Technical 1

OVERVIEW

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.

INTRODUCTION:
Frost damage to agricultural crops of all kinds continues to cost growers millions of dollars each year.

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.

HEAT TRANSFER:
The earth absorbs heat from the sun during the day and releases it into a 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 release of heat creates an 'inversion layer' of warmer air which can be found from 10-50 metres above the ground.

FROST FANS:
Frost Fans have become the central element in most frost protection strategies. They use the warmer "inversion layer" air 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.

The main principles involved are:

‣The removal of heavy cold air to prevent stratification and retard frost formation;
‣Allow warmer inversion layer air in to replace the colder air at crop level;
‣Some friction generated heat as the air moves past foliage to replace the 'skin' of colder ambient air.

Frost fans have had a considerable amount of technological experimentation and development leading to the knowledge that:

‣Large, slow moving fans are superior to small, fast-moving fans because they produce a large diameter 'cone' of slower-moving less turbulent air which does not produce desiccation.
‣The fan assembly rotates to face all parts of the property every 5-7 minutes to prevent re-stratification of cold air;
‣The frost fan is positioned to accommodate prevailing drift conditions.

Frost Fans are turned on before a frost occurs. Usually 0.5-1.5°C above freezing. There is documented proof that turning on frost fans early and shutting them down late results in considerably better crop protection performance. It is much easier to maintain stable temperatures than to try to correct a worsening situation. 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 method; even under severe conditions.

In severe conditions, the use of orchard heaters in conjunction with Frost Fans strengthen the useable inversion layer that the Frost Fan then mixes and distributes over the crop thereby increasing the protection radius and providing better protection.

The initial expense of protecting a large area with frost fans can be significant, but the comparative costs for their maintenance and operation are lower than for most other types of frost protection systems.

Depending on the type of crop, the geography, local climate and plot layout a single frost fans will provide adequate protection up to 6 - 8 hectares. Multiple frost fans installations will increase the effective range of individual units.