4 ms·

I'd be surprised if they use diesel engines. Seems like dealing with the exhaust in a closed environment like that would add too much risk and complexity.

jaclaz 9y ago
Right, though the issue is not much about risk and complexity, but rather by sheer power needed to push the air in a tunnel. The smaller the tunnel is, the smaller is the size of the ventilation tubes that can be used, and thus you need more power to push the air inside. The amount of air needed by an electric motor is "0", whilst the amount of air needed by a combustion engine is something that needs to be evaluated on a case by case basis, normally the rule of the thumb is 4 cubic meters per minute for each Diesel HP (i.e. with kW's 5.4/KW). And you have to assure anywyay some air to the people, usually 3 cubic meters per person per minute is used. And there is anywyay a limit to the pressure with which you can pump air in, because the "return speed" is usually limited to a maximum of 0.5 m/s (as higher speed may cause the transportation of dust and particles). And then of course the longer the tunnel is the more powerful must the ventilating fans be, we are talking of several hundreds of kW to power these fans.
- Dylan16807 9y ago
  So that's enough airflow to make the engine exhaust be half a percent CO2, which is right about at workplace limits. And extreme overkill on the per-person air if everything is mixed together. (If it's not mixed together it makes sense to over-ventilate the humans for comfort.)
  - jaclaz 9y ago
    I am not sure to understand what you mean "not mixed together", the tunnel is a single environment to which you provide air. The method generally used is "positive pressure". I.e. there is a single pipe or duct through which the air is pushed till the excavation front. The air, having no possible way out is forced to go back towards the tunnel opening. Since usually the most activities are near the excavation front there is concentrated the clean air, while the one returning back is "contaminated" by the CO2 and other fumes at the excavation front. The 3 m3/min per person and 4 m3/min per HP are common rules of the thumb to calculate the amount of air, and is in practice a "large" allowance since it is calculated with the maximum possible number of people and with the theoretical power of diesel engines (that never run at 100% throttle), and it has to take into account the said effect of contamination so that people working in the tunnel (not a the the excavation front) still get enough clean air. Moreover it is not a "continuous fine regulation", fans have normally a finite number of speeds, so you are always using the speed (and the number of fans) calculated for the "longest stretch". As an example for the first 200 mt of excavation you have one fan at 1st speed, the next step, switching to 2nd speed is good up to (say) 400 meters, and you switch to 2nd as soon as you get past 200 mt, and so on.
    - Dylan16807 9y ago
      So yes it's all mixed together from the second the air leaves the duct. Which makes it interesting that the 4 cubic meters per HP is about 10 times the allowance for contamination as the 3 cubic meters per person. Everyone shares the higher contamination level whenever the engines are active.
      - jaclaz 9y ago
        Not really everyone. In such a ventilation scheme during the works most personnel is near or in the immediate vicinity of the excavation front, they have "non-contaminated" "fresh air" at all times. You get some "contaminated" air only during the time it takes from the tunnel entrance to the excavation front, and in some circumstances people making maintenance or other works far from the excavation front. As said the quantity of fresh air fanned in is much more than what actually "needed", there are sensors for the contamination and all in all the air you breath in any modern tunnel during construction is much, much better than what anyone breaths everyday in a trafficked city.

jaclaz 9y ago

Right, though the issue is not much about risk and complexity, but rather by sheer power needed to push the air in a tunnel. The smaller the tunnel is, the smaller is the size of the ventilation tubes that can be used, and thus you need more power to push the air inside. The amount of air needed by an electric motor is "0", whilst the amount of air needed by a combustion engine is something that needs to be evaluated on a case by case basis, normally the rule of the thumb is 4 cubic meters per minute for each Diesel HP (i.e. with kW's 5.4/KW). And you have to assure anywyay some air to the people, usually 3 cubic meters per person per minute is used. And there is anywyay a limit to the pressure with which you can pump air in, because the "return speed" is usually limited to a maximum of 0.5 m/s (as higher speed may cause the transportation of dust and particles). And then of course the longer the tunnel is the more powerful must the ventilating fans be, we are talking of several hundreds of kW to power these fans.

Dylan16807 9y ago
So that's enough airflow to make the engine exhaust be half a percent CO2, which is right about at workplace limits. And extreme overkill on the per-person air if everything is mixed together. (If it's not mixed together it makes sense to over-ventilate the humans for comfort.)
- jaclaz 9y ago
  I am not sure to understand what you mean "not mixed together", the tunnel is a single environment to which you provide air. The method generally used is "positive pressure". I.e. there is a single pipe or duct through which the air is pushed till the excavation front. The air, having no possible way out is forced to go back towards the tunnel opening. Since usually the most activities are near the excavation front there is concentrated the clean air, while the one returning back is "contaminated" by the CO2 and other fumes at the excavation front. The 3 m3/min per person and 4 m3/min per HP are common rules of the thumb to calculate the amount of air, and is in practice a "large" allowance since it is calculated with the maximum possible number of people and with the theoretical power of diesel engines (that never run at 100% throttle), and it has to take into account the said effect of contamination so that people working in the tunnel (not a the the excavation front) still get enough clean air. Moreover it is not a "continuous fine regulation", fans have normally a finite number of speeds, so you are always using the speed (and the number of fans) calculated for the "longest stretch". As an example for the first 200 mt of excavation you have one fan at 1st speed, the next step, switching to 2nd speed is good up to (say) 400 meters, and you switch to 2nd as soon as you get past 200 mt, and so on.
  - Dylan16807 9y ago
    So yes it's all mixed together from the second the air leaves the duct. Which makes it interesting that the 4 cubic meters per HP is about 10 times the allowance for contamination as the 3 cubic meters per person. Everyone shares the higher contamination level whenever the engines are active.
    - jaclaz 9y ago
      Not really everyone. In such a ventilation scheme during the works most personnel is near or in the immediate vicinity of the excavation front, they have "non-contaminated" "fresh air" at all times. You get some "contaminated" air only during the time it takes from the tunnel entrance to the excavation front, and in some circumstances people making maintenance or other works far from the excavation front. As said the quantity of fresh air fanned in is much more than what actually "needed", there are sensors for the contamination and all in all the air you breath in any modern tunnel during construction is much, much better than what anyone breaths everyday in a trafficked city.