Frost Protection - UTL Repository

MECHANISMS OF ENERGY TRANSFER
If the air and surface cool sufficiently, the surface temperature can fall to T d and
water vapour begins to condense as liquid (i.e. dew) or to T i and water vapour
begins to deposit as ice (i.e. frost). This phase change converts latent to sensible
heat at the surface and partially replaces energy losses to net radiation.
Consequently, when dew or frost form on the surface, the additional sensible heat
supplied by conversion from latent heat reduces the rate of temperature drop.
FIGURE 3.11
Corresponding wet-bulb (T w ), frost-bulb (T f ), ice point (T i ) and dew-point (T d )
temperatures as a function of dew-point temperature at an elevation of 250 m above
mean sea level (i.e. air pressure (P b ) = 98 kPa) with an air temperature T a = 0°C.
T E M P E R A T U R E ( ° C )
0.0
-3.0
-6.0
-9.0
T d
-12.0
0.0 -3.0 -6.0 -9.0 -12.0
D E W - P O I N T T E M P E R A T U R E ( ° C )
Ti
T w
Tf
A good measure of the total heat content of the air is the “equivalent” temperature
(T e ), which is the temperature the air would have if all of the latent heat were
converted to sensible heat. The formula to calculate T e (°C) from air temperature
T a (°C), vapour pressure e (kPa) and the psychrometric constant γ (kPa °C -1 ) is:
e
T e = T a + °C Eq. 3.10
γ
Calculated T e values for a range of T a and T i are given in Table 3.1 and for a range
of T a and T d in Table 3.2. Values for T d and T i depend only on the water vapour
content of the air and hence the latent heat content of the air. When T d or T i is high,
then T e is often considerably higher than the air temperature, which implies higher
total heat content (i.e. higher enthalpy). Therefore, when T e is close to T a , the air is
dry, there is less heat in the air and there is more chance of frost damage.
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