Tunnel ventilation

Ventilation of tunnels is necessary to remove pollution or heat and to control smoke in the event of fire. Pollution in road tun­nels is caused by the emission of carbon monoxide, diesel smoke and the oxides of nitrogen from vehicles. In rail tunnels, it can come from locomotive diesel smoke while, in metros, heat removal requirements are far greater than those for pro­viding fresh air for people.

Heat removal does not normally affect the ventilation require­ments of a road tunnel as the calculated rate is far less than the ventilation rate required for removal of pollution. Heat removal can, however, affect the ventilation rate needed for rail tunnels

— when a train is running in a long tunnel, air may be pushed through the tunnel at nearly the same speed as the train. This causes a local temperature increase around the train and over­heating of the engine.

Control of the effects of fire is extremely important in tunnels. A crucial aspect is determining the design fire size as this will in­fluence the ventilation requirements. In a road tunnel, the source of fire is the vehicles, i. e. the fuel and cargo and the flammability of the vehicle itself. In rail tunnels, the situation is similar, but there is also the possibility of electrical fires. For ur­ban metros, electrical fires are important, as are general fires that can occur in buildings.

Ventilation and smoke control in metros

Ventilation of metros can be divided into two areas: tunnels and stations. At stations, heat generated by people, electrical equipment and trains needs to be removed. This is done by nor­mal HVAC methods, by supply and extract ducts, by through- ventilation or by air conditioning. Tunnels are ventilated by lon­gitudinal methods to remove heat from the trains and to avoid excessive air movement caused by trains.

For smoke control in stations, the system would be similar to that in a normal building except that access to external atmo­sphere for smoke extraction or escape is more restricted. The station has to be divided into compartments from which smoke can be extracted using high temperature fans and ducting.

Smoke should be extracted as close to the source of fire as pos­sible and so it is important to know the design fire size and the amount and temperature of the smoke that will be emitted. The fire theories developed originally by the UK Fire Research Sta­
tion (FRS),can be used for this purpose. The smoke tempera­ture calculated should be the maximum. If the smoke is diluted with fresh air the temperature will drop. A difficult problem to overcome is the natural buoyancy of the smoke, which causes it to rise even against opposing airflow.

For tunnel ventilation, total heat input from the trains is used to calculate the ventilation requirements. Heat is usually removed by extracting air from the tunnels, with replacement air coming partly from the station and also from supply fans close to the stations or from natural vent shafts. Natural vents or fans are also often used to reduce the airflow caused by the piston effect of the train.

For smoke control in a tunnel, the principle is to provide suffi­cient velocity to blow the smoke away from escape routes. This is normally not difficult as the fires are usually small, about 15MW. Research has established the rate at which smoke is produced and travels away from a fire. If air velocity exceeds this figure, the smoke will move away from the fire. Fora 15MW fire, 3m/s in the tunnel cross-section is sufficient to ensure this. The fans are usually fully reversible to enable the closest es­cape route to be used.

Although large areas are conventionally ventilated by using large fans to supply and extract air, jet fans can also be used (see Figure 21.25). These induce airflow by providing a high ve­locity jet of air, which diffuses and entrains the air through which it passes. The energy of the jet converts to static pressure, which causes a general air movement. Jet fans can be used in tunnels or large spaces such as covered stations. See Figure 21.26.

Ventilation of mainline rail tunnels

Rail tunnels are generally simpler to ventilate than metro tun­nels. There is usually a single tunnel open at each end, with a central extract/supply shaft. As these tunnels are not usually connected to others, there is no need for complex network cal­culations involving movement of trains or combinations of fans.

In most tunnels everyday ventilation is provided by the passage of trains. However, in some tunnels, ventilation will be required to remove diesel smoke or to ensure sufficient air movement past the trains to cool them. In all but the shortest tunnels, there should be ventilation for controlling smoke in case of fire.

Tunnel ventilation

Figure 21.25 A 2500 mm reversible main ventilating fan for Western Harbour Crossing Tunnel, Hong Kong

Tunnel ventilation

Figure 21.26 Three 1250 mm jet fans used for ventilating the Nanterre-Orgeval Tunnel, France

For rail tunnels, the only practical ventilation system is longitu­dinal air movement along the tunnels. This can be provided ei­ther by large, reversible fans extracting/supplying at the centre of a tunnel, or by jet fans. If air is extracted or supplied at the centre of a tunnel, however, it enters or leaves by the easiest path, usually through the empty tunnel, not the section contain­ing the train. So the relative system resistances must be taken into account when calculating the flow past the train.

With a jet fan system, air enters through one portal and exits through the other so all the air has to pass the train. The fans can be 100 % reversible to direct airflow away from the escape route. However, with a longitudinal system, if there is a fire in the centre of the train, the smoke has to be directed past parts of the train where there may be passengers.

Smoke control is again based on providing sufficient velocity to ensure that smoke only goes to one side of the fire, and on choosing the direction of flow to provide safety for the greatest number of people. Fans used for smoke control in rail tunnels would normally be rated for 150°C or 250°C for one hour.

