Pulsating approach to spray control

Tom Shelley reports on a digital way of controlling sprays over a wide range – and a clever trick to close their valves more quickly

Sprays for coatings, whether for the food industry or industrial products, can be controlled over much wider speed ranges by adopting pulse width modulation. This requires quick ‘on’ and ‘off’ turns - and the latter can be greatly accelerated by a sharp, negative pulse. This ensures more efficient application of coatings, at constant drop size, and also offers a way of switching solenoid operated valves more quickly for other applications. Just turning down the flow rate to a spray nozzle results in a larger drop size and different spray pattern. Raoul De Winne of Spraying Systems has been describing how his company considered using needle vales in nozzles, and even variable geometry nozzles, but then decided their new solution was far better. The solution to which he refers is the ‘PulsaJet’, which relies on PWM – Pulse Width Modulation – to turn a spray on and off very quickly to produce a sequence of short bursts, so the spray pattern remains the same, but the amount of spray can be varied. According to De Winne, a typical conventional nozzle turndown ratio is only about 1.5:1 before the spray pattern starts to deteriorate. But, by changing the ratio of ‘off’ to ‘on’ periods, a 5% duty cycle is easily achievable, and, in an extreme case, he says, 1:1000 is no particular problem. The facility to pulse nozzles on and off is very useful for spraying objects coming along a conveyor, he adds, with ‘on’ periods triggered by the detected presence of objects to be sprayed. This also allows items to be sprayed with even coatings, regardless of the speed that the conveyor is running at. Slower running simply requires longer ‘off’ periods for the spray. The maximum rate at which nozzles can be turned on and off is presently 167Hz, or 10,000 cycles per minute, although a new generation of systems is being brought out capable of being switched at 20,000 cycles per minute. The sprays are switched on and off by a needle valve in the tip, with power applied to an electromagnetic actuator to initiate spraying and a spring return to close it off. Left to its own devices, closure would take 8 to 12 milliseconds, so the controller applies a brief negative pulse at the end of each positive pulse train to ensure a quick cancelling of the magnetic field energy in the actuator, resulting in a turn-off in 3 to 4 milliseconds. PulsaJet systems can deliver up to 60 litres per minute, at pressure of up to 24 bar. Intrinsically safe systems are available for the spraying of flammable liquids. The company produces more than 80,000 different nozzles and sells more than 10 million annually. Design is assisted in all cases by extensive use of Ansys Fluent. Those with a sweet tooth might be gratified to hear of the system’s ability to spray hot, liquid chocolate in such a way as to produce confectionery more rapidly – without wasting a single drop! Pointers * By using pulse width modulation, it is possible to turn down spray ratios by 20:1 or more, without adversely affecting spray patterns * By applying a brief negative pulse, it is possible to cancel the magnetic field energy in an electromagnetic actuator, allowing a spring return to close the valve in about one-third of the time