Drip irrigation emitters vary from elaborate, variable –flow rate types to simple orifices or even punched, drilled or burned holes in the pipe. In general, the flow rate through the emitters is controlled by the hydraulic pressure at the emitter and flow path dimension of the emitter.
There are three Major Groups of Emitter Types:
a) The orifice or nozzle emitter
b) Long flow path emitter, and
c) Pressure compensating emitter.
a) The officer or nozzle type has fixed emitter geometry so that flow area is constant. The emitter flow and hydraulic pressure, theoretically can be shown as,
q= is emitter flow rate in lit/hr
c= is constant and
h= is the pressure head in meters
b) The long flow path emitter type will be considered as flow in a small micro-tube. If the area of the flow path is fixed the emitter flow function can be given as,
x= 1 for laminar flow,
x= 0.57 when flow is considered as turbulent flow in smooth pipe, and
x= 0.50 when the flow is considered as fully turbulent in the small tube.
The pressure compensating emitter is designed so the flow area ( orifice or nozzle) adjusts as pressure changes. For example, the flow area will decrease when the pressure increases.
c) The flow area and the hydraulic pressure relationship can be shown as
a= is the orifice area
b and y= are two constant in the power function.
d) The emitter flow function for this special type of emitter can be shown as
q= ch 0.5
This show that, in the power function of equation 2) the x-value can be made less than 0.5. If the y-values are 0.5, then x value will be zero; this means the emitter is fully pressure compensating, giving a constant flow rate and no changes with respect to the variation of hydraulic pressure.