Some very viscous fluids like sour cream are not that sticky while others, like honey, are very sticky. As such heated water jackets are often used to maintain a stable fluid temperature right up to the nozzle orifice.Īlthough many viscous fluids are very sticky, technically viscosity is not the same as stickiness. Undesired cooling of only a few degrees prior to spraying can impair the spraying process. A few degrees in temperature difference can mean the difference between a 1000cps fluid (not really practical for spraying) and a 100cps fluid which is able to be sprayed.Īs increases in viscosity can be very rapid for even small temperature differences care must be given to such spraying systems. At a little above melting point chocolate is still too viscous for practical spraying but raise its temperature a bit more and spraying becomes practical. At room temperature it is solid and so clearly cannot be sprayed, but when heated it will become a liquid. A practical example of this is chocolate. Conversely if certain liquids cool they will become more viscous. The viscosity of a fluid will tend to lessen as it is heated. These mix the air with the fluid stream after exit from the nozzle and so will not encounter the problems faced by internal mix atomisers. To overcome this problem external mix air atomisers should be deployed. This can lead to blockages and a severely degraded spray pattern. The larger droplet sizes and bubbles that form in the chamber severely impede the spray formation. This is perfectly acceptable for low viscosity fluids like water, but with fluids much above 100cps problems occur. Many air atomising nozzles mix the fluid and air inside the nozzle before ejection from the orifice. In most applications other methods of atomisation for viscous fluids are required so air atomisers are used to achieve good droplet sizes but without the corresponding increases in pressures and flow rates. Resulting flow rate increase at this new pressureĪs can be seen from the table, droplet sizes increase dramatically with viscosity and there is not really any practical way to compensate for this as any pressure increase will have a knock on effect of increasing flow rate. Required Pressure increase to compensate foo droplet size Using the above formula we can produce this table for some common food sprays : Note: the exponent of 0.5 for P may vary depending on the nozzle used V f being the viscosity of the fluid in Cp as water has a Cp of 1 A nozzle spraying water that will form a droplet size of D w will form a droplet size of D f according to the formula below: Viscous fluids will tend to form larger droplet sizes. The final column shows the resulting increased flow rate. to achieve the same droplet size as one would expect when spraying water. The table below shows the droplet size increase compared to water and the corresponding required increase in fluid pressure to compensate, i.e. It may be possible to compensate for this by increasing the fluid pressure as increased pressure will reduce the droplet size, but spraying at higher pressures also increases the flow rates. This would mean the spray would have 2.6 times lower surface area to volume ratio making it less likely to adhere to a surface and form an even layer. For example, double cream (Cps 120) sprayed would form droplets about 2.6 times the size of that of water sprayed from the same nozzle. This effect is relatively small for fluids with viscosities below 10cps but becomes more pronounced with higher cps. As such, finer sprays are better able to spread out on their target surface. This is because sprays consisting of smaller droplets have a much larger surface area per volume than those made up of larger droplets. It may also affect how well the coating adheres to the target. For very viscous fluids this will have a detrimental effect on coating applications as small droplet sizes help to ensure an even coating. water has a Cp of about 1 at room temperature.Īs the viscosity of a fluid increases, it will tend to form larger droplets when sprayed. This is most commonly redefined as the unit Centipoise (cps) with 1 cps being equivalent to 1 millipascal second, i.e. Viscosity is measured in pascal seconds with water at 20 degrees C having a viscosity of 1.002 mPa.s. It is an approximate measure of the 'thickness' of a liquid. Viscosity is a measure of a fluid's resistance to shear stress. In the food industry there are many viscous fluids that need to be sprayed so clearly this a key concern in this sector. The spraying of viscous fluids can represent some challenges.
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