Spray nozzle
Encyclopedia
A spray nozzle is a precision device that facilitates dispersion of liquid into a spray
. Nozzles are used for three purposes: to distribute a liquid over an area, to increase liquid surface area, and create impact force on a solid surface. A wide variety of spray nozzle applications use a number of spray characteristics
to describe the spray.
Spray nozzles can be categorized based on the energy input used to cause atomization
, the breakup of the fluid into drops. Spray nozzles can have one or more outlets; a multiple outlet nozzle is known as a compound nozzle.
Spiral design variant
The impingement surface can be formed in a spiral to yield a spiral shaped sheet approximating a full cone spray pattern; or, by changing the geometry, a hollow-cone spray pattern.
The spiral design of nozzle offers two main advantages over other direct pressure nozzles.
1- For a given pressure and flow rate they will generally form smaller droplets than pressure swirl type nozzles.
2- They have a large free passage making them naturally clog resistant and suitable for spraying "lumpy" fluids
A potential drawback is the relatively uneven spray pattern making them unsuitable for applications such as coating.
This design of nozzle was originally patented by John Bete of BETE Fog Nozzle in 1950 but, since patent expiry, is now manufactured by a number of other companies. Common applications include gas scrubbing applications (e.g. flue-gas desulfurization where the smaller droplets often offer superior performance) and fire fighting (where the mix of droplet densities allow spray penetration through strong thermal currents).
A spill-return pressure-swirl single-fluid nozzle is one variety of pressure swirl nozzle includes a controlled return of fluid from the swirl chamber to the feed system. This allows the nozzle pressure drop to remain high while allowing a wide range of operating rates.
Control of two-fluid nozzles
Many applications use two-fluid nozzles to achieve a controlled small drop size over a range of operation. Each nozzle has a performance curve, and the liquid and gas flow rates determine the drop size. Excessive drop size can lead to catastrophic equipment failure or may have an adverse effect on the process or product. For example, the gas conditioning tower in a cement plant often utilizes evaporative cooling caused by water atomized by two-fluid nozzles into the dust laden gas. If drops do not completely evaporate and strike a vessel wall dust will accumulate, resulting in the potential for flow restriction in the outlet duct, disrupting the plant operation.
is the ratio of the mass of a given volume of liquid to the mass of the same volume of water. In spraying, the main effect of the specific gravity Sg of a liquid other than water is on the capacity of the spray nozzle. All vendor-supplied performance data for nozzles are based on spraying water. To determine the volumetric flowrate Q, of a liquid other than water the following equation should be used.
is defined as the property of a liquid that resists change in the shape or arrangement of its elements during flow. Liquid viscosity primarily affects spray pattern formation and drop size. Liquids with a high viscosity require a higher minimum pressure to begin spray pattern formation and yield narrower spray angles compared to water.
of a liquid tends to assume the smallest possible size, acting as a membrane under tension. Any portion of the liquid surface exerts a tension upon adjacent portions or upon other objects that it contacts. This force is in the plane of the surface, and its amount per unit of length is surface tension. The value for water is about 0.073 N/m at 21° C. The main effects of surface tension are on minimum operating pressure, spray angle, and drop size. Surface tension is more apparent at low operating pressures. A higher surface tension reduces the spray angle, particularly on hollow cone nozzles. Low surface tensions can allow nozzles to be operated at lower pressures.
, Stainless steel
, and nickel alloys, but plastics such as PTFE and PVC
and ceramics (alumina and silicon carbide
) are also used. Several factors must be considered, including erosive wear, chemical attack, and the effects of high temperature.
See: Nozzles for ULV and CDA
Spray (liquid drop)
A spray is a dynamic collection of drops dispersed in a gas. The process of forming a spray is known as atomization. A spray nozzle is the device used to generate a spray. The two main uses of sprays are to distribute material over a cross-section and to generate liquid surface area. There are...
. Nozzles are used for three purposes: to distribute a liquid over an area, to increase liquid surface area, and create impact force on a solid surface. A wide variety of spray nozzle applications use a number of spray characteristics
Spray characteristics
Spray nozzles are designed to perform under various operating conditions. The following characteristicsshould be considered when selecting a nozzle:* Spray Pattern* Capacity* Spray impact* Spray Angle* Drop Size- Spray Pattern :...
to describe the spray.
Spray nozzles can be categorized based on the energy input used to cause atomization
Atomization
Atomization or Atomizer may refer to:* The conversion of a vaporized sample into atomic components in atomic spectroscopy* An apparatus using an atomizer nozzle* Atomizer Geyser, a cone geyser in Yellowstone National Park...
, the breakup of the fluid into drops. Spray nozzles can have one or more outlets; a multiple outlet nozzle is known as a compound nozzle.
