The science behind it
- Heat loss from the plant to the surrounding air and soil can be replaced by the heat released by the water changing from a liquid state to a solid state, or simply by the heat contained in water that is occasionally slightly warmer than the air. Using an energy balance and considering heat transfer by conduction through an ice layer on the surface of a spherical fruit, the heat transfer of the fruit can be defined by the heat transfer due to conduction, evaporation, forced convection and radiation.
- Unpublished data from Bartholic et al. suggests that a citrus crop will have a net loss on the order of 18 W/m2 of energy on an average radiation frost night (Hasey et al., 1994).
- The objective of a frost protection method is to replace the lost energy using, alone or together, enhanced conduction, convection, downwards radiation or, in this present study, using latent heat conversion. When fine water droplets are introduced into the air surrounding the fruit bearing plants and is condensed and frozen, large amounts of latent heat stored in the water droplets is converted to sensible heat and slows the temperature drop. The latent heat of fusion of water is 334 kJ/kg (CRC, 1981). This represents 93 W/kg or 351 W/gallon when considered over a 3-hour constant water-freezing event.
- The application of a thin layer of ice on the fruit crop by a continuous misting of a fine water film prevents the crop from reaching temperatures below 0°C (Issa, 2012), thus slowing the temperature drop and keeping the fruit above its critical damage temperature. Once the water application is stopped or the temperature drop becomes severe, the fruit will freeze. It is essential to recognize that this method only prevents the temperature of the protected plant from falling below the freezing point. It does not warm the plant nor does it raise the air temperature (Bootsma & Brown, 1985).
- Fruit crops tend to have high sugar contents relative to the rest of the plant, such as stems and leaves, and can therefore further withstand cold temperatures for longer periods of time. Each fruit crop will have a critical damage temperature, depending on its sugar content. The crop’s critical temperature indicates the temperature at which the frost prevention methods need to be initiated. Table 2 reveals various critical temperatures for freeze damage according to the type of crop.
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