Rethinking nozzle selection with PWM

Pulse width modulation (PWM) control of spray applications is becoming more widely available in the UK, with several manufacturers offering purposedesigned
or retrofitted systems on their sprayers.

Operators report the system, which uses electric solenoids to manage the flow rate through each nozzle via intermittent shut off of the nozzle flow, offers increased efficacy. Additionally, it provides greater flexibility to spray in tight windows of opportunity afforded by our unpredictable climate and once the system becomes familiar, setting up and adjusting applications is also said to be considerably more straightforward.

Uncertainty

Yet a degree of uncertainty and perhaps even confusion surrounds the selection of nozzles to work with PWM systems.

Air inclusion nozzles have become an important standby to reduce drift, but many are incompatible with PWM as the mixing chamber and air inlet can fill with water as the PWM valve cycles. This can then result in water escaping out of the air inlet holes, which can lead to poor distribution.

Consultant Dr Tom Wolf specialises in spraying research and has worked alongside manufacturers to help operators get the best from PWM systems, including
Canadian company Capstan Ag, which supplies UK manufacturers Bateman and Sands.

“Capstan’s system was adopted by Case IH for its self-propelled sprayers in the mid-90s, marketed as AimCommand, with nozzles supplied by Wilger  Industries,” Dr Wolf explains.

Wilger nozzles, proven with the Case IH sprayers, have been successfully used with Capstan systems on other brands, including in the UK, but there is one drawback.

Dr Wolf says: “The nozzle cap does not fit Teejet-style lugs without an adapter, so while the Wilger nozzles are a good solution for AimCommand and PWM in general, for many operators not being able to use the nozzles they are familiar with is a problem.”

Development

A relatively new development in the UK, John Deere’s ExactApply system, which combines PWM with auto nozzle switching, is established in North America, with an estimated 80% of the company’s sprayers sold in Western Canada, for example, fitted with the technology.

“There have been gaps in nozzle availability for ExactApply and operators may need to compromise, although co-operation with Hypro has given more options.

“The 3D produces a finer spray quality than a traditional low drift nozzle, but Hypro’s LD Max, developed for ExactApply, has been designed to meet this requirement.

However, it produces a coarser spray quality, so spray pressure needs to be increased to maintain coverage.”

Dr Wolf points to the Pulzar nozzle from Billericay Farm Services as a useful development.

“Derived from the Billericay Air Bubble Jet but without air inclusion, the Pulzar gives a medium to coarse spray quality that works for PWM applications.”

Learning curve

Nozzle structure is part of the learning curve for operators, as PWM nozzles tend to be made in two parts, with a pre-orifice and an external orifice.

Some, such as the Pulzar, have a removable pre-orifice which makes for easier cleaning but needs periodic checking to ensure it has not inadvertently come apart. On LD Max and 3D nozzles the pre-orifice is not removable so a blockage may mean swapping to a new nozzle.

Nozzle size selection is based on the familiar criteria of the application volume, travel speed, and nozzle spacing but a complication is that for PWM they are larger than in conventional systems.

“Flow is controlled via the duty cycle which allows the operator to expand the speed range or precisely set pressure, but as operation is generally at 60-80% duty cycle, the nozzle accordingly has to be oversized by 20-40%,” explains Dr Wolf.

“Most traditional nozzle charts are not set up for this, so operators may have to rely on specialist help to produce bespoke charts.”

Apart from Teejet, which uses flow rate-tested data to give its recommendations, manufacturers’ charts are calculated mathematically, he points out.

“There’s about a 5% tolerance, so once the operator has chosen a nozzle, the PWM control system will alter the duty cycle to adapt.”

The lack of suitable nozzle sizes is a concern, says Dr Wolf, although smaller companies, such as Billericay Farm Services, are able to react to new demand, extending the top of their range from an 06 to an 08.

“It’s an area that needs development – companies will have multiple nozzle choices for air induction but just one or two for PWM. No nozzles have as yet been purpose-designed for this application,” he says.

There are also regulatory issues in Europe – German testing any PWM nozzles for low drift applications, while the Local Environment Risk Assessment for
Pesticides only recognises them at 100% duty cycle, rather than in normal pulsing operation.

“This is a limitation in Europe – in Canada we estimate that 35-50% of all sprayers currently in operation have PWM fitted.  Yet there are so many benefits – it’s affordable, precise technology that offers quantifiable savings,” says Dr Wolf.

Reductions in overlaps alone, a consequence of individual nozzle shut off control, have been shown to save 4-5% on the annual chemical bill, which would pay for the system in one to two years on a larger farm, he points out.

“Environmental concerns also favour the use of PWM, and I would think that is especially relevant in the UK where agriculture is much more in the public eye.”

His advice to operators: “Do your homework and be prepared to make some adjustments to get the best from your system.”