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Craig Stewart

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  1. Battery Electric Vehicle (BEV) heat should be based on the continuous power output (CPO) of the machine. Peak power output is another figure sometimes quoted in specifications, but should be avoided for calculating likely heat as it is only emitted in short bursts. The sensible heat output therefore should be based on the total CPO output divided by motor and battery efficiency (estimated at 90-95% efficient for motor, battery). However, you still need to apply an average power utilisation factor to this – typically I use only 30% for ramp haul diesel trucks (uphill, downhill, rest average), which conservatively could be used for electric trucks too. In fact, because trucks have regenerative braking, and a more efficient power train, it is likely to be a lower power utilisation factor - maybe in the order of 20%-25% or even less. As a worked example, if the total CPO of a truck is 500kW, the calculated average Sensible Heat to include in Ventsim can be 500kW / 95% x 25% = 132kW. Ensure this is entered as Sensible Heat, not Diesel Motor Power in Ventsim. A future feature in Ventsim may have a specific BEV feature for this entry. One last consideration, is McPherson states that moisture generated by (diesel) mobile machines has been measured at several times the theoretical moisture emitted from the exhaust, likely due to the moist loads carried by many machines, and the accelerated evaporation of moisture around the machine activity. There is no reason to think BEV's would be any different. Therefore, placing all heat as Sensible Heat only may produce overly dry and hot air temperature results. Entering the heat as 50% Sensible and 50% Latent heat, or by adding 20ml/sec of moisture per 100kW of Sensible heat will inject some moisture into the model to produce more realistic results, although this amount should be ideally calibrated against measurements. For the above example, the 132kW could be entered at 66kW Sensible + 66kW Latent, or alternatively 132kW Sensible + 26ml/sec of moisture (which the simulation will evaporate and convert from sensible to latent) When simulated, you'll find significantly less heat and temperature increase should be generated by BEV for similar engine outputs. Ultimately, BEV's have motor outputs 90-95% efficient (plus downhill and braking regeneration), versus only 30-35% efficiency for diesel engines.
  2. Hi Thomas Nice summary. Remember that any kW calculation is an energy flow rate which depends on both the contained Sigma Heat and the mass flow of air. If you have a solution that decreases airflow, this will also show decreased energy flow kW, but this does not necessarily decrease contained energy or the air per kg (Sigma Heat) which is the most important consideration for the cooling power of air on human physiology
  3. Hi Thomas 1. Total Heat Summary is the positive Heat Inputs into a model. It excludes negative heat sinks such as refrigeration. Heat Added to Network is the net balance of positive and negative inputs. Perhaps the wording can be improved in Ventsim ...... 2. "Exhaust Heat Difference" is part of the auditing tool that ensures no significant heat is being lost in the model calculations. Models are simulated to converge to a solution with an acceptable error (for example with 0.1m3/s airflow imbalance, or with 0.1C temperature imbalance). If these limits are too high, or the model has lots of bias towards to extreme of this limit, some heat energy can be lost between junctions due the imbalance. Imbalances of more than a few percent or 0.1C warrant further checks to ensure limits are not too high - any imbalance can be reduced by decreased acceptable error limits (in the simulation airflow (flow) and simulation heat (zero mass flow, temperature) settings, at the expense of longer simulation times. 3. The figure you are probably after is the total sigma heat (similar to enthalpy) change. The summary doesn't directly provide this, but you can simply divide the HEAT BALANCE TOTAL by the TOTAL MASS FLOW and look at the difference between the figures in the different simulations. You'll find increasing airflow will increase the total heat absorbed from the rock, but not at the same rate as the airflow increase - therefore you'll likely end up with lower sigma heat (and therefore wetbulb) temperatures. In most cases, with warm rock and limited airflow and cooling, the best design option is to limit or stop airflow to active work areas, unless there is a need to frequently re-enter non-worked areas (which may require some time for the ventilation to cool again)
