Sprayer speed versus good coverage: how fast is too fast?

It seems simple: the faster you drive the sprayer, the more area you cover.

This makes higher travel speeds a seductive method for improving productivity. Sprayer manufacturers knew this 25 years ago when pull-type sprayers first received bigger, suspended outrigger wheels.

Since then, they’ve delivered a growing number of self-propelled units, more powerful engines, better hydraulic motors, smoother suspensions, better guidance and cruise control.

Each of these innovations still required the operator to consider the relationship between travel speed, pressure, nozzle choice and the desired output per acre.

However, now we have rate controllers, and we don’t have to think about such mundane things anymore. Or do we?

Do we still do a good job if we go faster? What exactly happens when we speed up?

Before considering the role of the rate controller, you have to decide on an overall target-speed range.

Charts, apps or online tools can help select nozzles that are sized to deliver your application volume at a given speed and pressure. This initial travel speed decision requires an understanding of how spray is delivered to the target.

Let’s start with the spray boom.

The oncoming air does three things to the spray as the boom moves through air:

  • Shears the spray, making it a bit finer.
  • Scrubs the smallest droplets from the pattern, leaving them in the wake of the boom.
  • Negative pressure behind the pattern sucks even more fine spray into the sprayer’s wake.

Collectively, these create the dreaded spray plume that hangs behind the spray boom.

The faster we move, the greater the proportion of spray that ends up in the plume: one to 15 percent. Once formed, the plume moves with the prevailing winds.

Today’s sprayers have wide booms and faster travel speeds, which often requires us to keep booms higher to prevent hitting the ground.

However, higher booms reduce our control over the spray’s direction.

For example, we have begun to use angled sprays when spraying vertical targets, such as wheat heads, but droplets lose momentum quickly. The further they are from the target, the more likely they are to slow or even fall vertically before they reach the target. That means higher booms often negate the benefit of angled sprays.

Still not convinced of the perils of high speeds? Think about the aerodynamics of the sprayer itself.

The sprayer, the boom and the spray pattern disrupt the air around it as travel speed increases.

Visualize a sprayer in a wind tunnel with smoke tracer lines. The nice pattern created by the boom gets really messy in a turbulent environment. This can cause a loss of deposit uniformity, resulting in a reduction of overall effectiveness.

For example, we can set overall speeds of eight, 12 or 16 m.p.h., choose a nozzle and then think about the effects of changes in travel speed within that targeted speed range.

Operators know that even small travel speed changes can result in large pressure variations. It’s because travel speed and pressure enjoy a square root relationship. If you double travel speed, your rate controller needs to quadruple the spray pressure to meet the new flow.

Even minor changes in speed to adapt to field conditions can lead to big fluctuations in pressure, which change average droplet size and affect coverage and drift potential. Severe pressure fluctuations are more likely with a faster average travel speed.

That’s perhaps why pulse-width modulation, which decouples spray pressure from travel speed and replaces it with a solenoid duty cycle, has a growing role in fast, self-propelled sprayers.

To minimize pressure fluctuations, use your pressure gauge as your speedometer. Have the boom pressure displayed prominently in your sprayer cab and try to operate at speeds that result in a pressure that is optimal for the job you’re trying to do.

The effects of increased travel speed:

Pros:

  • More area covered per hour.
  • Better contact with vertical targets, if the booms are kept low.

Cons:

  • More drift.
  • Less uniform deposition.
  • Wider pressure fluctuations.

So, how fast is too fast? We won’t draw a line in the sand, but we will emphasize how important it is to consider as much information as possible before deciding on a travel speed. Don’t rely on the rate-controller to think for you — it doesn’t have all the information.

Our next column will look at improving sprayer productivity without driving faster.

For more information, contact Deveau and Wolf via Twitter @spray_guy and @nozzle_guy or email jason.deveau@ontario.ca or agrimetrix@gmail.com.

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