DISTRIBUTION OF PRESSURE THROUGH A SYSTEM OF DUCT WORK
Air flowing through a duct exerts two types of pressure on its surroundings. The first is dynamic pressure due to its motion or kinetic energy; this is velocity pressure (VP). The second is the bursting pressure of the air trying to escape from the enclosure or of the surrounding air trying to enter the enclosing duct. This is static pressure (SP) and it acts in all directions. The sum of these two pressures is the total pressure (TP).
STATIC PRESSURE (SP)
Static pressure is the outward force of air within a duct. This pressure is measured in mm of water (mmH2O). The static pressure within a duct is similar to the pressure within an automobile tire. It is basically an inactive pressure.
VELOCITY PRESSURE (VP)
Velocity pressure is the forward-moving force of air within a duct. This pressure is measured in inches of water. The velocity of the air and the weight of the air create velocity pressure. The static pressures balance each other and the velocity pressure is the difference. The velocity pressure is comparable to the rush of air from a punctured tire.
TOTAL PRESSURE (TP)
Total pressure is the sum of velocity pressure and static pressure, otherwise known as impact pressure. This pressure is expressed in inches of water. The total pressure is associated directly with the sound level of the outlet. Therefore, anything that increases the total pressure, such as under sizing of outlets or increasing the speed of the blower will also increase the sound level. The total pressure of a duct can be measured with a manometer applied a little differently. Notice that the velocity component or probe of the manometer is positioned so that the air is directed into the end of the tube. This will register the static and velocity pressures.
AIR MEASURING INSTRUMENTS FOR DUCT SYSTEMS
To establish the fan performance and to trouble-shoot air distribution systems faults it is necessary to measure static and velocity pressures in ducts and air systems. The pressure involved in air systems is very small, hence it is measured in terms of water gauge ( 704mm of water column represents one psi pressure). The water manometer has been mentioned as an air pressure measuring instrument. An instrument used to measure the actual air velocity is the velometer. This instrument actually measures how fast the air is moving past a particular point in the system. When the average velocity of the air (in m/s) passing a point in the duct can be determined by using instruments, the volume of the air can be determined by using the area of the duct in m2. These instruments should be used as the particular manufacturer suggests.A special device called a pitot tube was developed many years ago and is used with special manometers for checking duct air pressure at most pressure levels.
PRESSURE CHANGES AT FANS
The fan produces a rise of total pressure in the whole system. This rise is the fan total pressure (Pa). The fan total pressure rise is equal to the drop of total pressure due to friction in the ductwork system. The fan total pressure is calculated from the total pressure in the fan outlet duct minus the total pressure in the fan inlet duct.
The fan velocity pressure is defined as the velocity pressure in the discharge area from the fan. The fan static pressure is defined as fan total pressure minus fan velocity pressure. Note that this is not necessarily the same as the change in static pressure across the fan connections.