Railroad Engineering: Ventilation, Draft Relief, HVAC





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1. Is ventilation a problem on railroads?

For surface railroads running mainly out in the 'fresh air' it would not appear, at first sight, that ventilation is a concern for today's railroad engineer.

This is mainly so, particularly where some form of electric traction is used and where there are only short tunnels or covered ways.

However, where parts of any railroad go into tunnel of lengths exceeding about half a mile, then ventilation is an aspect that must be given consideration. For Metros which are mainly underground, good ventilation should be a major consideration.

In the early days of railroads, before the introduction of electric traction, ventilation proved to be of great concern where long tunnels existed.

The author has in his possession a bound edition of the Minutes and Proceedings of the Institution of Civil Engineers Vol. XLIV for 1876. This volume contains a most interesting paper presented to a meeting of the Institution on 18th January 1876 by Gabriel J Morrison entitled 'The Ventilation and Working of Railroad Tunnels'. Incidentally the chair at the meeting was taken by the then President George Robert Stephenson, none the less! Clearly from the content of the paper and the reported discussion that followed, there was much concern at that time, both on the part of the public and by many practicing railroad engineers. This related to the health risks and safety hazards produced by 'impurities generated in tunnels by combustion of fuel'. The main cause for concern was the large amount of carbonic acid coupled with a lesser amount of sulphuric acid that is discharged into the atmosphere by coal fired steam locomotives. The paper includes reference to the crew on a goods train in the Mont Cenis Tunnel having fainted because of 'foulness of air' and to public complaint about the stale air on the Underground section of the Metropolitan Railroad in London.

Clearly, travelling on the footplate of a steam locomotive, or for that matter in following carriages, was not pleasant in long tunnels.

Modern railroads do not now use locomotives which burn coal or coke or run on steam. However, diesel power will continue to be in use for many more years. Where railroads are diesel powered and operate underground for part of their route, careful thought needs to be given to the problem of proper ventilation of the exhaust gasses, particularly where trains have to stand for any length of time with engines running. Even large terminal stations which have large 'train shed' roofs can suffer from diesel fume pollution caused by trains waiting to depart.

This section includes brief reference to the factors that are involved in tunnel ventilation which can be applied to longer tunnels and covered ways on surface and sub-surface railroads as well as to railroads which are completely underground.

2. Movement of air

The movement of airflow in and around a railroad system which is at least partially underground, caused by the piston effect and drag/thrust of the trains, is not an exact science due to the number of variables that exist.

As would be expected, the maximum piston effect of trains is in tunnels which only carry a single track. The minimum clearance between the rolling stock and the tunnel, and the shape of the front of the train, will both effect the amount of air that is pushed along by the train.

For main line trains, the blockage ratio in single line tunnels could be expected to be about 0.5 but for tube trains it can be as high as 0.7. Air movement in twin track tunnels is considerably less.

On an underground system, the amount of air displaced into a station by the piston effect of a single train entering is in most cases no more than the combined cubic capacity of the passageways, escalator shaft and ticket hall. The same train when leaving would tend to suck back after it, the same foul air that it had pushed into the station. It can be seen therefore that this would result in little air change, even in the upper levels and ticket hall, and hardly any in the running tunnels.

The movement of air through the tunnel caused by trains is taken into account when deciding the position of tunnel cooling fans. For an exhaust type ventilation system, fan shafts are usually placed near to the entry end of the platform. This ensures that air taken up the shaft and out to atmosphere has passed through the tunnel from the preceding station and has received a maximum amount of heat from the tunnels. Similarly, pressure type ventilation blows air in at the departing end of the platform so that it is taken into the running tunnels by departing trains and has a cooling and cleansing effect.

Controlled movement and gradual changing of air in long tunnels and underground railroads is important for a number of reasons which include the following:

  • Removal of 'stale' air with some tunnel dust and introduction of 'fresh' air containing more oxygen.


  • Removal of excessive heat generated by train equipment, human bodies, lighting, night engineering works and radiated from surrounding ground and the tunnel walls. This involves cooling tunnel air to a desirable average ambient temperature around 21°C.


