Although jet fans have been widely adopted for normal ventilation of road tunnels, they have limitations when it comes to controlling the products of fire and other contaminating incidents. Therefore it is often necessary to have additional extractor fans which allow the tunnel to be zoned for better control during fire-fighting and recovery operations.

As many of today’s traffic tunnels are within urban environments it is important that the design of fan installations pay due consideration to the tunnel’s neighbours such as residential areas, hospitals, educational establishments etc, by being unobtrusive and having low noise emissions. This applies to both the extractor fans associated with longitudinal ventilation and to the fans for lateral ventilation systems.

Another trend is the recognition of the power requirements of safe traffic tunnels, and their increasing cost. Thus, as with lighting, more attention is now paid to reducing power requirements both by fan selection and installation design.

Recent fan installations

Recent installation examples of Howden low-noise fans include the Ramsgate harbour approach tunnel in south-east England, for which smoke tests were conducted earlier this year (T&TI July).

The tunnel’s normal longitudinal ventilation system uses 18 reversible jet type APR-710 fans mounted in pairs and 85m intervals. These all have 3-D silencers three times their diameter to minimise sound emitted from the portals and the eastern adit shaft. During normal operation air is extracted to the tunnel’s western portal.

The design specification included four axial

extractor fans for emergency use, two of which were positioned horizontally in the western adit fan room, and two vertically in the eastern adit ventilation shaft. However, this design was eventually modified to replace the pair of fans in the eastern adit shaft with a single, large diameter Howden SX fan.

The Ramsgate tunnel eastern adit shaft building was limited to 4m in height as it had to be sited in a public park but the conventional axial fan installation planned would have required 5.5m, including transition pieces and silencers. An additional 1.5m would have been required for an exhaust plenum, and this was unacceptable to the local authority.

Use of the single Howden SX fan instead allowed transition pieces and silencers to be eliminated, saving on costs and space. The final Ramsgate installation has a total height of 2.15m which fits easily within the 4m high shaft enclosure. The developed noise level of the 4m diameter fan requires no attenuation. Compared with a high-speed conventional axial-flow fan, according to Howden, the SX is 15-25dB(A) quieter, even with silencers and transition pieces on the conventional fans.

Table 1, overleaf, gives a data comparison between the twin fans installed in the Ramsgate Western Adit installation, and the Howden SX fan with grp blades installed in the Eastern Adit.

It can be seen that the single 3.96m diameter SX fan eliminates the need for transition pieces, resulting in greatly reduced pressure losses. The single fan has an absorbed shaft power of 14kW instead of the two fans absorbing 36kW. As the SX fan has a low noise level (89dB(A)), silencers were not required, also reducing the space requirements and cost.

The SX model fan has limited applications for tunnel applications but prior consideration of ventilation in tunnel design could increase the opportunities for use. The designed volume flow needs to be relatively high (80-500m3/s), and the total pressure at a rather low 500Pa. As the Ramsgate eastern adit shaft is situated directly above the tunnel, the total pressure requirement was low, allowing use of the SX.

All the ventilation equipment at Ramsgate is designed for operation at 250º C for at least two hours. The fans themselves have been tested for this fire performance under the independent inspection bureau, Verita. Aerodynamic and acoustic performances have been proven in the long-standing heat exchanger and cooling tower applications.

Smoke testing

Although the benefits of the new type of fan installation are clear, the smoke tests at Ramsgate were disappointing. However tests on all three extractor fans showed that performance justified their selection and met contractual obligations. The western adit duo had a total measured capacity of 119.8m3/s compared to the rated 2x50m3/s, and the eastern adit SX fan, also rated at 100m3/s, had a measured capacity of 119.8m3/s. Performances were at lower system design pressures than calculated.

Using smoke formed by heated "atomised" vegetable oil, 14 reversed jet fans alone cleared the tunnel within three minutes at higher air velocities than calculated in the design. In a later test, the Western Adit fans were also put into operation. There was also a natural draft from west to east of approximately 2m/s. Smoke was not extracted from the Western Adit. According to Wil Hofstede of Howden this was because the jet fans air velocity was too high (6-7m/s). As an alternative only jet fans 11 and 12 were then operated, which improved extraction, but not totally satisfactorily. It was also concluded that a tunnel air velocity of 1.5m/s produced the best extraction result, which was below the natural draft at the time.

In another test, the single eastern adit fan was put into operation when the natural draft velocity from west to east had increased to 3m/s. Due to results of the last test, only jet fans 17 and 18 were operated, but the longitudinal air velocity in the tunnel was still too high, according to Howden, and smoke extraction was not optimal.

It was concluded that the ventilation system scenarios would have to be redesigned in terms of the number and direction of jet fans in operation, in order to achieve better smoke extraction. Some observers at the tests also noted that better extraction might be aided by smoother inlet profile into the adits from the tunnel, rather than the existing 90º bends.

The Howden SX fan design was originally developed for cooling tower applications, and features grp polyester composite blades which rotate with a low tip speed, hence the low noise emission and low power requirements. In theory it is possible to produce fans of up to 18m in diameter, although the maximum manufactured so far is 6.24m for a Swiss application.

Recent material developments have made the fan suitable for possible fire situations, as in transport tunnels.

Other applications

Howden Systems is actively bidding for other suitable tunnel and metro projects in which the SX fan can be used.

These include the short Spiertunnel in the Luzern canton, Switzerland for which Howden is building one fan (SX5) of 6.24m diameter for a single extraction point. This will use 90kW power to draw 360m3/s. The fan installation has been developed with EWI Engineering of Zurich.

Another likely application is the Bangkok Metro north line for which a letter of intent has been issued.

The low pressure requirements of shallow tunnels make them likely applications for such fans, especially if the tunnel design takes the possibility into consideration. The absence of a substantial structure makes the fans easy to install. For example, the Ramsgate SX fan was bolted into the housing in only three hours.

Future development

Howden anticipates that the SX fan installation design can be further improved by the development of low speed electric motors (24- or 30-pole), which could allow the elimination of reduction gearboxes. This would obviously reduce space requirements even more and the number of moving parts.