Jet thrust is the number that decides how many jet fans a tunnel needs. If you are buying a jet thrust fan, this is the figure to pin down first. It is also the number most often quoted wrongly — because "thrust" in a tunnel specification can mean three different things. This article explains how jet fan thrust is defined, how it is measured, how to calculate it for a given tunnel, and what to check before you accept a supplier's figure.
1. What is jet fan thrust?
The term jet thrust fan is used interchangeably with jet fan in most specifications, and the duty it describes is the same: a fan whose useful output is measured in newtons of thrust, not in m³/s of ducted airflow. That is why a jet thrust fan is selected from a thrust figure and a throw distance rather than from a fan curve in the usual sense.
A jet fan (also called an impulse ventilator, or jet vent fan) does not push air through a duct. It sits in the tunnel, usually near the ceiling, and fires a high-velocity jet along the tunnel axis. The momentum of that jet drags the surrounding air with it, producing a longitudinal airflow through the tunnel.
Thrust is the reaction force produced by that jet. It is measured in newtons (N) and it is the basis of every jet fan selection: the total fan thrust must overcome the tunnel's aerodynamic resistance so the required air velocity (typically 2–6 m/s for normal operation, or higher for smoke control) is achieved.
The basic relationship is momentum flux:
F = ρ · Q · v = ρ · A · v²
where:
- F = thrust (N)
- ρ = air density (kg/m³) — standard reference is 1.20 kg/m³
- Q = volume flow through the fan (m³/s)
- v = jet exit velocity (m/s)
- A = nozzle exit area (m²)
This is why a jet fan with a large diameter and a fast exit velocity produces more thrust: thrust scales with area and with velocity squared. It is also why thrust is measured at a defined air density — if a supplier quotes thrust without stating the density, the figure is not comparable.
2. The three meanings of "thrust" — and why it matters
This is the single most common source of disagreement between tunnel designers and fan suppliers. When a specification says "thrust shall be at least 1,280 N", it can mean any of these:
| Definition | What it means | Typical use |
|---|---|---|
| Nominal thrust, F₀ at v_t = 0 | Thrust measured with no tunnel airflow — the fan firing into still air, per ISO 13350 force-balance method. | The standard bench test figure. This is what most manufacturers publish and what test rigs measure. |
| Effective thrust, F(v_t) | Thrust actually contributed to the tunnel airflow when the tunnel already has a longitudinal velocity v_t (typically 8–12 m/s). | Used in the tunnel ventilation calculation to size the installation. |
| Installation thrust | Effective thrust further reduced by installation effects — proximity to ceiling, wall clearance, upstream obstructions, grouping. | Used in the final design verification. |
The gap between nominal and effective thrust is not small. As tunnel air velocity rises, the jet's momentum transfer becomes less efficient, and effective thrust falls. For a fan sized at the low-velocity end of a tunnel duty, the nominal thrust needed to deliver a given effective thrust can be roughly 40% higher than the effective figure itself.
Practical consequence: within a diameter limit, a fan that meets the specification interpreted as nominal thrust may not meet it interpreted as effective thrust — and the required machine can become one or two sizes larger, or fall outside the allowable diameter altogether. Always confirm which definition applies before the machine is sized.
3. How thrust is measured — ISO 13350
ISO 13350 defines the test method for jet fans. The two common approaches are:
- Force-balance (load cell) method. The fan is mounted on a measuring frame supported by load cells; the reaction force of the jet is read directly. This is the method most specifications reference, and it is the most direct.
- Traverse / velocity-area method. The jet velocity profile is measured downstream of the nozzle with a pitot or hot-wire traverse, and thrust is derived from ρ·Q·v. Requires a well-defined measurement plane and careful area integration.
Key conditions that must be stated with any test result:
- Air density — normally normalised to 1.20 kg/m³. Thrust scales linearly with density, so a test performed at 1,150 m altitude (ρ ≈ 0.90) will read about 25% lower.
- Tunnel air velocity at test — zero for nominal thrust F₀.
- Whether measuring stations, silencers or flow straighteners were fitted. A bellmouth inlet and a straightener add a small loss but improve jet quality; silencers reduce thrust slightly.
- Nozzle velocity and flow, so the result can be cross-checked against the momentum formula.
4. Calculating how many fans a tunnel needs
The number of jet fans is derived from the thrust required to overcome tunnel resistance at the design velocity:
n · F_eff(v_t) = A_tunnel · (Δp_friction + Δp_local + Δp_other)
In practice the tunnel resistance comprises:
- Friction — wall friction along the tunnel length, which rises with velocity squared.
- Local losses — portals, bends, sign gantries, niches, ventilation shafts, vehicle blockage.
- Piston effect — traffic-induced airflow, which can either help or oppose the fans depending on direction and congestion.
- Buoyancy and meteorological effects — significant for smoke control cases.
Because F_eff depends on v_t, and v_t depends on how much thrust is installed, the calculation is normally iterative.
A worked sanity check is worth doing on any offered fan: take the stated nozzle velocity and flow from the performance sheet, compute ρ·Q·v at ρ = 1.20, and compare with the claimed thrust. If the two differ by more than a few percent, ask which measurement basis was used. A fan quoted at high thrust with a low nozzle velocity is usually being quoted on a different definition than you assumed.
