Spray (liquid drop)

A spray is a dynamic collection of drops dispersed in a gas.^[1] 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 thousands of applications in which sprays allow material to be used most efficiently. The spray characteristics required must be understood in order to select the most appropriate technology, optimal device and size.^[2]

Spray formation

Spray atomization can be formed by several methods. The most common method is through a spray nozzle which typically has a fluid passage that is acted upon by different mechanical forces that atomize the liquid. The first atomization nozzle was invented by Thomas A. DeVilbiss of Toledo, Ohio in the late 1800s His invention was bulb atomizer that used pressure to impinge upon a liquid, breaking the liquid into a fine mist. Spray formation has taken on several forms, the most common being, pressure sprayers, centrifugal, electrostatic and ultrasonic nozzle.

Fuel sprays

Sprays of hydrocarbon liquids (fossil fuels) are among the most economically significant applications of sprays. Examples include fuel injectors for gasoline and diesel engines, atomizers for jet engines (gas turbines),^[3] atomizers for injecting heavy fuel oil into combustion air in steam boiler injectors, and rocket engine injectors. Drop size is critical because the large surface area of a finely atomized spray enhances fuel evaporation rate. Dispersion of the fuel into the combustion air is critical to maximize the efficiency of these systems and minimize emissions of pollutants (soot, NOx, CO).^[4]

Industrial sprays

Electrical power generation

Limestone slurry is sprayed with single fluid spray nozzles to control acid gas emissions especially sulfur dioxide (SO2) emissions from coal-fired power plants with liquid scrubbers. Calcium hydroxide (lime) is atomized into a spray dryer absorber to remove acid gases (SO2 and HCl) from coal-fired power plants. Water is sprayed to remove particulate solids using a spray tower or a cyclonic spray scrubber^[5] Cooling towers use spray nozzles to distribute water.

Food and beverage

• Sprays are used to wash fruits and vegetables.
• Spray drying ^[6][7] is used to produce hundreds of food products, including instant coffee, powdered soups, and flavor concentrates.
• Coating of food products with flavorings and surface additives.
• Cleaning and sanitizing storage tanks, and process equipment single fluid nozzles are used to rinse and wash away materials. These specialized tank-cleaning nozzles often have a fluid powered rotary motion to increase cleaning effectiveness.

Manufacturing

Sprays are used extensively in manufacturing.^[8] Some typical applications are applying adhesive, lubricating bearings, and cooling tools in machining operations.

• Cleaning components with sprays of hot water and detergent sprays for degreasing, electric motor rebuilding, diesel engine rebuilding, plant maintenance, steel mill bearings, railroad bearings and engine rebuilding.^[9]
• High pressure sprays are used to de-burr machined parts.
• Spray painting is broadly used in many manufacturing processes; for example, for automobiles, appliances, office furniture.

Paper making

• High pressure debarking
• Coating paper
• Cleaning rolls
• Trimming paper

Electronics

• Spraying etching chemicals and printed circuit boards flux agents

Fire protection

• Spraying water from fixed sprinklers
• High pressure water misting systems for expensive and delicate equipment, for example, marine engine rooms.
• Deluge systems for protecting assets or keeping potentially explosive materials cool in the event of fire (e.g. gas canisters)
• Water tunnel systems designed to ensure a safe "cool" corridor to allow people to escape in the event of fire.

Mining

• Water sprays are critical to reducing coal dust during mining^[10]
• Water is sprayed to control dust emission produced during grinding; spray nozzles are also used for washing gravel in screening plants.

Line and cement

• Suppressing dust from raw materials.
• Feeding fuel to high temperature calcining rotary kilns.
• Cooling and conditioning gas.

Steel industry

• High pressure water is used to remove scale (iron oxide) from red hot steel during the process of rolling into sheet or strip forms.
• Sprays are used in the continuous casing process and quenching of hot gases.
• Quenching coke from coke-ovens
• Cooling metal extrusions
• Pickling solutions and rinsing pickling solutions

Chemical, petrochemical, and pharmaceutical

• Spraying reagents to enhance dispersion and to increase liquid-gas mass transfer. Many systems are used, including spray towers.
• Spray drying fluid cracking catalyst for oil refining
• Washing and rinsing solids in filters and centrifuges
• Applying coating to many pharmaceutical tablets

Waste treatment

• Single fluid nozzles are used to break foam in activated sludge wastewater aeration basins and to apply antifoams.
• Water is spray mixed with material being composted.
• Liquid waste is injected into high temperature incinerators.

Agricultural applications

Spray application of herbicides, insecticides, and pesticides is essential to distribute these materials over the intended target surface. Pre-emergent herbicides are sprayed onto soil, but many materials are applied to the plant leaf surface. Agricultural sprays include the spraying of cropland, forest, turf grass, and orchards. The sprayer may be a hand nozzle, on a ground vehicle, or on an aircraft. Herbicides, insecticides and pesticides are spray applied to soil or plant foliage to distribute and disperse these materials. See aerial application, pesticide application, sprayer. The control of spray characteristics is critical to provide the coverage of foliage and to minimize off target drifting of the spray to adjacent areas. (pesticide drift). Spray drift is managed by applying only in appropriate wind conditions and humidity, and by controlling drop size and drop size distribution. Minimizing the height of the spray boom above the crop reduces drift. The spray nozzle type and size and the operating pressure provide the correct application rate of the material and control the amount of driftable fines. Spays, single fluid nozzles, are also used to cool animals

Consumer products

Atomizers are used with pump-operated sprays of household cleaning products. The function of these nozzles is to distribute the product over an area. see Aerosol spray and spray can

• Water and detergent sprays for the car washing

References

1. ASTM standard E-1620 Standard Terminology Relating to Liquid Particles and Atomization
2. Lipp, Charles W. , Practical Spray Technology: Fundamentals and Practice , 2012, ISBN 978-0-578-10090-6
3. Lefebvre, A.H. Gas Turbine Combustion, 1999, ISBN 1-56032-673-5
4. Reitz, Rolf D, Modeling atomization processes in high-pressure vaporizing sprays, Atomization and Spray Technology (ISSN 0266-3481), vol. 3, no. 4, 1987, p. 309-337.
5. R H Perry, C H Chilton, C W Green (Ed), Perry's Chemical Engineers' Handbook (7th Ed), McGraw-Hill (2007), sections 12.23, ISBN 978-0-07-142294-9
6. K. Masters, Spray Drying Second Ed, 1976, ISBN 0-7114-4921-X
7. N. Ashgriz, Handbook of Atomization and Sprays, 2011, ISBN 978-1-4419-7263-7
8. G.G. Nasr, A.J. Yuhl, L. Bendig, Industrial Sprays and Atomization, 2002, ISBN 1-85233-460-6
9. Spray Uses in Various Industrial Cleaning Applications http://www.stingraypartswasher.com/Parts_Washer_Cleaning_Application_Solutions.html
10. C. D. Taylor and J. A. Zimmer, Effects of Water Sprays Used with a Machine-Mounted Scubber on Face Methane Concentrations, SME Annual Meeting Feb 26-28 Denver CO 2001, (http://www.cdc.gov/niosh/mining/pubs/pdfs/eowsu.pdf)