Spray nozzle

A spray nozzle is a device that facilitates the formation of spray. When a liquid is dispersed as a stream of droplets (atomization), it is called a spray. The typical purpose of the spray is to maximize the effect of the liquid by increasing the total surface area for better dispersion.

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Applications of sprays

There are hundreds of applications where there is a need of spray systems to apply or use the liquid efficiently. Stated below are the few industrial applications of the spray:-

Lime & cement industry:Spraying liquid for gas cooling and conditioning or spraying fluidized fuel in the kilns for burning.

Energy/ environmental applications: Slurry is sprayed to control SO2 emission and Chemical solution is sprayed to control NOx

Food industry: Molten butter is glazed on dough before/after baking.

Beverage industry: Water jet is used to clean storage tanks

Chemical industry: Spraying reagents to enhance dispersion and to increase liquid-gas mass transfer .

Pharmaceutical industry: Spray drying application.

Electronic industry: For efficient coating of electronic parts

Mining industry: Water is sprayed to control dust emission produced during grinding

Automotive industry: Water and detergent sprays for the car washing

Steel industry: High pressure water is sprayed on red hot steel to remove slag scales and to quench it

Agricultural industry: To disburse insecticides and pesticides including water efficiently in fields. See sprayer, aerial application.

Paint industry: To evenly distribute paint on to surfaces

Waste treatment industry: To remove excessive heat from the biodegraded waste sludge by aerating and keeping sludge cool for optimum temperature for microbes

Aerospace industry: To produce a very high ratio of surface area to mass of a liquid. This expedites evaporation promoting high rates of combustion.

Types of spray

Sprays can be categorized into several types:

1. Hydraulic spray:

The hydraulic spray nozzle utilizes the liquid pressure as the energy source to break the liquid into droplets. This type of spray is less energy consuming than a gas atomized or twin-fluid spray nozzle. As the fluid pressure increases the flow increases and the drop size decreases. But this leads to problems in selecting a droplet size and to achieve a certain flow rate at a given pressure. To overcome this situation a special hydraulic nozzle ([Lechler] Spillback Nozzle) has been developed. This nozzle can vary the liquid flow rate at a particular droplet size and pressure. This nozzle creates a better and optimum control on the liquid spray and in certain applications can eliminate the need of expensive compressed air.

2. Gas (air) atomized spray: Air, steam or other gases can break up a liquid to form a spray, with the gas providing a souce of energy.

I). Internal mixing (fluids mix inside the nozzle.)

The gas atomized spray utilizes a gaseous source to break the liquid to the droplets. The internal mixed twin fluid spray can utilize two different ways for spraying liquid:

In the first type, the liquid impinges upon a surface for impact to break the liquid stream and then the air is mixed to atomize it. The advantage of this process is to reduce the amount of air required to generate the droplets but the downside is that the over time the impact surface becomes eroded and effects the spray droplet size pattern. The nozzle life can be very short if the liquid has impurities in it.

In the second type, the liquid is broken into droplets by using only gas. The advantage of this type is that the nozzle lasts longer but the downside is that this type of spray needs more gas to generate the same size of droplets.

II). External mixing (fluids mix outside the nozzle) This type of spray nozzle may require more air, but the mixing and atomization of liquid takes places outside the nozzle. If a liquid is atomized using any gas which may react with the liquid, it is possible that the reaction may damage the inside of the nozzle. This type of nozzle is most beneficial for a liquid which may evaporate inside the mixing chamber of an internal mix nozzle or using steam to atomize the liquid. Also this nozzle is suitable of spraying viscous fluid.

3. Other: Ultrasonic, rotary disk, ...

Control issue

Each nozzle has a performance curve and they produce droplet size based on the liquid and the gas stream pressure and flow rate. If a certain process is sensitive to the droplet size; then controlling the liquid and gases becomes a critical issue. Without a good control over the liquid and/ or gas flow rates, the nozzle will not generate droplet size in the region for which is selected and designed for. This can lead to failure of the equipment or could have an adverse effect on process and could cause catastrophic failure.

Material of construction

The material of construction is selected based on the fluid properties of the liquid which is to be sprayed and the environment. Spray nozzles are most commonly fabricated from metals, but plastics and ceramics are also used. For spraying slurry in a Fluidized Gas De-sulfurizer (FGD) the most preferable material is Silicon alloys such as Stellite which are hard but are not as abrasion resistant as Silicon Carbide. For corrosion resistance material such as Hastelloy are used. The material wear in the nozzle has adverse effect on the sprays. They can generate streaks of bigger droplets’ size exiting out of the nozzle which can have an undesirable effect on the process. For example: In a gas conditioning tower in a Cement Plant, the objective of liquid spray is to humidify and cool the gas before it enters the particulate matter abatement equipment. This application requires a particular droplet size for it to work. If the droplet size increases beyond a certain threshold they will not completely evaporate and will not cool the gas to the desired temperature. The un-evaporated droplet will then hit the bottom of the tower. As the dust laden gases hit the tower the dust will stick to the wet surface and will create major problems with increased maintenance. If not maintained they can even clog the gas outlet duct and can disrupt the plant operation.

See also

• Nozzle
• ilass Institute for Liquid Atomization and Spray Systems

References

Lefebvre, Arthur, Atomization and Sprays, Hemisphere, 1989, ISBN 0-89116-603-3

(ILASS) Institute for Liquid Atomization and Spray Systems