b'SPRAY TRANSFER EFFICIENCIESSpray Transfer Efficiency onto a High-Speed Web: Boundary Layer Effects and ManipulationBy B. Drummond, K. M. Bade, and R. J. Schick, Spray Analysis and Research Services, Spraying System Co., USAABSTRACT are used in conjunction with manyequipment or floors become coated in Moving webs are used frequently in thedifferent applications such as beltthe spray solution that was intended industrial manufacturing world as belts,cleaning, surface coating, andto have been applied to the web. In fabrics, or fibre-based products. Incooling. As the web (belt) moves,industry, when operators witness this, high speed applications, which can bea drag force is applied to the airsome type of deflector is created to moving upwards of 5000 ft/min, thesejust above the web surface and aslow the incoming boundary layer speeds create a significant boundaryboundary layer is formed over theair and shelter the sprays in an layer of moving air. This effect cantop of the web. As the web speedattempt to increase transfer efficiency. prevent unwanted buildup on the webincreases, the boundary layer mayAside from anecdotal evidence, surface, but when spraying a liquidgrow as additional momentum islittle rigorous examination has been solution onto the web surface, suchtransferred to the surrounding air. Atconducted to compare these air as adding lotion in a tissue makinglow speeds, spray droplets may havebarrier mitigation devices to quantify process, this air barrier can presentenough momentum to pass throughtheir effects on the process. For the an undesirable challenge. This studythe boundary layer and interact withpurposes of this investigation and characterises the air barrier, examining:the web surface. At higher speeds,discussion, the term boundary layer speed, thickness, and shape; andup to 7200 ft/min (36.6 m/s) [1]refers to the well-defined, momentum-explores techniques to manipulatecan be used for tissue making, thisdriven air stream close to the web the boundary for easier applicationboundary layer air provides sufficientsurface, while the term air-barrier of solution. A test setup with a high- drag to overpower the spray dropletis given to identify the qualitative speed belt was constructed, and thetrajectories and redirect themtransfer efficiency reducing air flow boundary later is evaluated usingdownstream, effectively preventing anear the web surface.Particle Image Velocimetry (PIV) withsignificant percentage of the droplets and without manipulation. Dropletfrom reaching the target web surface.The following research effort mimics trajectories and interactions with theThis significantly lowers the transferthe spraying of an additive onto liner-belt are investigated. Boundary layerefficiency of the sprays and willboard which is produced to become manipulation techniques are applied,increase the operating costs of thethe outside sheets of cardboard and the resulting downstream airoverall manufacturing process. Whileshipping boxes. These lines run barrier is investigated with and withoutlittle work has been done to rigorouslybetween 1000 ft/min to 4000 ft/min a spray. Physical measurementsexamine air-barrier interaction in thewind speedsdepending on the age of the machine, of the volume of fluid reaching thepaper-making industry, Zhu et al. [2]even lessand due to the very low application belt surface are used to quantify theand Guler et al. [3] demonstratedthan 1000rates of the additive, external-mix air-transfer efficiency and the PIV resultsthat for agricultural applications, windfpm (5 m/s)atomising style nozzles are typically are used to explain the changes inspeeds even less than 1000 fpm (5can haveused. Air atomising nozzles utilise transfer efficiency across the various airm/s) can have significant effects onsignificantcompressed air to break up the liquid barrier mitigation methods. drift and droplet transfer efficiency,effects ondroplets to create a desired spray which is 1-7 times lower than paperdrift andpattern. These nozzles spray the liquid INTRODUCTION industry conveyor speeds. A keydropletfrom a round orifice that is typically In the industrial manufacturing worldindicator of sprays with low transfertransferbetween 0.010 - 0.040 (0.254 - of moving conveyers, liquid spraysefficiency is when surroundingefficiency 1.016 mm) in diameter [4]. Directly 74 WORLD PULP&PAPER'