Vessel media fill calculator
Pick a FlowShell FRP vessel (or enter custom dimensions) and a filter media; get the settled-bed fill in litres and kilograms, the bed depth, the freeboard the media needs for backwash expansion, and the backwash flow that vessel demands.
Clack quotes 37–49 m/h backwash for fine (0.45–0.55 mm) sand with 20% freeboard minimum, which matches conventional rapid-sand practice; the default here is the bottom of that band. Coarser and denser gradings need MORE upflow to fluidise, not less, and cold water needs less than warm for the same expansion — so confirm the rate against your actual grading and the coldest water the filter will see, then check the valve and the drain can pass it. DWT standard fills allow ~30% freeboard.
Results
| Usable vessel volume | ~65 L |
|---|---|
| Media fill (settled bed) | ~50 L (77% of vessel volume) ≈ 80 kg |
| Bed depth at that fill | ~991 mm |
| Freeboard the media needs | ≥30% of bed depth (bed expands ~20%) |
| Backwash flow for this vessel | 1,87 m3/h (31 L/min) at 37 m/h |
The fill is calculated by mass and settled volume — fill to the weight, not to a line on the vessel. Support fine media on a graded gravel underbed unless the vessel has a false-bottom distributor (the bottom dome typically holds the gravel), and backwash new media to drain before first service. If the installation cannot deliver the backwash flow above, pick a smaller vessel or a media with a lower backwash demand.
Spec sheet: FlowShell FRP1054-2.5 technical data sheet (PDF)
Formula used
V = [π/4 × d² × (h − d) + π/6 × d³] × 1000
fill = V × media fill fraction · rest = freeboard
bed depth ≈ fill ÷ 1000 ÷ A · A = π × d² ÷ 4
Qb = A × vb
- V
- usable vessel volume (shell + domes), L — the bracket is m3, ×1000 gives litres
- d
- vessel internal diameter, m
- h
- vessel overall height, m
- A
- bed cross-sectional area, m2
- Qb
- backwash flow the installation must deliver, m3/h
- vb
- design backwash velocity for the media, m/h
These are geometric estimates. Volume assumes a plain cylinder with hemispherical ends on the nominal overall height, and bed depth divides that volume by the cylinder area, so it ignores the dome the bed actually sits in; neither accounts for the distributor, riser and support gravel that displace media. Final fills come from the manufacturer capacity figure and the real available expansion height. Vessel dimensions are nominal trade designations; FlowShell figures follow DWT's published spec sheets (DWT-DS-I-001 to -010). Fill fractions, freeboard, expansion, backwash velocities and densities are per-media design values from the manufacturers' data sheets (Clack, Dryden Aqua, DMI-65, Watch Water, Rotocarb, Calgon, Jacobi, Purolite, Extrepure, Inversand, DWT) — always confirm against the specific media's datasheet, and note that Siliphos-type dosing media do not belong in a backwashed vessel bed at all.
Equipment for this duty
Send the water analysis and duty point, and the branch confirms exact selections, pricing and lead times — usually within one working day.
Request a quoteResults are indicative engineering estimates based on standard formulas and typical assumptions. They are not a design service; confirm any design against a full water analysis and the relevant manufacturer’s data.