Dosing for coating applications

Accurate dosing is critical in coating applications.  If too much coating is applied then money is being wasted and product quality may suffer.  If too little is applied then product quality will almost certainly suffer resulting in rejected batches. Dosing is particularly challenging when the process line is fast moving.  Any small changes in spray consistency can dramatically increase or decrease the coating being applied.

Controlling dosing for conveyor based systems

Dosing on a conveyor system can be controlled in three ways:

Conveyor speed

Simply by speeding up or slowing down the rate at which the product moves under the spray bar will increase or decrease the level of coating applied.  Conveyor speed has the advantage of being completely independent of the spray fluid properties so is often the easiest way to control dosing.

Flow rate

The flow rate through a nozzle can be increased by either using bigger orifice nozzles or by increasing the pressure at which the fluid is sprayed.  Both options will also affect the consistency of the spray.  Larger nozzles will tend to produce sprays with bigger droplet sizes whereas a flow increase induced by higher pressure will produce a spray with smaller droplets.  As droplet size can affect the coating quality care needs to be taken when controlling dosing through flow rate.

With air atomising nozzles that have external air mix configurations (the atomising air is mixed externally with the fluid) the flow rate through the nozzle is independent of the air pressure.  So flow can be controlled by fluid pressure alone.  What this means is that any changes in droplet size caused by an increase in fluid pressure/flow can be compensated for by varying the atomising air flow/pressure. However, it should be noted that in standard air atomisers this can vary the shape of the pattern unless a nozzle with an independent spray shaping air flow is used.

Nozzles such as the SAM have independent air inlets for spray shape, atomisation and nozzle actuation. In this way droplet size and spray shape can be controlled independently of flow rate.

Pulse width modulation

With electrically actuated nozzles dosing can be controlled via pulse width modulation. These nozzles can be cycled on and off up to 150 times per second.  This means that if very rapid pulsing of on/off cycles is programmed the nozzles will deliver less fluid but, because the cycle is so rapid, there will be no noticeable difference in the continuity of the spray.  So, a continual even coating will still be delivered.  By using this technique, the same nozzles spraying at the same fluid pressure can deliver variable flow rates. As the fluid pressure remains the same, other spray characteristics, such as droplet size, remain constant.  

Pluse width modulation spray control



Accurate coverage 

Getting a coating spray precisely on target is of vital importance.  With over-spraying potentially expensive coatings are wasted or worse may contaminate areas where they are not needed.  With under-spraying the coating will not be complete and this will affect product quality.  So ensuring the emerging spray is of the correct angle and that it remains consistent along a spray line is clearly of the upmost importance.

On the face of it ensuring spray remains on target is a matter of simple trigonometry.  The spray angle of the nozzle can be used to calculate where the spray will be at a given distance from the target.  Then by using basic trigonometry the correct height of the bar can be determined to ensure coverage.  There are, however, several points to consider over and above a simplistic basic trigonometry calculation.

1- Sprays are not even.  Most sprays do not deliver their fluid evenly along the pattern line.  In fan spray nozzles there will tend to be a tapering of fluid distribution towards each end of the spray.  As such, sprays may need to be overlapped to ensure a consistent spray.

spray bar overlapping

2- Spray angles vary a bit with pressure.  For many nozzles this variation is quite minor but the spray angle shown in a nozzle datasheet is never 100% constant for all pressures.  As pressure increases internal 'whirl chamber' full cone type nozzles will see their spray angle decrease.  Conversely, though most flat fan nozzles will see their spray angle increase with increase pressure.  Air atomising nozzles will have a more complex relationship between spray angle and pressure as both fluid and air pressure will affect the spray shape.

3- Fluid properties will affect the spray angle.  The spray angles given in nozzle datasheets will be for water.  If spraying fluids with different specific gravities, viscosities or surface tensions to water then the spray angle delivered may vary.  For more viscous fluids the spray angle tends to decrease but it is hard to give hard and fast rules as to how much by.  The increased viscosity will also affect the spray distribution as the ability to form fine droplets is impaired.

4- Gravity.  Most nozzles will only maintain an approximate geometric shaped pattern at a distance of 300mm from the nozzle.  After this point, particularly for finely atomised sprays, the pattern tends to be drawn in by the effects of gravity.  For many spraying applications the target will be close enough to the nozzle to ignore this effect but for some this many not be possible.  As such, the coverage calculation and the spacing of nozzles on the spray bar may need to take this effect into account.

Bakeries key applications

Bakeries engineering considerations