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Films suitable for covering greenhouses
With the progress of science, the ways of food production enter a new stage every day, so that by eliminating traditional methods and combining modern technologies with modern human needs, today all kinds of edible plants can be produced, while the possibility of In the past, such a development was confined to an unattainable dream; For example, until a few decades ago, it seemed impossible to produce a product like strawberries in winter…
Abstract
With the progress of science, the ways of food production enter a new stage every day, so that by eliminating traditional methods and combining modern technologies with modern human needs, today all kinds of edible plants can be produced, while the possibility of In the past, such a development was confined to an unattainable dream; For example, until a few decades ago, it seemed impossible to produce a product like strawberries in winter, but today it can be produced easily and in almost any weather conditions using modern greenhouse cultivation systems. Therefore, the latter method can be considered as one of the new and effective integrated methods, which currently has a special place in the matter of food production, increasing the health of society, reducing inflation, business prosperity and various important economic indicators. The most important advantage of greenhouse production is that the desired product can be supplied to the target community in a shorter period of time, with high quality and marketability. For the commercial production and cultivation of high-quality plants throughout the year, favorable environmental conditions must be created along with the control of damaging factors such as wind, destructive storms, cold, frost, etc. provided to grow. Due to the ever-increasing needs of the market, both in terms of producing flowers and ornamental plants and in terms of vegetables and summer herbs out of season, this production method has become one of the most profitable sectors of agriculture today.
Introduction
Not only polyethylene films and other films, but all the coatings used for greenhouses must have all the characteristics of a good coating for the greenhouse and maintain its resistance against environmental factors and other factors, and take care of the cultivated products. act
Films suitable for covering greenhouses
This is a serious and important question for farmers and greenhouse owners who do not have to make a wrong choice between their needs and their low costs and proper performance, and they calculate the cost of buying film and plan to protect their crops. . These films are placed in natural environments throughout the year and in one area in front of the wind and in another point in several months of the year the weather is below the freezing point and in another area it is constantly bombarded by solar UV rays. Choosing a suitable film is not a simple process and all factors must be considered to be insurance for your products.
Resistance to UV rays in winter or through several years of work and service causes general damage to the film, and in general, films can be divided into different grades 1, 2, 3 and according to the geographical location where it is used. They can be used for an approximate period of time, and where there is a lot of sunlight, a film that lasts for 3 years can be used for 2 years, and where the sunlight shines at an average level, films for a duration of 3 years can be used. The time of 2 years can be effective in 3 years and in many other cases, using greenhouse films for the summer months is not a good choice if you do not have cooling equipment and it includes the costs of installing this equipment. The main thing to remember is that greenhouse films designed for this purpose are not suitable for general or construction use and are not recommended for general and unpredictable reasons.
Using films without UV stabilizer puts the products at risk. Another feature of multi-layer greenhouse film is that they are made of polyethylene copolymer materials, and copolymers are stronger and more flexible than polyethylene, and below we mention its important advantages:
• Resistant to wind, snow and hail
• Greater strength against folding and better resistance against separation or fragmentation
• Better durability and more flexible at lower temperatures
The use of double-walled pillow films with a thickness of 180-220 microns are recommended for greenhouses, while films of 150 microns with less thickness are not suitable and endanger the crops. The strength of the films is very dependent on the thickness, especially in its opening (film width), which should be more than 20 feet, and a film with a thickness of 180 microns is more reliable. And some manufacturers suggest that the outer layer is 180 microns thick and the inner layer is 150 microns thick to save costs. Firstly, all aluminum systems and clips are not designed for this size, and reducing the thickness to less than 270 microns may prevent more air from entering the pillows, and secondly, films with a thickness of 180 microns are naturally stronger and the resistance in They all have tough conditions.
A common misconception about greenhouse films is that a film with a thickness of 180 microns is half as resistant as a film with a thickness of 150 microns, and a film with a thickness of 300 microns is twice as resistant as a film with a thickness of 150 microns. and this is while films with a thickness of 180 microns may have twice the resistance to UV rays than films with a thickness of 150 microns. Films with high durability and resistance depend on its stabilizing and stabilizing materials and the thickness and strength of the film. Single-layer films are not suitable especially in favorable weather conditions, and strong single-layer films should be used. For example, the contraction that exists in the temperature difference during the day and night may cause the film to break or fragment, and in these types of films, there is no support layer for protection. On the other hand, double-walled pillow films that are filled with wind, with a thickness of 180-220 microns, are extremely suitable and strong for greenhouses, and they are more durable in strong winds and under snow loads, and this is due to their good and strong structure. In this structure, the air inside them acts like a pillow and is a membrane to transfer stress from one point to many points.
In the setting of these double-walled pillow films, tension is not only extended in one area but in the entire film, and the tension is not felt only in one layer. This ability to transfer tension around increases the ability of double-walled greenhouse films. It requires a strong structure.
