Frost Protection - UTL Repository

MECHANISMS OF ENERGY TRANSFER
By substituting the air (T a ), wet-bulb (T w ) or dew-point (T d ) temperature for T
in Equation 3.3, one obtains the saturation vapour pressure at the air (e a ), wetbulb
(e w ) or dew-point (e d ) temperature, respectively.
If the water surface is frozen, Tetens (1930) presented an equation for
saturation vapour pressure (e s ) over a flat surface of ice at subzero temperature
(T) in °C as:
e s
⎛ 21.875T ⎞
= 0.6108 exp ⎜ ⎟ kPa Eq. 3.4
⎝ T + 265 . 5 ⎠
where e s is the saturation vapour pressure (kPa) at subzero air temperature T (°C).
By substituting the frost-bulb (T f ) or ice point (T i ) temperature for T in Equation
3.4, one obtains the saturation vapour pressure at the frost-bulb (e f ) or at the ice
point (e i ) temperature, respectively.
The latent heat content of air increases with the absolute humidity (or density
of water vapour) in kg m -3 . However, rather than using absolute humidity,
humidity is often expressed in terms of the vapour pressure. Vapour pressure is
commonly determined using a psychrometer (Figure 3.9) to measure wet-bulb
(T w ) and dry-bulb (T a ) temperatures. The dry-bulb temperature is the air
temperature measured with a thermometer that is ventilated at the same wind
velocity as the wet-bulb thermometer for measuring the wet-bulb temperature.
An equation to estimate the vapour pressure from T w and T a is:
where
e w
( − )
e = − γ kPa Eq. 3.5
T a
T w
γ
= 0.000660 (1 + 0.00115T w )P b kPa °C -1 Eq. 3.6
is the psychrometric constant (kPa °C -1 ) adjusted for the wet-bulb temperature
(T w ), the saturation vapour pressure at the wet-bulb temperature (e w ) is
calculated by substituting T w for T in Equation 3.3, and P b (kPa) is the
barometric pressure, where all temperatures are in °C (Fritschen and Gay, 1979).
Alternatively, one can find the value for e w corresponding to the wet-bulb
temperature in Tables A3.1 and A3.2 (see Appendix 3 of Volume I).
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