# Fittings and Σ K

Source: https://meptools.com/tools/duct-pressure-drop-calculator/docs/fittings/

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.

## Manual Sigma 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.

## Schedule

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.

- **Override** takes precedence over the library default whenever it holds a finite number.
  Clear it to fall back to the default.
- **Custom fitting** has no default K, so a row using it is invalid until you supply an
  override. Any row that resolves to no K value blocks the whole calculation.
- Quantities and K values may not be negative.

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](/tools/duct-pressure-drop-calculator/docs/exporting-and-saving)
> 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 mode` row
> to see which figure was actually applied.

## The default K library

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.

## Choosing between the modes

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.