HVAC Systems and Equipment
System families
Know the categories: constant-volume versus VAV, single-zone versus terminal reheat, chilled water versus DX, water-cooled versus air-cooled condensers, central plant versus packaged rooftop. Each has a part-load story; VAV saves fan energy but must maintain minimum airflow for ventilation. Chillers are rated at AHRI conditions and lose capacity and efficiency as condenser water warms or chilled-water supply drops. Boilers are rated by input and output, with combustion efficiency and standby loss.
Fans and pumps
The affinity laws for geometrically similar machines at constant density: flow varies with speed, head with speed squared, power with speed cubed. Pumps in series add head at the same flow; pumps in parallel add flow at the same head. The operating point is where the fan curve meets the system curve. Distinguish static from total pressure and correct for density in hot or high-altitude air.
Heat rejection
Cooling tower range is the water temperature drop; approach is leaving water temperature minus entering wet-bulb. Approach drives tower size and cannot reach zero; the leaving water can never be colder than the wet-bulb.
Traps
Applying fan laws across different fan sizes or across a density change without correction; ignoring fan and pump heat in load calculations; and sizing ducts by equal friction while forgetting fitting losses.
Worked example
A fan draws 10 hp at 900 rpm. At 990 rpm (10% faster), power = 10 × 1.1³ = 13.3 hp, flow rises 10%, and static pressure rises 21%.