It is critical to the occupant’s health and wellbeing that internal heat levels do not reach fatal levels; this is achieved by controlling the temperature within a refuge chamber.
In a sealed environment, heat can increase rapidly due to internal and external sources. It is vital to have an adequate cooling system to ensure the internal temperature does not reach a critical point.
The Impact of Heat on Emergency Refuge
A refuge chamber is a sealed environment with several heat sources. A continuous build-up of heat and humidity is caused by the occupant’s own metabolic activity, scrubbing processes, electrical equipment and any ambient (external) heat affecting the refuge chamber’s internal temperature.
Unless external temperatures are very low, the temperature in a refuge chamber is controlled through artificial cooling and dehumidifying. Therefore, preventing the shelter’s interior from becoming too hot and humid to support life.
A cooling system with a minimum capacity of 130 watts per person is recommended to mitigate the heat load of chamber occupants.
MineARC Systems uses a combination of thermodynamic modelling, manned and simulated tests, and manufacturer’s specifications to correctly determine cooling requirements, taking into consideration the mechanisms of heat transfer and internal heat load.
Effects of Heat on the Human Body
If the body’s cooling mechanisms cannot dissipate heat sufficiently, several heat-related illnesses can occur, including the following:
- Transient Heat Fatigue – loss of alertness; sensations of general illness and fatigue; generally not life-threatening.
- Heat Syncope or Heat Fainting – temporary loss of consciousness resulting from insufficient blood supply to the brain caused by the dilation of peripheral blood vessels; occurs typically after prolonged periods of extreme heat; recovery is usually rapid and complete.
- Heat Cramps – painful muscle contractions in the arms, legs and abdomen, resulting from excessive fluid loss; rest and administration of fluids are usually an effective treatment.
- Heat Exhaustion – a general term for many heat-related symptoms including tiredness, thirst, dizziness, numbness, tingling in the fingers and toes, breathlessness, palpitations, low blood pressure, blurred vision, headache, nausea, and fainting; rest in a cooler area and administration of fluids is usually an effective treatment; if the individual is unconscious, heat stress should be assumed, and medical attention sought immediately.
- Heat Stroke – the most severe form of heat-related illness, immediately life-threatening; perspiration ceases, and the skin is hot, with blotchy red or blue colouration; body temperature begins to rise rapidly and uncontrollably; the victim may be delirious, disoriented, aggressive or unconscious; shivering and uncontrollable muscular contractions may occur, along with loss of bodily functions; immediate medical attention is required. Individual susceptibility to these conditions varies greatly, depending on age, physical condition, hydration, and acclimatisation to hot conditions.
How Do Air Conditioners Control Temperature in a Refuge Chamber?
Air conditioning is vital to combat the potentially fatal effects of heat stress inside an occupied refuge chamber. All refuge chambers for sale should include a quality reverse cycle split-system to cool and dehumidify the interior.
All air conditioning systems are sized based on the internal heat loads and sensible and latent cooling capacities per ASHRAE Standards.
Mitsubishi Air Conditioning systems are rated from 2.5kW to 8kW capacities depending on the cooling capacity required. Multiple units are used simultaneously for large occupancy permanent shelters (calculations performed by qualified MineARC engineers).
Understanding the Refrigeration Cycle
The refrigeration cycle occurs when the heat is transported from a colder location to a hotter area. The refrigerant is used as a medium that absorbs and removes heat from the space to be cooled and rejects that heat elsewhere.
Room temperature, liquid refrigerant is let into the evaporator. From here, some refrigerant vaporises under the evaporator’s low pressure and cools the remainder to the desired refrigerating temperature. As the remaining refrigerant evaporates, it removes heat from the evaporator and the internal environment.
The amount of heat absorbed is the total latent heat of vaporisation. The amount of heat absorbed from the environment is the effective latent heat.
The refrigerant vapour formed during evaporation passes down the suction line. As this happens, the vapour decreases in pressure slightly and increases in temperature. The superheated vapour is then compressed to a high temperature, high-pressure vapour. The superheated vapour passes to the condenser, and heat is transferred out into the atmosphere. As the vapour loses some latent heat, it condenses into a liquid. After it has become a liquid, the refrigerant goes to the refrigerant control, where the pressure is further reduced. The refrigerant is cooled as it vaporises, and the cycle is repeated.
This is represented graphically to the right on a pressure-enthalpy chart.
Measuring Occupants Health with Heat Indices
There are many different available heat indices used to measure occupant comfort. MineARC references 30 CFR Refuge Alternatives for Underground Coal Mines. This standard specifies that the temperature inside a refuge chamber should not exceed an apparent temperature of 35°C (95°F) or equivalent wet bulb temperature of 28.9°C (84°F).
The recommended heat indices and maximum levels are based on the National Institute of Occupational Safety & Health’s research and medical evidence.
Apparent temperature is a measure of relative discomfort due to the combined effect of heat and humidity. The likelihood of adverse health effects of heat varies per person. However, apparent temperatures greater than 26.7°C (80°F) are generally associated with some discomfort. Core body temperatures above 40°C (104°F) are considered life-threatening.
Wet Bulb Temperature
The wet-bulb temperature is the temperature a parcel of air would have if it were cooled to saturation (100% relative humidity) by the evaporation of water into it. The parcel supplied the latent heat.
Influences on Cooling Requirements for Safe Refuge
Mechanisms of Heat Transfer
To establish when artificial cooling is required, MineARC uses proprietary models to determine the internal temperature and humidity changes and metabolic heat generated within a refuge chamber.
All models assume metabolic heat generates a total of 400btu (117 Watts) per hour per person. This total can be broken down into the following quotients: Sensible (80W) and Latent (37W).
Heat in Portable Refuge Chambers
Thermal conduction and convection, and moisture latent heat phase changes are examined. Analysis of the natural convective heat transfer from air inside the chamber to the steel structure’s inner surface, through the steel sheet, and from the steel’s outer surface to air outside is conducted.
Relative humidity is assumed to be 100% as real-life testing has shown the refuge chamber will reach saturation quickly. The latent heat generated by the influence of condensation on internal chamber walls is taken into account in heat transfer calculations.
Heat in Permanent Refuge Chambers
The heat transferred through the refuge chamber entry wall structure (steel or concrete) to the outside atmosphere, and from the air inside the refuge chamber into surrounding strata walls are examined. The starting relative humidity is assumed and the increase is calculated based on the assumption that each occupant produces 1.5L of moisture per day (from MSHA 30 CFR Refuge Alternative Simulated Testing)
Calculating Internal Heat Load and Cooling
MineARC’s Production Pre-Build automatically calculates the air conditioner’s size required using the calculated internal heat load and cooling data supplied by Mitsubishi. The internal heat load considers metabolic heat, electrical heat, and the heat from the soda-lime chemical reaction in the CO2 scrubber.
The total sensible heat load is 1,816W, and the total latent heat is 740W for a 20 person Standard Design MineSAFE chamber. To ensure that both the sensible and latent cooling requirements can be adequately met, the heat loads are compared against both the air conditioners’ sensible and latent cooling capacities.
For cases where the external temperature is likely to be higher than the internal, the refuge structure requires insulation.