Road tunnel ventilation

Road tunnel ventilation rates are based either on pollution con­trol for comfort or the control of smoke in case of afire. For com­fort, the rate depends on the levels of carbon monoxide, diesel smoke or the oxides of nitrogen.

The pollutant emission levels of vehicles can be obtained from the handbook issued by the World Road Association (previ­ously known as PIARC), which is formed from representatives of many nations. The information is in the form of base data with factors that are used to represent the differing conditions in each country. This base is then modified for vehicle speed, the gradient of the road and the altitude of the tunnel.

It is also necessary to know the number and weight of vehicles in the tunnel. The number of vehicles is calculated from the length of the tunnel, the number of lanes and the number of ve­hicles/kilometre/lane. This information, from an earlier edition of the PIARC handbook, is shown in Figure 21.27.

Revised figures presented at the Montreal Conference of PIARC in 1995 and later conferences give lower levels based on’the improved emission rates of modem vehicles. For global

Tunnel ventilation

0 10 20 30 40 SO 60 70 80

Vehicle speed (km/h)

Figure 21.27 Maximum vehicle statistics

Installations, and taking into account the emissions from older vehicles, the rates shown are still recommended for tunnels in any country other than the developed world.

The ventilation rate required is calculated from the number of vehicles in the tunnel multiplied by the weight of the vehicles, times the specific emission (CO, NOx, and smoke), divided by the required increase in the level of emission (CO, NOx and smoke). Each emission will give a different ventilation rate, so the highest must be used.

With the poisonous gases

Carbon monoxide is a short-term poison that is easily absorbed into the body, but is also quickly given up. The normal levels of dilution designed for are 100 to 250 ppm. Diesel smoke ob­scures vision and therefore the levels have to be set to ensure sufficient visibility to drive safely. The allowable levels of obscu­rity are between 0.005 m_1 for high speed conditions to 0.009 nr1 for lower speeds. The actual level set will depend on the country, speed, traffic conditions and lighting. Oxides of nitro­gen are long-term poisons and are not normally important un­less background levels are already high.

When establishing the ventilation rate for these gases, allow­ance must be made for the pollution level of the “fresh air". Any air supplied to the tunnel may already be partially polluted. Ex­amples are: a city centre ortwin tunnels, when the discharge air from one tunnel may partially recirculate into the inlet of the tun­nel in the opposite direction. If the air is already polluted it could lead to a large increase in the required ventilation rate.

Control of smoke and hot gases

In recent years the effects of fires in tunnels has received con­siderable attention, following some well-publicised catastro­phes. The ventilation necessary to control smoke and hot gases in the event of fire has been the subject of much re­search.

Two methods may be used. The smoke is either extracted at high level and removed through a ducted system, or air is blown towards the fire to force the smoke and hot gases in the desired direction. Tests have been carried out by PIARC to establish the volume needed in either of the cases described above.

Ventilation systems

For road tunnels, a number of types of ventilation system can be used, either singly or in combination.

• Fully transverse

• Semi-transverse

• Mixed

• Longitudinal

A fully transverse system has ducted supply and extract air. The supply is normally at low level from a duct underneath the road­way, so that the fresh air mixes with the exhaust fumes from the vehicles. The polluted air, which is buoyant, is extracted at high level, normally through a ducted system above the roadway, see Figure 21.28. It is technically the most exact system as it is not affected by wind pressure on the portals or traffic move­ment. It is normally used on long, congested or two-way tun­nels. The costs of the civil construction and the mechanical equipment is high and the air distribution needs to be balanced carefully.

Tunnel ventilation

Figure 21.28 Fully transverse ventilation system

The fans used would usually be large axial flow fans operating in parallel, normally of fixed pitch design, with the total volume being controlled by switching fans on and off. Additional volume steps and reduced running costs can be achieved by using two-speed fans. Variable pitch and variable speed fans can also be used. The fans can be installed with their shafts vertical or horizontal. Splitter silencers are usually needed on both the system and atmospheric side of the fans. Two-speed fans can offer lower noise levels at night, when ventilation needs are nor­mally lower.

When a fire occurs in a tunnel without longitudinal movement of air through the tunnel, it draws in fresh air from either side and produces a plume of smoke and hot gases. The plume stratifies above the fresh air and spread sideways along the tunnel, typi­cally for 300m on each side of the fire. Beyond this distance, it cools sufficiently to lose its buoyancy and begins to mix with the fresh air being drawn in by the fire.

For effective smoke control, smoke must be extracted before mixing occurs. The system may extract air from the correct point, but it may not extract sufficient to capture all the smoke produced by the fire. The volume of air removed from close to the fire must therefore be increased. Often, additional extract points with dampers are fitted to the system. Those near to the fire can then be opened and the others closed to ensure air close to the fire is extracted.

If the tunnel is for one-way traffic, it is preferable to blow smoke and hot gases away from stationary vehicles. To achieve this, the supply and extract fans in different parts of the tunnel can be reversed to create the required longitudinal velocity.

