Agriculture Reference
In-Depth Information
Nowadays, ventilation can be auto-
mated. It is necessary to monitor condi-
tions with an anemometer, vane and rain
detector to close the vents in case of exces-
sive winds (depending on their direction)
or in case of rain (For more details, see
Appendix 1).
In median latitudes, the vents are usu-
ally continuous, along the greenhouse,
preferably on the ridge, with a recom-
mended opening area index of 15-25%
(ASAE, 1988) under high radiation condi-
tions. The most efficient and versatile ven-
tilation systems have vents on both sides of
the ridge and in the sidewalls. When the
vents have screens these open area ratios
must be increased.
To maximize ventilation it is essential
that the arrangement of the vents comple-
ments the convective movements inside the
greenhouse with the pressure differences in
the walls generated by the wind.
The sidewall ventilation is very impor-
tant in small greenhouses, contributing
equally or even more than the roof vents to
the air exchange, but in wide greenhouses
(width over 35 m) roof ventilation predomi-
nates (Pérez-Parra
et al
., 2003b).
In single- or double-span tunnels, the
most efficient ventilation is achieved com-
bining ridge vents with sidewall vents (in
the proportion 1.5 to 1.0). The chimney
effect, using sidewall and roof openings, is
of special interest if the wind is less than
1 m s
−1
, multiplying by three the efficiency
of a single opening (Fig. 8.8).With medium
or strong wind, the roof ventilation is
sufficient.
In multi-span greenhouses of large area
the roof vents located in both sides of each
8.4.3
Characteristics of the openings
The openings can be characterized by the
opening area and by their positions. The
opening area, in the case of hinged vents, is
at most that of the frame (hole) (Fig. 8.7).
In the case of a long vent of continuous
shutter, the maximum opening is achieved
with an angle of 60° (Wacquent, 2000).
The opening area index, which relates
the total ventilator opening area to the
ground area of the greenhouse, expressed
as a percentage allows for a comparison
between different greenhouses.
The air exchange rate increases with
the opening area ratio. There is an opti-
mum value for this ratio, above which a
complementary opening is less efficient
(Fig. 8.8 and Plate 14). This optimum
opening area ratio, in unscreened vents,
ranges between 15 and 20% for tunnels
(with well positioned vents) and between
25 and 33% for multi-span greenhouses
(Wacquant, 2000).
B
F
C
A
E
a
D
Fig. 8.7.
The useful ventilation area is, at its maximum, the frame of the vent (ABCD in the figure). A small
opening angle (
a
) limits the useful ventilation area. In the figure, the useful area is that formed by the
rectangle eFCD plus the triangles AeD and BFC, as long as the area of the frame of the vent is not
exceeded (ABCD) (adapted from Wacquant, 2000).
