PLASTIC - FILM - GREENHOUSES
DISADVANTAGES - DEMANDS - TYPES
Christian von Zabeltitz
Institut für Technik in Gartenbau und Landwirtschaft
Universität Hannover
Hannover
Germany, F.R.
1 . Introduction
If we speak and think about the development of plastic-film-greenhouses we have to consider:
- the construction,
- the film,
- the fastening of the film,
- the stretching of the film on the construction
- the ventilation as part of the construction.
Existing plastic-film-greenhouses have some disadvantages. But especially for vegetable growers glassgreenhouses become more and more too
expensive and uneconomic.
For Mediterranean areas and also for middle Europe we have to search
for greenhouses, which are cheap enough and suitable for part-time production during the winter in the south and during the summer in the
north. These can be plastic-film-greenhouses. But we have to overcome
the existing disadvantages.
In this paper the disadvantages will be pointed out and some demands
for the future development of plastic-film-greenhouses will be derived.
Then some construction types, conventional examples and trend of development for some countries will be shown. In this paper only the construction and not the other above mentioned parts will be considered .
2. Disadvantages of existing plastic-film-greenhouses
The following disadvantages can be mentioned.
- Labor expense for erection and changing the film.
- Decreasing film firmness by global radiation and friction against
structure .
- Fluttering film on the construction. Susceptible to storm.
- Drop fall and decrease of light transmissivity by condensation water.
- Inadequate ventilation in multispan greenhouses. Consequently insufficient qualities.
3. Demands for plastic-film-greenhouses
- Low costs for building up and maintenance.
- Simple film change with low labor costs. Deconnectible fastening devices integrated in the construction.
- Avoidance of film destruction on the structure by fluttering in the
wind. Use of simple stretch devices. Film must be stretched tightly.
- Avoidance of contact between film and parts of the structure, which
are heated by solar radiation.
Acta Horticulturae 154, 1984
Plastics in Mediterranean countries
305
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Possibility for covering alternative with film or rigid plastic.
Effective ventilation.
Long-term durability of the film.
Increased light transmissivity of the film for covering with double
layer film.
- Avoidance of drop fall of condensation water by choice of roof slope,
film treatment or new anti-fog-films.
- Possibility for easy removal or for scraping together the film during
periods of no production.
- High sidewalls and few construction elements inside the house for application of machines.
4. Greenhouse constructions
Now for some countries existing conventional types of plastic-filmgreenhouse constructions and trends of development for the construction
will be shown. The most used shapes of film greenhouses are saddle roof
a;-shed-roof b; round arch c; round arch with vertical side-wall d;
pointed arch with sloping side-walls e and pointed arch with vertical
side-walls (figure 1). Curved roofs are more qualified for stretching
the film than straight roofs.
In the Almeria area in the south of Spain most of the greenhouses
are cheap constructions. Figure 2 shows one type consisting of wooden
posts. The tops of the posts are connected by wires. On these wires are
two nets, one wire net 25 x 25 cm inside and a plastic rope net 40 x 40
cm outside. Between the nets lies the plastic film. This construction
is used because the average windspeed is relatively high.
The growers in south Portugal use cheap wooden constructions, shown
in figure 3- They are built from untreatened wood sticks. On the roof
the distance between the sticks is about 50 cm and the film lies alternately above and below the sticks. New developed constructions have
round treatened beams form Eukalyptus trees for the trusses and also
round beams at the roof, figure 4. For this greenhouse two nets of wire
inside and nylon outside will be used for holding the plastic film. The
nets do not avoid the fluttering of the film by wind and give more contact between film and construction .
Most of the greenhouses in Portugal are single-span buildings for
the purpose of better ventilation. The distance between the greenhouses
is 1,5 to 2 m or 0,5 to 1,0 m. In the second case, having a small distance between two greenhouses, a plastic-film is layed out on the
ground as a drain for collecting the rain water.
In Sicily they mostly use also saddle-roof constructions with rectangular concrete or round wooden trusses, figure 5- The roof construction is made of wood and very often they have double film nailed with
ledges inside and outside against the roof construction. The double
layer film is used for less heat loss at night.