17.5.5.2 Semi-transverse system

This system is similar to both the supply or extract system of a fully transverse system. If it is an extract system, (see Figure 21.29) the air is extracted at high level and the fresh air enters at the portals and is extracted through grilles at high level. The air becomes progressively more polluted towards the centre of the tunnel, so the ventilation rate must be increased to compen­sate.

Tunnel ventilation

Figure 21.29 Semi-transverse (extract) ventilation system

Tunnel ventilation

Figure 21.30 Semi-transverse (supply) ventilation system

In a supply system (see Figure 21.30), fresh air is supplied at low level and polluted air exits from the portals. All the air would be at the maximum allowable pollution level so there is no need for the ventilation rate to be increased. The air ducts can be above or below the road surface, or at the side of the carriage­ways.

A semi-transverse system is technically less exact than a fully transverse system as it relies on longitudinal air movement along the tunnel, which will be affected by wind pressure on the portals and by the movement of traffic. The ventilation rate, therefore, has to be increased to take into account the reduc­tion of airflow in a particular direction due to these effects.

A semi-transverse system would be used for long, congested or two-way tunnels, although it may be limited by a maximum ve­locity at the portals. To reduce the portal velocity, a semi-trans­verse system may be used for the end sections of a tunnel with a fully transverse system for the centre section. Like the fully transverse system, a semi-transverse system can be expen­sive to construct and install.

For smoke control, a semi-transverse system has to operate in the same way as a fully transverse system, with extraction at high level. If it is a supply system, the fans must be reversible and able to exhaust from high level, with dampers to close off the low-level grilles. Fan installations are as for fully transverse systems but, as there is a longitudinal movement of air along the tunnel, the resulting pressure loss must be added to the loss in the ducted system.

21.4.5.3 Mixed system

A supply duct may feed part of the length of a tunnel section and an extract duct draw from the remaining length, with longitudi­nal flow between sections. In long tunnels through mountains, a longitudinal system can be used provided there are shafts to the surface to supply and extract air at regular intervals.

In a longitudinal ventilation system, air movement is along the length of the tunnel so there is no need fo r distribution ducts in the tunnel. The air can enter at one portal and leave at another, or be supplied or extracted from locations in the tunnel. Air movement can be induced either using large fans at these loca­tions, by jet fans, or a combination of both types.

All the polluted air from a longitudinally ventilated tunnel exits from the portals. Large fans can be installed at these points to extract and discharge the air at high level. If the design velocity in the tunnel is very low, the fresh air may not be turbulent enough to dilute exhaust fumes from one car before they enter the air intake of the next. However, if the ventilation rate is de­signed to cope with fire, this is unlikely to occur.

If the tunnel is in an exposed position, the wind effects on the portals may be higher than the pressure that can be overcome using jet fans. In this case, the fans can be 100 % reversible to ensure that the airflow through the tunnel can follow the same direction as the wind pressure. The ventilation rate is deter­mined eitherfrom the rate needed to dilute pollutants, or to blow smoke away from a fire.

Jet fans are usually installed in multiples spaced along the tun­nel. Fan running time can be low because the air flow induced by traffic movement often provides sufficient ventilation. The fans are, therefore, sequenced to ensure uniform operating hours. It is important to note that the airflow through the tunnel is due to the pressure rise caused by the jet fan, which creates thrust by ejecting a jet of high velocity air. As this air deceler­ates, it transfers its energy to the general flowrate, causing a pressure increase equal to the fan thrust divided by the cross­sectional area of the tunnel. This pressure pushes the airflow through the tunnel and overcomes the drop in tunnel pressure.

As the deceleration of the air occurs gradually, if the longitudinal distance between fans is insufficient the deceleration will be in­complete and the increased velocity will affect the performance of the next set of fans. It is common to have ten tunnel diame­ters between sets of fans to eliminate this problem.

Other effects to be taken into account are the velocity in the tun­nel and the proximity of the fan to the walls and ceiling. Fan thrust is measured under still conditions and is due to the change of momentum of air passing through the fan. If the air is already moving at the inlet to the fan, the change of momentum is reduced. Figure 21.31 shows the magnitude of this effect.

Tunnel ventilation

Fan Velocity m/s

If a jet fan is installed close to a wall, there will be an additional loss due to the friction between the jet and the wall. PIARC sug­gested that the approximate effect of this is as shown in Figure 21.32. If the fan is close to a wall and a ceiling, the effect must be applied for both.

Reversible fans are slightly less efficient and slightly noisier than uni-directional fans but they are more flexible. They allow one-way tunnels to be used for two-way traffic when required and the normal direction of airflow to be reversed in fire condi­tions.

For tunnel ventilation schemes, jet fans offer lower capital and operating costs. They are often used with other systems, an ex­ample being to provide airflow at the required velocity in the event of fire or to provide the first levels of ventilation at reduced operating costs. Jet fans provide the performance required by a longitudinal ventilation system, which is the system generally accepted within Europe as the designer’s first choice.

