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Air Flow — Pressure & velocity

Air Flow tool step 1 — duct pressure and velocity in the same two-column grid as sensible. Volume is the shared CFM / L/s pair (no second row).

Layout

• One grid, Imperial left and Metric right. SA | RA under the grid is duct pressure sign only (not heating/cooling).
• SP / TP / VP / FPM / Area — duct fields (in. w.c. or Pa, FPM or m/s, sq.in. or cm²). Formulas use CFM and in. w.c. internally.
• Results update as you type (no Calculate button). The CFM/L/s toggle is not on this step.

SA | RA toggle

• Located under the field grid on the Air Flow step.
• SA (supply) — filled SP and TP are set positive.
• RA (return) — filled SP and TP are set negative (typical return gauges).
• Switching SA ↔ RA rewrites the sign on SP and TP. Empty boxes stay empty. VP stays the same magnitude (TP − SP).
• VP is usually positive (from TP − SP) on both supply and return when gauges are consistent.

Live update

• The last box you commit is held. Every other box that a formula can compute from that new number is rewritten (even if it already had a value).
• Empty boxes with no formula stay empty. Area is typed only and is never overwritten.
• The field you are typing is never overwritten. Numpad C leaves that box empty.
• Zero is not valid on these fields (leave blank instead of entering 0).

How the engine relates the blocks

VP and FPM are the same velocity (standard air). CFM and L/s are the same volume as the sensible block. Area already in the box is kept. Changing SP keeps TP and updates VP → FPM → CFM → BTU.

• FPM = 4005 × √|VP| and reverse VP = (FPM ÷ 4005)² (VP in in. w.c.).
• Chain: SP ── TP ── VP ── FPM ── Area ── CFM ── L/s ── BTU
• CFM + Area updates FPM and VP. FPM + Area updates CFM. Changing CFM does not invent SP/TP.

Example (supply): Enter SP = 0.45 and TP = 0.85 → VP = 0.40 → FPM ≈ 2533. If Area = 144 sq.in., the shared CFM / L/s update to match FPM × (144 ÷ 144).

Example (return): Enter SP = −0.80 and TP = −0.50 → VP = +0.30 → FPM updates from √0.30; Area and shared CFM track if Area or CFM is set.

Formulas

• VP = TP − SP (also SP = TP − VP, TP = SP + VP).
• FPM = 4005 × √|VP| (sea-level standard air).
• CFM = FPM × (Area sq.in. ÷ 144) — enter duct area in square inches.
• L/s ≈ CFM × 0.472 (1 CFM = 0.4719 L/s).

Disclaimer: Field guide for technicians — not a substitute for nameplate data, traverse standards, or local Code. Constants assume standard air density.