Spraying It Right

June 15, 2002

Atomization, the breakdown of liquid into drops, usually is accomplished with high-pressure nozzles or rotary atomizers. Nozzle atomization is used widely in the chemical process industries, in everything from chemical spray drying to foam and fog suppression to combustion of waste streams and the dispersion of chemical agents.

Unfortunately, atomization also is an area that is neglected. This column provides some pointers to optimize nozzle atomization.

The major types of nozzles include simple orifice, pressure swirl and fan designs. These are used in approximately 80 percent to 90 percent of all nozzle applications. The pressure drop generates the flow velocity necessary for atomization.

Various ways exist for measuring the quality of the atomization. Pattern and drop size methods receive the most attention. However, drop size is not easily measured.

Spray angle and penetration also are important in nozzle applications. Spray angle is accounted for in the spray pattern. Penetration is a measure of how far the spray travels, an important issue in spray drying, spray painting and fuel combustion. Lower spray angles result in a higher penetration.

The exit stream from the nozzle can take on different shapes, based on what is desired. Flat-sheet, hollow-cone, full-cone and even "square"-cone sprays are possible. A square cone pattern can deposit material in a more uniform manner than can a hollow- or full-cone spray pattern.

At the nozzle exit, a liquid sheet often is formed, which collapses into ligaments and finally into drops. Drop size is determined by the liquid sheet thickness. Thinner liquid sheets provide smaller drops. The pressure drop, nozzle geometry, material type and volumetric flow rate determine the liquid sheet thickness. Certain materials form only ligaments ," usually as a result of the non-Newtonian properties of the liquid.

A spray pattern might look fine at first glance, but it could be misaligned in reality. To ensure good performance, nozzle users should check the spray pattern from time to time. A patternator ," a collection of test tubes arranged to catch drops directly in the spray ," can be used. The different heights of the collected liquid in the various test tubes provide the drop accumulation pattern that directly illustrates the spray pattern.

Spray misalignment can be caused by improper nozzle orientation in which the nozzle is not pointed in the right direction. Misalignment also can be caused by wear or material buildup inside the nozzle. Nozzle alignment needs to be checked with a patternator for alignment. Spray edge effects can cause quality problems that are difficult to eliminate. Plugging might require the material to be filtered before spraying.

Flow rate, pressure drop and spray angle should be checked from time to time, with inspection frequency determined by the conditions in which the nozzle operates. Nozzle replacement might be necessary on a regular basis. Abrasives systems are particularly difficult to process with nozzles and are better handled with rotary atomizers.

Other nozzles include assisted atomizers where a second flow, usually a gas, atomizes the liquid flow. The controlling factor in this type of atomizer is the mass flow rate ratio of gas to liquid, called the ALR. This ratio usually has to be above two or three for the gas flow rate to have an appreciable influence and to prevent dripping.

Typically, assisted nozzles are used in the combustion of viscous waste streams in chemical and petrochemical plants. Often, the nozzles are "plant made." Their operation under plant conditions is not understood, and they drip. Many of these waste streams are hazardous, and the dripping leaves pools of hazardous waste in the combustion chamber. This results in unnecessary downtime while cleanup is accomplished. Nozzles with the proper ALR do not drip; improper operation does not have to be accepted.

A flashing atomizer also is possible ," one phase vaporizes and pushes the other phase through the nozzle at very high velocities. This typically occurs in flash drums.

Vapor explosions are highly effective in material atomization. Common examples of flashing atomization include popping popcorn and water drop explosions in hot grease.

By examining the quality of their nozzle atomization applications ," and taking the necessary actions to fix any problems ," plants will be able to improve performance and reduce downtime.

Tatterson is a technical editor for Chemical Processing. He is a professor at North Carolina A&T State University in Greensboro. Contact him at [email protected]. He also teaches short courses for the Center for Professional Advancement, www.cfpa.com.

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