However, tunnel length can be a limiting factor as there is a practical maximum for the air velocity in the tunnel. For safety, this should be below 10 m/s; velocities above 7 m/s are rare. This system is also not satisfactory for tunnels of over 300 m with urban two-way traffic, unless there are emergency exhaust fans for smoke venting and protecting passenger escape routes. Potential fire loads of 10 MW and greater precluded the use of earlier designs of jet fans where the average air tempera­ture could exceed 300 °C. But, in the last few years, these fans have been tested and certified for operation at 400 °C for two hours. This will enable people to be evacuated safely before fan failure.

Axial flow fans for vehicular tunnels

The most common type of axial flow fan has an impeller de­signed to operate mainly in the forward direction but is capable of being reversed. In order to improve performance in the re­verse direction it does not have guide vanes. Such a fan would give approximately 65% of the forward flow in reverse and 50% of the pressure development. If the performance in reverse is less important, fans can be fitted with guide vanes to increase pressure development.

When air passes through a fan, the impeller turns the air and gives it a rotating component. The guide vanes turn this compo­nent into the axial direction with a regain of the kinetic energy from the rotating velocity. The increase in pressure depends on the type of fan but it could be 5-50%. The performance in re­verse would typically be 55% of the volume in the forward direc­tion, with 35% of the pressure.

Tunnel ventilation

Figure 21.32 Tunnel fan installation factors

подпись: figure 21.32 tunnel fan installation factorsIf a fan is needed to give the same performance in both direc­tions, this can be achieved in two ways. If first cost is the most important criterion, a reversible impeller can be manufactured by turning alternate blades through 180°. In this case, the blades running in the forward direction do most of the work but there is additional drag from the blades operating in reverse. This type of fan produces about 85% of the volume of a stan­dard fan without guide vanes and 70% of the pressure. If perfor­mance and efficiency are important, a fan with 100% reversible blade sections is selected. This fan gives about 90% of the vol­ume of a standard fan and 85% of the pressure.

Separation factor 2z/(DT — DF)

подпись: separation factor 2z/(dt - df)A 100% reversible section consists of two top surfaces of an aerofoil section back-to-back in the reverse direction. The top surfaces of an aerofoil section creates nearly all the lift of the section by reason of the accelerated flow, which gives a nega­tive pressure. When the fan operates in one direction, the top surface in the forward direction produces its normal lift although there will be additional drag dues to the extra thickness at the trailing edge from the reversed top surface. When the fan oper­ates in the reverse direction, the other surface operates in its normal direction and an identical performance is produced. This type of section increases the fan efficiency by at least 5% compared to a fan with reversed blades.

The other type of fan commonly used for tunnels is a contra-ro­tating axial flow fan. This consists of two similar fans operating in series with the impellers rotating in opposite directions. The first impeller rotates the air in one direction and the second in the opposite direction so that it leaves the fan nearly axially. Thus, the contra-rotating fan develops up to three times the to­tal pressure of a single fan at a very high efficiency.

Flowrate control

The technical considerations for flowrate control are detailed in Chapter 6. Flowrate control for tunnels is not required to make the ventilation system operate correctly. It is, however, used to reduce the operating costs of the tunnel since only four to six steps of ventilation are necessary.

The methods of flowrate control are:

• Variable speed

• Variable pitch

• Fans in parallel Variable speed

The variable speed fan is a simple concept and usually employs induction motors driven by inverters. This method gives a good reduction of power with volume because, as a first approxima­tion, the power reduces with the cube of volume. However, there is an initial penalty in the additional losses in the motor be­cause the supply from the inverter is not truly sinusoidal and there is the loss in the inverter itself. The latter is an extra heat input into the plant room, which must be removed by ventilation.

The use of an inverter reduces starting current and so gives the best ratio of starting current to starting time. Care must be taken over the design of the system and input and output filters incor­porated as necessary to eliminate problems with electro-mag­netic compatibility and avoid affecting the mains supply. Also, larger inverters in some cases cost more than the complete fan.

Operating at a reduced speed reduces bearing wear and the temperature rise of a motor, although the non-sinusoidal wave form does give rise to additional magnetic forces in the winding.

The reliability of the winding, therefore, would not necessarily be improved by this decrease in temperature. The noise level of the impeller will be reduced by about 17dB at half speed, but ad­ditional magnetic noise may be apparent, particularly at the lowest speeds.

Variable pitch

A variable pitch fan is fitted with an electric, pneumatic or hy­draulic actuator, which enables the impeller pitch angle to be changed while the fan is running. The fan volume flow can thus be reduced by reducing the pitch angle. Again, the first approxi­mation to power reduction is that it reduces by the cube of the change of flowrate; however, the fan is normally selected for high efficiency at the maximum duty point and so, as the flowrate is reduced, so is the fan efficiency. The sound level re­duces as the volume is initially reduced but can increase again at very low angles when the fan efficiency is very low.

