In a typical cross-flow ventilation system, air is moved across the barn from one side to the other. However, in a tunnel ventilation system, a similar volume of air is moved the length of the barn, from one end to the other. The entire air inlet is concentrated on one end wall of the barn (Figure 1) with all of the exhaust fans located on the opposite end (Figure 2). This creates a high velocity wall of air moving through the building like a wind tunnel, and typically increases the air speed over the animals or birds by a factor of 2 to 4 times that measured with cross-flow ventilation. Tunnel ventilation provides a beneficial wind chill effect that cools the animals by convection. The net result is reduced heat stress, increased animal comfort, and the ability to maintain productivity in hot weather.

Figure 1. A 6-foot high tunnel inlet door is located across the end wall of each floor.

Figure 2. Six tunnel exhaust fans are located across the opposite end wall of each floor.

In a cross-flow ventilation system with a slot-type air inlet, the air speed at bird level will be in the range of 0.25-0.50 m/sec (50-100 ft/min). With tunnel ventilation, the goal is to achieve an air speed between 0.5-1.5 m/s (100-300 ft/min) over the animals. Most producers install sufficient fan capacity to achieve the upper portion of this range. However, some barns are using even higher air speeds with very good results.

Figure 3. Effective temperature reduction due to wind chill over the birds.

Figure 3 shows a wind chill curve developed by the USDA Poultry Research Laboratory at Mississippi State University for seven-week-old broiler chickens. It indicates that an air speed of 1 m/s (200 ft/min) over the birds will result in similar growth performance to birds raised in still or slow moving air that is 1°C cooler. Similarly, an air speed of 2 m/s (400 ft/min) creates a wind chill effect of 3.7°C. The birds in this trial grown with tunnel ventilation at 2 m/s were 0.5 kg heavier at 7 weeks of age than the birds grown in still air.

These results support the trend seen in Ontario barns towards higher tunnel air speeds for larger birds. However, extra equipment cost and inlet size requirements must be considered. Of course, the same wind chill effect eliminates the use of tunnel ventilation for cooler weather (less than 15°C outside) or for younger animals. When tunnel ventilation is being designed for a new building, the cross-flow system is usually sized for 40%-50% of the summer ventilation requirements to accommodate the 3 cooler seasons and small, sensitive stock.

Fan Capacity for Tunnel Ventilation
Tunnel ventilation is sized on the basis of creating a wall of air moving the length of the building at a velocity of 1.0-2.0 m/s (200-400 ft/min or 4-8 km/hr). Therefore, the required fan capacity is simply the useable barn cross-sectional area multiplied by the desired air speed. Note that floor-reared birds occupy the lower 0.3 m (1 ft) of the barn cross-section, so it is not useable for air movement. Similarly, cattle stalls, swine pen partitioning, etc. will reduce the net cross-sectional area available for airflow.

Another common sizing guideline is to achieve a complete barn air exchange every minute. Thus, the barn volume equals the tunnel fan capacity. Caged layer buildings usually require a greater air exchange rate due to the very high bird density.

Example Fan Sizing
What is the fan capacity required for a 18 x 91 m (60 x 300 ft) chicken broiler barn with a 2.7 m (9 ft) ceiling height and a desired tunnel air speed of 1.5 m/s (300 ft/min)?

In Metric

Q = (V x A) x 1000
= 1.5 x 18 x (2.7 - 0.3) x 1000
= 64800 L/s

In Imperial

Q = V x A
= 300 x 60 x (9 - 1)
= 144000 CFM

The most common fan size used for tunnel ventilation is a 1200 mm (48 in.) diameter fan due to its high-energy efficiency. On average, these fans deliver about 9400 L/s (20000 CFM) each. Therefore, the number of fans required is:

In Metric

64800/9400 = 7 fans

In Imperial

144000/20000 = 7 fans

Other fan sizes, or a combination of fan sizes, can be used and may be desired for a particular situation. Note that if there is insufficient room on the end wall to place all of the tunnel fans, it is acceptable to move them around the corner onto the sidewall as shown in Figure 4.

Air Intake for Tunnel Ventilation
Tunnel ventilation inlets can be motorized louvres, curtains, panels or large doors. These openings are usually sized on the basis of 2 m2 per 5000 L/s (2 ft2/1000 CFM) of fan capacity. Since air does not like to turn corners, this type of inlet is best suited across the end wall so that the incoming fresh air travels straight into the barn in the correct airflow direction. As shown in Figure 1, a 1.5-1.8 m (5-6 ft) high panel across the end wall will provide the necessary cross-sectional area for dairy stables and poultry houses. Some barns are equipped with a combination of panels and a roll-up door to achieve the required inlet opening.

Example Air Intake Sizing
What is the required air intake size for the example chicken broiler barn already considered?

