Pneumatic sprayer

Technical presentation

The operating principle of the pneumatic sprayer has nothing in common with the other two systems. Here, air is used not only to transport the droplet to the target, but also to break up the liquid stream. The operation of a pneumatic sprayer can be compared to that of a carburettor.

On the first devices, the very low pressure in the liquid circuit was only used to dose the quantity of spray mixture to be introduced into a venturi cone or similar system. Nowadays, a pressurised spray mixture combined with a pellet or nozzle system (used to calibrate the flow rate only) is the standard method. At the venturi, contact between the liquid and a stream of air at a speed of around 300 km/hour (when everything is going well) causes the liquid to break up into very fine droplets (50 to 150 microns). The smaller the quantity of liquid introduced into the air and the higher the speed of the air stream, the finer the drops will be.

S21 pneumatic sprayer

Spray liquid diffuser or splitter

Here's the main difference with a mounted jet:

BERTHOUD pneumatic diffuser

The fineness of the drop is created by the liquid breaking up on a splitter (see photo opposite) thanks to the speed of the air flow. For good fragmentation, the air flow is generally higher than that of an air-blast sprayer.
As you can see, there are no nozzles at this point on the machine, unlike with a hand-held sprayer. On the other hand, the calibration or flow rate of the spray liquid in L/min is done upstream of this device, either by pellets (very often) or by nozzles.

Each brand of sprayer has its own model of diffuser.

The surface of the nozzle must be clean and smooth. The presence of product deposits or a damaged surface will not ensure optimum spray quality (characterised by drops of irregular diameter).

Pressure and power take-off

BERTHOUD pneumatic sprayer boom

The working pressure is between 1 and 3 bars.

Remember that a pressure of 1 bar corresponds roughly to a height of 10 m in the water column and to 1 kg/cm².

For pneumatic spraying, it is essential to work at a PTO speed of 540 rpm to benefit from a high air flow (and therefore a high speed at the air/liquid meeting point) and thus obtain a good spray fineness.

However, it is not a good idea to have droplets that are too fine, as there are considerable losses through aerial drift, but also through desiccation. The higher the air temperature, the lower the humidity and the greater the distance between the diffuser and the target, the greater the desiccation.