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MINEARC TECH TOPIC: TUNNELLING DESIGN CHALLENGES

 

What are the principle safety design features of a refuge chamber for tunnelling projects? This MineARC Tech Topic explains how the MineARC TunnelSAFE Refuge Chamber is built to overcome the challenges of atunnelling emergency.

Did you know? MineARC Refuge Chambers have been successfully used around the world in multiple real-life tunnelling emergencies to save lives.

 

What is a MineARC TunnelSAFE Refuge Chamber?

MineARC TunnelSAFE Refuge Chambers are sealed steel containers with an artificially controlled atmosphere. The chambers maintain a habitable environment, to support life during an underground tunnelling emergency.

The principles of life support focus on controlling the following:

  • A secure sealed internal volume positively pressurised to ensure contaminants are unable to enter the occupied space.
  • Respirable air with multiple redundancies; required within a closed circuit breathing apparatus that is the refuge chamber.
  • Temperature and environmental humidity.
  • Electrical redundancy, with the use of a significant 24 hour uninterruptable power supply, to operate scrubbing equipment, air conditioners, lighting and gas monitoring equipment.

The methods by which these are achieved can be compared to those found in submerged submarines and have been around for some time. However, refuge chambers for the tunnelling industry possess several unique challenges that require further development of specialised features in all key areas.

Design Challenges and Solutions

 

Challenge: Respirable Atmosphere

Externally compressed air is usually the primary source for respirable air within a TunnelSAFE Refuge Chamber. MineARC Systems explored operational patterns of underground compressors as a primary breathing air supply, the risks posed by the potential supply of polluted compressed air both during occupied and non-occupied scenarios, and methods to remotely monitor and control the contaminated air from entering the sealed space. With an underground fire being one of the key breathing contaminant sources and reasons to evacuate to a refuge chamber, considerations around compressed, oxygen deprived, high carbon monoxide and high temperature air were used to begin design work for this life support system.

Solution: Compressed Air Management System (CAMS)

A key feature of MineARC’s TunnelSAFE Refuge Chamber is the Compressed Air Management System (CAMS); a feature specifically designed around the challenge of maintaining a respirable atmosphere within an enclosed space. An oxygen sensor is used to monitor incoming compressed air and will de-energise a normally closed solenoid in the event of low oxygen. Air that is sourced from an area experiencing fire or an environment that is likely to contain carbon monoxide as a result of incomplete combustion will also have reduced levels of oxygen. This system allows the internal volume of a refuge chamber to remain respirable with or without occupants.

Additionally, refuge chambers require the internal volume to be positively pressurised to prevent ingress of breathing contaminants. This is commonly achieved by bleeding the same breathable compressed air source into the chamber continuously while in standby (non-emergency periods). To prevent excessive compressed air wastage, a differential pressure controller is used to introduce compressed air only when required; therefore significantly reducing the demand on the compressors used to supply them.

Challenge: Temperature and Humidity

In tunnelling applications, high ambient conditions for conventional air conditioning units are often considered. Most common air conditioning heat exchangers will be unable to reject heat higher than an ambient temperature of 47°C. “High temperature” air conditioners are often rated to 60°C but will have a larger power draw and lower effective cooling capacity.

Solution: Computational Fluid Dynamics (CFD)

MineARC have existing high temperature and insulated designs that incorporate Computational Fluid Dynamics (CFD) modelling and analysis to ensure occupant safety. Furthermore, MineARC is able to employ MARCiS (Liquid Carbon Dioxide) and thermal storage cooling methods within atmospheres as high as 300°C.

Computational Fluid Dynamics (CFD) Examples

A baseline must first be established in order to identify if air conditioning or insulation is required.  Metabolic heat output is quantified as 117W, though 130W is used in these conditions. In Figure 1 below, it is clear that the chamber is already reaching uninhabitable environmental conditions within the first 30 minutes of entrapment. If the chamber detailed in Figure 1 were to reach saturation, the heat index would exceed survivable limits within 30 minutes.

Figure 1: Cross section: Scenario: Air Conditioning – No Insulation at 16 minutes.

 

Figure 2: Cross section: Standard Temperature Scenario: Air Conditioning – Scrubbing Fan Mixture.

Figure 3: ISO2: Standard Temperature Scenario: Air Conditioning – Scrubbing Fan Mixture.

Figure 2 and Figure 3: Computed with our standard operating conditions, show the significant mixing ability by drawing in contaminated air through chemical scrubbing cartridges. Though this scenario does not consider the high ambient temperatures, it does add weight to the mixing and cooling ability of a standard chamber in high temperature conditions.

 

For more information on temperature control and heat loads within refuge chambers, please view the MineARC Tech Topic: Temperature Control and MineARC Tech Topic: Heat Loads articles. For further information regarding MineARC’s range of Refuge Chambers, please email info@minearc.com.au to speak to a MineARC representative.