HVAC Commercial HVAC Systems 4 — Questions and Answers
Question 1: What is the primary purpose of an expansion tank in a closed hydronic HVAC system?
- To store extra chilled water for peak demand
- To accommodate the volume change of water as it heats and cools, maintaining safe pressure (Correct answer)
- To separate air bubbles from the water circuit
- To boost water pressure to upper floors
Correct answer: To accommodate the volume change of water as it heats and cools, maintaining safe pressure
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?
- Equipment energy consumption rating
- Control precision and stability by eliminating offset and minimizing overshoot (Correct answer)
- Safety shutdown response time
- Equipment operating noise levels
Correct answer: Control precision and stability by eliminating offset and minimizing overshoot
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?
- Cooling the compressor discharge gas below saturation temperature
- Cooling liquid refrigerant below its condensing temperature to increase system capacity and prevent flash gas (Correct answer)
- Reducing suction pressure below design conditions
- Lowering evaporator temperature for dehumidification
Correct answer: Cooling liquid refrigerant below its condensing temperature to increase system capacity and prevent flash gas
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?
- Reciprocating chillers for 10–50 tons
- Screw chillers for 100–500 tons (Correct answer)
- Centrifugal chillers for 20–100 tons
- Absorption chillers for 5–50 tons
Correct answer: Screw chillers for 100–500 tons
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?
- Condenser fan motor
- Electronic expansion valve (EEV) or thermostatic expansion valve (TXV) (Correct answer)
- Economizer damper actuator
- Supply air static pressure sensor
Correct answer: Electronic expansion valve (EEV) or thermostatic expansion valve (TXV)
The expansion valve meters refrigerant into the evaporator, maintaining the correct superheat and preventing liquid flood-back to the compressor.
Question 6: What is the function of an 'air separator' (air eliminator) in a commercial chilled water system?
- To separate outdoor air from return air in the AHU
- To remove dissolved and entrained air from the hydronic loop, preventing corrosion and noise (Correct answer)
- To divide airflow between multiple zones
- To filter particulates from the chilled water supply
Correct answer: To remove dissolved and entrained air from the hydronic loop, preventing corrosion and noise
Air separators remove dissolved air from hydronic systems, preventing air binding in coils, corrosion, and flow noise.
Question 7: A commercial building's HVAC system is described as having a 'primary-secondary' chilled water loop. What does the secondary loop do?
- Serves as a backup if the primary chiller fails
- Distributes variable flow to building loads while the primary loop maintains constant flow through chillers (Correct answer)
- Provides hot water heating during winter months
- Connects the cooling tower to the condenser water pumps
Correct answer: Distributes variable flow to building loads while the primary loop maintains constant flow through chillers
In a primary-secondary arrangement, the secondary (distribution) loop provides variable flow to loads while the primary loop maintains constant flow through chillers for stability.
HVAC Commercial HVAC Systems 4
What is the primary purpose of an expansion tank in a closed hydronic HVAC system?