Separate advertised fan airflow from installed airflow
A quote can list a fan model and airflow without identifying inner duct dimensions, elbows, filter loading or the pressure basis of the performance curve.
This worksheet helps owners of restaurants, small workshops and homes compare two quotes at the same verified target flow and ask for missing evidence.
A calculated intersection is a candidate from the supplied model, rather than a commissioning measurement.
Review the numbers before paying for another replacement
Start with a confirmed target, then review section friction, fitting and device losses.
A verified fan total-pressure curve enables a pressure comparison at target and an operating-point candidate.
Missing fan evidence can leave valid required-pressure results available, which helps you request a specific performance curve.
Keep price, installation quality and commissioning responsibilities in the same discussion.
Compare alternatives for one complete serial path
Supported scope
All sections carry the same airflow from the complete inlet to the outlet.
Split different cross sections into separate rows and identify the reference section for each fitting K.
Verify constant density and fixed Darcy factors over the reviewed airflow interval.
Systems requiring separate analysis
Branches, leakage, buoyancy, wind, imposed pressure differences, multiple fans and speed control are outside this model.
Confirm inlet, outlet, transition and verified system-effect losses before calling the total a complete system requirement.
A and B are alternatives; their airflow, losses and prices are not added together.
Evidence missing from one alternative does not hide valid results for the other.
A partial duct list describes the entered portion and should not be treated as the entire installation.
Understand inputs, missing values and zero
Inputs, evidence and maximum rows per quote; shared inputs use a dash| Input | Evidence to check | Before entering | Max rows/quote |
|---|
| Target and reference | Verified design or operating flow, m³/h or L/s | Distinguish free-air ratings and CADR | — |
| Density and velocity limit | Common air conditions and your comparison limit | No statutory or recommended limit supplied | — |
| Geometry and Darcy f | Inner dimensions, length and verified factor range | Do not use outer dimensions or Fanning f | 6 |
| K, quantity and section | Geometry and reference cross section | Use the correct local velocity pressure | 12 |
| Device and fan curves | Pressure basis, density, revision, filter state, rpm and test setup | Compare only within supplied ranges | 4 + 1 |
| Total quote and scope | Comparable KRW totals including tax, delivery and installation | Missing money is not zero | 1 |
Required numbers start blank; only the hypothetical-example button supplies example values.
Editing preserves the raw numeric draft, while blanks remain missing.
Verified zero length or K is allowed; target flow, inner dimensions, density and Darcy f must be positive.
A blank price is unknown; zero is an entered zero quote.
An empty device list requires explicit confirmation that no device losses are omitted.
Each quote supports 1–6 duct sections, 0–12 fitting rows and 0–4 devices.
Deleting a section clears every fitting reference so that K values cannot silently move to another area.
Numeric fields constrain committed values to their supported ranges; recheck displayed values after pasting.
These are computational limits rather than engineering recommendations.
Record inner geometry and coefficient evidence together
Area and hydraulic diameter
Use the actual inner width and height for rectangular ducts and inner diameter for round ducts.
Hydraulic diameter is 2ab/(a+b) for a rectangle and d for a circle.
Insulated outer dimensions, nominal sizes and an equivalent round diameter from another sizing method are different inputs.
Fitting reference sections
Read each K with its documented cross section and geometry.
Select that section so its velocity pressure is multiplied by K and quantity.
Keep the supplier reference instead of transferring a familiar coefficient from a different elbow or transition.
No material-name lookup generates Darcy factors or fitting coefficients.
The calculation holds your verified f constant over the reviewed interval; check its applicability when flow changes substantially.
No proprietary fitting database or recommended default coefficient is supplied.
Distinguish fan static pressure from total pressure
This worksheet uses total-pressure losses
Do not substitute a static-pressure curve into this total-pressure comparison.
Static or unknown pressure bases withhold curve comparisons without an automatic conversion.
Mismatched density also withholds a curve: request evidence for matching conditions from the manufacturer.
Enter two nonnegative numbers per line: airflow, pressure.
Commas, TABs and spaces are supported separators; exclude headers, thousands separators and exponent notation.
Supply 2–20 points in strictly increasing airflow order, with nonincreasing fan pressure or nondecreasing device loss.
For a fan with a rising or stall region, obtain the confirmed allowed monotonic interval rather than flattening the original data.
Units and evidence range
Use m³/h or L/s for flow, and Pa or conventional mmH₂O for pressure.
1 conventional mmH₂O = 9.80665 Pa, and 250 L/s = 900 m³/h.
Unit changes do not convert existing numbers: re-enter all fan and device coordinates in the selected units.
Endpoints are included; values outside a curve are not extrapolated or held at the last recorded loss.
How velocities, losses and intersections are calculated
Q(m³/s) = Q(m³/h) / 3600 = Q(L/s) / 1000
A = ab or πd²/4 · Dh = 2ab/(a+b) or d
v = Q/A · pv = ρv²/2
Δpsection = (fL/Dh + ΣK)ρv²/2
Psystem(Q) = Σfriction + Σfittings + Σdevice curves(Q)
Pfan(Q) − Psystem(Q) = 0
The fixed-f/K duct and fitting portion scales with airflow squared.
Device losses and fan pressure use linear interpolation between adjacent supplied coordinates.
The numerical intersection search stays within the overlap of the fan and every device curve.
No overlap or no crossing in that interval withholds a point without asserting zero actual airflow.
A coincident flat interval has no unique point and remains pending.
A unique zero-flow intersection is distinguished from a positive operating point.
Comparisons use unrounded values, correcting only machine representation noise; no hidden safety factor is added.
Calculated pressures above the 1,000,000 Pa support limit are withheld for an input and unit review.
