An efficient ventilation survey entails determining the amount of air, its pressure, temperature, moisture content, and leakage rates, and from these readings, identifying losses and confirming whether the proper ventilation rate is provided. The main ventilation circuit of a large mine needs to be established as a through-flow network, one that has adequate intake air, little resistance, and mostly parallels in the ventilation network. Here are the crucial steps to conduct a ventilation survey for a large underground mine.
Set the survey objectives
First, define what information the survey should provide: whether there is adequate airflow, locations of primary pressure drops, any leakage and recirculation, and the capability of the ventilation system to accommodate present and future production levels. The survey will also offer information that will help determine fan size, location of regulators, booster fans, and expansion plans. In larger mines, this process is not a one-time event but needs to be done on a regular basis.
Build the airway map
A modern ventilation diagram must be used, which indicates shafts, declines, raises, returns, regulators, doors, stoppings, fans, and key workplaces; otherwise, the survey will have to take into account the actual flow of air in the mine. In this case, it is necessary that the mining map allow for pinpointing the places where the air flows diverge, converge, or change their course, as they represent the most critical stations of the survey. In large mines, proper planning is particularly significant.
Select survey stations
Select locations for the stations in equal intervals along the main airways, as well as at strategic locations like divergences, convergences, regulators, fans, shaft junctions, and return junctions. The Bureau of Mines technique suggests placing the stations not exceeding 2,000 feet apart and additional stations wherever there is a significant variation in airway geometry and conditions. In a large-scale mining operation, you should select stations on the intake leg, return leg, and important crosscuts or high resistance zones.
Measure the right variables
The quantity of flow or speed, barometric pressure, dry and wet bulb temperature, altitude, and the pressure difference between two control devices, when necessary, are some of the parameters that need to be measured at each station. The following parameters were included in the example of CDC field study conducted for a large opening mine, which include barometric pressure, humidity, temperature, quantity of airflow, direction of airflow, and cross-sectional area.
Control for time effects
Since the atmospheric pressure will fluctuate during the day, continuous measurements from the base station should be taken while measurements of the other stations are being done by the roving party. The conventional pressure survey technique either involves simultaneous measurement of the two stations or base station and roving measurement. Although the latter is more preferable especially in big survey projects due to time-saving reasons, the former works best especially when the elevation difference between the two stations is large. Note all fan adjustment, cage operation, or interruptions.
Calculate pressures and losses
Transform the readings into values for total pressure, pressure losses due to static pressure, corrections for velocity pressure, and resistance per airway section, and develop a graph of the pressure gradient from intake to return. According to the information provided by CDC and the Bureau of Mines, the pressure measurements are important on their own, but the true utility is derived from the correlation between pressure and quantity measurements to identify regions of high resistance and measure the impact of any modification.
Define the primary circuit
The main ventilation circuit refers to the entire system of through-flow throughout the entire mine, which channels fresh air from the intake shafts into the main workings and exhausts it via return shafts. According to the WA recommendation, the main ventilation circuit is usually designed to operate in parallel lines through the mining regions, connecting to the exhaust shafts or raises, as shorter and more direct ventilation lines have lower resistance and require less energy.
Size the intake and exhaust paths
One of the most common errors in system design occurs with inadequate sizing of the intake capacity because high velocities in intake and main development airways result in dust production, energy losses, and high costs due to energy consumption. The WA standard mentions that air velocities in excess of around 6 meters per second in the main intake airway can cause problems in work and movement zones. The main circulation system must therefore be designed to provide for adequate volume transport at tolerable velocities rather than minimal construction costs.
Choose fans and controls
Choose the primary fans from the resistance characteristics of the mine and the duty of the airflow, and ensure that the operating point lies on the characteristic curve of the fan. The design will further indicate the location of the regulators, doors, boosters, and even secondary or circuit fans, since they control the distribution of the primary flow through time. In case of large mines, it is highly advisable to carry out computer networking simulations to test various developmental phases and production arrangements.
Verify, document, and update
After the designing process, it is necessary to compare the airflow model with field survey data, resolve any discrepancies that may arise, and modify the ventilation system and its control. Monitoring should be carried out on a constant basis in terms of volume, pressure, temperature, pollutants, and faults in the ventilation circuit, as this will change during the course of the mining operation.
Illustration: for instance, for mines with high thermal burden and long haulage distances, the most appropriate system for the mine would be the surface exhaust system or a combination thereof providing supply air to multiple parallel intake-return circuits with only booster fans where they make more efficient contribution to increasing total airflow in the circuit compared to regulators. This is in line with the recommendation that primary systems must provide maximum parallel flows, low resistance and the capacity for air diversion.
