Frost Protection - UTL Repository

MECHANISMS OF ENERGY TRANSFER
Water vapour flux density (E) is the flux of water molecules per unit time
per unit area (i.e. kg s -1 m -2 ). When multiplied by the latent heat of
vaporization (L ≈ 2.501 × 10 6 J kg -1 at 0 °C), the water vapour flux density
is expressed in energy units (i.e. W m -2 ). Evaporation is important for all frost
protection methods involving the use of water. The ratio of the latent heat of
vaporization to the latent heat of fusion is 7.5, so considerably more water
must be frozen than is vaporized to have a net gain of energy when using
sprinklers for frost protection.
It is common for fruit growers to experience problems with spots of damage
on the skin of fruit. While this may not damage the fruit to the point where it is
completely lost, the spot damage reduces the value of fruit for table
consumption. This problem is probably due to water droplets being on the fruit
before going into a night with subzero air temperature. For example, if a light
rain, fog or irrigation occurs during the day so that the fruit is covered by spots
of water, this water will evaporate during the night and the fruit flesh near water
droplets can cool as low as the wet-bulb or frost-bulb temperature, which is
lower than the air temperature. As a result, damage can occur where there were
water droplets on the fruit. If the dew-point temperature is low, damage can
occur to sensitive crops, even if the air temperature remains above 0 °C.
Additional resources on energy balance
Readers who want more rigorous and detailed information on energy balance as
it relates to frost protection are referred to Rossi et al. (2002), Barfield and
Gerber (1979) and Kalma et al. (1992).
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