HVAC Commercial HVAC Systems 4

Question 1

What is the primary purpose of an expansion tank in a closed hydronic HVAC system?

Accepted Answer

To accommodate the volume change of water as it heats and cools, maintaining safe pressure

Answer

An expansion tank absorbs the thermal expansion of water in a closed loop, preventing dangerous overpressure conditions.

Question 2

In commercial HVAC controls, what does 'proportional-integral-derivative (PID) control' primarily improve over simple on/off control?

Accepted Answer

Control precision and stability by eliminating offset and minimizing overshoot

Answer

PID control continuously adjusts outputs based on error, accumulated error, and rate of change, producing stable, accurate control without the cycling of on/off control.

Question 3

What is 'subcooling' in a commercial refrigeration circuit and why is it beneficial?

Accepted Answer

Cooling liquid refrigerant below its condensing temperature to increase system capacity and prevent flash gas

Answer

Subcooling liquid refrigerant prevents flash gas formation at the expansion device, increasing refrigerating effect and system capacity.

Question 4

Which type of commercial chiller uses a screw compressor and is typically best suited for which capacity range?

Accepted Answer

Screw chillers for 100–500 tons

Answer

Screw (helical rotary) chillers are most efficient in the 100–500 ton range, filling the gap between reciprocating and centrifugal equipment.

Question 5

In a commercial rooftop unit, what component is responsible for controlling the refrigerant flow rate to the evaporator coil?

Accepted Answer

Electronic expansion valve (EEV) or thermostatic expansion valve (TXV)

Answer

The expansion valve meters refrigerant into the evaporator, maintaining the correct superheat and preventing liquid flood-back to the compressor.

HVAC Practice Test

HVAC Practice Test

HVAC Commercial HVAC Systems 4