5.2 Airflow Continuity, Standard Air, Density, and Velocity

Airflow, Area, Density, and Velocity Pressure

Volumetric flow

Airflow is mean velocity normal to a cross-section multiplied by net area:

Q = V × A

Use feet per minute and square feet to obtain CFM. Net internal area excludes liner, structural obstructions, and other area the method requires removed. A 30-inch by 18-inch internally clear rectangular duct has area (30/12) × (18/12) = 3.75 sq ft. At 1,280 FPM, flow is 4,800 CFM.

Do not use external sheet-metal dimensions when liner reduces the flow area. For round duct, A = pi × D squared / 4, with D converted to feet. For irregular shapes, divide the section into measured subareas or use the procedure's equivalent method.

Velocity pressure

Under standard-air conditions used by the NEBB fundamental formula chart:

V = 4005 × square root of Pv

where V is FPM and Pv is inches of water gauge. Because of the square root, convert every traverse point to velocity before averaging unless the selected method explicitly provides an equivalent square-root workflow.

Example: Pv values of 0.04 and 0.16 in. w.g. correspond to about 801 and 1,602 FPM. For two equal-area points the mean velocity is about 1,202 FPM. Averaging pressure first gives a different and incorrect equal-area result.

Density correction

The general relationship in the NEBB formula chart is:

V = 1096 × square root of (Pv / air density)

with density in lb/ft3. At standard density 0.075 lb/ft3, the constant becomes approximately 4005. For the same measured velocity pressure, lower-density air has greater velocity than the uncorrected standard-air calculation.

Determine density using the project-approved method from barometric pressure, temperature, humidity, and altitude as needed. Distinguish actual volumetric airflow from standard airflow or mass flow. Fan curves, code ventilation quantities, and instruments may reference different conditions. Apply a correction when the governing method, instrument, or required uncertainty calls for it; do not assert an unsupported universal altitude or five-percent trigger.

Equal-area weighting

An arithmetic average is valid when readings represent equal areas. If cells differ, use an area-weighted sum:

Qtotal = sum of each local velocity × its represented area

A centerline value is usually high and cannot replace a traverse. Likewise, averaging readings from a dense cluster near one wall biases the result unless the coordinate method assigns appropriate weights.

Worked examples

For a 40-inch by 12-inch clear duct, area is (40 × 12)/144 = 3.333 sq ft. If representative mean velocity is 1,878.5 FPM, flow is about 6,262 CFM.

For Pv = 0.25 in. w.g. at standard density, V = 4005 × 0.5 = about 2,003 FPM. If actual density is 0.060 lb/ft3, V = 1096 × square root of (0.25/0.060) = about 2,237 FPM. The reported result must state which density basis was used.

Field workflow

1. establish the operating mode and measurement boundary;
2. measure clear internal dimensions and calculate net area;
3. select the point layout and instrument range;
4. zero and inspect the instrument;
5. collect signed, stable readings at every coordinate;
6. convert point pressure to velocity and apply correct weighting;
7. apply density treatment consistently; and
8. compare with an independent total or terminal sum.

Error checks

Common mistakes include mixing inches and feet, using gross instead of net area, averaging velocity pressure, applying 4005 twice, using a density ratio upside down, and reporting a standard-flow value as actual CFM. Preserve intermediate units. A result with many decimals is not more accurate than the dimensions, profile, and instrument allow.

Dimensional reasonableness

Estimate before calculating. A 2 ft by 2 ft duct is 4 sq ft, so 1,000 FPM should produce roughly 4,000 CFM. A velocity pressure of 1 in. w.g. under standard air corresponds to about 4,005 FPM, while 0.25 in. w.g. corresponds to half that velocity because its square root is 0.5. These anchors catch misplaced decimals and incorrect unit conversions.

Density changes the velocity inferred from a pressure signal, not the tape-measured area. Keep density in lb/ft3 in the general formula and label the chosen reference condition. If a fan curve is stated at standard density while the field result is actual volume, perform the comparison using the curve manufacturer's correction method.

Reporting the reference condition

Label every final airflow as actual, standard, or another defined reference condition. Record the density inputs and correction method beside the result. This prevents a later reviewer from comparing unlike quantities or applying the same density correction a second time.