Longitudinal tunnel ventilation by jet fans

This system of ventilation was first tried in Italy about 40 years ago. Ventilation cost is greatly influenced by the section length between access points at which fresh air may be supplied and polluted air exhausted. Longitudinal ventilation systems with­out ducts, in which the whole of the required airflow moves through the tunnel at constant velocity have become increas­ingly popular. To provide a positive longitudinal pressure differ­ence, jet fans (Figure 1.52) are suspended from the tunnel roof and blow in the same direction as the traffic (normally one way) though they are often capable of reversal according to traffic density or for emergency smoke ventilation.

The lower fan efficiency can often be more than offset by the re­duction in the pressure required due to the absence of a ducting system. Tunnels with lengths exceeding 1 km in length become increasingly difficult to ventilate by this method, as the tunnel air velocity becomes excessive. Hybrid systems of longitudinal and extract ventilation have, therefore, been developed.

Many hundreds of kilometres of road tunnel in Italy have been ventilated by the longitudinal induction method, including the Naples Tangenziale, the Lecco-Colico Super Strada around Lake Como, and the Frejus IV tunnel. The method has also been used in the UK for tunnels on the M25 London Orbital Motorway, the A55 North Wales Expressway and the A20, A27 and A38 trunk roads.

Barcelona, the principal commercial city in Spain, staged the 1992 Olympic Games. In order to relieve the current and antici­pated congestion, the government built a new 12 km express­way, almost 3 km of which is underground in cut-and-cover tun­nels.

Smoke eihamted I

 

Sangatte

 

Shaktspear Cliff

 

Pistorveflect relief duct

 

Tram on fire

 

Croat-pat sage door with one-way

 

► Normal air flow • Emergency air flow

 

Air lock at

 

Longitudinal tunnel ventilation by jet fans

Figure 1.50 The Channel Tunnel ventilation system

Longitudinal tunnel ventilation by jet fans

Longitudinal tunnel ventilation by jet fans

Figure 1.51 4.5 m SVS axial flow fans for the Channel Tunnel

подпись: figure 1.51 4.5 m svs axial flow fans for the channel tunnel

Longitudinal tunnel ventilation by jet fans

Figure 1.52 Typical jet fan

Comprising five tunnels, four single way and one for two way traffic, ventilation in these tunnels was designed on the longitu­dinal system, using main and jet fans. The longest tunnel, Vallvidrera, at 2.5 km includes three shafts, each having a 2.8 m aerofoil axial flow fan for smoke venting only. A “Galeria” pro­vides a means of escape and 10 fans of 610 mm diameter, 2 speed, maintain pressure across each door to prevent smoke passing through. 30 purpose-designed jet fans of 1.6 m diame­ter and truly reversible (Figure 1.53) are grouped in 5 rows of 3,

Figure 1.53 Purpose-designed jet fans

15 each at either end of the tunnel. In the 4 remaining shorter tunnels a total of 710 mm uni-directional jet fans are used.

One of the strategic plans for the regeneration of London’s old docklands area, made redundant by the sea container revolu­tion, was the provision of the 1.6 km Limehouse Link road. This is believed to be the most expensive ever constructed on a per length basis.

A major challenge was to design a ventilation system which could deal with a disaster such as a 50 MW fire as well as the pollution caused by very heavy traffic flows. Other factors in­cluded the effect of noise on nearby residents and traffic control in the tunnel. The road was designed for a maximum of 1800 vehicles per hour per lane for free flowing traffic and ventilation is achieved by a system of 128,710 mm jet fans mounted at in­tervals across the tunnel roof in groups of four (Figure 1.54).

Air is propelled in the direction of the traffic flow and then ex­hausted at the portals through grilles in the roof of the tunnel. From there it is ejected through exhaust chimneys by 8 2.8 m di­ameter and 4 x 1.5 m diameter axial fans (Figure 3.55) mounted on the roofs of the service buildings. The ventilation system was complex because of the road junctions.

Extensive computer modelling studies were carried out in order to analyse fire and smoke control in the case of fire or accident. For this exercise, the tunnel ventilation system is divided into

When the late Mr Maurice Woods came to Colchester in 1909, he had previous experience of operating an electrical generat­ing station in Hampstead, London. His main interest was in the design and development of electrical machines and so he set up his company with premises at the Hythe and a total workforce of 6 people.

Figure 1.54 Jet fans used in the Limehouse Link road

подпись: 
figure 1.54 jet fans used in the limehouse link road
At that time electrical voltages and frequencies throughout the United Kingdom werefarfrom standardized and there was con­siderable scope for small manufacturers to provide the many special machines required. Although the majority of motors were wound for DC supplies, Mr Woods built up his business and reputation by competently producing AC single phase ma­chines for 10Ov 100Hz, 300v 400Hz and even 105v 77Hz AC. It was not long before the motors were being applied to ceiling and propeller fans so that by the 1930s, electrically-driven fans were the sole product (Figure 1.56).

Longitudinal tunnel ventilation by jet fans

Figure 1.56 Early production of electrically driven Woods fans

Longitudinal tunnel ventilation by jet fans

Figure 1.55 Axial exhaust fans for the Limehouse Link road

Six areas and the size of blaze anticipated is equivalent to a me — dium-size petrol tanker catching light. The level of ventilation has to be balanced between allowing people to move with safety and the need to blow the smoke away.

Posted in Fans Ventilation A Practical Guide


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