Five steps for reviewing a quote
- Set matching requirements.
Enter the verified target and reference, common density and your velocity limit, then confirm the complete serial boundary.
- Build the duct and fitting ledger.
Record inner dimensions, lengths, Darcy factors and each K reference section and quantity; check inlet, outlet and transition completeness.
- Copy the original curve evidence.
Record pressure basis, density, units, revision, rpm, filter state and test setup, then confirm the supplied monotonic interval.
- Read target and operating-point results separately.
Review required pressure, fan pressure difference and section velocities at target, then the candidate flow difference and shortfall.
- Request matching evidence with the worksheet.
Enter comparable total quotes and save TXT or print; send pending reasons, missing items and commissioning questions with the original evidence.
Hypothetical example: two different duct widths
Assume 900 m³/h target, density 1.2 kg/m³ and a user velocity limit of 4 m/s.
Both alternatives have length 10 m, Darcy f = 0.02 and two K = 0.5 fittings plus one K = 1 fitting.
The device curve is (0,0), (1800,100), and the fan total-pressure curve is (0,180), (1800,0), in m³/h and Pa.
Every coefficient, curve, limit and price here is hypothetical, rather than a product specification or design recommendation.
Target and operating-point comparison for hypothetical duct quotes A and B| Item | A: 200×250 mm | B: 300×250 mm |
|---|
| Target velocity m/s | 5 | 3.333 |
| Friction Pa | 13.5 | 4.889 |
| Fittings Pa | 30 | 13.333 |
| Devices Pa | 50 | 50 |
| Required total pressure Pa | 93.5 | 68.222 |
| Fan pressure at target Pa | 90 | 90 |
| Pressure difference Pa | −3.5 | +21.778 |
| Candidate flow m³/h | 886.082 | 1,009.702 |
| Operating total pressure Pa | 91.392 | 79.030 |
| Total quote KRW | 1,200,000 | 1,450,000 |
A lacks 3.5 Pa at target and its candidate flow is about 13.918 m³/h short.
B has about 109.702 m³/h of candidate flow margin, without an automatic purchase recommendation.
Confirmed comparable quotes differ by KRW 250,000; review that alongside space, constructability and the allowed operating interval.
Real quotes with different fitting geometry or filter condition need their own evidence.
Separate shortfall, pending evidence and velocity exceedance
Pressure margin at target
Subtract required total pressure from fan total pressure at the same target flow.
Negative is a shortfall, positive a surplus, and zero equal.
A dash outside the fan evidence range is a withheld comparison, rather than a free-air rating substituted for that target.
Candidate flow and velocity
Read intersection flow minus target flow, shortfall and percentage margin.
Check velocities again at that point: flow surplus can coexist with a section above your limit.
Use both target and operating-point tables.
Pending means missing evidence or unsupported model scope, rather than established installed underperformance.
Static curves, density mismatches, missing devices, duplicate flows and unsupported curve direction need evidence review.
No intersection in the supplied range does not establish zero actual airflow.
Check rpm, frequency, density, test setup and the allowed interval before replacing a curve.
Practical review situations and worksheet use
A shop ventilation replacement
If one supplier replaces only the fan and another widens the duct, enter their paths and scope independently.
Ask for losses including filters, inlet/outlet and fittings at the same target, instead of comparing advertised fan airflow alone.
A home or workshop evidence review
Check that the path is serial; do not compress several room branches or leakage into one row.
Recalculate with the appropriate loaded-filter curve when the filter scenario changes.
- TXT retains original units and curve text, references, confirmations, example origin, pending reasons and results.
Send the original quote and evidence with it so the assumptions remain traceable.
- Print the loss table and candidate with an agreed measurement location and operating mode.
Do not label a calculated intersection as a measured installed result.
- Review occupant-based outdoor-air demand separately in the CO₂ outdoor airflow calculator.
This result does not determine particle-cleaner CADR, statutory ventilation, fire/smoke control, hazardous-gas, infection prevention or noise design suitability.
Frequently asked questions
What if I only have a static-pressure specification?
Calculate required total pressure from valid system evidence, then request fan total-pressure data or a manufacturer-verified conversion for the same boundaries and conditions.
Can one maximum-flow number produce an operating point?
At least two verified total-pressure coordinates are required; one maximum-flow or free-air rating cannot establish installed operation.
Can I use one filter loss at every flow?
A single point is not a device curve; request coordinates covering the reviewed flow interval and the relevant clean or loaded condition.
Why are there no default friction or fitting coefficients?
Verified material, joints, geometry and flow range matter, so no recommended coefficient or proprietary fitting database is built in.
Are curve and field density differences corrected automatically?
No; obtain fan and device evidence with density and operating conditions matching the common inputs.
Does no intersection mean the fan stops?
It means no intersection was established inside the supplied evidence range, without determining zero actual airflow or a stopped fan.
Does velocity below my limit confirm noise or legal compliance?
It only compares your entered limit; noise, statutory ventilation and safety need separate design and measurement checks.
Should I buy the higher-flow candidate if prices are close?
No purchase ranking is supplied; confirm matching scope, allowed operation, constructability and commissioning responsibilities.
Primary technical references and the next evidence request
Method checked: 2026-10-11 · No statutory design thresholds applied
General technical references do not replace current product coefficients and curves.
Update inputs and results when revision, f applicability, K reference section, filter state or system boundaries change.
Review current facility and jurisdiction requirements separately for statutory ventilation, fire protection or safety.
Request matching conditions with a traceable loss table
Combine target flow, section losses and operating-point candidates in the worksheet, using pending reasons as an evidence request list.
Obtain curves for matching pressure basis, density and filter state plus a commissioning measurement plan, then compare complete quote scope.
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