Frost Protection - UTL Repository

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F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S
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FIGURE 3.10
Saturation vapour pressure over water (upper curve) and over ice (lower curve)
versus temperature
V A P O R P R E S S U R E ( k P a )
0.70
0.60
0.50
0.40
0.30
water
ice
e d
e w
e f
Td
ei
Ti
0.20
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
T E M P E R A T U R E ( ° C )
Figure 3.11 shows the corresponding air, wet-bulb, frost-bulb, ice point and
dew-point temperatures at sea level for a range of dew-point temperature with
an air temperature T a = 0 °C. If the dew-point is T d = -6 °C at T a = 0°C,
both the wet-bulb and frost-bulb temperature are near -2 °C. In fact, there is
little difference between the wet bulb and frost bulb temperatures for a given
dew-point temperature in the range of temperature important for frost
protection. However, the ice point and dew-point temperatures deviate as the
water vapour content of the air (i.e. the dew-point) decreases. Because there is
little difference between the wet-bulb and frost-bulb temperature, there is little
need to differentiate between the two parameters. Therefore, only the wet bulb
temperature will be used in further discussions.
The total heat content of the air is important for frost protection because
damage is less likely when the air has higher total heat content. During a frost
night, the temperature falls as sensible heat content of the air decreases. Sensible
heat content (and temperature) decreases within the volume of air from the soil
surface to the top of the inversion because the sum of (1) sensible heat transfer
downward from the air aloft, (2) soil heat flux upward to the soil surface and
(3) transfer of heat stored within the vegetation to the plant surfaces is insufficient
to replace the sensible heat content losses resulting from net radiation energy losses.
Tw
Tf
ea
T
e
52