Mechanical Engineering - Ventilate an Underground Tunnel

Where is the trick?

A few people will work, the rest will be drilling. How big is "the rest". Here some thumb rule: a too big fan will make the workers fly away.

Has the tunnel 2 open ends? Or under construction with only 1 access hole?

You will need to get rid of the dust and weld fumes. Before putting a figure on your fan or duct system tells us some of the processes and their air contaminants.

Where are you drilling in? What and how much do you need to weld? How thick are the seems?

Thanks for replying. Tunnel has two linear openings only on either sides ie entrance - tunnel - exit, are along one line. We will know the number of persons & welding sets to be permitted at a time, according to airflow permissable, of course at high velocity, due restricted size, say @335meters/minute(1100FPM). Trench is 10meters below ground

Small capacity high velocity suction at the weld point will take care of fumes. For general ventilation 20 air changes /hour may be sufficient. Any local statutory requirements must be complied with.

I'd go with the rate required by your requirement.

You should first evaluate the nature and quantity of dust produced too.

Welding smoke first goes up, until cooled down. The way of exhausting is very important. As a basic principle get it at the source. If you are drilling on rock, watch the composition carefully and protect your people. You don't want that dust in their longs. Welding smoke produces also toxic components.

Just cycloning the air out in the whole will not solve the problem in this case.

People will become sick because of the "breeze" if that is too high (read comfortable air velocity)

Getting the contaminants and catching these in a stream, high enough to exhaust and separate will automatically result in "fresh source" air to replace the dust.

A central cyclone unit will only reduce the exposure time to the health risks if 20/h by a factor 20 and nothing more. It will contribute to visibility at the dust production but contaminate the air in the entire tunnel, creating unnecessary exposure to different working persons, everywhere in the tunnel.

Without knowing most of (read all + eventualities) the parameters, I wouldn't even consider giving a flow, airspeed or refreshing rate in general.

Thanks for raising the issue of dust. The geologist will confirm type of existing soil & gases at various depths. We expect water & moisture mostly. Where can I buy codes on ventilation of tunnel during construction? The constraint is small size of tunnel coupled with only two ends permitted for all works including air intake & exhaust, material & personnel movement etc. This restriction will result in high air velocity as well as guarding of workers from direct breeze

A robust exhaust fan should be available to suck out smoke and dust from this tunnel, as a safety precaution at least. Sounds like you are trying to have a recirculating air system, requiring filters, when you need to think about just sucking the hot and dirty air out and dumping it in the atmosphere so the fresh and cool air can get into the tunnel. A big blower turned on from time to time should do. Mining ventilation codes is where to look for an answer.

Thanks folks. Mining ventilation recommends air velocity @137mps(450fpm). Problem is the small dia of tunnel (3600mm) & hence air has to be removed at high velocity, location will be humid-part rocky soil. Dust will be a problem. I shall look for mining codes.

You could focus the exhaust air flow through some small conduits directed at the sources of smoke, dust, etc. Like a vacuum cleaner with a bunch of hoses that could be unblocked when needed. These localized point sinks of low pressure could be put next to a welding job to suck out the smoke, along with the heat. High velocity at this point sink is good for entraining the dust and smoke, but, as you say, high velocity for air flow through the whole tunnel may be too much.

The exhaust fan would also be a redundant ventilation fan, to back up the main ventilation fan should it fail. For cooling the tunnel, you would want to just exhaust the humid air instead of trying to recirculate it through an HVAC system. Same for discos as for mines and other tight spaces.

Only passages available are two ends of tunnel for all supply & exhaust air flows & hence the problem of high air velocity. No other tubes for supply or point exhaust are permitted. In case of high dust level, workmen may have to come out periodically till dust is cleared from entire length