HVAC Air Distribution cheat sheet

Total pressure equals static pressure plus velocity pressure: TP = SP + VP; velocity pressure VP = (V/4005)^2 when V is in fpm and VP in in. w.c. (standard air, 0.075 lb/ft^3).

Duct friction loss is commonly estimated with the Darcy-Weisbach form or the equal-friction chart: friction rate depends on velocity, duct roughness, and hydraulic diameter; doubling velocity roughly quadruples friction loss per unit length.

Equivalent length converts fittings to straight duct: total pressure drop = (straight length + sum of fitting equivalent lengths) x friction rate per 100 ft / 100.

Velocity pressure recovery at a transition is limited by the pressure recovery coefficient; a gradual expansion recovers more static pressure than an abrupt one, which dissipates energy in turbulence.

Airflow continuity: Q = V x A. For a rectangular duct A = W x H; for round duct A = pi x D^2 / 4; keep units consistent (cfm, fpm, ft^2).

Hydraulic diameter for a rectangular duct: D_h = 1.30 x (W x H)^0.625 / (W + H)^0.25; used to read round-duct friction charts for rectangular ducts.

Aspect ratio W/H above about 4:1 raises friction, increases surface area and insulation cost, and worsens fitting performance; design targets are usually 2:1 or lower.

Fan total pressure must equal the sum of supply and return/relief path total pressure losses plus any system effect; system effect arises from poor connections at the fan inlet and outlet.

Face velocity at a return or exhaust grille is limited by noise and entrainment: typical design face velocities are about 300-500 fpm for occupied spaces and higher for non-critical areas.

Throw, drop, and spread characterize supply diffuser performance; a jet with higher initial velocity and smaller effective area produces greater throw but also higher noise (NC level).

Occupancy comfort criteria commonly cited: occupied-zone air speed below about 50 fpm in heating and 40-50 fpm in cooling, with neck-level velocity targets around 25-35 fpm.

The Air Diffusion Performance Index (ADPI) relates effective draft temperature to local velocity; ADPI of 80% or higher is a common design target for cooling operation.

Effective draft temperature combines local air temperature and velocity: EDT = (T_local - T_room) - 0.07 x (V - 30), with V in fpm; EDT between about -3 and +2 F indicates acceptable comfort.

Air change rate: ACH = Q x 60 / room volume; used for ventilation, dilution, and contaminant control rather than for thermal load alone.

Mixing ventilation relies on high induction and entrainment; displacement ventilation supplies low-velocity air near the floor and relies on buoyant stratification, requiring careful supply temperature control.

Duct leakage classes (e.g., per SMACNA) specify allowable leakage as a percentage of design flow at a test pressure; seal class A covers all transverse joints and seams.

Duct static pressure reset: reduce fan speed (and thus power, approximately with the cube of speed) when the critical-zone damper is nearly open; a VAV box damper below about 90% open indicates excess static pressure.

Fan laws: flow varies with speed (Q ~ N), pressure with the square of speed (P ~ N^2), and power with the cube of speed (kW ~ N^3); use them for balancing and commissioning estimates.

VAV box minimum and maximum airflow settings must satisfy ventilation requirements at minimum flow; ASHRAE 62.1 ventilation rate procedure sets the outdoor air intake based on zone ventilation efficiency.

Duct static pressure sensor placement should be about two-thirds of the way along the longest supply run, not at the fan discharge, to reflect the critical zone's needs.

Balancing dampers should be placed so they do not create excessive noise; a damper at part-closed position generates self-noise and should be avoided as a permanent flow-control device.

Return air path design must provide a relief path; a tightly sealed building with inadequate relief pressurizes spaces, causing door-opening problems and reduced outdoor air intake.

Fire and smoke dampers are required where ducts penetrate rated assemblies; access doors are required for inspection and testing of dampers and smoke detectors.

Duct insulation thickness depends on temperature difference, conductivity, and surface condensation control; vapor retarders must be on the warm side in cooling applications.

Flexible duct pressure drop per foot is much higher than rigid duct and increases sharply when compressed; keep bends gentle and avoid sharp radius bends and sagging.

Duct sizing methods: equal friction (constant pressure loss per length), static regain (sizing for pressure recovery at each branch), and velocity reduction (progressively lower velocities).

Static regain method sizes each downstream section so the regained static pressure equals the loss of the preceding section, aiming for uniform static pressure along the run.

Noise control: duct velocity limits are commonly about 1,000-1,200 fpm in main supply ducts near occupied spaces to keep regenerated noise low; branch velocities are lower.

Return air plenum design must limit plenum velocity and pressure drop across the ceiling; a plenum with high velocity causes ceiling tile lift and dirt staining.

Air terminal unit selection: a VAV box's minimum and maximum flows, pressure drop at design flow, and sound power level at the design static pressure determine its suitability for a zone.

Commissioning airflow measurement: use a calibrated flow hood at diffusers; pitot-tube traverse in ducts requires adequate straight duct length (often 7.5 diameters upstream, 2.5 downstream) for accuracy.