Breathable air merely is air that can support life, has enough oxygen in its mix, and doesn’t contain contaminants like carbon monoxide, carbon dioxide, water, or any other toxins that will poison refuge chamber occupants during entrapment.
Multiple sources of breathable air within their refuge chambers need to be supplied to accommodate a range of situations.
Air Quality Standards for Refuge Chambers
Within Australia and New Zealand, to provide a true source of breathable air, a refuge chamber’s filtration system must comply with AS/NZS 1716:2003, and clause 9.16 from the air supply standard AS/NZS 1715:2009 or their international equivalents. This standard states that the air supplied to the refuge chambers shall:
- Have no objectionable or nauseating odour; and
- Contain no less than 19.5% and not more than 22% by volume of oxygen.
Additionally, at 15°C and 100 kPa absolute, the air must:
- Contain no more than 11 mg/m3 (10 p.p.m. by volume) of carbon monoxide;
- Contain no more than 1400 mg/m3 (800 p.p.m. by volume) of carbon dioxide;
- Contain no more than 1 mg/m3 of oil, and
- Regular testing of the air at the respirator should be undertaken to verify the quality of the air, with records kept of these tests.
Untreated, this air supply standard is generally not attainable, due to the nature and location of the compressors and plumbing network. As such, air treatment and secondary breathable air sources are required in order to maintain a constant supply of safe, breathable air for refuge chamber occupants in an emergency.
Sources of Breathable Air in a Refuge Chamber
The Western Australian Department of Mines and Petroleum’s Guidelines on Refuge Chambers in Underground Mines stipulates a minimum of two independent sources of breathable air are to be supplied to a refuge chamber. MineARC Systems recommends a third source of breathable air wherever possible.
As a refuge chamber is essentially an airtight box that restricts natural air from entering the internal atmosphere, with no breathing supply occupants would quickly suffocate. Rough calculations show this would occur within 2-3 hours.
The two standard sources of breathable air within a MineARC refuge chamber are:
- Incoming, filtered and regulated compressed air from an external source. Usually, this is a large surface compressor.
- A scrubbing system used in conjunction with oxygen cylinders. The scrubbing system removes toxins (carbon dioxide and carbon monoxide) and the oxygen bottles reintroduce oxygen at the rate it is being consumed.
An optional, but recommended third source, is chemically produced oxygen from an oxygen candle. 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.
Source One: Compressed Air
Compressed air is simply the atmospheric “air” from above ground being pushed through pipelines down to the refuge chamber via a compressor.
The flow rate of the compressed air into the chamber is adjusted with a regulator. 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.
Before compressed air can be considered suitable for breathing, it needs to be filtered from the following contaminants:
- Water that has been compressed from humidity in the air (airborne water particles). If water is not removed, it will increase the humidity within the refuge chamber to the point where the heat index increases to an uninhabitable level.
- Oil (as a liquid), drawn from the lubricants of the mechanical elements in the compressor, or smoke from oil or lubricants that are burnt during the compressing process. Oil mist can cause breathing discomfort as well as sickness and pneumonia.
- Pollutants such as carbon monoxide and hydrocarbons from diesel exhausts aren’t uncommon in a compressor’s airline. Carbon monoxide only needs to exist at 30 parts per million to cause physical effects. Also diesel particulate can cause breathing discomfort and irreversible lung damage.
The Filtration Order
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) is tested to ISO 12500-1 and has been specifically designed to meet AS/NZS 1716:2003.
Click here for more information on MineARC’s Compressed Air Management System (CAMS).
Stage 1: Water Separator
The CAMS flood protector prevents large amounts of water from passing any further through the filtration system, while the centripetal water separator spins smaller water droplets out of the air and lets them collect in the bottom of the filter housing, to be disposed of via the auto drain.
Stage 2: Pre-Filter
The pre-filter is specifically designed to remove large particles of water and oil from the incoming compressed air source. The element materials are galvanised steel inner and outer sleeved tubes which create turbulence without a significant pressure drop. Micro glass fabric sheets are positioned between these tubes.
Stage 3: Coalescing Filter
The coalescent filter is used to remove the finer particles of oil and water from the incoming compressed air. The borosilicate micro glass element is able to absorb both water and oil particles while a glass scrim allows the water to flow to the auto drain.
Stage 4: Absorption Filter
The absorption filter is designed to remove diesel particulate, odours, volatile organic compounds, small levels of carbon monoxide and very fine oil particles. The element material is a charcoal based activated carbon sheeting that is designed to absorb contaminants until saturation occurs.
Source Two: Scrubbing & Oxygen Cylinders
In the event that the compressed air source fails, the refuge chamber must have an independent source of oxygen as well as the ability to remove the toxins in order to keep the atmosphere inhabitable. MineARC utilises compressed oxygen from cylinders and a chemical scrubbing system to filter the air of carbon dioxide (CO2) and carbon monoxide (CO).
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, which is 0.5-0.6L per person per minute. MineARC recommends the use of the largest medical grade oxygen cylinder – ‘G size’ – which contains 8200L of breathable oxygen.
Air scrubbing is the process of chemically removing contaminants or undesired gases from the air. Inside a MineARC refuge chamber, the scrubbing systems can absorb 1400L of CO2 using a soda lime product which generates a chemical reaction converting CO2 into an inert substance. Each occupant will produce 30.6L/hour of CO2 which will need to be removed by recirculating the chamber’s air through the chemical bed. As the chemical reaction comes to an end, the cartridge is changed out and the reaction starts again.
Scrubbing differs from filtration, in that a filter is a mesh that screens the air and captures solid particles, whereas scrubbing uses chemical reactions to change the composition of gases as they pass through the system.Continue reading about Refuge Chamber Scrubbing Systems in this related article
Optional Third Source: Oxygen Candles (Recommended)
Using a chemical reaction to generate oxygen, the oxygen candle is the third and final method of breathable air supply. 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 and once ignited, it will produce oxygen at a rate of approx. 28 Litres per minute until it is exhausted. Being the third independent source of oxygen, it would only be used as a last resort.