Technical 2

ABOUT FROSTS

Radiative frost

‣only occurs at night.
‣caused by intense, long wave radiational cooling
‣has calm, clear and dry atmospheric conditions.
‣temperature increases with height above the ground until the inversion cap is reached
‣from which temperatures either stay the same or slowly decrease again.
‣the coldest air is the thin layer closest to the ground ie a surface inversion.

Advective frost

‣occurs during the day OR night.
‣with large scale invasion of cold air from the polar regions.
‣with moderate to strong winds
‣well mixed atmospheric conditions.
‣temperature trace decreases with height above the ground.
‣temperature of the entire air mass is well below 0c.
‣large scale protective action in horticulture is NOT feasible.

ABOUT FROST FANS

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.

It is the wind momentum that draws down the warm air from the inversion, mixing it with and replacing the colder air as it flows through the orchard or vineyard.

The more momentum the wind stream has, the further it penetrates into the orchard or vineyard.

What do you mean by 'wind momentum'?

The wind momentum is like the inertia of a car rolling down a hill. The faster the car rolls down the hill, the harder it is to stop. And a heavy car is harder to stop than a light one.

So the inertia of the car is the product of its mass and its speed.

Just like the inertia, or momentum, of the wind is the product of the mass of air being moved and the speed of the air.

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 pull down into the orchard. Now with a helicopter, the thrust from 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.So 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 is the wind flow rate for the FrostBoss Wind Machine?

During the calculation of the wind momentum for the FrostBoss the wind flow rate was calculated to be 373 cubic metres per second (790,330 cubic feet per minute).

This calculation was based on wind speed measurements recorded at 7 metres from the fan and air flow rates when the frost fan was operating at 1700 rpm.

But don't confuse this measurement (of air flow) to the often quoted 'air volume mixed' rates claimed by other frost fan manufacturers. Wind flow comparisons are only meaningful when the calculations are based on the same measurement formula, and when the tests are conducted in identical conditions.

Our measurements are recorded just 7 metres from the fan, are not influenced by wind drift, and are conducted in similar if not identical conditions. Furthermore, these tests can be replicated at any time - subject to favourable weather conditions.

Other frost fan Manufacturers refer to ‘air volume mixed rates’ and this is totally different to the wind flow rate calculated above and typically includes wind drift.

What's the pro and cons of 2 and 4 blade wind machines?
The major source of noise from a wind machine stems from the tip speed of the fan so the longer the blades and the higher the revs the more noise is produced.

New Zealand Frost Fans Ltd designed the 4 blade wind machine in responce to requests for quieter machines without losing performance. The blades have been designed and pitched to match the maximum torque of the engine and because of this we are able to run the fan at lower revs (1700-1900rpms) and achieve more air movement with less noise.

New Zealand Frost Fans Ltd also manufacture a 2-Blade machine based on conventional technology for use where noise is not a major issue. The FrostBoss 2-Blade wind machine achieves its maximum output at relatively low revs (2100rpm) compared to conventional 2-Blade machines due to the design and pitch of the blades.

Are 4-Blade fans really that much quieter?

Yes they are. Not only is the fan running much slower, but the engine is running slower too. So both the aerodynamic (fan tip) and the mechanical noises are reduced.

Furthermore, the low frequency 'thwack, thwack, thwack' beating sound is not present with the FrostBoss 4-Blade fan. And it is this sound that is most annoying on still nights because it is a low frequency sound, and it carries much further than high frequency sounds. This is just like your neighbour playing music really loud, and all you can hear is the base beating away and no music to go with it.

Is the wind momentum of a wind machine significantly affected when it is operated at less than its design speed in order to meet District Plan Noise Rules?

Yes it is. As the engine speed is reduced from its design speed of say 2400rpm down to 1800rpm, the power required to turn the 2-blade fan at this lower speed, is only a fraction of what it used to be at full speed. As a result the wind momentum is at least halved.

If you have ever driven a jet boat, or a jet ski, you will know how dramatically the thrust from the jet pump drops, as you back-off the throttle from full power. This thrust is the reaction created by the momentum of the water leaving the jet pump. The jet boat has the same balance of forces that the frost fan has - only the Wind Machine is putting the momentum to work while standing still, whereas the jet boat puts the thrust to work to create motion.

Comparative fan testing showed that the new FB 405 4-Blade fan produced significantly more wind momentum than all other fans when the noise emission levels were similar.

FEATURES OF THE C49 FAN

Q. What is the outstanding feature of the C49 fan?

Fan efficiency. The C49 provides the greatest coverage for the lowest fuel consumption of any frost fan on the market. In a continental climate, like Australia, with big temperature inversion and low katabatic wind speed, the C49 can achieve coverage of approximately 10ha. The same fan in an island climate, like New Zealand, in a cold valley with a stronger katabatic drift and a small temperature inversion will achieve a smaller coverage of approximately 6 – 7 ha.

The noise signature from the C49 is smooth, and at a low level without the distinctive “Iroquois chop” of the 2-bladed fans.

The C49 turns more of your fuel into solid wind and less into turbulence and noise.

Q. What is the benefit of a 4-blade fan?

The 4-blade fan was developed 10 years ago in response to the need to produce a quieter fan, that doesn’t have the distinctive “Iroquois chop” 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.

Anecdotal evidence from actual frost events show that 4-blade fans have a greater penetration capability than 2-blade fans, this giving them greater coverage.

Q. 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 enough pitch variation (called twist) to achieve a uniform wind momentum. Instead, the blade tips end up creating all the wind momentum, and this mixes with the slower air in the middle of the fan blast, introducing unnecessary energy absorbing mixing. 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.

Q. Why do 4-blade fans spin much more when they are not running?

Four blade fans spin very easily when an ambient wind hits them, from any direction, other than edge-on to the fan plane. 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 conditions, resulting in damage to the blade attachment assembly.

Q. Why do 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.

Many people have commented about how elegant the C49 fan looks, and for owners who live on their properties, this is a bonus for them as they would see the fans every day. Some have even commented that the fans could pass as a kinetic sculpture, so they not only get a piece of equipment that performs on frost nights, but they also get a kinetic sculpture during the day.