HVAC System Design 4 — Questions and Answers
Question 1: When sizing a refrigerant suction line, the primary concern is avoiding excessive:
- Liquid subcooling
- Pressure drop that reduces compressor capacity (Correct answer)
- Discharge temperature rise
- Condenser fan speed
Correct answer: Pressure drop that reduces compressor capacity
Excessive suction line pressure drop lowers suction pressure at the compressor, reducing capacity and efficiency, and can cause liquid refrigerant slugging.
Question 2: What is the 'effective length' of a duct fitting used for in duct design?
- The actual measured length of the fitting
- The equivalent straight duct length that produces the same pressure drop as the fitting (Correct answer)
- The length of duct upstream of the fitting
- The distance between supply registers
Correct answer: The equivalent straight duct length that produces the same pressure drop as the fitting
Effective length (or equivalent length) converts the pressure drop of a fitting into an equivalent straight duct length to simplify total system friction calculations.
Question 3: In a chilled water plant, what does 'delta T' (ΔT) across the cooling coil represent?
- Temperature rise of the chilled water supply
- Temperature difference between entering and leaving chilled water (Correct answer)
- Outdoor air to supply air temperature drop
- Refrigerant evaporating temperature range
Correct answer: Temperature difference between entering and leaving chilled water
Chilled water ΔT is the difference between return water temperature and supply water temperature, typically designed at 10–12°F for optimal plant efficiency.
Question 4: Which system type is best suited for a large open-plan office requiring individual occupant control without ductwork to each desk?
- Single-zone rooftop unit
- Variable air volume (VAV) with perimeter radiation
- Underfloor air distribution (UFAD) with floor diffusers (Correct answer)
- Fan-coil units at each perimeter window
Correct answer: Underfloor air distribution (UFAD) with floor diffusers
UFAD systems deliver conditioned air through raised-floor diffusers near occupants, offering flexible zone control and good air quality without overhead ductwork to every work area.
Question 5: The Coefficient of Performance (COP) of a heat pump in heating mode is always:
- Less than 1.0
- Equal to the SEER rating divided by 3.412
- Greater than 1.0 because it moves heat rather than generating it (Correct answer)
- Equal to the cooling COP minus duct losses
Correct answer: Greater than 1.0 because it moves heat rather than generating it
A heat pump's heating COP exceeds 1.0 because it moves (transfers) heat from outdoor air into the building, delivering more energy as heat than it consumes as electricity.
Question 6: In HVAC design, an 'economizer' cycle saves energy by:
- Recovering heat from exhaust air in winter
- Using cool outdoor air directly for cooling when conditions permit (Correct answer)
- Reducing compressor staging during partial loads
- Increasing refrigerant subcooling
Correct answer: Using cool outdoor air directly for cooling when conditions permit
An air-side economizer uses 100% outdoor air for cooling when the outdoor temperature and/or enthalpy is below a setpoint, reducing or eliminating mechanical cooling energy.
Question 7: What does the 'leaving air temperature' (LAT) of a cooling coil determine in system design?
- The required refrigerant condensing temperature
- The supply air temperature delivered to the space after the coil (Correct answer)
- The outdoor air wet-bulb temperature entering the unit
- The minimum duct insulation R-value needed
Correct answer: The supply air temperature delivered to the space after the coil
The LAT is the temperature of air exiting the cooling coil, which becomes the supply air temperature delivered to conditioned spaces to offset the sensible cooling load.
HVAC System Design 4
When sizing a refrigerant suction line, the primary concern is avoiding excessive: