Tunnel Jet Fan Motor: Ventilation of Long Tunnels and Motor Power

A tunnel jet fan motor is sized by how the total thrust needed along the tunnel is divided among the fans: moving air at 3 m/s through a 1,500 m tunnel with a 60 m² cross-section needs about 2,980 N of thrust; if this is done with 8 jet fans of 1,000 mm diameter, each fan draws about 10 kW of shaft power and runs with a 4-pole motor in the 11–15 kW class. In one manufacturer's catalogue, tunnel jet fans have motor powers of 2.2–30 kW, speeds of 1,440–2,950 rpm and diameters of 560–1250 mm [1][2].

Besides power, the second question is the duty the motor serves. Most tunnel jet fans also direct smoke during a fire; in these fans the motor is part of a set certified together with the fan to EN 12101-3, and it is replaced with the fan manufacturer's certified motor. DRG Motor is a manufacturer, supplier and seller of electric motors; our standard motors are not sold with smoke extraction certification. The DRG suggestions in this article are only for fans that have no smoke duty and perform normal ventilation; models for that scope are among the 120 models on the Buy Electric Motors Online page.

What is a tunnel jet fan and how does it differ from a car park jet fan?

A tunnel jet fan is an axial fan with a cylindrical casing open at both ends, hung from the tunnel ceiling or side wall. The term describes a purpose: the operating principle is axial and the installation is a free unit without ducts. Silencers are fitted on both sides of the casing and the motor sits in the middle of the casing, inside the airstream; in one manufacturer's catalogue the junction box is placed outside the casing [1].

The principle is the same as a car park jet fan, but the scale and the job differ. In a car park it is enough to carry air a few tens of metres to the exhaust shaft; in a tunnel, an air mass hundreds of metres long must be moved in one direction. That is why tunnel fans have larger diameters, higher thrust and are mostly 4-pole; they are arranged along the tunnel in pairs or groups of three at set distances. Each group is a thrust ring that re-accelerates the slowing jet of the previous group.

The thrust chain along the tunnel: how many fans, how many kW?

In tunnel ventilation, fan selection starts not from a single fan but from the total resistance of the tunnel. Air rubs against the tunnel walls and loses pressure at entry and exit; adverse wind at the portal and traffic also oppose the flow. Multiplying the sum of these pressure losses by the tunnel cross-section gives the total thrust required. In the example below the values are assumptions; the actual project calculation is the tunnel engineer's job.

Thrust per fan is found from T = ρ · Q · v. However, not all of it is transferred to the tunnel: as the fan sits close to the ceiling, part of the jet rubs against the wall (an installation factor of 0.85 is used here), and as tunnel air already flows at 3 m/s, only the jet's excess velocity does useful work, which enters as the factor (1 − u/v). As a result a 543 N fan does about 404 N of work in the tunnel, and 8 fans are needed for 2,980 N.

Motor power of one fan

• Outlet area: π × 1.0² / 4 = 0.785 m²; airflow: 0.785 × 24 = 18.85 m³/s (about 67,860 m³/h)
• Dynamic pressure of the jet: 0.5 × 1.2 × 24² ≈ 346 Pa
• Shaft power: P_shaft = Q·Δp / η_fan; assuming a fan efficiency of 0.65 including silencers, 18.85 × 346 / 0.65 ≈ 10.0 kW
• With a 10% margin, 11.0 kW: it fits an 11 kW motor on paper but leaves no reserve.
• In winter at −10 °C, air density is about 1.34 kg/m³; shaft power rises by the same ratio to about 11.2 kW.

So in this example a 15 kW motor or a lower operating point comes into question; the fan manufacturer decides using its own curve. A 1,000 mm, 4-pole catalogue model close to our example is listed with an 11 kW motor and 580 N of thrust [2]; our calculation being of the same order confirms the method, not the exact value. To cross-check with your own values, use the Pump & Fan Motor Power calculator.

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Speed and poles: why are tunnel fans mostly 4-pole?

In a tunnel fan, pole selection is limited by blade tip speed. A 1,000 mm impeller turning at 1,470 rpm has a blade tip speed of about 77 m/s; if the same impeller turned at 2,950 rpm with a 2-pole motor, tip speed would rise to 154 m/s. For a large impeller this speed is unacceptable in terms of blade stress, balance and noise. That is why in one manufacturer's catalogue 2-pole models are limited to 560–710 mm, while the larger models from 630 mm up to 1250 mm are 4-pole [2]. Small-diameter 2-pole fans suit narrow or short tunnels; large-diameter 4-pole fans suit long road tunnels.

Noise matters too: in the same catalogue, the sound levels of 4-pole models are clearly lower than those of 2-pole models with similar thrust [2]. This difference is important near tunnel portals or in galleries where maintenance staff work.

Silenced casing, two directions and high temperature

Tunnel fans differ from other axial fans in three features:

• Silenced casing: Silencers are added to both ends of the fan. This slightly increases the fan's resistance and therefore the power required; the fan efficiency used in the motor calculation must include the silencers.
• Two directions: Because of traffic direction, wind or the fire scenario, air may be pushed either way. A reversible impeller with symmetrical blades gives similar thrust in both directions; the motor's direction is changed by swapping two phases or via an inverter. As the large impeller has high inertia, it must come to a full stop before reversing, and this waiting time is written into the project scenario. Panel work must be done by a qualified electrician.
• High-temperature classes: Fans with a fire duty are offered in classes such as 200, 300 or 400 °C for 2 hours [1]. EN 12101-3 defines the test conditions of these classes [3]. These classes only make sense when fan and motor are certified together; one cannot say a standard motor "will withstand it" by looking at its insulation class.

In long tunnels fans can also be two-speed [1]: low speed is enough in daily traffic, and full speed is used as the load rises. Instead of a two-speed motor, a single-speed motor and an inverter can be used; the inverter's role in the fire scenario is defined in the tunnel project.

Data needed for motor selection

• Fan nameplate: model, diameter, thrust (N), speed, installed power, direction (one/two directions).
• Old motor nameplate: kW, rpm, voltage and connection (is it 400/690 V?), IEC frame, mounting type, shaft diameter and length.
• The fan's duty: does it have a role in the smoke scenario? If so, the certification class and the fan manufacturer's spare motor information.
• Starting: direct, star-delta, soft starter or inverter.
• Environment: minimum and maximum temperature inside the tunnel, exhaust soot, washing water, salt and vibration.
• Access: maintenance of a ceiling fan requires lane closure; the motor's weight and removal direction matter.

Responsibilities are separate. The tunnel ventilation calculation, number and layout of fans, fan curve, impeller and silenced casing design are the work of the fan manufacturer and the tunnel project engineer. The motor supplier's job is to provide a motor that suits the given shaft power, speed, mounting and ambient conditions.

Mounting and motor connection

The motor is fixed to the support in the middle of the casing and the impeller is mounted directly on the motor shaft; there is no belt or coupling. Depending on the fan model, the connection to the support is foot or flange mounted; the exact IEC mounting type of the motor is taken from the fan manufacturer's drawing. Hanging the fan from the tunnel ceiling and its anti-vibration mounts are a separate matter. DRG motors are supplied in B3/B5/B14/B35 mounting as specified in the order note.

In the DRG range, motors of 4 kW and above have 400/690 V windings; they are delta-connected on a 400 V supply and are suitable for star-delta starting. The large impeller of a tunnel fan lengthens run-up time, so overload setting and starting method must be considered together. For options, see the star-delta starting tool.

Matching with DRG motors

The DRG range covers 0.25–11 kW in aluminium frames and 0.55–355 kW in cast iron, 1000/1500/3000 rpm, 3-phase, 230/400 V or 400/690 V; B3/B5/B14/B35 mounting is set by the order note. The 2.2–30 kW range of tunnel jet fans falls within this range, but this does not mean tunnel fans on the market are DRG products, or that a DRG motor can be fitted to every tunnel fan. The model and price table in the middle of this article is only for fans with no smoke duty: auxiliary galleries, service tunnels, or additional fans with no fire duty defined in the project.

For the 1,000 mm fan in our example, DRG-160M-4 at 11 kW is considered, or DRG-160L-4 at 15 kW if more reserve is needed. The shaft diameter and flange size must suit the impeller hub and the support; compare them with the old motor's dimensions. For general motor requirements in underground galleries and mine ventilation, see our underground mine and tunnel ventilation motors article, and for the effect of speed changes on power, our affinity laws article. All fan applications are on our Fan Motors page.

DRG motors for tunnel jet fans

• DRG-112M-4 – 4 kW – 1,450 rpm – 112M cast iron
• DRG-132M-4 – 7.5 kW – 1,460 rpm – 132M cast iron
• DRG-160M-4 – 11 kW – 1,465 rpm – 160M cast iron
• DRG-160L-4 – 15 kW – 1,465 rpm – 160L cast iron

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Tunnel jet fan motor: FAQ

How many kW is a tunnel jet fan motor?

There is no fixed value; diameter, jet velocity and number of fans decide it. In one manufacturer's catalogue, tunnel jet fan motor power is 2.2–30 kW [1]. For your own fan, check the power on the nameplate.

Why do tunnel jet fans run with 4-pole motors?

On a large impeller, 2-pole speed raises the blade tip speed too much. On a 1,000 mm impeller the tip speed is about 77 m/s with 4 poles and about 154 m/s with 2 poles. Small-diameter tunnel fans also use 2 poles.

Can a standard motor be fitted to a tunnel fan with a fire duty?

No. The motor is part of the set certified together with the fan to EN 12101-3; replacement is made with the fan manufacturer's certified motor. DRG's standard motors are not sold with this certification.

How many jet fans does a tunnel need?

Total pressure loss × tunnel cross-section gives the thrust required; this is divided by the effective thrust per fan. In our example 8 fans came out for a 1,500 m tunnel. The exact number is set by the tunnel project engineer.

Send us your fan nameplate and the old motor's nameplate, and we will determine the right model together: phone +90 (542) 666 11 11, e-mail [email protected], contact page. DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the date of delivery (warranty certificate). For current prices, see Buy Electric Motors Online.

Sources

1. Vitlo – TUNEL-AXF tunnel jet fan (product page) – 560–1250 mm, two-speed and reversible option, silenced casing, temperature classes.
2. Vitlo – TUNEL-AXF Tunnel Jet Fan (catalogue, PDF) – model table: 2.2–30 kW, 1,440–2,950 rpm, thrust and sound values.
3. BS EN 12101-3 – Smoke and heat control systems, Part 3: Powered smoke and heat control ventilators – high-temperature classes and test conditions.