Diesel Load Outruns the Auxiliary Ventilation
A single-heading decline at 350 m is ventilated by a 1.2 m diameter forcing column delivering 22 m3/s at the face; the fan is rated 30 m3/s at the collar. Working at the face: one 200 kW development jumbo and two 250 kW diesel loaders, all running during mucking. The heading is 6.0 m wide x 5.5 m high. The site standard requires a minimum of 0.05 m3/s of air per kW of installed diesel power at the face, and a minimum air velocity of 0.25 m/s in development headings. Operators report headaches and eye irritation by mid-shift. NO2 at the operator position reads 4.5 ppm against an 8-hour TWA limit of 3 ppm (ceiling 5 ppm). Rock temperature is 31 degrees C and the decline is dry.
1. Calculate the airflow required at the face and assess compliance.
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Diesel dilution requirement
- Installed diesel power at the face = 200 + 2 x 250 = 700 kW
- Required air = 700 x 0.05 = 35 m3/s
Velocity check
- Face area = 6.0 x 5.5 = 33 m2
- Current velocity = 22 / 33 = 0.67 m/s
- Velocity needed for 35 m3/s = 35 / 33 = 1.06 m/s
Assessment
- The heading passes the velocity rule (0.67 m/s is above the 0.25 m/s floor) and fails the diesel rule: 22 m3/s delivered against 35 m3/s required, a 13 m3/s (37%) shortfall.
- This is the trap in the question: velocity compliance is a floor for mixing, not a design criterion. Where diesel is the contaminant source, the kW-based rule governs.
- Note also that 35 m3/s is a contaminant criterion, not a thermal one. At 31 degrees C with 700 kW of diesel dumping heat into the airway, the heat balance may require more air again - check it before sizing the fan.
2. The fan is rated 30 m3/s at the collar but only 22 m3/s reaches the face. What do you do?
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First, diagnose the loss
- 8 m3/s lost over 350 m = 27% loss, about 0.077% per metre. Well-maintained ducting runs at 0.02-0.03%/m (a few percent to about 10% total).
- Suspect torn or holed sections, badly made or taped joints, crushed duct behind the jumbo, tight bends, and a baggy column running with excess internal pressure. Survey and repair.
Then recognise that repair alone is not enough Even a perfect column gives 30 m3/s, still short of 35 m3/s. So you have two levers:
- Reduce the diesel kW at the face - restrict to one loader at a time at the face (200 + 250 = 450 kW, needing 22.5 m3/s, which a repaired system nearly delivers), park the second unit on the fresh-air side, or move to an electric loader. This is usually the fastest and cheapest intervention and buys time.
- Upsize the system - larger duct (1.4-1.6 m), higher-pressure fan, or a dedicated secondary circuit. Beware bolting a booster onto the same flexible column: pressure rating and recirculation are real failure modes if intake and return are close.
Do not keep running by relying on the re-entry timer alone - the exposure is during the shift, not after the blast.
3. NO2 at the operator position is 4.5 ppm. Where is that coming from and what controls it?
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Source and mechanism
- Diesel engines are the source; the forcing column delivers air but the jet has not mixed at the muck pile, so the loader operator works in a pocket of exhaust downstream of the machine while nominally 'ventilated'.
- NO2 is the acute problem (headaches, eye irritation, risk of methaemoglobin effects and, at higher concentrations, delayed pulmonary oedema). It is not fixed by adding airflow alone; the geometry matters.
Controls, in order of effectiveness
- Geometry: extend the duct to within about 10-15 m of the face and orient it to sweep the muck pile and the operator's position; keep the loader working on the fresh-air side with the exhaust discharging downwind.
- Source reduction: low-NOx engines, correct injector maintenance and tuning, diesel particulate filters or scrubbers, and reduced idling.
- Substitution: electric loader where the infrastructure allows.
- Monitoring: personal NO2 monitors with an alarm and a stop-work trigger; occupational hygiene sampling to confirm the TWA, not a one-off reading.
- Blasting: base the re-entry time on measured NO2 clearance rather than a fixed clock, and record it.
The 4.5 ppm reading is already close to the 5 ppm ceiling and comfortably above the 3 ppm TWA - treat it as a reportable over-exposure and investigate the whole shift, not just the sampling instant.