Application of jet fans in tunnel ventilation systems

Posted by JArunKumar on March 9th, 2016

Tunnels have very severe air quality constraints that typically require a good ventilation system. It is important to bring in fresh natural air into the tunnels either to remove the exhaust gases under natural conditions or get rid of smoke in case of fire emergencies. Generally, tunnel ventilation systems have complex designs and higher installation & operational costs because they consume significant amount of power in several megawatts, and require expensive structural space and electrical facilities. Hence, there is an increasing interest in optimizing the tunnel ventilation system design to bring down the power consumption, boost efficiency and minimize infrastructural costs.

Even though the fresh air requirement is different for each tunnel, the main layout parameter for all ventilation systems is the good air quality. This can be efficiently achieved by installing jet fans in road tunnels in order to move the air by giving it an impulse (measured as thrust in Newton) in the desired direction.

The possibility of air extraction in the tunnel basically depends on two factors -
• type of ventilation (longitudinal, semi-transverse or transverse ventilation)
• loss factor that is reducing the air speed

Standard Jet fans for tunnel ventilation:
Jet fans are technology intensive products with base design of casing that holds a silencer, a rotor, a motor as power source and a stator that fixes the motor to the casing. In addition to being energy efficient, most of the jet fans are made of compact design so that the ventilation system won't protrude into traffic space when installed inside the tunnel.

• Jet fans have been in use from many years for tunnel ventilation to limit the concentration of Carbon Monoxide (CO) to acceptable levels and reduce the other likely pollutants like nitric oxide (NO), nitrogen dioxide (NO2), sulphur dioxide (SO2) and provide fresh air without any contaminants. They work the way similar to axial fans but rotate at high speed to discharge great quantities of air.

• Jet fans are easy to maintain when compared to other ducted systems because jet fans allow closely spaced fan installations, which reduce the cabling and operational costs.

• Jet fans have re-engineered design to operate at very low noise because of the silencers on the two sides of the system. The fans also have reversibility feature to discharge smoke in both directions if necessary in case of fire accidents inside the tunnels. Reversible (symmetric) and unidirectional (asymmetric) jet fans can be installed based on the requirement of airflow direction, which are available with various customized options for size, casing thickness, impeller type etc.

• To increase the output efficiency, tunnel jet fan installations have to consider loss factors that are incurred by wall friction, impulse, background velocity, losses in corners and bends, portal losses at inlet & outlet etc. These losses can be reduced either by inclining the jet away from the wall or pitch the fan by pointing its outlet towards middle of the downstream cross-section of the tunnel.

How to improve the ventilation efficiency of jet fans?
The main objective of any tunnel ventilation system is to optimize the operating range of fans for more efficiency and minimize the installation and functional costs. However, ventilation systems in tunnels demand large amount of energy to move the air, sometimes up to several megawatts for full load because 30 to 40 % of the thrust produced by each jet fan is wasted in several aerodynamic losses.

Minimizing losses means decrease in the required thrust and increase in the jet flow for the same fan power input; in other words, there is a reduction in the power consumption and energy cost. One of the effective ways to improve the ventilation efficiency of jet fans is by inclining the jet away from the restrictive surfaces using silencers or ducts. This approach of positioning ventilation equipment with the bent outlet helps to cut down the losses by 25 to 50%, thereby fewer fans are needed to get the desired output, and consequently installation and operational costs are lowered.

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