Generating Negative Pressure in Tunnel Ventilation: Can Gable Fans Perform Effectively?

Tunnel ventilation is widely used for ventilating livestock farms, especially in hot climates. In pig production, it provides the necessary cooling for the animals through pad cooling and air velocity. Achieving the right climate with appropriate air velocity is crucial for batch uniformity, high productivity, and animal welfare.

In tunnel houses, there's typically a temperature difference between the front and rear of the house. Several factors influence this difference, including air velocity, leaks, and building insulation. If the actual temperature inside the house is too high, experienced temperature can be lowered using air velocity. The experienced temperature is calculated as:

The Experienced Temperature = Actual Temperature – (Air Velocity X Chill Factor) 

It's essential that the air velocity feels like a light breeze for the pigs – too much airflow can create a draught that negatively impacts animal welfare.

The Role of Gable Fans

One critical factor in achieving sufficient air velocity is ensuring that your gable fans can extract enough air and create the necessary negative pressure. As air moves through the house, there are several factors that influence pressure:

• Cooling Pads
  The pressure drops as air passes through cooling pads.
• Tunnel Door
  The pressure drops as air enters through the door opening.
• Transitional Zones
  Pressure drops occur when air changes direction upon entering the house.
• Longitudinal Pressure Drop
  As air moves through the house, pressure gradually decreases. Longer houses experience more significant pressure drops.

In addition, different types of production and buildings have varying obstacles that impact pressure loss within the house, including:

• Type and growth stage of pigs – either sows, studs or piglets 
• Production setups such as, flooring system as well as penning and stall systems 
• Elements within livestock buildings like purlins, pillars, roofing, feeding and lighting systems

Pressure Drop Scenario 

In a broiler house that's 100 meters long with floor production and no obstacles at an air velocity of 3.5 m/sec and 4.0 m/sec (688 ft/min and 790 ft/min), here's what happens:

Increasing air velocity from 3.5 m/sec to 4.0 m/sec results in an increase in required pressure from 44 Pa to 51 Pa according to the table above. The cooling pads and tunnel door are designed to handle the pressure effectively, with the right sizing in place. When turning, there is a drop in pressure due to increased air velocity leading to turbulence, requiring more pressure to redirect the airflow. The added velocity also results in higher resistance as the air moves from front to back.

If you aim to boost air velocity from 3.5 m/sec to 4.0 m/sec, like in the example above, you'll need fans capable of producing greater negative pressure. Generally, higher air velocity means increased resistance and thus demands more pressure.

Factors for Choosing the Right Fan

Selecting the appropriate fan for your needs is crucial as fans serve different functions. Some prioritise energy efficiency while others focus on maintaining stable pressure levels. Striking a balance between energy efficiency and pressure stability is key for creating an optimal environment for animals and ensuring productivity.

Opting for an energy efficient fan can be advantageous as it provides adequate pressure while conserving energy over its lifespan. However, there are scenarios where using such fans may not be feasible due to high pressure requirements. In certain situations, such as in a long building with many obstacles or when animals need strong air circulation for cooling, energy-efficient fans may not be adequate. 

Understanding the correlation between pressure and the amount of air extracted per hour from a house is also critical in choosing the right fan, for example: 

In a breeder house measuring 140 meters (459 ft) that requires an air velocity of 3.5 m/sec (688 ft/min), about 75 Pa (0.3 inH2O) is needed. This necessitates a system where fans can expel 460,000 m3/h (270,800 CFM) of air.

• An energy-efficient fan can move up to 35,000 m3/h (20,600 CFM) per fan. To reach close to the required hourly air volume, approximately 13 fans are required to expel 455,000 m3/h (267,800 CFM) of air
• A pressure stable fan can handle up to 58,000 cubic meters per hour (34,150 cubic feet per minute) of air. To meet the required hourly air exhaust amount, eight fans will be needed to expel 58,000 m3/h (273,200 CFM) of air

Ensuring that gable fans can generate sufficient negative pressure is crucial for effective tunnel ventilation. Big Dutchman offers a range of fans that are ideal in tunnel ventilation systems, these include the highly pressure resistant AirMaster Blue series, as well as the more energy-efficient AirMaster Evo that is powered by inverter technology. These fans are not only proven to perform but are also built for use in demanding environments with a long service life. 

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