HVAC Systems
Loads and air quantities
Cooling loads come from conduction through the envelope, solar gain, internal gains (people, lights, equipment), infiltration, and ventilation. Use CLTD/CLF or radiant-time-series methods, but always account for fan heat and duct gain as part of the coil load. Ventilation is not the same as supply air: ASHRAE 62.1 gives Vbz = Rp·Pz + Ra·Az, then adjusts for zone air distribution and system ventilation efficiency.
System types and controls
Constant-volume with terminal reheat is simple but energy-hungry; VAV varies flow with load and needs a minimum airflow for ventilation and box stability. Economizers use cool outdoor air when the enthalpy or dry-bulb comparison favors it, and must lock out appropriately. DOAS decouples ventilation and dehumidification from sensible cooling. Air distribution uses equal-friction or static-regain duct sizing, with velocities limited for noise (roughly 1,000–1,500 fpm in mains).
Traps in exam items
Using occupant density without diversity when ventilation is aggregated; sizing a coil on sensible load only when latent load exists; mismatching the coil SHR to the space SHR; forgetting that VAV boxes reduce flow but not the ventilation fraction; and ignoring duct leakage or fan static that inflates brake horsepower.
Worked example
An office zone has 50 occupants, 2,000 ft², Rp = 5 cfm/person, Ra = 0.06 cfm/ft². Vbz = 5(50) + 0.06(2,000) = 250 + 120 = 370 cfm of breathing-zone outdoor air, before system-level efficiency adjustment.
Rule of thumb: comfort cooling air quantities run about 400 cfm per ton, and typical office floor area needs roughly 1 cfm/ft² of supply air.