5. Thrust, power and efficiency — what the ratio tells you
Thrust per unit of electrical power (N/kW) is a compact way to compare jet fans of different sizes. Larger diameters are more efficient: the same thrust costs less power, because the jet is produced with more area and less velocity.
From our own design work across a 500–1,800 mm range at 4-pole / 1,460 rpm, the trend is consistent:
| Diameter | Thrust at maximum pitch | Thrust per kW |
|---|---|---|
| Ø500 mm | ≈ 117 N | ≈ 57 N/kW |
| Ø800 mm | ≈ 359 N | ≈ 52 N/kW |
| Ø1250 mm | ≈ 1,102 N | ≈ 48 N/kW |
| Ø1400 mm | ≈ 1,477 N | ≈ 47 N/kW |
| Ø1800 mm | ≈ 2,607 N | ≈ 46 N/kW |
Two useful readings from this table:
- A diameter limit is a thrust limit. If the tunnel geometry caps the fan diameter at, say, 1,450 mm, the maximum thrust achievable from a given blade technology is also capped. Within that limit, a Ø1250 fan of a given blade family cannot be tuned to reach the thrust of a Ø1400 fan with the same family — the disc area is simply smaller. This is geometry, not tuning.
- Tailoring pitch is the last knob, not the first. Blade pitch adjusts thrust within roughly ±30% for a given diameter. If the required thrust lies outside that band, no pitch setting will reach it — the diameter must change.
6. Reversible jet fans and the reverse thrust question
Many tunnel specifications require reversible fans (for smoke control, or for tunnels with bi-directional traffic), and state a limit on the difference between forward and reverse thrust — often 5% or less.
There are two ways to build a reversible jet fan:
- By control — the fan reverses rotation. With a cambered (asymmetric) blade, the blade is efficient in one direction only, so reverse thrust is lower, and the fan must be oversized forward to make the reverse figure.
- By geometry — the blade is symmetrical, with no camber and no sweep. In this case forward and reverse thrust are equivalent as a matter of geometry, because the blade section is identical in either direction.
The geometric approach is materially simpler and cheaper: no oversizing is required for the reverse case, no controller logic is needed to compensate, and the reverse-thrust acceptance criterion is satisfied by design rather than by tuning. Its cost is that a symmetrical blade is slightly less efficient at its design point than a cambered blade optimised for one direction — which is usually the right trade for a tunnel fan that must work both ways.
One practical caution: reversibility of thrust does not imply reversibility of noise. The acoustic behaviour differs between directions because the motor and struts are upstream in one case and downstream in the other. If a specification limits sound power in both directions, the two cases should be declared separately rather than assumed equal.
6b. Jet thrust fan selection: per fan or for the tunnel?
Specifications quote jet thrust per fan and total installed thrust as if they were the same quantity. They are not. Total thrust is the sum of the per-fan values only if the jets do not interfere; in a real tunnel with fans mounted in groups the second fan in a group works in air that is already moving, so its effective contribution is lower than its bench figure. Design practice is to apply an installation factor, typically 0.7 to 0.9, when converting jet thrust per fan into tunnel thrust.
Two consequences for buyers:
- Ask for the thrust at the installed condition, not just the bench value. The bench figure is measured to ISO 13350 with the fan in free air; the tunnel is not free air.
- Check the throw. A jet thrust fan with high thrust but a short throw needs more units to cover the same tunnel length, and the extra units cost more than the thrust saved.
7. A checklist before accepting a thrust figure
- Is the figure nominal (v_t = 0) or effective? At what tunnel velocity?
- At what air density — and is it normalised to 1.20 kg/m³?
- Measured by which method — force balance per ISO 13350, or derived from a velocity traverse?
- With or without silencers, measuring stations and flow straighteners?
- Does the stated nozzle velocity and flow cross-check with ρ·Q·v at the quoted thrust?
- For reversible fans, is reverse thrust guaranteed at the same value, and is the reverse noise declared separately?
- Does the figure account for installation effects — ceiling proximity, wall clearance, grouping?
- What is the thrust at the actual tunnel operating velocity, not just at standstill?
Any supplier who can answer all eight, with a test report to match, is worth taking seriously. Any supplier who quotes a bare number without the basis is quoting a number you cannot verify.
8. How we approach it
SINDFAN designs and manufactures jet fan impellers and complete tunnel fans in Tianjin, China — Ø500 to 1,600 mm, with thrust from 152 N up to roughly 1,700 N for our standard range and higher for our larger platforms. We test to ISO 13350 and ISO 5801.
For our current tunnel development programme we work with symmetrical blades, so that reversibility is delivered by geometry. Our aerodynamic models are calibrated against ANSYS CFX full three-dimensional CFD and against measured points, and we issue a thrust envelope against blade pitch for every diameter, so that a tunnel engineer can see immediately whether a required thrust is reachable inside a given diameter limit.
If you are working on a tunnel specification and want a thrust envelope for your diameter and duty, send us the duty point — flow, thrust required, diameter limit, voltage and frequency — and we will come back with the achievable performance rather than a single number taken on faith.
Send your duty point: WhatsApp +852 9524 8434 · info@sind-tech.com — we reply within 24 hours.