Agriculture Reference
In-Depth Information
T
S
- T
F
= Temperature difference between
the surface and the fluid outside the contact
zone (°C)
q
conv
= Heat transmitted by convection per
unit time (W) from a surface
S
to the
fluid.
(Fig. 5.2). The net transmission is expressed
(Urban, 1997a):
4
4
Q SF Ts ee
=× × − ××
(
)
12 1 12
−
−
1 2 1 2
(5.7)
Q
1-2
= Heat exchanged by radiation from
body 1 that reaches body 2 (W)
S
1
= Body surface (m
2
)
F
1-2
= Energy fraction emitted from body 1
that reaches body 2 (dimensionless)
T
1
and
T
2
= Surface temperatures of bodies
1 and 2 (K)
s
= Stefan-Boltzman constant (5.67 10
−8
W m
−2
K
−4
)
e
= Emissivity of bodies 1 and 2
To calculate the night radiation heat
losses through a greenhouse cover, if we con-
sider the whole greenhouse as one of the bod-
ies (body 1) and the sky as the other (body 2),
the equation would be (Montero
et al
., 1998):
A.4.3
Evaporation and condensation
The heat transmitted when water evapo-
rates or condenses is given by (Montero
et al
., 1998):
h
( )
c
−
(5.5)
Q H
=∆ × ×
SX X
S A
C
P
where:
Q
= Heat transmitted per time unit (W)
DH
= Latent heat of vapourization (J kg
−1
)
h
c
= Heat transmission convection coeffi-
cient (W m
−2
°C
−1
)
C
p
= Dry air specific heat (J kg
−1
°C
−1
)
S
= Evaporation or condensation surface (m
2
)
X
S
= Absolute humidity on the surface
(kg kg
−1
)
X
A
= Absolute humidity in the greenhouse
air (kg kg
−1
)
F es
4
4
(5.8)
−
=× −
(
TT
)
12 1
2 1
where
T
2
is the sky temperature (a supposed
black body), as
e
2
= 1 and
F
1
-
2
= 1.
The sky temperature may be estimated
(
T
2
) (under clear sky conditions and low
ambient humidity):
T
2
= 0.055 ×
T
a
1.5
(5.9)
where
T
a
is the air temperature (K).
A.4.4
Radiation
A.4.5
Air renewal
The radiation emitted by a body depends on
its surface temperature and its nature,
according to the Stefan-Boltzman law
(Monteith and Unsworth, 1990).
F
rad
=
e
×
s
×
T
4
The heat losses by air renewal in a green-
house (
Q
ren
) can be approximately calcu-
lated (Montero
et al
., 1998):
Q
ren
=
m
×
C
p
(
T
i
-
T
e
)
(5.6)
(5.10)
F
rad
= Energy flux (W m
−2
)
e
= Emissivity of the surface (dimen-
sionless)
s
= Stefan-Boltzman constant (5.67 10
−8
W m
−2
K
−4
)
T
= Surface temperature, in Kelvin (K)
At low temperatures the energy emitted
will be low, increasing as the temperature
increases.
It is admitted that the radiation emitted
by a body has the same intensity in all
directions. If two bodies are at different
temperatures, they will exchange radiation
Q
ren
is in watts where:
m
= Air mass renewed (kg s
−1
)
C
p
= Air specific heat (J kg
−1
°C
−1
)
T
i
= Internal air temperature (°C)
T
e
= External air temperature (°C)
The air mass can be calculated knowing
the air exchange rates (volumes of green-
house air renewed per hour), the greenhouse
volume and the air density.
The most difficult thing to establish is
the air exchange rate. In greenhouses that are
closed tight it is less than 1, in greenhouses
