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Alternative ventilation comments
Authors respond to Feedback comments from William G. Connell of WSP USA and present the attached pdf
for further details.See the full Feedback contributions at the bottom of this article and on the Feedback page.
Comments and feedback about the alternative ventilation concept article focus on concerns about the control of smoke and heat in the event of an in- tunnel fire. Control of fires in tunnels is being highlighted as more critical than controlling pollutants in tunnels and that more attention would be needed to develop the concept in this regard.
TunnelTECH
An alternative air injection ventilation method 07 Dec 2017
A new system of forced air ventilation of road tunnels in operation, and similar in concept to the ventilation systems used during construction, is proposed as an alternative to longitudinal ventilation systems using ceiling anchored jet fans. Developer and patent-holder Ahmad Reza Ghavami, together with Yasaman Ghavami, Project Manager, and Thomas McMahan, Marketing Specialist, of Naghsh E Shahr Consulting Engineer (NSCE) of Iran and the USA, describe the concept and the potential advantages. A patent for the concept has been granted in the USA.
Standard longitudinal ventilation has been the popular design for traffic tunnels over several decades. There are however many disadvantages and operational difficulties with these systems including:
- Expensive financial investments
- Heavy ceiling jet fans and costly steel hangers
- Extensive cabling and power consumption required for normal and backup supplies
- Maintenance of electrical consumption of the units
- Substantial costs for maintaining spare parts inventories and their storage and management
- Necessary warranties and specifications and manufacturer guarantees for the parts
- Required high-level skilled technical labor, high wages and large annual maintenance budgets
- High levels of noise pollution and reflectance of sound during jet-fan operation
- Limitations regarding unit inspection, maintenance, setup and repair that may become very dangerous for the crew
- Hidden danger factors from the weight and vibration of the units
- Considerable cost if redesign of the fan system is needed
- Inability of jet-fan systems to filter sand or purify pollutants in the air
The flaws, cost and operational inefficiency of the current approach demanded a new solution that is cost effective and has fewer working parts and easier repairs.
Conventional longitudinal ventilation
Fig 1. Proposed new system with air handling units, ducts and diffusers
The injection method
The alternative solution proposed results from years of technical research on tunnel ventilation and is based on continuous partial positive pressure, which applies to the total air volume of the tunnel. One, two or three medium-pressure centrifugal air-handling units are installed at both portals and a one-piece round steel section duct runs along the length of the tunnel to connect the units to each other (Fig 1). A case-specific number of diffusers are placed at certain points along the length of the duct at certain air-throwing angles for a calculated volume of outlet air (Fig 2).
The basic foundation of the method is an experiment that used a horizontal pipe, partially sealed at both ends and measuring about 10cm diameter x 2m long, to represent the tunnel. Colored water is inserted into the pipe to represent the air pollution. Using a syringe, clean water is injected into the middle of the pipe to simulate fresh air. The introduction of the clean water forces the colored water to either side of the main pipe, indicating the elimination of polluted air inside the tunnel. The same basic rules of physics can be applied to the real conditions inside tunnels for forcibly ventilating air pollution.
Ducts and diffusers supply fresh air throughout the tunnel with diffusers located based on pollution density (Fig 2 right)
As an example, two sets of fans could be stationed at the portals of the tunnel with filters to remove sand, particles, toxins or chemicals from the air, each set with three to five sets of airfoil fan blades. Each one is equipped with an inverter, controller and processor for controlling the energy consumed or used, based on need and usage. Each motor has a variable speed, as well as a bi-directional version for reverse rotation in case of a fire. Each unit has a bag to filter fresh air for each fan. Each unit has an anti-vibration and silencer to reduce noise and vibration. Detectors with optical or spectrometer sensors identify and analyze CO and CO2 and other gases, particles and chemicals. These results go to the processor and controller to adjust the fans or registers or openings, or to adjust the position and tilt of the fans with respect to the horizontal axis or vertical plane. The controller also decides which combination of fans and units are operational at any given time.
This system minimizes the pollution and particle concentrations in the tunnel depending on traffic and concentration. It is flexible, dynamic and changeable. Government mandates or recommendations for particle limits (for example, those set forth by the EPA, UN, PIARC, ASHRAE, ISAVT, local governments and federal rules) are used to determine the airflow and filtering. The average exchange rate of the air through a tunnel space is about five to ten air changes/hr. At the maximum injection of air, the average speed to exit from each of the tunnel diffusers is about 53m/min to 76m/min.
Fig 3. Rendering sections of air inlet horns for high, medium and low pollution areas
The benefits of the injection system include:
- Most parts for running the system can be made within most countries
- Much lighter and more efficient materials
- No special skills or extra costs are needed for installation
- Nonstop operation (unless there are problems with the outside air-handling units)
- A lower power consumption when compared to numerous jet-fans
- Simple upkeep and storage of spare parts
- Minimal noise pollution compared to loud jet fans
- No inside maintenance required, with all air-handling unit maintenance occurring outside
- No heavy equipment within the tunnel, thus no danger imposed by falling equipment
- Design and maintenance modifications are practical
- Air filtration capabilities are possible and practical
- Units that can be reversed to exhaust smoke in case of fire
The efficiency of the system translates to cleaner air in the tunnel, which prevents sickness from air toxicity or death by fatal accidents in the tunnel caused by dizziness of drivers due to air pollution. Short- and long-term effects on the health of drivers and passengers are significant, particularly for children, who are sensitive to polluted air, or for people with asthma or allergies.
