Agriculture Reference
In-Depth Information
8.11
Summary
•
In greenhouses with a good ventilator
area when the wind velocity is low
(less than 2 m s
−1
) natural ventilation
depends, primarily, on the buoyancy
effect. With higher wind velocities, the
wind effect is more important.
The opening area ratio is the propor-
•
In order to avoid excessive tempera-
tures inside the greenhouse the energy
inputs must be decreased and the heat
losses maximized. The reduction of
natural inputs is achieved by limiting
solar radiation and by means of shad-
ing. The increase in energy losses, as a
first step, is achieved with ventilation,
natural or forced. If the interior temper-
ature must be further decreased, active
cooling methods, most commonly by
evaporating water, will have to be
used.
Greenhouse
•
tion of the total ventilator area of a
greenhouse with respect to the area
covered by soil, expressed as a percent-
age. The recommended values for the
opening area ratio in high radiation
conditions range from 15 to 25%.
The use of anti-insect screens in the
•
greenhouse vents to avoid or limit the
entrance of insects, notably reduces
ventilation, thus the opening area ratio
must be increased where screens are
used.
In small greenhouses the combination
•
natural
ventilation
(exchanging interior air for exterior
air) allows for the evacuation of the
excess heat and decrease of the tem-
perature, modifying the humidity and
the gas content of the greenhouse
atmosphere.
The air exchange rate (
•
of sidewall and roof vents is more effi-
cient for natural ventilation than the
use of only roof vents.
Mechanical or forced ventilation, blow-
•
R
) of a green-
house is the ratio between the volume
of air exchanged per hour and the total
volume of the greenhouse.
A greenhouse is not an airtight struc-
•
ing air into or extracting air from the
greenhouse, allow for high ventilation
rates to be achieved.
Cooling by water evaporation may be
•
ture, having leaks of air whose value
depends on the construction quality of
the greenhouse and on the exterior
wind speed.
Natural ventilation of the greenhouse
•
done by means of 'pad and fan' systems
(injection of external air cooled as it
passes through wet pads) or by means
of water fogging or misting inside the
greenhouse,
•
takes place through permanent or
temporary openings (normally vents)
in the roof, the sidewalls or front
walls. The efficiency of ventilation
depends, in the first place, on the
exterior wind intensity and direction
(the wind effect) and the temperature
differences between the interior and
exterior air (the thermal or buoyancy
effect). These two effects generate
pressure
renewing
the
interior
humid air in both systems.
The fog-mist systems can be of different
•
types: low or high water pressure, and
air/water systems.
Water transpiration through the plants
•
is the simplest method to evaporate
water in a crop, but it involves a non-
restrictive water supply and efficient
ventilation.
Shading allows for a permanent high
differences
which
move
•
the air.
The characteristics of the openings
temperature decrease but it involves
limiting the PAR, which decreases
photosynthesis and yield. Therefore, it
is advisable to use mobile screens, to
deploy them only when they are
needed.
Shading devices can be internal or
•
(vents) and the vegetation (height and
plant
arrangement)
also
influence
ventilation.
The efficiency of the roof ventilation
•
depends on the greenhouse height. Due
to the chimney effect, tall greenhouses
ventilate better.
•
external to the greenhouse, and perma-
nent
(fixed)
or
mobile
(movable).
