Agriculture Reference
In-Depth Information
S
= Total ventilation area of the greenhouse
C
d
= Discharge coefficient of the vent
C
w
= Global wind effect coefficient
U
= External mean wind velocity
is placed on a greenhouse vent
As a first approach, the reduction of the
air flow through a screen can be quanti-
fied as follows (J.I. Montero, personal
communication):
ventilation
Φ
=−
Φ
( )
e
2
e
(8.7)
m
With the same previous simplification (half
of the ventilation area acts as an air inlet
and the other half as an outlet), the volumet-
ric flux of air caused by the thermal effect is
(Muñoz
et al
., 1999):
where:
F = Volumetric air flow through a vent with-
out screen
F
m
= Volumetric air flow through a vent
with screen
e
= Screen porosity (screen holes area per
unit area)
1
2
S
TH
Cg
T
∆
Φ= ×
2
×
(8.5)
T
d
2
4
where:
F
T
= Air volumetric flux caused by the ther-
mal effect
S
= Total ventilation area of the greenhouse
C
d
= Discharge coefficient of the vent
g
= Gravity acceleration
D
T
= External − internal air temperature
difference
T
= External air temperature
H
= Vertical height of the vent
The thermal effect in natural ventila-
tion usually has little relevance in coastal
areas, where the wind effect clearly pre-
dominates, due to the frequent presence of
wind.
A.7
Chapter 9
A.7.1
CO
2
units
The most commonly used units to quan-
tify the air CO
2
content are (Nederhoff,
1995):
Dimension
Unit
Air CO
2
index, in
volume:
1 vpm = 0.0001%
(volume)
Air CO
2
index, in moles:
1 ppm = 1
m
mol mol
−1
Partial pressure
1 Pa = 10 mbar
At normal greenhouse temperatures at
sea level, it is not necessary to make correc-
tions to convert CO
2
units. The equiva-
lences (at 20°C and 101.3 kPa) between
units are:
1 vpm = 1 ppm = 0.101 Pa
The wind loads over the greenhouse struc-
ture are calculated using the equation
(Zabeltitz, 1999):
F
w
=
A
×
C
×
P
w
(8.6)
where:
F
w
= Wind force
A
= Greenhouse area under wind pressure
or suction
C
= Wind pressure coefficient
P
w
= Wind dynamic pressure
The wind dynamic pressure depends
on the effective height of the greenhouse
(Zabeltitz, 1999). See Fig. 8.3.
A.8
Chapter 11
stress index
The crop water stress index (CWSI) is
based on the measurement of the crop can-
opy temperature and allows for knowing