This type of fan requires more maintenance and, because of the complexity of the design, is less reliable than a fixed pitch fan. The cost of a variable pitch fan would be between that of a fixed pitch fan and a fixed pitch fan plus inverter. Operating at reduced pitch angle and power will reduce the temperature rise of the motor and increase the life of the motor windings. Starting at reduced pitch angle will reduce the run-up time, particularly at reduced voltage.

Fans in parallel

Flowrate control can be most simply achieved by using more than one fan in parallel, i. e. the volume flow is divided between the number of fans used, all operating at the same pressure. The flowrate is controlled by switching off fans. As each fan is switched off, the flowrate through the total system reduces and so does the pressure. The fans therefore move down their fan characteristic curves actually giving less pressure but more flow from each.

The power does not reduce directly with the volume cubed as the losses in the parallel part of the installation (the fan, damper and connecting duct work) do not reduce because the volume through this section does not reduce. But if two-speed motors are used with full and three-quarter speeds, the power on the lower speed will be only 42% of that on high speed for a volume of 75%. Subsequent volume and power reductions can be made from this low level.

The other advantage of using a number of fans in parallel is that the fans are smaller, more standard and can be factory assem­bled and shipped ready to install. A higher percentage of stand-by in case of a fan failure is also achieved.

Calculation of jet tunnel fan requirements

Fresh air requirements

Fresh air required will depend on the following factors:

• The specified maximum permitted carbon monoxide and diesel smoke level. (Nitrous oxides have not been consid­ered to be significant in the past, but in recent times levels are now being specified.)

• The number of vehicles/hour and the number of these which are diesel-fuelled

• Speed of traffic

• Gradients

• Altitude

In practice, it is accepted that the maximum air requirements occur when traffic is heavily congested at a speed of 10-15km/h. The figures in Tables 21.4 and 21.5 are based on vehicle emission data published in 1987 by PI ARC following the XVIIIth World Road Congress.

Type of tunnel

CO at peak traffic (ppm) congested traffic or standstill

Urban tunnels (used to capacity)

Daily congestion

100-150

Seldom congested

250

Inter-urban tunnels

250

(highway or mountain)

Table 21.4 Recommended CO levels (PIARC 1987)

Permissible visibility limited

Type of tunnel

Klim m’1

Urban tunnel with dense rapid traffic

0.005

Congested traffic

0.009

When K = 0.12 m’1 the tunnel must be closed

Table 21.5 Recommended visibility limits (PIARC 1987)

The recommended CO levels are shown (see also Figures 21.33 and 21.34). The figures in Table 21.5 are also based on diesel vehicle emission data published in 1987 by PIARC. The

Tunnel ventilation

Parts per million CO Figure 21.33 Fresh air requirements for CO dilution at sea level

Tunnel ventilation

Parts per million CO Figure 21.34 Fresh air requirements for CO dilution at 800 m altitude

= P

подпись: = p

Where:

P

Q

A

подпись: where:
p
q
a
Tunnel ventilation

— io m Axial velocity (m/s)

подпись: — io m axial velocity (m/s)

10 trucks per km at 10 km/h 15 tonnes at 800 m altitude

Tunnel ventilation

Visibility limit M-1

Figure 21.35 Fresh air requirements for smoke dilution at 800 m altitude

10 trucks per km at 10 km/h 15 tonnes at sea level

Tunnel ventilation

.015 .012 .009 .0075 .005

Visibility limit M*1

Figure 21.36 Fresh air requirements for smoke dilution at sea level

Charts assume congested conditions with 10% of the vehicles having diesel engines, and having an average weight of 15 tonnes. (Figures 21.35 and 21.36 give the recommended fresh air requirements for smoke dilution.)

Tunnel thrust requirements

When the airflow requirements have been established it is nec­essary to calculate how much thrust is required to overcome the resistance of:

• Inlet and outlet loss of the tunnel The combined loss is generally assumed to be about 1.5 times the tunnel air dy­namic pressure.

• Tunnel surface friction The loss associated with sus­pended fittings and road direction signs.

1 ? L

Pressure loss =-pVT f— Equ21.5

2 T DH

Where:

VT = tunnel velocity (m/s)

L = length of tunnel (m)

DH = hydraulic diameter (m)

(circular equivalent to tunnel cross-section)

F = friction factor

P = air density (kg/m3)

The value of “f” is generally taken as 0.025, but can vary from

0. 02 to 0.04 depending on surface roughness and physical ob­structions like lights, signs, etc.

There will be drag resistance where the traffic speed is lower than the average tunnel velocity in a single-direction tunnel. In a two-way tunnel, the traffic speed in the opposite direction to the tunnel air velocity must also be considered. Wind or tempera­ture and barometer pressure difference between tunnel entry and exit must be considered.

The actual total thrust required from the fans is:

Total thrust = P x AT (Newtons)

Where:

P = summation of pressures (Pa)

AT = tunnel area (m2)

The rating of a jet fan is commonly identified in terms of thrust applied to the air. The basic thrust rating is equal to the change of momentum between the fan inlet and outlet which is the prod­uct of the mass flow and some average velocity.