Single-Fluid Nozzle
Single-fluid or hydraulic spray nozzles utilize the kinetic energy of the liquid to break it up into droplets. This most widely used type of spray nozzle is more energy efficient at producing surface area than most other types. As the fluid pressure increases, the flow through the nozzle increases, and the drop size decreases. Many configurations of single fluid nozzles are used depending on the spray characteristics desired.Plain orifice type nozzle
The simplest single fluid nozzle is a plain orifice nozzle as shown in the diagram. This nozzle often produces little if any atomization, but directs the stream of liquid. If the pressure drop is high, at least 25 bar, the material is often finely atomized, as in a diesel injector. At lower pressures, this type of nozzle is often used for tank cleaning, either as a fixed position compound spray nozzle or as a rotary nozzle.Shaped orifice nozzle
The shaped orifice uses a hemispherical shaped inlet and a “V” notched outlet to cause the flow to spread out on the axis of the V notch. A flat fan spray results which is useful for many spray applications, such as spray painting.Surface impingement single fluid nozzle
A surface impingement nozzle causes a stream of liquid to impinge on a surface resulting in a sheet of liquid that breaks up into drops. This flat fan spray pattern nozzle is used in many applications ranging from applying agricultural herbicides to row crop to painting.Spiral design variant
The impingement surface can be formed in a spiral to yield a spiral shaped sheet approximating a full cone spray pattern; or, by changing the geometry, a hollow-cone spray pattern.
The spiral design of nozzle offers two main advantages over other direct pressure nozzles.
1- For a given pressure and flow rate they will generally form smaller droplets than pressure swirl type nozzles.
2- They have a large free passage making them naturally clog resistant and suitable for spraying "lumpy" fluids
A potential drawback is the relatively uneven spray pattern making them unsuitable for applications such as coating.
This design of nozzle was originally patented by John Bete of BETE Fog Nozzle in 1950 but, since patent expiry, is now manufactured by a number of other companies. Common applications include gas scrubbing applications (e.g. flue-gas desulfurization where the smaller droplets often offer superior performance) and fire fighting (where the mix of droplet densities allow spray penetration through strong thermal currents).
Pressure-swirl single fluid spray nozzle
Pressure-swirl spray nozzles are high-performance (small drop size) devices with one configuration shown. The stationary core induces a rotary fluid motion which causes the swirling of the fluid in the swirl chamber. A film is discharged from the perimeter of the outlet orifice producing a characteristic hollow cone spray pattern. Air or other surrounding gas is drawn inside the swirl chamber to form an air core within the swirling liquid. Many configurations of fluid inlets are used to produce this hollow cone pattern depending on the nozzle capacity and materials of construction. The uses of this nozzle include evaporative cooling and spray drying.A spill-return pressure-swirl single-fluid nozzle is one variety of pressure swirl nozzle includes a controlled return of fluid from the swirl chamber to the feed system. This allows the nozzle pressure drop to remain high while allowing a wide range of operating rates.
Solid cone single-fluid nozzle
One of the configurations of the solid cone spray nozzle is shown in a schematic diagram. A swirling liquid motion is induced with the vane structure, however; the discharge flow fills the entire outlet orifice. For the same capacity and pressure drop, a full cone nozzle will produce a larger drop size than a hollow cone nozzle. The coverage is the desired feature for such a nozzle, which is often used for applications to distribute fluid over an area.Compound nozzle
A compound nozzle is a type of nozzle in which several individual single or two fluid nozzles are incorporated into on nozzle body, as shown below. This allows design control of drop size and spray coverage angle.Two-fluid nozzles
Two-fluid nozzles atomize by causing the interaction of high velocity gas and liquid. Compressed air is most often used as the atomizing gas, but sometimes steam or other gases are used. The many varied designs of two-fluid nozzles can be grouped into internal mix or external mix depending on the mixing point of the gas and liquid streams relative to the nozzle face.Internal-mix two-fluid nozzles
Internal mix nozzles contact fluids inside the nozzle; one configuration is shown below. Shearing between high velocity gas and low velocity liquid disintegrates the liquid stream into droplets, producing a high velocity spray. This type of nozzle tends to use less atomizing gas than an external mix atomizer and is better suited to higher viscosity streams. Many compound internal-mix nozzles are commercially used for, e.g., fuel oil atomization.External-mix two-fluid nozzles
External mix nozzles contacts fluids outside the nozzle as shown in the schematic below. This type of spray nozzle may require more atomizing air and a higher atomizing air pressure drop because the mixing and atomization of liquid takes place outside the nozzle. The liquid pressure drop is lower for this type of nozzle, sometimes drawing liquid into the nozzle due to the suction caused by the atomizing air nozzles (siphon nozzle). If the liquid to be atomized contains solids an external mix atomizer may be preferred. This spray may be shaped to produce different spray patterns. A flat pattern is formed with additional air ports to flatten or reshape the circular spray cross-section discharge.Control of two-fluid nozzles
Many applications use two-fluid nozzles to achieve a controlled small drop size over a range of operation. Each nozzle has a performance curve, and the liquid and gas flow rates determine the drop size. Excessive drop size can lead to catastrophic equipment failure or may have an adverse effect on the process or product. For example, the gas conditioning tower in a cement plant often utilizes evaporative cooling caused by water atomized by two-fluid nozzles into the dust laden gas. If drops do not completely evaporate and strike a vessel wall dust will accumulate, resulting in the potential for flow restriction in the outlet duct, disrupting the plant operation.
