Operator`s manual

59
Manual
3.3 Right-Hand Terms of the Priestley-Taylor Equation
gradient of the saturated vapor pressure
The saturated vapor pressure is a function of the air temperature. It is expressed by the following equation:
e
sat
= 0.611 exp( 17.3 T
a
/ (T
a
+ 237.3))
in which:
e
sat
= saturated vapor pressure [kPa]
T
a
= air temperature [°C]
The Priesley-Taylor approach uses the gradient of the saturated vapor pressure versus temperature curve, which
is represented by , and can be expressed with the below formula:
= 4098 x e
s
/ (237.3 + T
a
)
2
in which:
= gradient of the saturated vapor pressure [kPa °C-
1
]
T
a
= air temperature [°C]
e
s
= saturated vapor pressure [kPa]
BOX 2: Calculation of D for air temperature of 20 °C
, the gradient of the saturated vapor pressure, is a function of the temperature T, as follows:
e
s
= 0.611 * exp( 17.3 T
a
/ (237.3 + T
a
)
e
s
= 2.34 [kPa]
Insert this value into the equation for the gradient of the saturated vapor pressure, as shown below:
= 4098 x e
s
/ (237.3 + T
a
)
2
= 4098 x 2.34 / (237.3 + 20)
2
= 0.145 [kPa °C-
1
)
Psychrometric constant y
The psychrometric constant y is a function of air temperature and the barometric pressure. It is expressed by the
following formula:
y = c
a
x P / 6.62 x
v
in which:
c
a
= heat capacity of air = 0.24 X 4.1868 [J g-
1
°C
-1
]
P = barometric pressure [kPa]
v
= is Latent Heat of Vaporization [J g
-1
]
Annexes