Frost Protection - UTL Repository

ACTIVE PROTECTION METHODS
funnels that retain heat. Also, when fires are too big or hot, the warmed air can
break through the top of the inversion, there is less circulation in the inversion
layer and the heaters are less efficient at warming the air (Figure 7.3).
Because heaters warm the air, the air inside a protected crop is generally rising
and cold air outside is being drawn in from the edges to replace the lifted air.
Consequently, more frost damage occurs and hence more heaters are needed on
the borders. Kepner (1951) reported on the importance of inversion strength
and placing more heaters on borders. He studied a 6.0 ha citrus orchard that was
warmed with 112 chimney heaters burning 2.8 litre h -1 with an average
consumption of 315 l ha -1 h -l . The unprotected minimum air temperature was
1.7 °C, but the results are similar to what one expects on a radiation frost night.
The orchard was square and the easterly wind varied from 0.7 m s -1 to 0.9 m s -1
(2.5 km h -1 to 3.2 km h -1 ). Figure 7.4 shows how the temperature varied in a
transect across the centre of the orchard. The wind direction was from the left.
The upper graph (A) shows the effects of heater operation on temperature during
two nights with differing inversion strength. The lower graph (B) shows the
benefits from using twice the number of heaters on the upwind border.
FIGURE 7.3
Diagram of a frost night temperature profile and the influence of heater output
on heat distribution and loss from an orchard
100
lapse condition
H E I G H T ( m )
75
50
ceiling
25
inversion condition
0
-4 -2 0 2 4 6 8
T E M P E R A T U R E ( ° C)
147