Theoretical thrust = air density xair volume xair velocity Q2

Equ21.6

= air density (kg/m3)

= airflow volume (m3/s) = fan outlet area (m2)

In the simplified equation 21.6 the nominal outlet velocity has been used, i. e. the fan flowrate divided by the fan outlet area. With a jet fan this is far from correct as the velocity varies con­siderably at the outlet plane (Figure 21.37).

Tunnel ventilation

Figure 21.37 Axial velocity profile along a jet fan

Strictly speaking, the local thrust at varying radii should be inte­grated. Immediately on the fan outlet this is difficult and the thrust is therefore measured on a rig in accordance with ISO 13350.

Measured thrusts have been found to vary between about 85% and 105% of this theoretical value, depending on the blade de­sign and resultant velocity distribution, distance from impeller to silencer outlet, effects of swirl, etc.

Equ21.7

The entry and exit loss, PEn ex. is normally assumed to equate to about 1.5 times the tunnel air dynamic pressure. With careful design by the use of a “streamlined bellmouth” opening to the tunnel and gradual diffusion at the exit, these losses can be re­duced.

1

PdT =цPvt

Where:

PdT

VT

P

Note:

VT =

Tunnel air dynamic pressure (Pa) average tunnel air velocity (m/s) air density (kg/m3)

QT

(N)

Equ21.11

The value of “f can vary from about 0.02 minimum to 0.04 maxi­mum. It is dependent on the surface roughness of the tunnel surfaces and the size and number of the tunnel fittings. In the absence of information to the contrary, a reasonable figure to use is f = 0.025

Total tunnel thrust — TT

The total thrust required from the jet fan is numerically equal to the losses in the tunnel

TT = pTAT (N)

Where pT is the summation of the pressure losses in items 1.0 to 4.0 i. e.,

Pt = Penex + Pdrag — Pstack + Pl Equ21.10

Jet fan thrust

To assist in the design of longitudinal ventilation systems by jet fans, it is convenient to rate them in terms of the thrust applied to the air, which they develop.

The basic thrust is equal to the change of air momentum be­tween the fan inlet and outlet. This is the product of the mass air flowrate and the “average” air velocity at the fan inlet/outlet. The theoretical fan thrust is given by:

Tm = air density xair volume flowrate xair velocity

Where:

VT

=

Average tunnel air velocity (m/s)

L

=

Length of tunnel (m)

Dh

=

Hydraulic diameter (m)

F

=

Friction factor

P

=

Air density (kg/m3)

Note:

Dh =

4AT

Pt

Where:

Pt

=

Tunnel periphery at a section (m)

At

=

Cross-sectional area of the tunne

Where:

QVF — fan air volume flowrate (m3/s)

VF = average velocity at fan outlet (m/s)

AF = cross-sectional area of fan (m2)

It should be noted, however, that this formula is only correct for a uniform velocity. The velocity profile at the fan outlet is far

=pqvFvF „ 2 _pqyF

1 2fL

P. =-pvT f —

2 Du

Pdrag C

Av

AT

Mountainous environments. There may then be differences in wind velocity/direction, air temperature and barometric pres­sure, leading to stack effects either adding to or detracting from the tunnel resistance. The difference in barometric pressure at the two tunnel ends, if measured in Pascals is simply added to the system loss. This effect is known as pstack.

Tunnel surface friction

Tunnel surface friction, together with the loss associated with suspended fittings such as lighting, road direction signs etc needs to be established. This pressure loss can be calculated from:

 

Tunnel ventilation
Tunnel ventilation
Tunnel ventilation

Equ 21.9

 

Tunnel ventilation

Where:

QT = tunnel air volume flowrate (m3/s)

AT = tunnel cross sectional area (m2)

Traffic drag or resistance

There will be a drag resistance in a single direction tunnel where the vehicle speeds are lower than the average tunnel air velocity. In a two-way tunnel, the traffic speed in the opposite di­rection to the tunnel air velocity must also be considered.

In recent times it has been recognised that jet fans can be used in the event of fire for the control of smoke. Depending on the predicted fire size, the resultant drag from a large number of stationary vehicles could be higher than that for moving traffic, when there would be fewer vehicles in the tunnel.

Traffic drag loss in a bi-directional tunnel may be approximated as:

 

Tunnel ventilation

(NC1 + NTi)(vv1 + VT)2

—(NC2 + Nt2)|Vv2 “ VT|(VV2 “ VT)

 

Equ 21.8

 

Where:

Pdrag

Cd

Av

Nci

 

Pressure loss due to traffic drag (Pa)

Vehicle coefficient of drag (1.0)

Frontal area of vehicles (cars 2 m2, trucks 6 m2) (m2)

Number of cars in tunnel moving against airflow

Number of trucks in tunnel moving against airflow.

Number of cars in tunnel moving with airflow

Number of trucks in tunnel moving with airflow

Vehicle speed of traffic moving against airflow (m/s)

Vehicle speed of traffic moving with airflow (m/s)

 

*C2

 

Vv1

 

Vv2

 

In a unidirectional tunnel the second term within the square brackets becomes zero.

