Agriculture Reference
In-Depth Information
vents. The most simple models establish
relationships between the opening angle
and the air exchange rate, depending on the
wind conditions and the temperature differ-
ences. Normally, their use involves previous
adaptation to the local conditions (green-
house characteristics, orientation, etc.).
diameter (Cabrera, F.J., 2008, personal
communication).
The porosity of the screen is the rela-
tionship (per unit or percentage) between
the holes area and the total area. The poros-
ity depends on the diameter of the thread
and the number of threads per unit area and
determines the decrease in ventilation rate
when the screen is placed in the vents (see
Appendix 1 section A.6.4).
When covering a vent with a screen,
the useful ventilation area of the vent is
restricted to the area free of threads (net
hole area of the screen), which must be
taken into account when calculating the
useful vent area, correcting it as a func-
tion of the screen's porosity. In some
cases, screens are used that are larger in
total area than the vent they cover, so the
area free of screen (net hole area of the
screen) equals or exceeds the ventilator
area.
When forced ventilation is used, and
screens are also present, then it may be
necessary to increase the performance of
the fans.
The holes of the screens tend to get
dirty and may be blocked very easily due to
their small size, so a periodical cleaning
must be done to avoid ventilation being lim-
ited even more.
Although in Mediterranean green-
houses a ventilation area (roof plus sidewall
vents) of 15-20% of the greenhouse area
has been described as sufficient for a well-
developed and irrigated crop (Montero and
Antón, 2000b; Pérez-Parra
et al
., 2003b), the
use of anti-insect screens in the vents makes
this value insufficient.
Anti-insect screens placed in the roof
vents decrease the ventilation rate by 20
and 33%, for flap or rolling vents, respec-
tively, for a 39% porosity screen in a
low-cost greenhouse (Pérez-Parra, 2002)
(Plate 15).
Screens decrease the ventilation rate by
around 40% in the case of anti-bug screens,
and 70-80% in the case of anti-thrips
screens (Muñoz, 1998; Montero and Antón,
2000a), although this decrease can be higher
if the wind velocity is very low (see
Appendix 1 section A.6.4).
8.4.6
Anti-insect screens
Anti-insect screens are made of uniform
threads which form the screen. The type
of screen to be used will depend in each
case on the size of the insect to be excluded
(Table 8.1). It may be that some biotypes
of insects have different sizes: for exam-
ple the white fly biotype found in Almeria,
is smaller than the American one, as it
only measures 240 mm (Cabrera, F.J., 2008,
personal communication). The nomencla-
ture of the screens in the 'mesh' scale des-
ignates the number of threads per inch in
each direction (Aldrich and Bartok, 1994).
In this way a 64-mesh screen has 64
threads per inch (2.54 cm) in each perpen-
dicular direction. To evaluate the size of
the hole the diameter of the threads must
be known.
The common nomenclature used in
Spain to designate the screens is imprecise,
as it does not express their exact character-
istics. For instance, a 20 × 10 screen indi-
cates that the screen has 20 threads per
centimetre in one direction and 10 threads
per centimetre in the perpendicular direc-
tion, but this does not specify the thickness
of the thread, which is usually around
0.27 mm, although the most commonly used
types range between 0.23 and 0.29 mm
Table 8.1.
Selection criteria for anti-insect screens
as a function of insect size and hole size in the
screen (Aldrich and Bartok, 1994).
Insect to be
excluded
Insect size
(
m
m)
Hole size
(
m
m)
Leaf miner
640
266 × 818
Melon bug
340
266 × 818
White fly
462
266 × 818
Thrips
192
150 × 150