  • Reduction of uncomfortable drafts.


  • Smoke control in the event of fire or smoldering.

3. Deciding on exhaust or pressure

As has already been mentioned, ventilation in tunnels can be effected by either exhaust or pressure, sucking out the foul air or blowing in the fresh.

Early tunnel ventilation schemes usually adopted a pressure approach which meant that all fans blew fresh air into the system and the stale air came out at station and tunnel openings. This arrangement meant that you were met by the foul air in your face as you entered a station, giving a false impression of a 'smelly' railroad.

In recent years the usual practice has changed to run fans to exhaust, which has in no way affected the general principal of cooling, but gives a considerable improvement for the passenger, who now enters the station with the fresh air drawn in by the fans. In winter, passengers pass smoothly from the cold outer air to the higher temperature at platforms, without usually even noticing that the temperature may have risen by as much as 15°C. Another point to remember is the need to ventilate lavatories, shops and staff rooms which are below surface level. This type of accommodation is usually ventilated by exhausting from the rooms and drawing the make- up air from the surrounding passages or circulating areas. If these passages are now filled with fresh air entering to feed the exhaust fans, then fresh air is available for the small ventilating plants for the station rooms referred to above. Blowing in cold air direct to running tunnels in the winter can also cause condensation which can effect track circuits and signaling. There are a number of reasons therefore why, generally speaking, it is best to run tunnel fans to exhaust and not to pressure.

4. The 'Piston' effect of trains on fans

When considering the construction of a fan with its housing and connecting airways, it is essential to ensure that all the components can withstand the fluctuating surge pressures caused by the passage of trains.

Non-return dampers are used with positive effect to stop the piston action of a passing train from reversing the fan. Without such basic precautions, the train's air volume would stall the fan, with probable disastrous effect on the fan motors and drives.

5. Design and operation of tunnel fans

Tunnel cooling fans generally range in size and duty from 10 m^3/s up to as large as 60 m^3/s. During normal operation the two main tasks of tunnel fans were met by the foul air in your face as you entered a station, giving a false impression of a 'smelly' railroad.

In recent years the usual practice has changed to run fans to exhaust, which has in no way affected the general principal of cooling, but gives a considerable improvement for the passenger, who now enters the station with the fresh air drawn in by the fans. In winter, passengers pass smoothly from the cold outer air to the higher temperature at platforms, without usually even noticing that the temperature may have risen by as much as 15°C.

Another point to remember is the need to ventilate lavatories, shops and staff rooms which are below surface level. This type of accommodation is usually ventilated by exhausting from the rooms and drawing the make- up air from the surrounding passages or circulating areas. If these passages are now filled with fresh air entering to feed the exhaust fans, then fresh air is available for the small ventilating plants for the station rooms referred to above. Blowing in cold air direct to running tunnels in the winter can also cause condensation which can effect track circuits and signaling. There are a number of reasons therefore why, generally speaking, it is best to run tunnel fans to exhaust and not to pressure.

4. The 'Piston' effect of trains on fans

When considering the construction of a fan with its housing and connecting airways, it is essential to ensure that all the components can withstand the fluctuating surge pressures caused by the passage of trains.

Non-return dampers are used with positive effect to stop the piston action of a passing train from reversing the fan. Without such basic precautions, the train's air volume would stall the fan, with probable disastrous effect on the fan motors and drives.

5. Design and operation of tunnel fans

Tunnel cooling fans generally range in size and duty from 10 m^3/s up to as large as 60 m^3/s. During normal operation the two main tasks of tunnel fans is to control the environment by removing heat at night or at other times when trains are not operating and to provide reasonable air movement when a train is stopped in a section of tunnel.

All fans should be designed to be capable of being reversed. With a normal high efficiency axial flow fan, the efficiency will be much less in reverse, possibly as little as 50% of normal running, due to the aerofoil shape of the fan blades. Even with this reduced efficiency, the facility of reversal is very important so that there is complete flexibility in times of emergency.

