Beware of Installing Fine Mesh Screens on Poultry House Air Inlets

Even though modern poultry house fans are much more powerful than those made 10 to 20 years ago, they are still considered relatively weak compared to other types of fans. The fact is that the air moving capacity of the typical tunnel fan drops 25% as the static pressure increases from 0.05" to 0.20" and essentially goes to zero as the static pressure approaches 0.30" (the amount of pressure required to pull water up a straw 0.30"). Though the modern poultry fan can be used successfully to venti...late our birds throughout the year, it is important to realize they are typically operating at pressures toward the upper range of their capability. For instance, when in inlet mode, poultry house fans typically operate at static pressures between 0.10" and 0.15". When in tunnel mode, static pressures often range between 0.15" and 0.20". In either ventilation mode, an increase in static pressure of just 0.05" or 0.10" could dramatically affect a producer’s ability to provide fresh air to their birds.

It is important to keep in mind the relative weakness of our exhaust fans because there is an increasing interest in using screens over side walls and/or tunnel inlets to reduce the amount of dust and/or insects entering a house. Although the benefits of doing so are questionable, the effect it will have on the air-moving capacity of our fans is far less questionable.
A study of various screens was conducted in a fan/inlet test chamber (Georgia Poultry - Newton Grove, North Carolina) to better quantify how the installation of screens on inlets could affect the performance of poultry house ventilation systems. The fan/inlet test chamber was used to precisely determine how static pressure varied with air speed through four different types of commercially available screens:
a) 1/4" hardware cloth
b) “Pet screen”
c) “Insect screen”
d) “70% shade cloth”

The testing found that if a house’s side wall inlets were wide open and the house was operating at a static pressure of 0.10" and a fine mesh screen were placed over the inlets, the static pressure would increase to between 0.27" and 0.34".
To minimize the negative effect on fan performance of installing fine mesh screens, the screen area would need to be made much larger than the area of the inlet it is being installed on. The greater the screen area relative to the inlet area, the lower the velocity of air moving through the screen, the lower the pressure generated, and the lower the effect the addition of fine mesh screens over a house’s inlets would have on exhaust fan performance. It is generally recommended that screens be sized so that when a house’s inlet system is operating at maximum (inlets fully opened), the additional pressure required to pull the air through screens is kept to a maximum 0.02", ideally 0.01". For the fine mesh screens tested, this would mean that the air speed through the screens should kept to a maximum of approximately 200 ft/min, ideally 100 ft/min.

What does this mean if you are considering installing screens on your house’s inlet system? At a static pressure of 0.10", the air will enter through the typical side wall inlet without fine mesh screens at a velocity of approximately 800 ft/min. To keep the additional pressure required to pull air through a fine mesh screen to ideally 0.01", the air speed through the screen should be kept at approximately 100 ft/min. This means the screen area should be at least eight times that of the side wall inlet area.
Of potentially greater concern when installing fine mesh screens on a poultry house is that even if the proper amount of screen area is installed, over time the screen will tend to become filled with dust, dramatically increasing the static pressure a house’s ventilation system is working against and thereby adversely affecting a producer's ability to maintain proper environmental conditions. This problem can be mitigated to some extent by further increasing the proportion of screen area to inlet area and performing frequent screen cleaning; however, the fact remains that placing fine mesh screens over a house’s inlet system is a challenging and a very potentially risky proposition.