  4. The following advice is a preliminary research paper investigation. Users considering lithium battery fires should undertake their own review. BEV fires are exothermic chemical fires that can be described in terms of heat release rates (HRR) like any other type of fire. They also release asphyxiant gases such as carbon monoxide like hydrocarbon fires although other gases may be more toxic. As such, there is no reason that they cannot be modelled in fire simulation providing an estimate of the HRR can be made, however some considerations are needed, and more industry test work and validation would be welcome. Also, remember a lithium battery fire can still spread to other more conventional fire fuel sources on the BEV such as tires, hydraulic hoses and oil etc which are still likely to be a greater hazard (by mass and heat) than the lithium battery itself. If you are using VentFIRE in Ventsim, the HRR is sourced from the heat of combustion (per kg) and the gases are sourced from the yield rates of the combustible material. For a lithium fire, the electrolyte chemistry and associated reactions are the main source of energy (which will change depending on the state of charge SOC), so a determination of the mass of the battery would have to be made by examining the likely HRR release rate versus the battery size. Lithium batteries typically only have around 10%-15% of the runaway heat compared to diesel fuel by mass, hence the heat of combustion is much smaller, however the batteries carried by most BEV's tend to be much heavier than an equivalent fuel machine. The heat release rate curve HRR is not dissimilar to a hydrocarbon fire - check literature and examples on the web for this. While not a lot of test work has been done on underground machines it should provide some insight into how long a fire could burn for. https://www.nature.com/articles/s41598-017-09784-z The release of gases; whilst CO gases are an asphyxiant , it is more likely that the fluoride gases released from battery fires will be more dangerous. Studies indicate HF (hydrogen fluoride) could be emitted at a yield of up to 1kg per 30kg of lithium battery mass (for a fully charge battery - less otherwise). Also important is that lithium battery fires don't consume large amounts of oxygen and may not be significantly impacted by low or no airflow, so set the oxygen yield consumption rate low in VentFIRE. While Ventsim VentFIRE does not currently have a specific fire gas for HF, one of the other unused fire gases (such as NH3 or CH4) could be used to supplant and simulate this at a substitute yield rate. Exposure of humans to HF gas is poorly researched, however the CDC states exposure to 50ppm for 30-60 minutes may be fatal. Short term (1 hour) exposure limits are recommended <8ppm
  5. Hi Evaporative cooling is easily simulated by just directly adding moisture to the airflow in the airway. You can do this in the EDIT BOX > HEAT TAB > Moisture which is added in the form of grams/second (for example 100g/second) evaporates 0.1l per second of water into the air. If you spray too much water for the available airflow to evaporate, the excess will be ignored. Once the water is evaporated, the ambient (dry bulb) air temperature will reduce in response. Wet bulb temperatures will remain the same. The scientific methodology behind this is essentially that sensible heat is being transferred latent heat, resulting in a decrease of sensible heat and an equivalent increase in latent heat. Indirect evaporative cooling is more complex. The moisture evaporated is kept separate from the main airflow via a close plate heat exchanger. In this case, while sensible heat is still being offset by latent heat, the latent heat is no longer being added to the air. If the heat exchanger was 100% efficient, you could simulate this by observing the sensible and latent heat exchange in the evaporate cooling circuit, and then entering the equivalent =ve sensible heat in the main airstream circuit.
  6. Another way to calibrate a drill machine is to use the HEAT CALCULATOR tool in Ventsim. If you can physically measure the airflow and temperature of the air going in and out of the job, you can enter the result in the HEAT CALCULATOR and it will report the amount of sensible heat and moisture to add. Just a caution though - some of this heat may be from surrounding rock, so keep the measurement of in and out airflow short.
