A giant hot-air blower designed to lay a warm blanket of air over crops during frosty weather is being trialled near Mt Pisa, north of Cromwell.

The device, called a ''heat ranger'' burns LPG, rotates 360degrees and can protect up to 15ha of vineyards, black currants or other lower-growing crops.

Hamilton company Heat Ranger Ltd is using a new cherry orchard to test one of its two prototypes, which director Fred Phillips described to the Otago Daily Times this week as a ''world first''.

''There's nothing like it in the world.''

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Phillips, an agricultural engineer, said the Cromwell site had been chosen for testing because it was an area with more reliable frosts than the Waikato or Christchurch, where the devices were built.

The heat ranger is designed to raise the air temperature by 2degC-4degC, Phillips said.

''In still-air conditions it creates a blanket over the top of the crop.

''In other words, it creates its own inversion layer that protects the crop.''

Phillips said the $200,000 heat ranger was not yet on the market, but the capital cost would be about a quarter to a third of the cost of installing a frost-fighting system that used water.

And, despite using eight 200kg LPG cylinders in a 12 hour period, he considered its operating costs would be competitive with other frost-fighting systems, such as diesel-powered wind machines.

Phillips and his partners began developing the device six years ago while trying to devise a frost-fighting system for a black currant crop near Christchurch for which there was a limited water supply.

The group decided to look at using warm air instead, in a similar way to outdated frost pots, and a design was drawn up on ''the back of a cigarette packet''.

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Phillips was reluctant to say how much had been spent since on its development, but it had been a ''huge job''.

Photo / Mark Price
Photo / Mark Price

''In any situation that requires research and development, the cheapest thing is to walk away, but that doesn't make much progress.

''If you think your idea has merit, you put your money where your mouth is.''

The trials near Cromwell were designed to prove the device's reliability and effectiveness.
''We are measuring what we call the radius of influence.''

Phillips believed the heat ranger had ''significant potential'' but ''it only takes one failure to lose your crop''.

He hoped the device would be on the market in time for the next frost season.
The main disadvantage of the Cromwell site was the distance from bulk supplies of LPG, with refilling done in Christchurch.

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''If these machines turn out to be as good as we hope they will be, then we will be talking to big gas suppliers about putting in a refuelling facility in Cromwell to cope with machines like this.''

The second machine was being trialled in Waipara, in North Canterbury.

'Heat ranger' prototype

• 5m tall

• Metal and fibreglass

• 1200kW LPG burner

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• 1300rpm fan driven by 2 litre LPG powered Subaru engine or power take-off

• Air speed at nozzle 130kmh

• Air temperature at nozzle 30degC

• LPG use, 8 200kg LPG bottles per 12 hours

• Can oscillate or rotate 360degrees

• Portable

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• Coverage 12ha-15ha

• Cost, about $200,000