A fundamental dictum in aeronautics states that high aspect ratio blades (long and narrow) are more efficient than low aspect ratio blades (short and wide). Blades manufactured by Orchard-Rite have the highest aspect ratio in the Industry!
Efficiency of Orchard-Rite 2-bladed frost fans
The distance a frost fan can cover is proportional to both the blade diameter and airspeed. As a rule of thumb, every 1% increase in blade diameter results in a 1% increase in reach.
Increasing the blade diameter not only improves reach but efficiency.
There are five manufactures of frost fans in the world. Four of these companies believe that a two-blade design is the design which gives the best coverage!
More blades, greater disturbance
The trouble with adding more blades is, with every blade you add, it generates a disturbance behind each blade, which in turn restricts the RPM the fan can run at.
On a four or five-blade fan if you run the fan too fast, you will generate less wind! You might save fuel in running at lower RPM, but it won’t push the air out at the same velocity and it won’t protect as big an area. So you won't save money on fuel if you work out fuel cost per acre protected.
Generally, the more blades the lower the RPM the fan is set at.
Scientifically the most efficient fan is a one blade fan, but how do you balance it? Under current technology one-blade fans can’t be used.
The next best design is a two-blade machine. Two-blade machines generate the most wind and protects the biggest crop area.
A two-blade fan is used for the best coverage.
This fan in most situations protects 16 acres of crop. (In some locations you can get coverage of a much larger area). This protection area will vary depending on several conditions, namely whether the crop is trees or vines, if it is trees the size of the trees, the terrain, the amount of drift etc.
This fan moves a lot of air—this movement of air does generate noise. The noise is 61 dB at 300 metres. It is recommended that this fan should be placed at over 400 metres from the closest house.
The speed of the rotation of the fan is a determining factor in the amount and distance of air which is moved. The slower the rotation the less thrust behind the blade, the smaller area the air is disturbed.
Pitch does affect the volume of air, but speed of the rotation is the more important factor.
The larger the diameter of the fan the greater amount of air can be moved. A small diameter fan just can’t push the same volume of air. Longer blades are more efficient.
To protect a bigger area from frost, you need a bigger amount of moving air. The more air that is disturbed, and the greater the thrust, the greater the area is protected, but a consequence is more noise.
The noise level although audible from a distance doesn’t generate much more noise than a passing large truck.
If the house in question is close to a road, there will be potentially more noise from road traffic than there would be from the frost fan.
Frost fans as a general rule are set to come on at 2 C when there is no wind. (When there is wind there is no need for the frost fan to come on).
Houses on a cold night would have the windows closed, which would negate the noise.
Frost fans are designed to protect crops from frost. Different crops require different protection.
Deciduous fruit trees such as apricots, peaches, plums need protection from frost at flowering which generally happens in late winter and early spring.
So, having a frost fan come on in midwinter would not be needed in these crops. However, citrus and avocado trees need protection in winter.
The average frost fan in Australia runs for 40 hours a year. But one bad frost can destroy an entire years’ worth of crop.
Best frost protection
The best protection from frosts available in Australia today, is the Orchard-Rite D2600 two-blade frost fan.
This fan is the biggest selling frost fan worldwide. This fan in the field has showed to protect a bigger area than the four or five-blade designs.
The D2600 is powered by a Caterpillar 7.1 litre motor set at 165 hp.
The three-blade fan was developed by Orchard-Rite to reduce noise where there are houses nearby. The three-blade fan generates less wind, so it generates less noise, but the consequence of this is that it protects a smaller area. It protects 11 acres of crop in most situations. (You can get coverage of a much larger area than what is quoted).
Protection area will vary as per the above conditions. This fan is a slight compromise between area covered and noise. It has been tested to cover 6% bigger area than a well known four-blade design, and still generate less noise.
The Orchard-Rite three-blade fan is 50 dBa at 300 metres. This meets the farming zone guidelines in the Victorian EPA. (If the zone is rural living [not farming] then a level of 40 dBa must be reached, so the fan would need to be about 400 m from the nearest house).
Contact Sunrise AG phone 03 5023 0284
Rose Ayling mobile 0428 221 848
See this article in Tree Fruit August 2018