Frosts are cold weather conditions, of relatively short duration, during which organic material may be damaged by exposure to surrounding air temperatures lower than 0°C. Frost damage to agricultural crops continues to cost growers millions of dollars each year.
The earth absorbs heat from the sun during the day and releases it into the colder atmosphere at night. The heat loss is greatest just before dawn and this is usually when the maximum danger of frosts and frost damage occurs.
This nocturnal release of heat creates an 'inversion layer' of warmer air, which can be found from 10-50 metres above the ground.
‣ only occurs at night.
‣ is caused by long wave radiation cooling from the earth to the sky.
‣ has calm, clear and dry atmospheric conditions.
‣ the coldest air is closest to the ground
‣ temperature increases with height above the ground until the inversion cap is reached
‣ the most common type of frost in spring and autumn.
‣ radiation frosts are far more common than advection frosts. Frost fans are an efficient and effective method to protect against frost damage and cold injury during radiation frost events.
‣ can occur during either the day or night.
‣ is caused by a large scale invasion of freezing air from the polar regions.
‣ is generally associated with moderate to strong winds
‣ fully mixed atmospheric conditions.
‣ the air temperature decreases with height above the ground.
‣ the temperature of the entire air mass is well below 0°C.
‣ large scale protective action in horticulture is not economically feasible.
‣ generally only occur in winter.
frost fans early and shutting them down late, results in considerably better crop protection performance. It is recommended that growers use this procedure, even when the temperature drops below freezing for several hours, and it may appear to be a 'lost cause'. Substantial crop salvage has been achieved with a “slow-freeze-slow-thaw” even under severe frost conditions.
In severe conditions, the use of orchard heaters in conjunction with frost fans strengthens the useable inversion layer which the frost fan then mixes and distributes over the crop, thereby providing better protection.
The initial expense of protecting a large area with frost fans can be significant, but the cost of capital, maintenance and operation is comparatively lower than other types of frost protection systems.
Depending on the type of crop, geography, local climate and crop layout, a single frost fan will provide protection for about 6 - 8 hectares. Multiple frost fan installations will increase the effective coverage of individual fans.
During a radiation frost, a frost fan is used to draw down the warmer air in the inversion layer and blow it into the orchard or vineyard. The frost fan needs to blow as much air as it can, to the greatest distance possible. This will give the most economical coverage for the frost fan.
In order to reach the greatest distance, the fan needs to produce a strong, uniform wind. The distance the wind can penetrate into the orchard is proportional to the momentum of the wind. The momentum of the wind or wind momentum is the product of the wind flow rate, the wind speed and the air density.
Frost fans have become the central element in most frost protection strategies. They use the warmer air in the inversion layer to protect a crop from frost damage. The frost fan is angled slightly downwards to pull this inversion layer down to ground level, to protect the crop from frost damage.
Frost fans are turned on before a frost occurs, usually 0.5 to 1.5°C above freezing. There is documented proof that turning on
> FEATURES OF THE C49 FROST FAN
What is the benefit of a C49 fan?
Fan Efficiency. The Frost Boss C49-4 blade fan provides the greatest coverage for the lowest fuel consumption of any frost fan on the market. In a continental climate, like Australia, with large temperature inversions and low katabatic wind speeds, the C49 can achieve coverage of approximately 10ha. The same fan in an island climate, like New Zealand, in a cold valley with a strong katabatic drift and a small temperature inversion will achieve coverage of approximately 6 – 7 ha.
The noise signature from the C49 is smooth, and at a low level, without the distinctive “Iroquois” chopping sound of the 2-bladed fans. The 4 blade fan, by virtue of its greater blade area, allows a higher blade pitch to be used without running the risk of the blades stalling. This allows the fan to run at a slower speed, thus making less noise, while still doing the same work.
It also converts more of your fuel into useful wind and less into turbulence and noise. Evidence from actual frost events shows that 4 blade fans have a greater penetration capability than 2 blade fans, thus giving them greater coverage.
Why does the C49 blade shape look so different?