Place the movie
While most of the manufacturers know the rules of installation in such a way that the connection of the first layer is at the corners and at appropriate intervals, and then the second layer is placed in its place so that both layers are mounted on each other. Closing the layers It usually ends in the door and is done along or on both sides.
When installing the roll of films, the roll is opened (gusseted version) so that it is folded at the bottom and can be closed along the high point, or it is closed along a gutter and then stretched and folded on the opposite side. and close from the bottom
The film should be stretched enough to prevent any creases and tightly closed and allowed to fully inflate and stretch. If the film is not strong, it may loosen on a hot day and the cold weather will cause the film to tear and crumble.
All about pillowcase films
A type of blower (squirrel cage blower) turns double-walled films into a bag full of air (pillow-like). And the general rule is that for every 1,000 square feet of cushioned area a 1.30 horsepower blower is required, and this can be attached to more than one pillow, and the pillows can also be attached to each other, and the ultimate measure of air pressure. Is.
To direct the air in two directions, T metal sheets are used as ducts for furnace materials. A flexible pipe with a diameter of 4 inches is used in one or two directions. And it is possible to attach the 4-inch tubes to the pillows in different ways, however, using methods that do not directly introduce air flow to the upper layer of the film works best. Constant blowing of air in the upper layer can cause premature destruction at that point.
A + shaped cut in the film where we need two 4 inch long slits to connect the air duct to the lower layers of the film. 4 The tab created can surround the air duct and close it with an adhesive tape. Clear plastic tape is desirable. An alternative method is to drill a round hole in the plastic that is slightly smaller than the outside diameter of the pipe and screw the pipe in place with adhesive tape.
Because the use of outside air, especially in northern places with minimal humidity and as a result, reduces the density between the layers of the pillow, it is desirable and used. Therefore, the blower fan must be mounted on the top of the wall. Remember to stabilize the air inside after the pillow is inflated and the fan will only maintain the pressure. In cold conditions, the outer layers of the film are removed close to the air temperature.
The air that is directed between the layers is warm and it will be hotter in the inner layer and will remain at approximately the same temperature inside the layer. If the air pipe inside the pillow is not fitted with a gutter for its flow, the risk of snow falling on it will cause it to close and shut down.
Since the air between the layers is constant, it is considered as a constant air space (air in a constant space) and such air acts as insulation and has a high value. And it is for good reason that the air pressure is controlled by closing the area from the blower and making a hole in the film. And finally, it is very important to measure the air pressure inside the pillow using a pressure gauge. A pressure gauge is a simple device for measuring air pressure that can be purchased, or you can use a piece of wood or a piece of plastic, a 1/4 tube and a ruler to mark the ruling marks.
Pressures above 0.3 are not desirable because the film is flexible and they can overinflate. And the main problem is that the stretching of the film makes it thinner and weaker, and pressures below 0.2 put enough stress on its functions under average stress on the film, and here it is useful to draw a diagram. Hand-made pressure gauges are fine, but not suitable for growers with large greenhouses. Regular measurement and adjustment of air pressure can be a good way to prolong the life of the film.
Fighting the elements
Natural enemies for a greenhouse film include UV rays, heat, wind, extreme cold, snow, and hail, and engineers have come a long way to combat these enemies, however, some practical ways to Growers are there to extend the life of the film. Greenhouse owners with experience know that the first thing that causes the film to fail and deteriorate is to be placed in a rotating cycle of heat and cold, which weakens the strength of the film, and this is normal. Film layers must withstand temperature differences of up to 100 degrees Fahrenheit during the day because they absorb heat during the day and emit it at night. When they are in such bad conditions and fixed amount, they should have more power.
One way to reduce this thermal stress is to put a layer of white reflective material between the columns and the film. There, the high temperatures during the day are significantly reduced (because the energy is reflected and not absorbed) and there are two practical ways to reduce the temperature: use white color or white tape. The white color in the layer of the horizontal part on the columns, etc., which is in contact with the film, is to reduce degradation. And make sure that if they use solvent-based paints, they allow the paint to dry completely. Latex paint not only dries faster, but also does not contain organic solvents that can chemically damage the film. White plastic tape (some with foam backing) is more expensive, but generally easier and less expensive overall. And another advantage is that it is smooth, which does not cause wear between the layers of the film.
Another enemy for greenhouse owners is snow. When the snow accumulates on the outer layer, it easily slides over it, and this is the best reaction to avoid snow on the outer layer. For this reaction, we can reduce some of the air in the pillows and since we put both layers under pressure and this pressure raises the internal temperature and as a result the temperature of the greenhouse increases and causes the snow to melt. and this is the optimal use of energy, and we prefer this work to the deterioration and failure of the film and the collapse of the greenhouse.
Some chemicals used in greenhouses are likely to attack HALS (hindered amine light stabilizers) used in clarifiers and UV inhibitors that are commonly provided in industry.