PulsaJet PWM for the control of spraying systems

Spraying Systems have found a unique solution to the problems of controlling spray volumes by using the principle of Pulse Width Modulation of the spray, ie changing the duty cycle of the spray action

In any conventional spray nozzle, flow rate is governed by the size of its orifice and the pressure of the liquid.

Flow can be varied by changing liquid pressure, but this will also change performance characteristics such as droplet size and spray pattern.

In many situations such changes are not desirable.

On the other hand, changing the nozzle itself is simply not practical if flow needs to be continuously varied.

A further limitation is that at very low flow rates many liquids are difficult to spray accurately, and if control is achieved by using a small orifice, there is also an increased risk of blockage.

Spraying Systems found a unique solution to these problems by using the principle of Pulse Width Modulation of the spray - Spraying Systems have developed spray nozzles, control equipment and software, and built integrated spray control systems that are now making critical improvements in industries as different as food processing, pharmaceuticals, steel, papermaking and wood panel manufacture.

Pulse Width Modulation (PWM) is a way of controlling the rate of liquid flow through a nozzle without changing the liquid spray pressure and without replacing one nozzle by another with a different size of orifice, or varying the orifice size.

PWM works by switching the nozzle 'ON' and 'OFF' repeatedly in a rapid controlled succession, so that the liquid volume delivered in the course of, say, 1 minute depends on the percentage of time the spray is ON.

This measurement is called the Duty Cycle.

The key elements in a PWM spray system are:.

1) An AutoJet control unit - purpose-designed and manufactured by Spraying Systems for controlling spray volumes and operation, incorporating dedicated control software.

2) Automatic spray nozzles from the Spraying Systems PulsaJet series, capable of up to 10,000 ON/OFF cycles in 1 minute.

The main PWM features are as follows:.

* Variable flow at constant pressure: The Duty Cycle chosen determines the liquid volume sprayed per unit of time, and the Duty Cycle can be precisely varied.

* Accurate coating weights at varying line speeds: Changes in line speed are the most common reason to adjust flow from a single nozzle - to maintain consistent coating weight, flow must increase and decrease at the same rate as line speed.

The PWM system can also respond to fluctuations in signals from a weighing hopper: flow rate responses can be almost instantaneous using PWM.

* Uniform coverage of moving objects: the rate of ON/OFF cycling (pulse frequency measure expressed in Hz) can be adjusted to match line speed.

This not only allows full coverage of objects on fast-moving lines, but also enables very accurate fault marking or spot dosing on e.g a continuous fabric process.

* Improved transfer efficiency: an important advantage of PWM over air atomising sprays at low flow rates is the reduction of misting.

Better transfer efficiency (with larger droplets) can reduce material wastage and in some circumstances also improve worker safety.

* PWM can be applied over a very wide working range: Duty Cycles can be adjustable between 100% flow and flow rates as low as 5%, giving a flow rate reduction of 20:1 within the range of a single nozzle.

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