Ambient conditions

At the tunnel entry and exit, ambient conditions may differ espe­cially where the tunnel is long and/or the altitude changes as in

 

From even. The degree of distortion is very much dependent on the fan design particularly the hub to tip ratio on the impeller, the basis on which the blades have been designed (free, forced or arbitrary vortex) effectiveness of fairings, motor obstruction etc.

The measured thrust is obtained from tests carried out in accor­dance with ISO 13350. It varies from between 0.85 and 1.05 times the value of the “theoretical” thrust. Other designs have been tested with values as low as 0.65 times the “theoretical” thrust.

The total thrust developed by a number of fans in a tunnel is the sum of the individual thrusts. Fans may be located in groups op­erating in parallel, or in series spaced lengthwise along the tun­nel, or any combination of the two.

General working rules are that fans in series should be spaced at ten or more tunnel diameters apart. Alternatively the spacing (m) can be taken as equal to the fan dynamic pressure (Pa) — h 10. Fans in parallel should have a minimum distance between centres of 2.0 times fan diameter.

The above rules are of necessity approximate only. More accu­rate calculations require knowledge of the Craya-Curlet Number for detailed tunnel designs.

The number of fans required:

Nf=-^ to the next whole number Equ21.12

Where:

TT = total tunnel thrust (N)

T, = installed fan thrust (N)

When installed in the tunnel the actual thrust transmitted to the tunnel air will be less than that measured under the laboratory conditions specified in ISO 13350. Thus:

Installed fan thrust"^ =Tmk., k2 k3 Equ21.13

Ki is a correction coefficient based on the fact that the tunnel air velocity “offloads” the fan as compared with still air conditions. This may be obtained from Figure 21.31.

K2 is a correction coefficient based on the knowledge that as the fans are eccentrically placed in the tunnel adjacent to one or two surfaces, some of the air will attach itself to the wall and or roof, rather than be directed into the main flow. This effect will be more severe the closer the fans are to these surfaces.

Figure 21.32 is for the case of no inclination i. e., the jet fan is parallel to the tunnel axis. In Figure 21.32:

Z = distance of jet axis to tunnel wall or ceiling

Df = jet fan diameter

DT = tunnel diameter (can use hydraulic diameter

For rectangular tunnel too)

Note: The corner factor applies to a fan installed equal dis­tances from wall and ceiling.

The horizontal axis is more complex than most graphs of this coefficient reflecting the effect of both tunnel and jet fan diame­ters. It is developed from work carried out at London South Bank University.

K3 as found from Figure 21.38 is a correction coefficient based on the knowledge that a small jet fan inclination of the fan can improve the installation performance.

Here, a family of curves have been presented for 4 separation factors (note the separation factor is the horizontal axis on Figure 21.32).

As expected, for the smallest separation, the jet clings to the wall, even at high inclination. The best result from jet fan inclina­tion is achieved at a separation factor of 0.16, where an inclina­tion of about 7 degrees gives an increase in thrust of 10%.

Separation Factor 2z/(D. — Df)

Tunnel ventilation

Figure 21.38 Effect of jet inclination on fan thrust

‘——- ‘

”1 !

— °F

1.5 D,

| 1-5 Dc

Figure 21.39 Typical tunnel niche

Clearly the optimum inclination angle differs with separation factor, increasing with decreasing separation.

Other factors which can affect the installed thrust capability are:

A) How close the first fan is mounted to the tunnel entry por­tal.

B) How close the last fan is mounted to the tunnel exitportal.

C) lnimmersedtubeorcutandcovertunnels, it iscommonto have limited headroom and to locate fans in areas that are locally heightened i. e., niches.

For the typical construction shown in Figure 21.39. The com­bined coefficient for k2. k3 for the jet fan installed with the centre of the discharge on the ceiling line is as follows:

Inclination angle (°)

K2 ■ k3

0

0.82

5

0.88

10

0.93

15

0.90

Testing for performance Air flow

The volume flowrates are measured using complete units, i. e. the fan with silencers and bellmouths fitted, but with the inlet bellmouths replaced by inlet measuring cones in accordance with ISO 5801. Single measurements are taken and corre­spond to the volume flow close to zero static pressure. The ve­locities are derived from the area calculated from the inlet/outlet diameter.

Thrust

To measure the thrust, the jetfoil units are mounted on a test rig. This consists of a platform, supported by lowfriction linear bear­ings, mounted on a frame. The bearings constrain the move­ment to that in the direction of the axis of rotation of the fan. The platform is restrained by a load cell which measures the force exerted by the fan.

The rig is installed centrally in a large building to ensure that the circulating velocities are low and that there are minimal effects from the proximity of walls, ceiling and floors. The rig is levelled and the force exerted by the fan measured when the thrust and
power reading have stabilised. This arrangement is one of the methods approved in ISO 13350.