Rotary Atomizers
Rotary atomizers use a high speed rotating disk, cup or wheel to discharge liquid at high speed to the perimeter, forming a hollow cone spray. The rotational speed controls the drop size. Spray drying and spray painting are the most important and common uses of this technology.Ultrasonic atomizers
This type of spray nozzle utilizes high frequency (20 kHz to 50 kHz) vibration to produce narrow drop-size distribution and low velocity spray from a liquid. The vibration of a piezoelectric crystal causes capillary waves on the nozzle surface liquid film.Electrostatic
Electrostatic charging of sprays is very useful for high transfer efficiency. Examples are the industrial spraying of coatings (paint) and applying lubricant oils. The charging is at high voltage (20 to 40 kV) but low current.Liquid Properties
Almost all drop size data supplied by nozzle manufacturers are based on spraying water under laboratory conditions, 70° F (21° C). The effect of liquid properties should be understood and accounted for when selecting a nozzle for a process that is drop size sensitive.Temperature
Liquid temperature changes do not directly affect nozzle performance, but can affect viscosity, surface tension, and specific gravity, which can then influence spray nozzle performance.Specific Gravity
Specific gravitySpecific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
is the ratio of the mass of a given volume of liquid to the mass of the same volume of water. In spraying, the main effect of the specific gravity Sg of a liquid other than water is on the capacity of the spray nozzle. All vendor-supplied performance data for nozzles are based on spraying water. To determine the volumetric flowrate Q, of a liquid other than water the following equation should be used.
Viscosity
Dynamic viscosityViscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
is defined as the property of a liquid that resists change in the shape or arrangement of its elements during flow. Liquid viscosity primarily affects spray pattern formation and drop size. Liquids with a high viscosity require a higher minimum pressure to begin spray pattern formation and yield narrower spray angles compared to water.
Surface Tension
The surface tensionSurface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
of a liquid tends to assume the smallest possible size, acting as a membrane under tension. Any portion of the liquid surface exerts a tension upon adjacent portions or upon other objects that it contacts. This force is in the plane of the surface, and its amount per unit of length is surface tension. The value for water is about 0.073 N/m at 21° C. The main effects of surface tension are on minimum operating pressure, spray angle, and drop size. Surface tension is more apparent at low operating pressures. A higher surface tension reduces the spray angle, particularly on hollow cone nozzles. Low surface tensions can allow nozzles to be operated at lower pressures.
Nozzle Wear
Nozzle wear is indicated by an increase in nozzle capacity and by a change in the spray pattern, in which the distribution (uniformity of spray pattern) deteriorates and increases drop size. Choice of a wear resistant material of construction increases nozzle life. Because many single fluid nozzles are used to meter flows, worn nozzles result in excessive liquid usage.Material of construction
The material of construction is selected based on the fluid properties of the liquid that is to be sprayed and the environment surrounding the nozzle. Spray nozzles are most commonly fabricated from metals, such as brassBrass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
, Stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
, and nickel alloys, but plastics such as PTFE and PVC
PVC
Polyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...
and ceramics (alumina and silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
) are also used. Several factors must be considered, including erosive wear, chemical attack, and the effects of high temperature.
See also
- NozzleNozzleA nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
- nebulizerNebulizerIn medicine, a nebulizer is a device used to administer medication in the form of a mist inhaled into the lungs....
- Pesticide applicationPesticide applicationPesticide application refers to the practical way in which pesticides, are delivered to their biological targets...
- ilassIlassThe Institute for Liquid Atomization and Spray Systems, , is an organization of researchers, industrial practitioners and students engaged in professional activities connected with the spraying of liquids and slurries. Annual technical conferences are organized by each of the ILASS organizations...
Institute for Liquid Atomization and Spray Systems
See: Nozzles for ULV and CDA