The C49 fan blade is twisted from the tip to the root in a progressive manner, whereas all other frost fans are twisted linearly. The progressive pitch of the blade gives it a distinctive 3 dimensional shape, especially towards the blade root. This combined with the progressive increase in blade width, and the rounded blade tips, gives improved performance and an aesthetically pleasing blade shape.
Is thrust important in measuring the performance of a fan?
Thrust is used to measure the performance of a propeller on an aircraft, and is not a good measure of the penetration capability of a frost fan. The parameter best used to measure a frost fan performance is the wind momentum, and in particular the uniformity of the wind momentum.
A uniform wind momentum is best achieved with a blade design that has the blade angle pitched progressively from the tip to the root. In this way, the air flows uniformly across the fan diameter, and creates a uniform blast of wind. The C49 fan has over 20 degrees of progressive pitch in the blade.
On the other hand, some 2 blade fans have only 5 degrees of pitch between the tip and the root of the blade. This is nowhere near enough pitch variation (called twist) to achieve a uniform wind momentum. Instead, the blade tips end up creating all the wind momentum, which mixes with the slower air in the middle of the fan blast, introducing unnecessary mixing, which absorbs energy from the wind stream. This in turn dilutes the momentum of the wind stream, before it even gets a chance to drive out into the orchard or vineyard.
Thrust is a useful indication of how much power the engine is putting into the fan, but it gives no indication about the uniformity and penetration capability of the wind stream. This is a bit like a car wheel-spinning … the power is going into the wheels (thrust) but the wheels are making a lot of noise and turbulence.
Why do C49 fans spin more easily in natural wind, when they are not running?
C49 fans spin very easily when an ambient wind hits them, from any direction other than edge-on to the fan plane. This quickly turns the fan out of the wind thus avoiding shock loading. Two blade fans, on the other hand, tend to rock on the teeter assembly for a bit, before spinning in the wind. Even if spinning slowly, a two blade fan will continue to rock on its teeter assembly. In areas that experience strong winds, large diameter, two blade fans can be subjected to considerable shock loading in changeable wind conditions, resulting in damage to the blade attachment assembly.
> FREQUENTLY ASKED QUESTIONS
What is better - wind volume or wind speed?
Neither - the most important aerodynamic property of a frost fan is the momentum of the wind stream it generates. This 'wind momentum' is the product of the wind flow rate, the wind speed and the air density. The more momentum the wind stream has, the further it penetrates into the orchard or vineyard.
Does this also apply to helicopters when they are used for frost protection?
Yes it does. The more thrust the helicopter can produce, the more wind momentum it will generate, and so the greater the amount of warmer inversion air it can blow down into the orchard. With a helicopter, the thrust developed by the rotor equals the weight of the helicopter, so the heavier the helicopter can be made, the more effective it will be at frost fighting. Thus any helicopter used for frost fighting, should be operating at its maximum safe operating weight, by carrying additional dead weight, in order to maximize the amount of wind momentum it can produce.
What are the pros and cons of 2 and 4 blade fans?
The main source of noise from a frost fan comes from the high tip speed of the fan, so the longer the blades are and the higher the fan speed, the more noise it makes.
NZ Frost Fans designed the 4 blade fan to be quieter without losing performance. The blades were designed and pitched, to match the maximum torque of the engine. Because of this, the engine and fan can operate at a lower speed (1700-1900rpm) and achieve better coverage and fuel economy with less noise.
NZ Frost Fans also manufacture a conventional 2 blade fan for use where noise is not a major issue. The FrostBoss C24 fan achieves its maximum output at a relatively low speed (2100rpm) compared to other 2 blade fans due to the design and pitch of its blades.
Are C49 fans really that much quieter?
Yes they are. Not only is the fan running much slower, but the engine is running slower as well, so both the aerodynamic and the mechanical noise is reduced.
Further, the low frequency chopping sound is not present with a C49 fan. It is this sound that is most annoying on still nights, because it is low frequency sound, and carries much farther than high frequency sounds. This is like your neighbour playing music really loud, and all you can hear is the bass beating and no music to go with it..
> FEATURES OF THE C59 FROST FAN
How does the FrostBoss C59 fan differ to the C49?
The C59 has 5 blades and operates at lower rpm, whilst covering the same area as the C49 with even lower noise levels.