Agriculture Reference
In-Depth Information
CFD simulations have also demonstrated
that the greenhouse roof slope has a signifi-
cant effect on ventilation rate; therefore, in
the south of Spain, traditional horizontal
roof greenhouses are being replaced with
symmetrical or asymmetrical greenhouses
with a near 30° roof angle (Castilla and
Montero, 2008). No further increase in ven-
tilation has been identified for roofs with
roof angles greater than 30° (Baeza, 2007).
The combination of side and roof vents
is more efficient than the use of a single
type of vents, of equal opening areas
(Montero and Antón, 2000a). The roof vents
located by the ridge are more efficient than
those located by the gutter (Muñoz
et al
.,
1999; Montero and Antón, 2000a, b). The
hinged vents (with a flap) oriented wind-
ward are more efficient than the leeward
vents (on the side protected from the wind)
(Plate 15), improving the air exchange rate
from 35 to 60%, when the wind ranges from
2 to 7 m s
−1
(Pérez-Parra, 2002), whereas in
rolling vents the air exchange rates do not
depend on the wind direction, at least in
low slope multi-span greenhouses.
In each case, specific locations of the
vents are used. For instance, if there are
regular winds (land or sea breezes, in coastal
areas) the greenhouse may be oriented with
its openings facing windward and leeward,
such is the case of the Mediterranean coastal
area greenhouses (Wacquant, 2000).
In tropical regions, the tunnels are ven-
tilated best by orienting them in the direc-
tion of the trade winds with the front walls
open, due to their small length.
In the case when sidewall vents are
available, wind flows that are too cold or
too dry which impinge directly on the plants
must be avoided, by avoiding too large an
opening.
For protection from insect damage, and
mostly, from the virus diseases transmitted
by them, the air can be filtered at the openings
by means of low porosity screens to avoid
their entrance. These screens reduce the air
flowing through the vent, notably decreasing
ventilation (Plate 15), so the opening area
ratio must be increased or a device with a
higher screen surface than that of the vent
itself must be adopted (see section 8.4.6).
50
Roof and sidewall
40
30
Sidewall
20
10
0
10 20
Opening area ratio (%)
(Vent area/Greenhouse soil area)
30
40
Fig. 8.8.
In a tunnel greenhouse, with weak wind,
when sidewall and roof vents are used, the increase
in the vent opening area (beyond a certain threshold
value) does not increase the air exchange. The
sidewall ventilation alone is less efficient than the
combination of roof and sidewall vents. In a multi-
tunnel greenhouse the opening area ratio must be
higher than in a simple tunnel with roof and side
ventilation, to achieve the same air exchange effects.
span facilitate alternate use depending on
the wind direction (Wacquant, 2000):
1.
With zero or weak wind - open vents of
both sides of each span.
2.
With moderate wind - preferably open
the vent protected from the wind (leeward),
in a first phase to profit from the suction
effect. The opening of the vent facing the
wind (windward) is delayed until the venti-
lation requirements are higher.
3.
With strong wind - the vent facing the
wind opens even less, or even closes if
the wind is very strong. In the case of extreme
winds, both sides close, to avoid breaks.
Computational Fluid Dynamics (CFD;
see section 8.4.5) analysis indicates that
windward ventilation is more efficient than
leeward ventilation, especially in green-
houses with limited total width. Therefore
new greenhouse constructions should have
larger openings oriented towards the pre-
vailing winds. In existing designs, outside
air may enter and leave the greenhouse
without mixing with the internal air. To
avoid this problem, the use of deflectors to
conduct the entering air through the crop
area is strongly recommended (Baeza, 2007).
