The two Σ K modes, the full default K library, and how to override a K value per row.
Fitting losses are calculated as Δp = Σ K · ½ρv², where v is the actual duct velocity.
Card 04 — Fittings offers two mutually exclusive ways to arrive at Σ K.
Enter one combined coefficient in Total fitting coefficient (Σ K). Use this when you
already have a Σ K from another source, or when you only want the straight-run friction
(leave it at the default 0).
Manual mode also hosts the Duct role (advisory) selector:
| Role | Advisory band |
|---|---|
| Main duct | 5–8 m/s |
| Branch / near terminal | 3–6 m/s |
| Not selected | no band, no verdict |
The band drives the velocity gauge and the band warnings only — it never changes the calculated pressure drop.
As-built quirk: the role selector is rendered only in Manual Σ K mode. Switching to Schedule hides it, and the role you last chose stays active in the gauge. To clear or change a role while working in Schedule mode, switch back to Manual, set the role, then switch across again.
Pick a fitting from the dropdown and select + Add row. Each row carries a quantity, the
library default K, an optional override, and its Σ contribution (K × quantity). The table
foot totals Computed Σ K (schedule) live, and the delete button removes a row.
Switching modes does not delete anything: your schedule rows survive a trip through Manual mode, and vice versa. Only the active mode's Σ K is applied.
As-built quirk: a PDF report prints the fitting schedule table whenever any rows exist — including while Manual Σ K is the active mode, where those rows contribute nothing. Check the report's
Σ K moderow to see which figure was actually applied.
Version mvp_default_v1. These are simplified typical figures for preliminary checking.
They are not reproduced from a named standard, are not specific to any manufacturer, and are
not adjusted for the geometry of your particular fitting. Override them per row for
manufacturer or project-specific data.
| Category | Fitting | Default K |
|---|---|---|
| Bends / elbows | 90 deg bend — long radius | 0.2 |
| Bends / elbows | 90 deg bend — short radius | 0.5 |
| Bends / elbows | 45 deg bend | 0.15 |
| Bends / elbows | 90 deg elbow — mitred | 1.3 |
| Tees / branches | Tee — straight through | 0.2 |
| Tees / branches | Tee — branch take-off | 1.0 |
| Tees / branches | Wye / lateral 45 deg | 0.3 |
| Transitions | Transition — gradual | 0.2 |
| Transitions | Transition — abrupt | 0.6 |
| Entry / exit | Entry — sharp-edged | 0.5 |
| Entry / exit | Entry — bellmouth | 0.05 |
| Entry / exit | Exit — to plenum/space | 1.0 |
| Dampers | Balancing damper (open) | 0.3 |
| Dampers | Fire damper | 1.5 |
| Dampers | Volume control damper | 0.6 |
| Silencers / attenuators | Silencer / attenuator | 1.0 |
| Flexible connections | Flexible connection (short) | 0.5 |
| Terminals | Grille / diffuser (typical) | 2.0 |
| Filters | Filter (clean, typical) | 2.0 |
| Custom | Custom fitting | — (override required) |
The library version is stamped into the PDF report and into every share link, so a saved calculation records which K set produced it.
Manual Σ K is faster and is the right choice when a figure is handed to you. The schedule is worth the extra typing when you need the audit trail: the PDF report lists every row with its quantity, effective K, contribution, and whether the value came from the library or from your override.
One caveat that applies to both: all K values are referenced to the actual duct velocity pressure of this single run. A fitting whose published K is referenced to a different velocity — a branch take-off referenced to the combined upstream flow, for example — needs that conversion done before the value is entered here.