HVAC Air Handler Units and Coils

Question 1

What is the primary function of an air handler unit (AHU) in an HVAC system?

Accepted Answer

Condition and circulate air throughout the building

Answer

An air handler unit (AHU) conditions and circulates air. It typically contains a blower, heating/cooling coils, filters, dampers, and controls to deliver conditioned air to occupied spaces.

Question 2

What causes 'coil freeze-up' on an evaporator coil?

Accepted Answer

Insufficient airflow or low refrigerant charge causing suction pressure to drop below freezing

Answer

Coil freeze-up occurs when suction pressure drops low enough that coil surface temperature falls below 32°F. Causes include low airflow (dirty filter, failed blower) or low refrigerant charge, both of which reduce suction pressure.

Question 3

What is the recommended face velocity for evaporator coils in air handling units?

Accepted Answer

300-500 FPM

Answer

The recommended face velocity for evaporator coils is typically 300-500 FPM (feet per minute). Higher velocities can cause moisture carryover; lower velocities reduce efficiency and increase coil size requirements.

Question 4

What type of coil is used in an air handler for the heating mode in a heat pump system?

Accepted Answer

The same evaporator coil, which acts as a condenser during heating mode

Answer

In a heat pump system, the indoor coil serves dual purposes — it acts as the evaporator in cooling mode and as the condenser in heating mode. The refrigerant flow is reversed by the reversing valve.

Question 5

What does the 'fin spacing' on a coil affect?

Accepted Answer

Heat transfer surface area and airflow resistance

Answer

Fin spacing (fins per inch) affects the coil's heat transfer surface area and air-side pressure drop. Tighter fin spacing increases surface area for better heat transfer but also increases airflow resistance.

HVAC - Heating, Ventilation, and Air Conditioning Practice Test

HVAC - Heating, Ventilation, and Air Conditioning Practice Test

HVAC Air Handler Units and Coils