Comparing the methods
A comparison of the energy consumption between the jet-fan and injection methods for a 2km long tunnel with a 45m2 cross section shows that the injection method is more efficient with regard to power consumption and usage. A jet-fan system would need a minimum of 30 pairs of duplex fans approximately 70m apart (or 60 single fans) using about 37kW of power. That totals 2,220kW of electricity consumed for all fans.
Fig 3. Rendering sections of air inlet horns for high, medium and low pollution areas
The new method would require four airfoil air-handling units with capacities of 2,265.3m3/min to 2831.7m3/min, 10,000m2 to 12,000m2 of spiral duct, 150 boxes of air supply register and about 1,500 aerodynamic horns (Figs 3 and 4) for air supply into the tunnel. The individual power consumption of each air-handling unit is about 75kW or 300kW of total consumption.
With more than a three-fold improvement in efficiency and cost savings, the injection method could save a country with many tunnels millions of dollars. The injection system requires less cabling for power and control systems within the tunnel, which also saves money in terms of repairs, installation and operation.
The air injection system is practical, economical and easy to implement. The cost of installation is lower and repairs and maintenance are safer and cheaper. It is safer than the jet-fan method because the vibration of fans on the ceiling causes them to loosen eventually without much warning, causing dangerous crashes, whereas diffusers have no mechanical parts, heavy fans or motors. Additionally, the injection method does not need preventive inspections or repairs, which are disruptive and dangerous for both traffic and the repair crew.
Authors' Reference
United States Patent No: 9534496 BI
Dated 3 January 2017
Download a pdf copy
References
- Exit spacing influence on ventilation choices – TunnelTalk, May 2012
Feedback
In response to the Feedback comments from William G. Connell of WSP USA (below), the authors of the TunnelTECH paper introducing the alternative road tunnel ventilation system present the following information and make available the attached pdf for further details.
The proposed positive partial pressure (P.P.P.) ventilation system prioritizes both fire and pollutants. It can be applied for project in both modern and undeveloped countries.
Both inject fresh air and exhaust polluted air.
- Cost a lot of time, energy and money to construct both the semi-transverse supply ventilation system the semi-transverse exhaust ventilation system.
- Both systems require installation of ducts either in the floor or on the ceiling.
- Both systems require vents or grates to be installed to inject fresh air into the tunnel or to exhaust the polluted air out of the tunnel.
- Utilizing either of the systems requires a false floor which reduces the overall height of the tunnel.
- Reduces cost by up to one third of the traditional systems in use today.
- The only cost will be the two outside fans and the diffusers, which are installed via a duct on the ceiling of the tunnel.
- No cabling.
- Reduced maintenance inside the tunnel. No fans or cabling to require maintenance inside the tunnel, thus reducing the need to close the tunnel.
- The fans will be equipped with an INVERTOR which will allow the fans to reverse in the event of fire and high pollutants.
- The fans will be equipped with an ECONOMIZER, controlling the revolution of the fan from 0-1400 rev/min, depending on the volume of traffic in the tunnel. This accelerates or decelerate the amount of power being used by the system thus saving significantly on energy consumption costs.
- The P.P.P system uses the tunnel itself as the vehicle for moving the fresh air in and the pollutants out.
Ahmad Reza Ghavami, Mechanical Engineer and CEO/Founder,
Yasaman Ghavami, Project Manager, and
Thomas McMahan, Marketing Specialist of
Naghsh E Shahr Consulting Engineer (NSCE), Iran and USA
Alternative ventilation method for road tunnels
Feedback from: William G. Connell, WSP, USA
The paper describes a semi-transverse supply ventilation system, as noted below (Fig 1), only the delivery of air to the tunnel is at the ceiling level. This type of system has been used in many road tunnels in the past but is less common now. Fact is, semi-supply systems work best for pollutant control when air is delivered lower along the roadway at about exhaust pipe level.
Fig 1. A typical semi-transverse supply ventilation system concept where air is delivered via a duct below the roadway and introduced to the tunnel at tailpipe level and is distributed with the piston effect of the traffic
Fig 2. Design of a typical semi-transverse exhaust ventilation system concept where fresh air is drawn into the tunnel and exhausted through ports into a duct above the roadway and then discharged to ambient
The concept is not new – but more importantly the system as described will not provide ventilation control of smoke and heat during a fire emergency which is actually the more important function of a tunnel’s ventilation system design these days. The emergency function of the system is not addressed. Because current day vehicles emit much lower levels of pollution, the critical criteria that drives the ventilation design in most modern road tunnels is the fire design criteria.
The extra emphasis on removal of pollutants appears to suggest a filtration plant (which will not likely work due to the large volume air flows - and will unnecessarily increase fan motor horsepower). The only tunnels that I have seen using any kind of filtration system were a few in the Nordics where they were needed to help for visibility issues created by the common use of studded tires which was creating pavement dust.
The article does not seem to account for the fan facility (building) required to house the fans at either end of the tunnel. These buildings – which will add a lot of cost – are not required for the jet fan longitudinal system being compared.
Feedback from: Petr Pospisil, Switzerland
This system will not be efficient, as explained in the Road Tunnel Ventilation Compendium. Some designers seem to make the same errors for decades.
Guidelines for road tunnel ventilation
Regards,
Petr Pospisil,
I-P Switzerland
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