In Metric

A = 2 x (Q/5000)
= 2 x (64800/5000)
= 25.9 m2
= 1.7 m x 16 m

In Imperial

A = 2 x (Q/1000)
= 2 x (144000/1000)
= 288 ft2
= 6 ft x 48 ft

Tunnel Ventilation Control
Tunnel ventilation is strictly a warm weather system and should only be activated above 21°C (70°F) with animals that are not overly sensitive to drafts, such as birds less than 3 weeks of age. As with normal cross-flow ventilation, tunnel ventilation should be automatically staged to operate as the barn temperature increases. Generally employ 2 or 3 tunnel stages so that Stage 1 operates the first 2 or 3 tunnel fans; Stage 2, another pair of fans and Stage 3, the remaining fans. This control strategy allows the wind chill effect to be less with younger animals and reduces the cooling effect during cooler night time conditions.

Normally the cross-flow ventilation system would be turned off completely - sidewall fans off and the baffle board air inlet closed to obtain the full effect of the tunnel ventilation. Remember that the main objective is to evacuate the hot air in the barn as quickly as possible without extra mixing and blending of hotter air located near the ceiling.

There is an upper critical temperature for tunnel ventilation. Excessive air movement above 40°C (104°F) actually increases heat stress because this air temperature is equal to or greater than the animals' body temperature. These high temperatures would be more common in the southern U.S. and some form of evaporative cooling would be incorporated into the system to reduce the effective temperature.

Potential Tunnel Airflow Challenges
Air Inlet Location
Air inlets should be located on the end wall if at all possible. Air will not turn 90-degree corners unless forced. If some air enters from the sidewall, it will generate airflow across the barn rather than along the length. This perpendicular airflow will eventually turn and move endwise to the exhaust fans, but will leave a dead air zone along the sidewall containing the inlet panel. Air deflectors have been used to assist turning this incoming air. See Figure 4.

Figure 4. Floor plan showing dead air zone due to placement of tunnel ventilation air inlets.

Service or Entrance Room Location
The service or entrance room should be located outside the main building as shown in Figure 1 to provide full end wall exposure for the tunnel air inlet. Besides reducing the available end wall opening, it can create a major wind shadow on that side of the room. An unconfined air stream will fan out at about a 10-degree angle, and therefore a 2.4 m (8 ft) wide service room will prevent tunnel air movement along that wall for at least 14 m (46 ft). (See Figure 5.) Smoke testing indicates that the actual affected distance can be even greater due to a slow, backward eddy-type flow in the wind shadow area. If an interior room is necessary, it should be located near the tunnel exhaust fans and opposite the winter air inlet to minimize its obstruction effect.

Figure 5. Floor plan showing dead air zone due to service room obstruction.

Outside Wind Effect
Significant summer winds blowing at an angle on the tunnel air inlet can skew the airflow to one side of the barn. (See Figure 6). This scenario can drastically reduce the airflow along the other side of the room. Compare an outside wind speed of 20 kph (12.5 mph) to a typical tunnel air speed of 6 kph (3.8 mph) to understand the potential for the wind to overpower the airflow pattern. Sometimes opening the side air inlet baffle board to introduce more air to the starved side can compensate for this problem. Air deflecting baffles located at the tunnel inlet have also reduced this problem.

Figure 6. Floor plan showing skewed air flow pattern due to wind effect.

Need for Migration Fences
Floor reared birds tend to migrate towards the incoming air stream, which can cause overcrowding in the inlet end of the building. This migration can be controlled by placing fences across the barn at approximately 30-m (100-ft) intervals. Generally these picket-style or open mesh fences are 300-450 mm (12-18 in.) high and are portable.

Bright Sunlight Effect
Many broiler chickens are grown under a specific lighting program and are not exposed to any bright lighting until tunnel ventilation begins. When the tunnel fans start and the tunnel air inlet opens, there is a significant increase in light level. Birds not conditioned to higher light levels may exhibit a fear response and quickly move away from both ends of the barn. This "light flight" is thought to be responsible for some bird losses due to smothering in the middle zone of the barn during this rapid movement away from the bright sunlight. Some veterinarians also believe this reaction is responsible for numerous skin scratches and subsequent infections. Remember to expose birds to some brighter lighting prior to switching to tunnel ventilation. Some producers introduce the birds to tunnel ventilation on a gradual basis prior to it truly being needed, to get them accustomed to the alternative ventilation system.

Ceiling Air Deflectors
A simple method of increasing the air speed at bird level without installing extra tunnel fans is to install drop curtains or other air deflectors from the ceiling every 12-25 m (40-80 ft) across the barn to reduce the barn cross-sectional area. This is an effective means to reduce the barn cross-section in older facilities with no ceiling. One should limit the increased air speed to a maximum of 2.5 or 3.0 m/s (500 or 600 ft/min) to prevent a reduction in fan performance.

Tunnel ventilation is an effective method of relieving heat stress and enhancing animal performance in hot weather, but it requires careful design in order to make it work effectively.