Large fans running at lower revolutions are usually much more effective than smaller ones running faster. Full life maintenance costs will also be less. It is always best to seek out a location for a larger fan and shaft rather than a number of small shafts with smaller overworked 'screamers'. Noise and vibration problems are also likely to be much less with larger, slower fans.

Air noise caused by increased velocity through ducts and tunnels can also cause problems where these reach the surface next to domestic buildings.

6. Smoke and dirty air in tunnels

With modern forms of electric traction, there should be no smoke in tunnels under normal operational conditions. Diesel traction does produce some smoke, however which should not be a problem in twin track tunnels although single bore tunnels of any length need to be carefully considered relating to ventilation for normal running.

Smoke on any railroad can be a serious hazard in the event of a train or some fixed piece of equipment catching fire. The hazard can affect other trains behind and in front on the same track and also those going in the other direction on an adjacent track or even in another interconnected tunnel. At interchange stations, smoke can also find its way through passageways and shafts to other lines of the same system.

Fans can be used to great effect in removing smoke from tunnels and in moving smoke away from an escape route being taken by passengers who have been detrained from a stalled train.

It is essential when dealing with smoke in tunnels to know the nature of the fire, its exact location in relation to the fixed railroad infrastructure and the location of all trains and passengers that could be affected.

Broadly, tunnel fires can be grouped as follows:

  • In a running tunnel.


  • On a train in a running tunnel.


  • On a train in a station tunnel.


  • In a station tunnel or passageways.


  • Under an escalator.


  • In a shaft.

As can be quickly appreciated, if all the relevant facts are not known or if incorrect information is given, it is easy to take the wrong action with fans and make the situation worse.

All modern railroads which have appreciable lengths of line in single track tunnels must have a disciplined system of reporting fires and relevant information to central control so that correct action can be taken with tunnel fans where they exist.

It is also essential that a comprehensive fire and smoke alarm system is installed to give early warning of any trouble in vulnerable locations like escalator machine chambers.

7. Draft relief

Trains travelling at speed through single track tunnels with small clearances will produce considerable drafts at stations unless some action is taken to reduce them to tolerable levels. Additionally, high air pressure will be experienced in leading cars or coaches of trains unless they are sealed from outside effect.

This effect can be considerably reduced by the introduction of small cross-passages between adjacent single track running tunnels at intervals a little over the length of the longest train using the tunnels. This then allows air to be pushed through the cross-passage into the adjacent tunnel and then back through another cross-passage behind the train as it passes, making a local circulatory path which relieves considerably air pressure ahead of the train.

Also draft relief shafts can be constructed over or alongside the running tunnel which allows air to escape to atmosphere as the train approaches and then to rush down the shaft as the train passes the shaft.

In this case, no fan or dampers are provided at the top of the shaft and strictly speaking it does not materially contribute to the tunnel ventilation system. Draft relief shafts are often located at stations where they have a good effect on reducing high air velocities on platforms. Sometimes shafts which are sunk from the surface to construct a tunnel can be left unfilled to act as draft relief.

On very long bores for railroads under water like the Channel Tunnel, ventilation becomes a major consideration. Careful thought needs to be given to the possible 'fire load' and the extent, duration and fierceness of possible fires underground.

In this case, it is very unlikely that intermediate shafts, either for ventilation or for draft relief, can be provided. There is also the problem of moving people from the scene of the fire quickly and safely if the train they are travelling in is on fire.

One solution is to drive a separate service tunnel parallel with the running tunnels and to which it is connected laterally at intervals by cross- passages.

Some form of air-lock or smoke door will be required in these cross- passages which are closed automatically as soon as smoke is detected.

In tunnels of this type which are more than say three miles long, the service tunnel should be large enough to allow emergency service vehicles to drive down and some form of transport to be provided to pick up detrained passengers. The service tunnel will also require a fire main with suitable take-off points to enable any fire or smoldering to be put out as soon as possible after detection.

8. Maintenance and inspection of fans

As with all other engineering services, tunnel fans require regular inspection and servicing. Every railroad authority needs to set up an organization to ensure that this is done and a system to ensure that control rooms are informed when fans are out of service for maintenance or major repair.

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