Understanding Static Pressure in a Tunnel-Ventilated House

Static pressure is essentially a measure of how hard the exhaust fans are working in a house utilizing negative pressure ventilation. The higher the static pressure, the harder the fans must work to ventilate a house. When operating in side wall inlet mode, the level of work required by the fans can be changed by simply changing the amount of side wall inlet area. The lower the amount of side wall inlet area available to the fans, the greater the amount of work required of the fans, and the high...er the measured static pressure will be. Since it takes essentially no work for exhaust fans to slowly draw the air to them once it is in house, the static pressure measured at the inlet will be the same as that measured by the exhaust fans.
Measuring static pressure in a tunnel-ventilated house is a little more complicated. When operating in tunnel ventilation mode, static pressure is a measure of the amount of work required to bring outside air to where you are standing in the house measuring it. Suppose you are standing in the pad room with a static pressure gauge and place the tube connected to the “high” pressure side of the gauge outside the house. In that case, you will simply be measuring the amount of work required to move the air through the pad. If a pad is clean, adequately sized, and all the fans are operating, this is typically around 0.05". It is essential to recognize that this represents only a small portion of the total workload required of fans in a tunnel-ventilated house. The incoming air still has a long way to go. Now, if you are standing in the house 20' or so from the end of the evaporative cooling pads and measure the static pressure, you are now measuring amount of work required to take air from outside the house and pull it through the pads, tunnel doors (inlet pressure), and into the cross-sectional area of the house (“funnel” pressure). It is essential to keep in mind that in a modern broiler house the evaporative cooling pad area is always larger than the cross-sectional area of a house, which means that work is required to “funnel” all the air into the smaller cross-sectional area of the house (Figure 1). Since additional work is required to pull the air through this additional restriction, the static pressure measured at this location will always be greater than that measured at the pads. Finally, if you are standing 20' or so from the tunnel fans and measure the static pressure, you are now measuring the total amount of work required of the tunnel fans, or in other words, the amount of work required to pull air from outside the house through the pads and the tunnel doors (inlet pressure), into the cross-sectional area of the house (funnel pressure), and down the length of the house (pipe pressure). The static pressure measured at the fans will always be greater than that measured at the pads or 20' from the pads, because additional work is required to move the air down the length of the house (Figure 2). The longer the house, the greater the amount of work.

The total pressure measured at the tunnel fans will increase if there are any additional restrictions along the path the air takes from the pads to the fans. Suppose the pads become dirty or the tunnel door opening is reduced. In that case, the fans will have to work harder to pull the air into the house, which will be reflected in an increase in the static pressure measured at the fans. Suppose the half-house curtain is not fully drawn up against the ceiling, or there are multiple deflector curtains; the amount of work required to pull the air down the length of the house will increase. The higher the level of work required of the fans, the greater the static pressure measured at the tunnel fans.

Another factor that increases the amount of work required of tunnel fans is the air speed in a tunnel house. If we add fans to a tunnel house in an attempt to increase air speed, the speed of the air moving through the pads will increase, resulting in an increase in work and pressure. The good news is that the work required of our fans to pull the air through the pads can be reduced by increasing the pad area. The bad news is that the air still has to be pulled into the relatively small cross-sectional area of the house, which can’t be increased. Although by increasing the pad area we can assure that the speed of the air moving through the pad remains at around 350 ft/min, thus keeping the work required to pull the air through the pads to a minimum, the air still has to be forced into the house’s cross-sectional area which can be as little as half that of the evaporative cooling pad area.

Air speed Funnel pressure
600 ft/min 0.045"
700 ft/min 0.06"
800 ft/min 0.08"

Table 1. Tunnel air speed Vs. Funnel pressure

The resulting increase in air speed as the air moves into the cross-sectional area of the house from 350 ft/min to 600, 700, or even 800 ft/min will result in a significant increase in the amount of work required of the fans (Table 1). So if the pressure measured by the pads is 0.06" (assuming the tunnel door is opened enough not to cause a significant increase in the level of work). The air speed is 700 ft/min the pressure measured 20' from the pads (inlet pressure + funnel pressure) will be approximately between 0.12" (0.06"Pad + 0.06"Funnel = 0.12"). The fact is that the amount of work required of the fans in a modern high air speed broiler house to pull the air into the cross-sectional area of the house is typically equal to or greater than that required to pull the air through a house’s evaporative cooling pads.

The work required to pull the air along the length of a tunnel house will also increase as air speed increases (Table 2). For instance, pulling air down a house at an air speed of 700 ft/min the static pressure will increase 0.012" for every 100' it travels from the pads to the fans. If there is 500' between the pads and the fans, the static pressure will increase 0.06" (0.012" X 5 = 0.06").

Air speed Pipe pressure - 100' of travel
600 ft/min 0.008
700 ft/min 0.012"
800 ft/min 0.016"

Table 2. Tunnel air speed Vs. Pipe pressure

The net result is that the higher the air speed you want in a tunnel house, the more work it will require of the tunnel fans, and the higher the static pressure you will measure at the fans. Ultimately, it’s the overall speed of the air in a house that determines the amount of work our tunnel fans require to perform. Although the pressure will, of course, vary from house to house, Table 3 provides an estimate of the expected static pressure at the tunnel fans in a well-designed and properly maintained tunnel house.

Air speed Static Pressure Expected at Fans
400 ft/min 0.07" - 0.09"
500 ft/min 0.10" - 0.12"
600 ft/min 0.13" - 0.15"
700 ft/min 0.16" - 0.18"
800 ft/min 0.18" - 0.20'

Table 3. Approximate total static pressure in the typically tunnel-ventilated broiler house as a function of air speed

In conclusion, static pressure serves as a vital indicator of the workload placed on exhaust fans in tunnel-ventilated poultry houses. It reflects the considerable effort required of the fans to move air through the pads, tunnel doors, into the cross-sectional area of the house, and down the length of the house. As air speed increases, so does the resistance to airflow, resulting in higher static pressures and greater demands on the fans. Understanding how each element contributes to overall static pressure can help us determine if a “high” static pressure reading is a problem or simply an indicator that we are simply doing a good job cooling our birds.