  7. Thanks for the question. Your function mixes different types of units and values so let's revisit radon exposure. Dosage/year is a function of the time exposure to ionizing radiation from the radioactive decay of radon gas and its progeny. Cumulative exposure (dosage) to all combined ionising radiation by humans is normally measured in WLM (Working Levels Months) or mSv (Milli Sieverts) and current allowable limits for workers are typically mandated by many countries at 4 WLM per year, or 20 mSv per year Ionizing radiation (consisting of alpha, beta particles and gamma radiation) is emitted from radon gas decay or one of the numerous progeny of radon (elements that have decayed from radon) which form elemental solids. In fact, it is the tiny solid particles of the progeny of radon (not radon itself - a gas) that gets caught in the lungs and does the most radiation damage. The ionizing radiation and therefore dosage is therefore not calculated directly from the concentration of radon gas (which is the contained radioactive decay energy potential) usually measured in Bq/litre, but as the decay rate of radon progeny measured as ionizing radiation uJ/m3(which changes over time depending on how long the radon has been decaying) A great resource for this is available from McPherson's book. It shows the calculations used by Ventsim and can be found at https://www.mvsengineering.com/files/Subsurface-Book/MVS-SVE_Chapter13.pdf
  8. It’s important to calibrate your model for heat with known measurements where possible, before extending the model deeper for future predictions. Your surface temperatures, pressures and rock parameters need to be entered into the SETTINGS > SIMULATION > ENVIRONMENT section. Ensure the SURFACE DATUM value is correctly referencing the Z Coordinate where you have taken the surface pressure and temperature. Once you enter these values, the surface AIR DENSITY and all associated underground densities are automatically calculated – you do not need to enter this. Rock parameters such as conductivity and diffusivity can be measured by a laboratory, however if this data is unavailable, examples of common rock properties are available in the Ventsim manual. Your measured underground rock temperatures need to be used to calculate the geothermal gradient (difference in temperature per 100m vertical) and project the temperature gradient back to the surface level to establish the SURFACE ROCK TEMPERATURE – which also needs to be entered in the SETTINGS > SIMULATION > ENVIRONMENT section also. Measuring the surface rock temperature after blasting is not ideal – it may be ok but it is likely to have cooled a little. The gold standard is to drill a hole in the side wall near the blasting face, and measure the temperature at the back of the hole with a probe (at least 3m deep). To calibrate the model to your actual underground temperatures, you will need to consider any diesel or electrical activity in the mine beyond just the rock strata, auto compression and fans (all of which Ventsim calculates automatically). A good guide to establishing the correct amount of diesel engine power to put in to the model is to estimate the diesel fuel usage by the mine per day, and then calculate the fuel usage per operating hour (for example if the mine uses 25,000 litres per day and works 12 hour shifts, then the diesel usage for operating times (let's assume 9 hours per shift) will be 25000 / 18 hours = 1388 l per hour) Use the HEAT CALCULATOR in the TOOLS menu in Ventsim to convert that to Diesel Engine kW, and then spread these heat sources through the working areas of the model using either a number of point sources, linear sources, fuel burn or activity tracks (these are all options in the HEAT section of the EDIT box). Focus on getting the WET BULB temperatures correct initially. Once you have a model that has good validation with the WET BULB measurements, you can then focus on the DRY BULB – the most significant factor affecting the DRY BULB is the WETNESS FRACTION assigned to the airways (or SETTINGS > SIMULATION > ENVIRONMENT if you haven’t directly set the wetness fraction in the airways). Increase the wetness fraction to increase evaporative cooling and decrease dry bulb. You will find this will not normally change wet bulb temperatures significantly and therefore you should be able get the dry bulb to change to match your measurements. You may need to adjust different airways in the mine with different wetness fractions. Once you have these factors correct you should then be in a good position to use the model with confidence for future planning. Hope that helps Craig
  9. PQ Surveys are measured with an accurate anemometer (airflow velocity meter), a manometer (differential pressure meter) or barometer (atmospheric pressure meter), and a distance measuring device (tape measure or laser measure). Once pressure and airflow are measured, Atkinson's formula can be used to determine resistance. The methods can vary with different practitioners, however a great summary is provided in McPherson's Subsurface Ventilation Engineering book available here https://www.mvsengineering.com/files/Subsurface-Book/MVS-SVE_Chapter06.pdf The attached file by John Rowland is also an excellent reference to barometric surveys. Barometric Surveys Resistance JR.pdf
  10. All working theory formulas for airflow and thermodynamic analysis in Ventsim can be found in the book "Subsurface Ventilation Engineering" by Malcolm McPherson, an excellent text that should be owned by all ventilation practitioners. We have extensively utilised the proven and peer reviewed methods presented in the book in Ventsim and validated our software with many of the examples presented in the book. MVS have kindly continued to make available free downloads of this text as PDF files, which includes several chapters on both the theory and application of ventilation principles in simulation. https://www.mvsengineering.com/index.php/downloads/publications
  11. Hi Deswik have made sure almost every Ventsim attribute can be mapped to Deswik if required. The Deswik exporter allows you to link these attributes. In most cases however at a minimum you may only need Layers (primary and secondary for perhaps zone and type of airway) Airways Sizes Airway Profiles Stages timing (if you want to export schedules) You may need to ask Deswik for more instruction on how to do the above, but when they showed me it looked fairly simple. This will allow you to export a Deswik design to a Ventsim file format that requires minimum changes in Ventsim. Regards Craig Stewart
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