11 April 1997


in plastic mould

Spray nozzle manufacture has changed over the years. Andy Collings discusses developments with the UKs leading supplier, Lurmark

ON the face of it, squibbing diluted chemical evenly on to a field of growing plant should be a straight forward operation. After all, there have been watering can roses for generations.

But when factors such as wind speed, droplet size, pressure, crop size and application rates are taken into consideration, the task takes on an increasingly complex format.

One of the key components of the spraying operation is, of course, the spray nozzle itself – its design dictating in many ways, the volume of liquid applied and the spray quality achieved. The British Crop Protection Council defines spray quality as fine, medium or coarse depending on the size of droplets produced.

Lurmark, based at Longstanton, Cambs, has been in the sprayer component business for over 40 years. It claims to have about 65% of the UKs spray nozzle market and a growing share of the industrial market.

It seems reasonable then, to discuss with such a company the developments which have occurred over the years in spray nozzle technology.

"The most radical changes have been in the material used to construct nozzles," says Ian Sutton, Lurmarks sales director. "Brass, stainless steel and ceramics were used extensively in the early years, but perhaps the biggest revolution came in 1982 when plastic nozzles were introduced."

Mr Sutton believes their introduction allowed a new degree of accuracy to be achieved. "However carefully metal or ceramic nozzles are machined, there will always be a slight variance in their manufacture," he says.

Injection moulded plastic nozzles are always consistent – direct copies of the original mould – which puts great emphasis on the accurate construction of the mould. In Lurmarks workshops production of a new design can take weeks to perform using some of the most sophisticated electronic processes.

The introduction of plastic nozzles also carried the bonus of being able to manufacture in different colours for different nozzle size recognition – colour coding.

Spray drift has been a contentious issue for a number of years. Along with several other manufacturers, Lurmark has endeavoured to produce a version which would help reduce drift.

"The secret is in producing a high percentage of large droplets," explains Mr Sutton. "You can only go so far with the initial designs, using experience gleaned from the development of other nozzles. But at the end of the day, its a matter of trying a new nozzle out and seeing just how it performs."

Lurmark has on its books the TurboDrop nozzle which uses a venturi system to draw air into the nozzle and mix it with the liquid. This produces large, air-filled droplets which are more resistant to drifting.

Visually, the TurboDrop produces a very coarse spray which the BCPC might find difficult to quantify in terms of its fine, medium coarse ratings.

With all the recent developments, one could believe there is little else to achieve on the nozzle front. But there remains at least one, as yet, unresolved problem – the development of a nozzle which can produce consistent spray quality and pattern over a wide range of pressures.

Increasing use of proportional application systems designed to maintain a selected application rate, irrespective of changes in operating speed, are restricted by the ability of current nozzles to handle vast changes in pressure.

If, for example, a sprayer is applying chemical at 3 bar pressure while travelling at 8kph an increase of speed to 16kph would mean the pressure has to rise to an impossible 12 bar to maintain application rate. Spray drift would perhaps be the least of the problems experienced.

Similarly, a drop in speed to 4kph would require the pressure to drop to 0.75 bar at which point, if the anti-drip shut off valves were not activated, spray pattern would have deteriorated drastically.

Extreme conditions indeed, but it can be seen that these operating pressures are beyond the capabilities of current nozzle design and limit the full exploitation of proportional application systems.

One solution could be to develop a nozzle with a variable orifice to enable larger or smaller volumes to be applied without changes in pressure or spray quality. Another could be to have a twin jet system where the second jet is phased in or out as volume requirements change.

Clearly, the problem remains a challenge for nozzle manufacturers to solve.

Nozzle-testing, demonstrated by Ian Sutton, is important in the process.

Injection moulding equipment has made a big difference to consistent accuracy.

New nozzle designs under way at Lurmark. An intensive operation,

it can be a number of weeks before the master dye is in production.