Oxygen is the most important gas for sustaining life. Our bodies are dependent on oxygen for survival; it is consumed by our bodies in order to produce energy.
Given that a refuge chamber is a sealed environment, it is vitally important to consider how oxygen levels inside a refuge chamber are managed. Oxygen levels of the “dead air space” will quickly be consumed and with no breathing supply, occupants would quickly suffocate within a matter of a few hours.
Monitoring and controlling the internal atmosphere as well as air treatment and multiple breathable air sources are therefore vital to refuge chamber occupants over the duration of entrapment.
Oxygen Levels for a Refuge Chamber
Amongst other things, ‘breathable air’ or air supplied to a refuge chamber, should contain no less than 19.5% and no greater than 22% by volume of oxygen (AS/NZS 1715:2009).
The normal lower working limit for oxygen levels is 19%. At 18% a slight increase in breathing effort will occur. At 16% a flame lamp will go out, but still, only a slight increase in breathing and heart rates would be noticeable and some cognitive impairment may occur. At 14%, impaired judgement and coordination are noticeable. However, as levels fall below 12%, cardiac damage and vomiting can occur. At 10% and below, a lapse into unconsciousness and death will occur.
On the contrary, oxygen levels higher than 23% presents a fire hazard. Though not itself flammable, oxygen promotes and accelerates combustion, so an oxygen-rich environment increases the combustibility of materials and is undesirable in any environment, particularly a confined space. Handling oxygen supply equipment, therefore, requires care in ensuring that no grease or oil come into contact with the oxygen gas flow path.
To maintain a level of oxygen that is both safe to breathe and non-hazardous, it is necessary to replace the oxygen as it is being consumed.
Oxygen Supply to a Refuge Chamber
The two standard sources of breathable air within a MineARC refuge chamber are incoming, filtered and regulated compressed air from an external source and oxygen cylinders. A recommended optional third source is an oxygen candle.
Primary Oxygen Supply in a Refuge Chamber: Compressed Air
Compressed air is simply the atmospheric “air” from above ground being pushed through pipelines down to the refuge chamber via a compressor. Before compressed air can be considered suitable for breathing, it needs to be filtered from airborne water particles, oil contaminants and pollutants such as carbon monoxide and hydrocarbons from diesel exhausts.
The order of a filtration system’s elements is critical in ensuring the compressed air supplied to a refuge chamber is safe for occupants to breathe. The MineARC Compressed Air Management System (CAMS) features a four-stage filtration process. CAMS is tested to ISO 12500-1 and has been specifically designed to meet AS/NZS 1716:2003.
The flow rate of the compressed air into the chamber is adjusted with a regulator and the compressed air required to sustain a breathable atmosphere within a chamber is calculated at 85L/m (3CFM) for the chamber and 85L/m (3CFM) per person thereafter.
Secondary Oxygen Supply in a Refuge Chamber: Oxygen Cylinders
In the event that the compressed air source fails, the refuge chamber must have an independent source of oxygen. MineARC utilises compressed oxygen from cylinders.
Oxygen cylinders reintroduce oxygen into the refuge chamber at the rate it is being consumed. Compressed oxygen from a cylinder is released into the chamber through a regulator that controls the flow rate. Flow is set at the rate of occupant consumption.
Oxygen consumption rates vary from person to person and with activity. MineARC uses 0.5 Litres per person per minute in calculations involving the consumption rate of oxygen in refuge chambers. This allows a safety margin and for variations in individual size and requirements of chamber occupants.
Using this rate of consumption, it is possible to work out oxygen supply requirements and how many oxygen cylinders are necessary to meet the total amount of oxygen required for an occupied refuge chamber. This is calculated as follows: number of chamber occupants x rate of consumption x minutes in an hour x number of hours duration
The flow rate can be calculated as follows: number of occupants x flow rate
MineARC recommends the use of the largest medical grade oxygen cylinder – ‘G size’ – which contains 8200L of breathable oxygen. Consideration also needs to be given to whether the regulator used can handle the required flow rate.
In conjunction with oxygen cylinders, air scrubbing is utilised to chemically remove contaminants or undesired gases like carbon dioxide (CO2) and carbon monoxide (CO) produced in normal human metabolic processes, and filter the internal atmosphere.
MineARC’S Automated Oxygen Delivery System
An option for MineARC Refuge Chambers is the Automated Oxygen Delivery System (AODS) which takes the guesswork out of correctly maintaining oxygen levels in the refuge chamber. Once the system is activated, the AODS continuously measures the internal oxygen level and will disperse metered amounts of oxygen from a compressed oxygen cylinder, to maintain oxygen levels between 18.5% and 23% inside the refuge chamber. The system’s flow rate is limited to 15 Litres per minute, which is suitable for up to 30 persons.
This is advantageous for a number of reasons:
- Ensures chamber will not become oxygen-rich
- Ensures oxygen levels do not drop too low
- Minimises wastage of oxygen in cylinders by only dispensing what is being consumed
- Reduces handling of oxygen supply equipment
- Reduces the chance of human error in equipment operation
Third Optional Oxygen Supply in a Refuge Chamber: Oxygen Candle
Using an exothermic chemical reaction to generate oxygen, the oxygen candle is the third and final method of breathable air supply. This would be used in an extreme emergency, in the event of failure of the first two systems, to either extend the duration or to buffer a response time to exit the chamber. The candle will burn for approximately 60-90 minutes, producing 2600L of oxygen. This can provide approximately 20 hours of breathable air for 4 people. An oxygen candle cannot have its oxygen production regulated or halted and once ignited, it will produce oxygen at a rate of approximately 28 Litres per minute until it is exhausted.