Heating, Ventilation, and Air Conditioning: Loads, Psychrometrics, and Air Systems
What this domain tests
HVAC questions cover heating and cooling load components, psychrometric properties, air distribution, and the equipment used to condition air. Expect items on sensible and latent heat, relative humidity, duct sizing, and the operation of coils, humidifiers, and air-handling units.
Core rules and concepts
- Sensible heat changes dry-bulb temperature; latent heat changes moisture content. Total load = sensible + latent.
- Psychrometrics: dry-bulb, wet-bulb, dew point, relative humidity, humidity ratio, and enthalpy. Cooling below dew point condenses moisture, so a cooling coil dehumidifies.
- Relative humidity is the ratio of actual vapour pressure to saturation vapour pressure at the same dry-bulb temperature. Warming air lowers RH without adding moisture.
- Air distribution: supply, return, and outdoor air; duct velocity affects friction and noise. Balancing dampers set branch flows.
- Equipment: boilers and furnaces for heating, chillers and DX coils for cooling, humidifiers (steam, evaporative) and dehumidifiers, and air-handling units with filters, coils, and fans.
Worked example
A room has a sensible load of 12 kW and a latent load of 3 kW. Supply air is 14 C and room air is 24 C. What supply airflow is required (using air density 1.2 kg/m3 and cp 1.005 kJ/kg.K)?
Reasoning: sensible heat = mass flow x cp x temperature difference. Mass flow = 12 / (1.005 x 10) = 1.194 kg/s. Volume flow = 1.194 / 1.2 = 0.995 m3/s, about 1.0 m3/s. The latent load determines the moisture that must be removed; if the coil cannot cool below dew point, latent load is unmet and room RH rises. Check that the coil's sensible heat ratio matches the load's.
Traps that cost marks
- Sizing on sensible load alone and ignoring latent load, which leaves humidity uncontrolled.
- Confusing wet-bulb with dew point; they coincide only at saturation.
- Assuming a cooling coil always dehumidifies; if the coil temperature is above the air dew point, it is dry.
- Ignoring outdoor air load, which can dominate latent load in humid climates.