Air duct system is designde at 1 static pressure but some load calculation programs say a total system of 4 static What is the difference of the two?
When a fluid/air is flowing though pipe/duct in one direction, the conduit surface will apply some resistance to the flow. And also the resistance is applied by its own molecules. So there are two types of friction losses. 1) static pressure and 2) by velocity pressure.
so the total pressure in a system = static pressure + velocity pressure
Velocity pressure resistance applied by the conduit while the air/fluid is in motion.
Static pressure means the force applied by the air/liquid in a pipe/duct at steady state, means at zero velocity. It is expressed as inches water gauge per 100foot. It is calculated by using friction loss charts for straight duct and friction loss coefficient for duct fittings. It is the total pressure that a fan shall overcome to throw the air till the last air outlet. It is calculated as per the duct layout of a project. So, its value differs from one project to another project. 1Static pressure means the duct layout has 1inch.w gage pressure and a fan shall be selected to over that pressure. Similar manner for 4static pressure.
A fan is selected on these two pressures. But for low velocity systems ( velocity of air below 2000fpm) the fan is selected on only Static pressure. But for high velocity systems (velocity of air greater than 2000fpm) will be selected on total pressure