Tunnel Ventilation for Livestock

Tunnel ventilation creates longitudinal airflow through livestock buildings by placing exhaust fans at one end and inlets at the opposite end, drawing air the entire building length at velocities of 400-800 ft/min. This high-velocity airflow provides significant convective cooling through increased heat transfer from animal surfaces, making tunnel ventilation the preferred system for hot climates and high-density housing.

Convective Cooling Fundamentals

Animals dissipate heat through sensible heat transfer when surface temperature exceeds air temperature:

$$q = h_c \cdot A \cdot (T_{surface} - T_{air})$$

The convective coefficient increases with air velocity:

$$h_c = h_{natural} + k \cdot v^{0.6}$$

where $k$ = 0.15-0.20 for livestock and $v$ is air velocity (ft/min).

Effective Temperature Reduction:

$$\Delta T_{effective} = k \cdot v^{0.6}$$

At 700 fpm, animals experience approximately 9-10°F cooling effect.

Design Airflow Rate

Velocity-Based Calculation:

$$Q = v_{design} \cdot A_{cross}$$

Heat Removal Calculation:

$$Q = \frac{H_{sensible}}{1.08 \cdot \Delta T}$$

Design Example - Swine Finishing Barn:

Building: 48 ft wide × 12 ft ceiling × 500 ft long Stocking: 2,400 finishing pigs, 200 lb average Target velocity: 700 fpm

Cross-sectional area: $A_{cross}$ = 48 × 12 = 576 ft² Required airflow: $Q$ = 700 × 576 = 403,200 CFM Per-animal rate: 403,200 / 2,400 = 168 CFM per pig

Fan Selection and Staging

System Static Pressure:

$$\Delta P_{total} = \Delta P_{inlet} + \Delta P_{building} + \Delta P_{exhaust}$$

Typical total: 0.08-0.18 in. w.c. (design for 0.125 in. w.c.)

Fan Sizing:

Individual fan capacity: 25,000-40,000 CFM at 1/8 in. w.c.

For 403,200 CFM: Number of fans = 403,200 / 36,000 = 12 fans

Staging Strategy:

Evaporative Cooling Integration

Cooling Effectiveness:

Pad efficiency: $\eta$ = (T_in - T_out) / (T_in - T_wb)

Typical cellulose pad efficiency: 75-85% at 350 fpm

Temperature Reduction:

$$T_{out} = T_{in} - \eta \cdot (T_{in} - T_{wb})$$

Example: 95°F dry bulb, 72°F wet bulb, 80% efficiency

$T_{out}$ = 95 - 0.80 × (95 - 72) = 76.6°F

Temperature drop: 18.4°F

Cooling Capacity:

$$Q_{cooling} = Q_{CFM} \cdot 1.08 \cdot \Delta T$$

For 403,200 CFM with 18.4°F drop: $Q_{cooling}$ = 403,200 × 1.08 × 18.4 = 8,011,622 BTU/hr = 667 tons

Water Consumption:

$$\dot{m}{water} = \frac{Q{cooling}}{h_{fg}}$$

Where $h_{fg}$ ≈ 1,050 BTU/lb:

$\dot{m}_{water}$ = 8,011,622 / 1,050 = 7,630 lb/hr = 916 gal/hr

Pad Sizing:

$$A_{pad} = \frac{Q_{cfm}}{v_{face}}$$

For 6-inch pads at 350 fpm face velocity: $A_{pad}$ = 403,200 / 350 = 1,152 ft²

Performance Monitoring

Key Metrics:

1. Air velocity: 700 ± 70 fpm throughout
2. Static pressure: -0.08 to -0.15 in. w.c.
3. Temperature rise: 5-8°F (exhaust vs inlet)
4. Pad efficiency: 75-85%
5. Animal response: respiration rate, activity, feed intake

Energy Consumption

Fan Power:

$$P = \frac{Q \cdot \Delta P}{6356 \cdot \eta}$$

For 12 fans at 36,000 CFM each, 0.125 in. w.c., 50% efficiency:

$P_{total}$ = (432,000 × 0.125) / (6356 × 0.50) = 17.0 HP

Operating cost: 17 × 0.746 × 24 × $0.10 = $30.43 per day

Annual cooling season (120 days): $3,652

Per pig cost: $1.52 for summer cooling

Economic Justification

Investment: Tunnel system for 2,400-head barn

• 12 fans @ $600: $7,200
• Evaporative pads (1,150 ft²) @ $12/ft²: $13,800
• Controls: $3,500
• Installation: $5,500
• Total: $30,000 = $12.50 per pig space

Returns:

• Heat stress mortality reduction: 1.5% saved = $5,400/year
• Feed efficiency improvement: $7,200-12,000/year
• Payback: 2.5-4.2 years

Tunnel ventilation provides high-velocity longitudinal airflow creating significant convective cooling enhanced by evaporative cooling pads, requiring careful fan selection, staging strategy, and inlet design to achieve uniform air distribution and target cooling performance.