Tunnel Ventilation and Heat Stress in a Broiler House
A broiler house is 120 m long, 12 m wide and 2.8 m high, holding 20,000 birds at 2.2 kg each. Summer outside temperature reaches 40 °C, and the target tunnel air speed is 2.5 m/s. Exhaust fans are rated at 40,000 m³/h at 25 Pa static pressure. The house currently has six fans, sidewall inlets, and no evaporative cooling pad. Birds show panting and wing spreading from midday, and mortality rises to 0.15% per day during heat waves. The farmer proposes adding two more fans but keeping sidewall inlets, and closing the house completely. The local power supply can support 10 fans. A pad supplier recommends 60 m² of 100 mm cellulose pad with 1.5 m/s face velocity.
1. Calculate the required airflow for 2.5 m/s tunnel speed and the number of fans needed. Assess whether six or eight fans will achieve the target.
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House cross-section = 12 m × 2.8 m = 33.6 m². Required airflow = 33.6 m² × 2.5 m/s = 84 m³/s. Convert to m³/h: 84 × 3,600 = 302,400 m³/h.
Fan number = 302,400 / 40,000 = 7.56, so 8 fans are needed.
With six fans: 6 × 40,000 = 240,000 m³/h = 66.7 m³/s. Air speed = 66.7 / 33.6 = 1.98 m/s. That is below the 2.5 m/s target and explains the panting and wing spreading at 40 °C.
With eight fans: 8 × 40,000 = 320,000 m³/h = 88.9 m³/s. Air speed = 88.9 / 33.6 = 2.65 m/s, which meets or slightly exceeds the target. Eight fans are sufficient, but only if the house is tight and the inlets are correctly managed.
2. Evaluate the farmer’s plan to add two fans while keeping sidewall inlets and closing the house completely. Calculate the pad face velocity for the supplier’s 60 m² pad.
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Adding two fans without changing the inlet system will not produce true tunnel ventilation. Sidewall inlets distribute air along the house and create lateral airflow; tunnel ventilation requires a large inlet at one end and exhaust fans at the opposite end, with the house otherwise sealed. Closing the house completely with only sidewall inlets can starve the fans, raise static pressure, reduce actual fan output, and create dead spots.
Pad face velocity = airflow / pad area = 84 m³/s / 60 m² = 1.4 m/s. The supplier’s 1.5 m/s figure is close; 60 m² gives about 1.4 m/s, which is within a reasonable range for 100 mm cellulose pad, though 56 m² would give exactly 1.5 m/s. The pad area is therefore adequate, but the pad must be installed with a proper tunnel inlet and water distribution system, not merely added to a sidewall-inlet house.
The correct plan is eight fans, a sealed tunnel inlet at the far end, and a 56–60 m² evaporative pad. The farmer should also check that the fans are rated at the actual static pressure of the pad and house; 25 Pa is low for a pad system, so actual airflow may be less than 40,000 m³/h per fan.
3. Recommend a heat-stress management plan and contingencies, given that power can support 10 fans and mortality is already 0.15% per day.
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The immediate priority is to reduce heat load and keep air moving:
- Install the eight-fan tunnel system with a proper tunnel inlet and 56–60 m² pad. Do not rely on sidewall inlets for tunnel operation.
- Use the two spare fan positions for backup or for additional exhaust capacity during extreme heat, but do not exceed the house’s static-pressure limits.
- Start the pad and fans before midday, not after birds are already panting. At 40 °C, evaporative cooling can lower incoming air by roughly 8–10 °C if the pad is properly wetted and the air is dry.
- Provide cool drinking water, extra drinker space, and feed during cooler hours. Reduce stocking density if possible; 20,000 birds at 2.2 kg in this house is about 15.3 kg/m², which is high for 40 °C conditions.
- Have a backup generator, alarms, and a contingency plan for power failure. If fans stop, open curtains immediately and use foggers or sprinklers cautiously to avoid wet litter.
- Monitor mortality and bird behaviour. A rise above 0.15% per day warrants emergency action: maximum ventilation, pad cooling, and possibly partial depopulation or market withdrawal.