Figure 21.41 Typical fan locations

подпись: 
figure 21.41 typical fan locations
“Real” thrust requirements

The total thrust developed by a number of fans in a tunnel is usually calculated as the sum of the individual thrusts. Fans may be located in parallel groups but should be spaced a num­ber of tunnel diameters apart length-ways to ensure that one fan does not affect the other. Alternatively, an empirical rule is that the spacing can be taken as:

Spacing (m) = fan dynamic pressure (mm)

Mounted in a tunnel, the fan will be off-loaded by the tunnel ve­locity, which will reduce the thrust available as seen in Figure 21.31. The higher the outlet velocity, the less is the fall-off in thrust. However, a high outlet velocity fan has a lower rating of Newtons/kilowatt. The distance of the fan from the tunnel wall or ceiling will also have an effect as seen in Figure 21.32. While work carried out in the late 1990s indicates that the ratio of fan diameter to tunnel diameter will also be of importance (Figure 21.40).

Tunnel ventilation

Figure 21.40 Variation in efficiency with separation ratio

.1 0.2 0.

Separation ratio

Log curve

Y = 97.536 +

Y =96,484+

Y « 96,831 +

Fit equations 10.201 ,LOG(x) 12.998,LOG(x) 18.363*LOG{x)

Installed fan thrust= N.,

подпись: installed fan thrust= n.,K^N^IC, Equ21.14

Where

Ni

= thrust measured under laboratory conditions (Newtons)

VT

= average tunnel velocity (m/s)

VF

= average fan outlet velocity (m/s)

Ki

= velocity coefficient (see Figure 21.31)

K2

= proximity of fan to tunnel wall coefficient (see Figure 21.32)

K3

= inclination of fan to tunnel coefficient (see Figure 21.38)

Other factors which can influence the result are:

VF

• The distance apart of adjacent fans

• The distance between successive fans down the length of the tunnel

• How close the first fan is mounted to the tunnel entry portal

• How close to the exit portal the last fan is mounted

Fans would typically be located as shown in Figure 21.41. Where the tunnel must cater for traffic in either direction, fully reversible fans may be necessary, with blades designed to give equal flow for either rotation. Uni-directional fans may be ac­ceptable and these can give about 60% of the forward thrust in reverse.

Tunnel ventilation

It is apparent that there is a degree of empiricism in the calcula­tion of such systems. They work, but there are a number of “co­efficients” used which disguise the lack of knowledge. In the major fan companies, these have been the result of extensive research programmes. They are unique to a particular design of fan and its consequent velocity profile.

Manufacturers look on them as closely guarded secrets and they are applicable to their product range. The alternative, and equally valid, statement is that they are unknown for some com­panies.

Guidelines for jet tunnel fan selection

When selecting jet fans the following guidelines should be fol­lowed:

• Choose the largest fans that can be fitted. Larger fans give a higher ratio of thrust to capital and installation costs than smaller fans.

• For the lowest operating cost, choose a low speed and/or pitch angle. The ratio of power to thrust is directly related to the fan outlet velocity, so for any given thrust requirements, the higher the velocity the higher the power consumption. Reducing the thrust for a given size of fan increases the number of fans and hence the capital cost, unless using a lower velocity also means that the length of silencers can be reduced of the silencers eliminated completely.

• The installation cost can be reduced by providing local niches or installing fans in car-only lanes.

Ventilation during construction

Ventilation is needed for construction as well as operation of tunnels. The requirements are for:

• sufficient fresh air for the people working in the tunnel

• removal of pollution from diesel powered vehicles

• removal of heat

• removal of dust caused by blasting.

The requirement for fresh air given in BS 6164 for working in tunnels which are freely ventilated, (i. e., not working in com­pressed air) is 9 m3/min/m2 of tunnel face. To this is added 1.9 m3/min per kilowatt of diesel powered vehicles.

Fresh airfor heat removal depends on heat input and allowable temperature. If the ventilation rate required is too high, cold wa­ter cooling systems can be used. The ventilation rate for blast­ing depends on the rate at which the fumes need to be cleared.

2 m3/min/kg of gelignite will clear the face in 20 minutes. The ventilation system used consists of flexible ducting at high level in the tunnel and axial fans fitted as close to the inlet portal as possible to reduce cabling costs. The ducting is used to supply fresh air close to the face, particularly important when blasting.

The air becomes progressively polluted as it moves away from the face, so any air added to the tunnel reduces the pollution, which is highest at the exit from the portal. The pressure loss of the ducting varies with its design from 0.01 to 0.03 x velocity
pressure x length/diameter. It is important to use the manufac­turer’s pressure loss data.

Multi-stage axial fans are ideal for this application. As the length of the ducting is increased, additional fans can be added to in­crease the pressure capability. By this means it is necessary only to install the number of fans needed to provide the required pressure, thus reducing operating and capital costs to a mini­mum. It is normally possible to select fans of a diameter similar to the ducting, which gives a very simple installation. The axial fans are light-weight and simple to support at high level in the tunnel. Fans can be added as required when the actual system pressure loss is known.

Posted in Fans Ventilation A Practical Guide


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