Agriculture Reference
In-Depth Information
provide large flows at low pressures
(Wacquant, 2000). These fans are built to
work at low rotation velocities, because if
their velocity is high they are quite noisy
and use a lot of electrical power.
Air velocities greater than 1 m s
−1
(which
may affect the plants) must be avoided, so
large-diameter fans must be used. They can
work blowing air in or sucking air out of the
greenhouse, with a pressure differential less
than 30 Pa. The air circulation is usually
horizontal.
The distance between two fans on
the same wall must be less than 8-10 m and
the distance between fans and vents must
be less than 30-40 m, avoiding obstacles in
the direction of air movement at a distance
shorter than 1.5 times the diameter of the fan
(ASAE, 2002). The vents must close auto-
matically, when the fan stops.
The total flow capacity of the fans
must be calculated to ensure 20-30 air
renewals h
−1
, in autumn and spring, and
40-80
8.4.7
Screenhouses
Recently, greenhouses covered with a screen
as cladding material are being used
(Plate 18). They are known as screenhouses.
Inside them, the greenhouse effect does not
occur and the windbreak and shading effects
prevail, besides their restriction to the
entrance of insects (depending on the size
of holes in the screen and the installation
conditions). In fact, they are a variant of the
shade houses (see Chapter 4 section 4.6.4).
In screenhouses, ventilation is perma-
nently ensured. Rainfall water penetrates
inside them, through the screen, which
restricts their use in areas where the average
rainfall is high.
More sophisticated variants of screen-
houses are greenhouses that have inter-
changeable roofs, in which the film cover
can be substituted by a screen cover
(Photo 8.2), and the retractable roof green-
houses (Photo 8.3).
air
renewals
h
−1
in
summer
(Wacquant, 2000).
The system generates certain heteroge-
neity in the temperatures inside the green-
house, and the energy consumption during
the summer is high.
8.5
Mechanical or Forced Ventilation
In order to inject or extract air from the
greenhouse helical fans are used, that
Photo 8.2.
Rolling roof greenhouse (plastic film and screen) that allows the cover material to be selected
depending on the climate conditions. (Source: J.I. Montero.)
