MINEARC TECH TOPIC: TEMPERATURE CONTROL
What are the sources of temperature control in a refuge chamber and why do we need them? This MineARC Tech Topic explains the importance of temperature control within refuge chambers.
A refuge chamber is a sealed environment with numerous 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, some form of artificial cooling and dehumidifying is required to prevent the refuge chamber 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, a number of 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; normally occurs 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 normally an effective treatment.
- Heat Exhaustion – general term for a number of 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 normally an effective treatment; if the victim is unconscious, heat stress should be assumed, and medical attention sought immediately.
- Heat Stroke – the most serious form of heat-related illness, immediately life-threatening; perspiration ceases, and the skin is hot, with blotchy red or blue coloration; body temperature begins to rise rapidly and uncontrollably; 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 acclimatization to hot conditions.
AIR CONDITIONING SYSTEM
Air conditioning is vital to combat the potentially fatal effects of heat stress inside an occupied refuge chamber. All MineSAFE Refuge Chambers include quality reverse cycle split-system Mitsubishi Air Conditioners to both cool and dehumidify the refuge chamber.
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. For large occupancy permanent shelters, multiple units are used simultaneously (calculations performed by qualified MineARC engineers).
THE REFRIGERATION CYCLE
In the refrigeration cycle, heat is transported from a colder location to a hotter area. The refrigerant is used as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere.
Room temperature liquid refrigerant is let into the evaporator. Some of the refrigerant vaporises under the low pressure in the evaporator and cools the remainder to the desired refrigerating temperature. As the remainder of the refrigerant evaporates, it removes heat from the evaporator and, resultantly, from the internal environment. The total 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 in to 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 vaporizes and the cycle is repeated.
This is represented graphically to the right on a pressure enthalpy chart.
There are a number of different available heat indices used to measure occupant comfort. MineARC refers to 30 CFR Refuge Alternatives for Underground Coal Mines, which specifies 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 research completed by the National Institute of Occupational Safety & Health and available medical evidence.
Apparent temperature is a measure of relative discomfort due to the combined effect of heat and humidity. The likelihood of adverse effects from heat vary per person, however, apparent temperatures greater than 26.7°C (80°F) are generally associated with some discomfort. Core body temperatures in excess of 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, with the latent heat being supplied by the parcel.
COOLING REQUIREMENT: INFLUENCES
Mechanisms of Heat Transfer
In order 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 in to the following quotients: Sensible (80W) and Latent (37W).
For 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.
For 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 is 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 Alterative Simulated Testing).
Internal Heat Load
MineARC’s Production Pre-Build automatically calculates the size of the air conditioner required using the calculated internal heat load and cooling data supplied by Mitsubishi. The internal heat load takes into account 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 sensible and latent cooling capacities for the air conditioners.
For cases where the external temperature is likely to be higher than the internal, the refuge structure requires insulation.
For further information regarding MineARC’s Refuge Chamber cooling systems and requirements, please email email@example.com to speak to a MineARC representative.