System and Equipment Fundamentals
Fans
Centrifugal, axial, and vane-axial fans all follow the fan laws for a fixed system: airflow varies with speed, static and total pressure with the square of speed, and brake horsepower with the cube of speed. For density changes at constant speed, airflow stays essentially constant while pressure and power vary directly with density. Distinguish fan static pressure from total pressure, and remember that the operating point is the intersection of the fan curve and the system curve, not a catalog rating. Poor inlet or outlet conditions (elbows at the fan discharge, abrupt transitions) create system effect losses that reduce actual performance; AMCA publication guidance describes the allowances. Avoid operating near the stall or surge region of the curve.
Pumps
Centrifugal pumps follow affinity laws for speed and impeller diameter. Cavitation and low NPSH margin cause noise, vibration, and erosion; a pump run far out on its curve can also cavitate. Verify that the installed impeller matches the submittal.
Coils
At standard air, sensible heat transfer is approximately Btu/h = 1.08 x cfm x delta T (dry bulb), and latent heat is about 0.68 x cfm x delta grains. Excess face velocity causes condensate carryover; too low a velocity reduces heat transfer. Coil bypass factor affects leaving air conditions.
Dampers and valves
Opposed-blade dampers give a more linear characteristic and are preferred for balancing; parallel-blade dampers suit two-position or control duty. Leakage classes matter on outside-air and smoke applications. Control valves are typically equal-percentage or linear; valve authority above roughly 0.3 keeps control stable. Three-way valves and PICVs change how a circuit is balanced.
Worked example
Doubling fan speed in a fixed system quadruples static pressure and increases power eightfold, which is why sheave changes need an engineering review.