Raised-Floor Tile Airflow and Plenum Pressure
Estimates standard and high-flow raised-floor delivery from the orifice relation.
Example
You enter
- Tile gross area (sq ft) 4
- Standard tile open area (%) 25
- Discharge coefficient 0.7
- Plenum static pressure (in w.c.) 0.05
- Total supply airflow (cfm) 96000
- Open standard tiles 150
- High-flow grate open area (%) 56
- Reduced plenum pressure (in w.c.) 0.02
- Increased open tile count 200
You get
- Standard tile airflow 627 cfm
- High-flow grate airflow 1,404 cfm (2.24x)
- Airflow at reduced pressure 396 cfm (-36.8 %)
- Required current pressure 0.052 in w.c.
- Increased count pressure change (%) -43.75
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeIncluding pressure sensitivity and the redistribution caused by opening more outlets at fixed fan airflow. Manufacturer curves and an under-floor pressure survey govern.
Formula or tableQ = 4,005 x discharge coefficient x free area in sq ft x sqrt(plenum pressure in w.c.); pressure at fixed geometry varies with flow squared.
Source and editionThe standard orifice airflow relation. Tile manufacturer measured flow curves and an under-floor pressure survey govern.
Free accessA public orifice relation and square-law inversion.
What governsEstimate. AHJ and licensed professional govern.
API field namestile_area_ft2, open_area_pct, discharge_coefficient, plenum_pressure_in_wc, total_supply_cfm, open_tile_count, high_flow_open_pct, reduced_pressure_in_wc, increased_tile_count, tile_airflow_cfm, high_flow_airflow_cfm, reduced_pressure_airflow_cfm, required_plenum_pressure_in_wc, increased_count_pressure_change_pct
- Discharge coefficient is entered a tile maker's chart is better than 0.7: Tate's DirectPerf 25% tile prints 357 CFM at 0.02 in. and 496 at 0.04, which fits about 0.62, so 0.7 reads about 13% highTate Airflow Panels & Controls, Performance Charts
- Free area gross tile area times entered open-area fractiontile manufacturer
- Discharge coefficient entered coefficient represents the selected tile geometrymanufacturer curve / field measurement