Agriculture Reference
In-Depth Information
presence of water over the leaves favours
the development of many fungal diseases
(e.g.
Botrytis
). Besides, the formation of con-
densation droplets on the internal surface of
the plastic films cladding the greenhouse
involves important reductions in light trans-
mission, and eventually dripping over the
crops with harmful effects on their health
(Photo 5.2).
inside, it decreases their global heat transfer
coefficient (section 5.4).
The 'anti-dripping' plastic films do not
avoid condensation, but avoid the forma-
tion of droplets. A proper roof slope and a
corresponding system of collection chan-
nels at the greenhouse gutter allows for the
effective removal of the condensation water
(see Chapter 4).
Effects of condensation on transmissivity
and the thermal balance
5.1.3
Radiation
In plastic greenhouses, when the water
condenses in the form of water drops,
there is a decrease in the radiation trans-
missivity, as the droplets reflect part of
the radiation. However, if the condensa-
tion occurs in a continuous film, not only
is there no reduction in transmissivity,
but in the case of special plastic films
treated with special surfactants there
can even be an increase (Fahnrich
et al
.,
1989).
Moreover, water condensation on the
plastic cover improves the thermal balance
of plastic greenhouses, and by avoiding
long-wave radiation escaping from the
All bodies with temperatures above − 273°C
(0 K) emit energy through their surface,
in the form of electromagnetic radiation
following the Stefan-Bolzman law (see
Appendix 1 section A.4.4). This radiant
energy is transformed into thermal energy
if it finds a body to absorb it.
The characteristics of the radiation
(wavelength) which a warm surface
emits depend on its temperature, follow-
ing Wien's law (see Appendix 1, section
A.1.4). The amount of energy of a certain
radiation decreases as the wavelength
increases.
Photo 5.2.
Condensation of water vapour on the internal face of the greenhouse cover is of great
importance (see text).
