Ventilation system design

Why System Design?

The system design aims at choosing the equipment which together with the energy cost gives the lowest total cost for the ventilation during the project.
The important part of the system design is the ventilator calculation which give following answers, necessary for choosing fans and ducting:

 

By the use of frequency inverters to control/start a fan, the fan speed can be set to any from 0 to full speed. This is achieved by the function of the inverter which outputs a frequency of 0-300 Hz. Normally, the output from the frequency inverter for a tunnelling fan is 10-60 Hz. Lower than 10 Hz, the cooling of the fan motors can be to low and higher than 60 Hz, the strain on the fan impeller starts to be too high.

Example: if the fan speed is 1500 rpm at 50 Hz, with the frequency reduced to 25 Hz, the fan speed is 750 rpm. The incoming power to the frequency inverter is always determined by the local electric grid, 50 Hz for some countries, 60 Hz for other.

The power load on a fan is in relation to the fan speed by cubed. Example: if a fan is running at full speed 1500 rpm and the power load is 100 kW, with the fan is running at half speed 750 rpm, the theoretical power load is 12,5 kW. A reduction of 87,5 kW! Since there are some energy losses in the frequency inverter composed mostly of heat, the practical power load at half speed is 15% compared to full speed, saving of 85%.

The airflow delivered by the fan is linearly in relation to the fan speed which means at half speed, the airflow is half compared to full speed.

In a drill & blast tunnel, normally, several works do not require so much airflow which means when those works are going on, the fan speed should be reduced. Those works are scaling, drilling, charging etc. Normally, the drill & blast tunnelling can be described as following ”cycle”: It starts with blasting. After blasting, mucking out the blasted rock starts. After the mucking is finished, scaling starts and after that, drilling and charging.

 

Swedfan´s recommendation for max energy saving is following:

immediately after the blast, the fan is started and run at full speed in order to ventilate the blasting fumes out quickest possible. (this normally takes 20-40 minutes)

After the blasting fumes are out, the mucking commences and also during this event, the fan is run in full speed since normally, a large amount of diesel fumes from trucks and loaders is polluting the air. (this normally takes 90-150 minutes)

After the mucking is completed the scaling, drilling and charging takes place and for these works, the fan speed is reduced to half speed. These works totally takes 180-240 minutes.

With the Swedfan experience, supplying ventilation systems to the international tunnelling market since 1990, our feedback from hundreds of tunnelling projects using drill & blast show that normally, the fan can be used 50-60% of the total construction time running at half speed. Considering that the fan is consuming only 15% running at half speed, 85% energy saving is possible for 50-60% of the tunnelling period.

In most tunnels, the energy cost saving using the above described method saves money exceeding the investment cost of both fans and ducting for the same project!

 

What does this mean in terms of money?

Energy consumption with fan 100% full speed: 6,5 hours x 220 kW x 80% = 1.144 kWh/6,5 hours

Energy consumption with fan running 50% full and 50% half speed: (2,5 hours x 220 kW x 80%) + (4,0 hours x 220 kW x 80% x 15%) = 546 kWh/6,5 hours

Energy saving per hour = 546 kWh/6,5 hours = 84 kw/hour

For a two year project where the power cost is 0,15 USD/kWh, this results in 15.000 hours x 0,15 USD x 84 =  189.000 USD

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