Spray nozzle
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When a liquid is dispersed as a stream of droplets is called a spray. The purpose of the spray is to maximize the use of liquid by increasing the total surface area for better dispersion. Spray nozzles facilitate formation of spray.
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[edit] 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 interaction.
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
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
[edit] Types of spray
Sprays can be categorized in to two types:
1. Hydraulic spray: The hydraulic spray utilized only the liquid pressure as the energy source to break the liquid into droplets.
This type of spray is less energy consuming than a twin-fluid spray nozzle as it does not require the additional air/gas. But the droplet size tends to be bigger than the twinfluid nozzle to a certain extent. There are two categories of hydraulic nozzles. In principle the droplet size reduces as the fluid pressure is increased. But as we go higher on pressure the flow through the orifice also increases. But again the droplet size is inversely proportionate the pressure at the nozzle tip. 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: The gas atomized spray utilizes like air, steam or other gases to facilitate the break up of liquid to form spray.
Again the gas atomized spray can be sub divided in to two segments.
I). Internal mixing (Fluids mix inside the nozzle.) The gas atomized spray utilizes a gaseous source to break the liquid to the droplets. Again 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 air. The advantage of this type is the nozzle is its longer life but the downside is that this type of spray needs more air 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 dry/evaporate inside the mixing chamber of an internal mix nozzle or using steam to atomize the liquid.
[edit] 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.
[edit] Material of construction
The material of construction is selected based on the fluid properties of the liquid which is to be sprayed. For spraying slurry in a Fluidized Gas De-sulpurizer (FGD) the most preferable material is Silicon Carbide. However the Silicon Carbide is brittle and is more sensitive to the tensile stress. There are other 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.
