Normally, ventilation systems are based on the principle of negative pressure. Air is pumped out of the house by ventilators, and comes into the house through the inlets and through all other openings. For bio-security reasons, it can be necessary to create a positive pressure ventilation system.
With this kind of ventilation system, air is not pumped out of the house by the ventilators, but the ventilators blow air into the house. In this way, the pressure inside the house will be higher than the outside thus air will escape from the house through the openings.
The advantage of this system is that air can't come into the house in an uncontrolled way. All air will come in through the ventilators, and as long as the pressure inside is higher than the outside, air will not come in anywhere. This gives full control over the incoming air, and with filters, the bio-security of this air can be controlled. The system is often called FAPP, which stands for Filtered Air in Positive Pressure.
When the system is designed and used, some aspects should be taken into consideration.
- The bio-security depends on the capacity to keep the house under positive pressure. Therefore, the capacity of the fans must be high enough to compensate when doors are opened, etc. A good method is to have the fans controlled on pressure, which means their speed will increase when pressure drops.
- As air will normally enter the house at one or only a few spots, good air mixing and even distribution is difficult.
- As all air normally will pass through filters, high capacity of ventilators will be needed, with high energy consumption.
- When the outside air is cold, it is difficult to prevent the air from falling straight on the birds, as the air will not have enough speed for proper mixing.
- It is very difficult, if not impossible, to create a significant air velocity over the birds. This means that cooling by wind chill does not contribute significantly to the heat transfer of the birds. To create an equal effective temperature for the birds, air temperature should be controlled to much lower level than in systems with negative pressure ventilation.