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Static pressure in a poultry house is the difference in pressure that a ventilation fan creates between the inside and outside of the poultry house and is the resistance to airflow. Static pressure (SP or Ps) is very important to a mechanical ventilation system since it is the driving force for air movement.

Increasing the static pressure a fan must operate against reduces the amount of airflow delivered by the fan, as shown below.

static pressure ventilation fansAlmost all manufacturers send fans to the Bess Lab at the University of Illinois for testing. As these test results above show, operating a fan at higher static pressures causes dramatic losses in airflow (CFM) and in electrical efficiency (CFM/watt).

For instance, in the above case, in a tunnel -ventilated house, increasing static pressure from 0.05″ to 0.20″ will tend to decrease the amount of air moved by this tunnel fan by roughly 17%, while at the same time increasing power usage of the fan by 11+%.

In a poultry house, the resistance to airflow that must be overcome by fans is affected by ventilation inlets and fan shutters and guards. Additional pieces of equipment, such as wind protection devices, evaporative pads or light traps, further restrict airflow.

Fan airflow capacity is influenced in turn by static pressure, which is most effective when kept at 0.05 to 0.08 inches water gauge across the poultry house inlets. This is monitored as part of the ventilation system control, but it only represents one component of the static pressure difference against which the fan must operate.

Total resistance along the airflow path from outside to building interior and back outside can be as high as 0.20 inches in the water gauge, if the fan is moving air through evaporative pads or exhausting air into strong winds. Most agricultural ventilation fans operate at 0.05 to 0.15 in. water static pressure and deliver a given airflow rate in cubic feet per minute (CFM) at a particular static pressure. Some agricultural ventilation fans can effectively operate at static pressures of up to 0.25 in. water by delivering a CFM airflow rate similar to the CFM rate at much lower static pressures.

The performance of a fan is defined as the amount of airflow in CFM at a given static pressure.

Static pressure is usually expressed as inches of water column (IWG).

The unit of measure for static pressure is often inches of water (in. water). The static pressure of one inch of water is the suction needed to draw water up a straw one inch. For comparison, 1 pound per square inch static pressure (1 psi) is equal to 27.7 in. water static pressure. Sometimes static pressure is given as Pascals (Pa). The static pressure of 0.1 inches of water (in. water) is equal to about 25 Pascals (Pa).