Emitters

Knowledge about the emitter characteristics and functioning is important for design, operation, and maintenance of microirrigation systems. The information given below should be supplemented with other background literature [11, 98, 123, 124].

General

Emitters are designed to dissipate pressure and to discharge a small uniform flow of water at a constant rate. The ideal emitter

• has a large opening to minimize plugging,

• has low discharge rate to maximize lateral length,

• is relatively insensitive to pressure and temperature variations,

• is simple and relatively inexpensive to manufacture to high uniformity,

• is durable and long-lived (sunlight resistant; minimal wearing of orifices or moving parts), and

• is resistant to insect and rodent damage.

All of these criteria cannot be met with a single design. In fact, several criteria create opposing conditions (i.e., large opening and small discharge rate). Manufacturers have developed many emitter designs. Table 5.10 summarizes some of these basic designs and their advantages and disadvantages. A large majority of the emitters in use are labyrinth or pressure-compensating for drip and orifices for microspray.

Table 5.10. Typical emitter types and their characteristics

Path Type

Description

Meritsa

Problemsb

Typical Discharge Exponent x

Microtube

Molded, long, smooth

Vortex

Tortuous

Porous pipe

Pressure-

compensating

Multiple flexible orifice

Orifice

Long, small-diameter a 2,5,6

spaghetti tube; laminar flow.

Long, smooth, coiled 2,4

or spiral passageway in a molded emitter body; laminar flow.

Water enters tangentially a,b,c 3,4

into a chamber in which it spins and then exits through a hole on the opposite side.

Labyrinth or zigzag c,d,e path; turbulent flow at some points in the passageways.

Very small holes in the tubing itself sweat or emit water.

Some type of flexible Possibly membrane, O-ring, b,c,d,e or other design is used to reduce the path size at higher pressures; quality is highly variable among manufacturers.

Water passes through d,e 1, possibly 7

several orifices in flexible membranes; particles caught in one orifice will create backpressure, expanding the orifice and moving the particle through.

A single, simple a,b,c hole, typical of microsprayers.

Possibly 1,4,6,7

a a, inexpensive; b, flow rate is insensitive to temperature changes; c, low manufacturing Cv (little variation between emitters); d, typically a large hole; e, less susceptible to plugging than other emitters with the same hole size.

b 1, expensive; 2, flow rate is sensitive to temperature changes; 3, typically a small hole; 4, relatively sensitive to plugging; 5, very sensitive to plugging; 6, large manufacturing Cv with some makes and models; 7, discharge characteristics of some makes and models may change after a few years.

Source: Adapted from [11].

Hydraulic Characteristics of Emitters

Hydraulically, most emitters can be classified as long-path, orifice, vortex, pressure-compensating, or porous-pipe emitters. The hydraulic characteristics of each emitter are related directly to the flow regime inside the emitter as characterized by the Reynolds number (Re). These flow regimes usually are characterized as laminar, Re < 2,000; unstable, 2,000 < Re < 4,000; partially turbulent 4,000 < Re < 10,000; and fully turbulent 10,000 > Re.

The flow regime in an orifice emitter is fully turbulent. The flow rate is given by where q is the emitter flow rate (L h-1), A is the orifice area (mm2), Co is the orifice coefficient (usually about 0.6), H is the pressure head at the orifice (m), and g is the acceleration of gravity, 9.81 m s-2. Because orifice flow is usually fully turbulent, small changes in fluid viscosity caused by fluid temperature changes usually do not affect emitter performance. Short-path emitters generally behave like orifice emitters. For twin-chamber tubing, with no external orifices for each orifice in the inner chamber, Eq. (5.155) also applies with appropriate modifications.

The flow in a long-flow-path emitter is through a small microtube. When the flow regime is turbulent, the emitter flow rate can be expressed as using the Darcy-Weisbach equation with q(L h-1), d is inside diameter (mm), L is microtube length (m), and f is the friction factor (dimensionless). The cross-sectional shape of the conduit will affect the hydraulic characteristics. For laminar flow, the emitter discharge becomes proportional to H and the effects of fluid temperature changes on viscosity can cause significant flow variation.

One of the most popular types of emitters is the tortuous path or labyrinth emitter. It allows the maximum opening size for a given flow rate. For example, one manufacturer's tortuous-path drip emitter that discharges 2 L h-1 at 100 kPa has a minimum path dimension of 1.4 mm and a path length of 160 mm. An orifice to give the same flow would have an opening diameter of only 0.3 mm (Eq. 5.155) and be much more susceptible to plugging. Flow in tortuous-path emitters is usually turbulent and thus is insensitive to water temperature, and discharge is given by a equation similar in form to Eq. (5.156).

The vortex emitter (or sprayer) has a flow path containing a round cell that causes circular flow. The circular motion is achieved by having the water enter tangentially to the outer wall. This produces a fast rotational motion, creating a vortex at the center of the cell. Consequently, both the resistance to the flow and the head loss in the vortex emitter are greater than for a simple orifice having the same diameter. Flow rate is usually given by

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