exchanger. This movement of air helps to increase the rate of heat transfer, as the forced airflow enhances the contact between the air and the heated surfaces. When the air passes over the hot surfaces of the heat exchanger, heat from the metal of the exchanger is transferred to the air through conduction—this occurs at the interface where the molecules of the air come into direct contact with the molecules of the exchanger’s material. However, since the air is being actively moved by a fan or blower, the heat transfer process significantly relies on forced convection. This process allows for a more effective and rapid transfer of heat compared to natural convection, which would involve the natural movement of air due to density differences caused by temperature variations. In summary, while conduction does initially occur at the heat exchanger's surface, the dominant mechanism by which heat is effectively distributed to the air is through forced convection due to the active movement of air facilitated by mechanical means.