From Italy comes the pointed arched steel-pipe construction, figure
6, with a simple film stretch device at the gutter. This is a simple
and good development for stretching the film over the roof.
In the Timbaki area of Crete they have combined steel-pipe and wood
constructions. The purlins are wooden beams and the bars steel-pipes,
figure 7.
Cyprus has now about 130 ha greenhouses. Most of the greenhouses in
Cyprus are in Cyprus are in the south-east Paralimni area and in the
south-west Paphos area. In the Paphos area predominates a low wooden
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structure, figure 8. But wood becomes more expensive and so growers
more and more turn over to metal structures. In the Paralimni area they
have two similar types without and with gutter, shown in figure 9. The
trusses are forced about 75 cm into the soil without foundation. A
stone under the truss has to prevent from sinking. Considering the construction without gutter, the film will be nailed on a small wooden
beam fixed on the top of the trusses. The other construction has a specially formed gutter, on which the film will be fixed in a round profile by steel-pipes. The round arched roof structure is prefabricated
from steel-pipes and has a length of 6,1 m. This roof construction will
be fastened at the gutter by screws.
In the Agricultural Research Institute in Nicosia they have developed another steel-pipe construction, figure 10, for single layer film
as well as for inflated double layer film. A new development from a manufacturer in Cyprus shows figure 11. The truss and the round arched
roof are made of one pipe. So the trusses under the gutter consist of
two parallel pipes. The side-walls are sloping with rolling ventilation
The film will be rolled around pipes and fastened at side-wall and gutter as shown in figure 11. The gutter is specially formed for fastening
the pipes.
The tent-type construction comes from Israel, figure 12. The crossing steel-pipe bars can be boosted for stretching the film.
A German manufacturer has developed a pointed arched type made out
of Aluminium pipes, figure 13- In the Institute for Horticultural Engineering Hannover we have developed the construction, shown in figure 14
The film is fastened at the gutter and is stretched by an inflated closed plastic-tube . So the film has no contact with the structure in the
roof area. The transparent plastic-tube has an inside pressure of about
35 mm water (3,5 mbar).
The construction consists of rectangular trusses with a gutter on
top. The tops of the trusses are connected by steel rods (12 mm diameter) and also sloped steel rods (16 mm diameter) are stretched at the
side-walls. Therefore in 'the trusses only pressure forces appear and in
the other construction parts tension forces only. This has advantages
for the construction itself and also for the built-up-time.
Discussion
Une séance spéciale a été réservée à la discussion de l'exposé de
M. von Zabeltitz. Après un rappel de son exposé, un document "Plastic
Film Greenhouses,,, reprenant les divers types de serres réalisés dans
les Pays à climat méditerranéen, a été très largement discuté sous la
direction de l'auteur.
Le but de cette discussion était de recueillir les idées des participants au Symposium sur les serres existantes, de tenter de préciser
leurs avantages et inconvénients et de voir dans quelle mesure il
serait possible de concevoir un (ou plusieurs) nouveau(x) type(s) de
serres "plastiques,,, plus performant (s) .
Il est malaisé et d'ailleurs prématuré de reprendre ici toutes les
idées qui ont été avancées au cours de cette discussion fort animée;
cet échange de vues s'est avéré très fructueux, mais il exige une certaine période de réflexion.
Il sera repris au cours de la "Working Party,, que le Professeur
von Zabeltitz organise, sous les auspices de la S.I.S.H., à Karlsruhe
début octobre 1984.
307
a
HolzBOMO
Stahlrohr20
s
Holz 100 'oder
Stahlrohr
SO0
Wood or steel
Pig. 7: Steel pipe construction from Crete
310
pipe
5m
P i g . 8: Wood
structure from Cyprus
prefabricated
roofele ments 6,1m Iona
steel
with
gutter
Pig. 9: Steel pipe construction Cyprus
311
steei
60
Fig. 10: Pointed arched
film
pipe
0
steel pipe construction Cyprus
fastening
"^¿Jf
V
film
&Jfastening
0,75 m
Fig.11: New developed
312
steel pipe construction Cyprus
Seil;
rope
Fig. 12: Tent type construction from Israel
Aluminiumrohr
—\
6j;8;10m
Fig. 1 3 : Aluminum construction from Germany
313
70'
314