MINEARC TECH TOPIC: HEAT LOADS
This MineARC Tech Topic explains why understanding heat loads is important to temperature control within refuge chambers.
Numerous heat sources affect the internal temperature of a refuge chamber. A continuous build-up of heat and humidity is caused by the metabolic activity of occupants, scrubbing processes, electrical equipment and any ambient (external) heat.
Due to the sealed environment, the heat generated from these sources cannot escape and the internal chamber temperature will rise quite rapidly. Eventually, without some form of mitigation, this will lead to the chamber becoming uninhabitable and posing a severe hazard for its occupants.
MineARC Systems take into consideration the mechanisms of heat transfer and internal heat load as well as a combination of thermodynamic modelling, manned and simulated tests, and manufacturer’s specifications to correctly determine refuge chamber cooling requirements.
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, in ascending order of seriousness:
Air conditioning is vital to combat the potentially fatal effects of heat stress inside an occupied refuge chamber. A cooling system with a minimum capacity of 130 watts per person is recommended to alleviate the heat load of chamber occupants. All MineARC Refuge Chambers include quality reverse cycle split-system Mitsubishi Air Conditioners to both cool and dehumidify the refuge chamber.
Air conditioning systems are sized based on the internal heat loads, and sensible and latent cooling capacities per American Society of Heating, Refrigerating and Air-Conditioning Engineers (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).
For more information on air conditioning and heat-related illnesses, please view the MineARC Tech Topic: Temperature Control article.
Humans produce heat as a result of metabolising energy. For heat calculations, MineARC allow 117 watts per person, comprising of 87 watts of sensible heat and 30 watts of latent heat. In this case, sensible heat is radiant heat loss and latent heat is evaporative heat loss. The heat lost by the body is gained by the refuge chamber, therefore it is important to consider that people are adding both heat and humidity to the refuge chamber.
OTHER HEAT SOURCES
Some electrical energy is lost in the form of heat, which will then dissipate into the chamber environment. While typically not a large amount of heat, consideration must be given to the amount of heat energy contributed to the refuge chamber by electrical equipment such as lighting, digital gas monitors and control systems. Allowances have been made for these heat sources in MineARC’s calculations.
The chemical reaction that occurs during both CO and CO2 scrubbing is exothermic, meaning heat is lost during the reaction. This heat will then be transferred to the chamber environment. The amount of heat lost varies with the number of occupants and the amount of CO or CO2 being scrubbed.
For more information on scrubbing systems, please view the MineARC Tech Topic: Scrubbing Systems article.
Chambers located outdoors will gain heat from solar radiation. Depending on location, this energy from the sun can be enormous, and very difficult for an air conditioner to control. In most cases outdoor refuge chambers require insulation to slow down the effect of external heat loads on the chamber space, in addition to upsized air conditioners to ensure they will remain habitable. In some cases, special high ambient temperature units will be required. Insulation will not entirely stop heat transfer, just limit the amount that will occur.
External ambient air and rock temperature can also impact the internal temperature of the refuge chamber. If the temperature outside the refuge chamber is above 30°C, it will add to the internal heat and therefore must be considered in cooling requirement calculations.
MECHANISMS OF HEAT TRANSFER
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 Alternative 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.
It is important to note that the calculations do not take into account heat being absorbed from the external atmosphere. As mentioned previously, 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.
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