The performance characteristics of the multi-span plastic film greenhouse.
The performance characteristics of the multi-span plastic film greenhouse.
(1) Sawtooth Type Greenhouse
The Sawtooth Type Greenhouse, due to its ventilation openings located at the top ridge, generally has a better natural ventilation effect than the arched-domed greenhouse. With the external shading cooperation, the natural ventilation effect of this type of greenhouse can basically reach a temperature difference of 1 to 3℃ between the indoor and outdoor. However, the sealing effect of the skylight of this type of Sawtooth Type Greenhouse is often poor. In southern regions of China Sawtooth Type Greenhouse where the summer temperature is high and the winter temperature is not very low, its promotion has good economic benefits. But it is not suitable for regions with hot summers and cold winters，Sawtooth Type Greenhouse. When choosing to use the Sawtooth Type Greenhouse, special attention should be paid to the local dominant wind direction, making the ventilation openings of the Sawtooth Type Greenhouse face the downwind direction to form a larger negative pressure ventilation and avoid cold air backflow.
(2) Double-layer heated greenhouse
The double-layer inflatable greenhouse and the traditional plastic film greenhouse have almost no significant differences except for the covering materials. The latter uses a single layer of plastic film, while the former employs a double-layer inflatable membrane. As a result, the insulation performance of the greenhouse has improved by more than 30%, but at the same time, the light transmittance has decreased by about 10%. In northern regions of China where there is abundant sunlight and low winter temperatures, it has good economic benefits. However, when used in areas south of the Yangtze River, due to insufficient sunlight in winter and high temperatures, the energy-saving effect of the double-layer inflatable greenhouse cannot compensate for the losses caused by insufficient light transmittance. Generally, it is not advisable to adopt it.
During the use of double-layer inflated greenhouse, if the air used for inflation comes from the interior, although the inflation temperature is relatively high, since the indoor air often has high temperature accompanied by high humidity, when this air is re-inflated between the two layers of membranes, it will encounter the outer layer membrane which is cooled down and is prone to condensation. The condensation droplets will remain and accumulate between the two layers. Over time, Sawtooth Type Greenhouse，water bubbles will form between the membranes, causing local stress on the inner layer membrane and leading to its damage. To reduce the accumulation of such condensation droplets, it is generally required that the suction port of the inflation fan be installed outdoors, because the relative humidity of outdoor air is lower than that of indoor air.
(3) Double-Arch greenhouse
The purpose of the double-layer structure greenhouse is also to achieve the energy-saving effect of the double-layer inflated arch greenhouse. However, in the structural treatment, a double-layer framework is adopted, which supports the two layers of film separately and eliminates the inflation fan between the two layers of film. Its advantages are that it saves the operating cost of inflation electricity consumption, avoids condensation between the two layers of inflated film, and the roll film ventilation can open or close the two layers of film respectively. According to the changes in outdoor light intensity and temperature, the plastic film can be opened or closed, so that the Arch greenhouse operates in an optimized management in terms of energy saving and light utilization, further reducing the operating energy consumption of the arch greenhouse and saving costs. It is especially suitable for use in regions in southern China with insufficient sunlight.
The double-layer structure arch greenhouse, due to the addition of an extra layer of structure, leads to an increase in the cost of the arch greenhouse and also raises the shading rate of the arch greenhouse frame. From the perspective of operational effect, its energy-saving performance is 5% to 10% higher than that of the double-layer insulated greenhouse. When used in Beijing, combined with indoor insulation curtains, without heating in winter, a temperature difference of over 10℃ between the indoor and outdoor can be achieved. The main reason is that when there is wind outside, the air flow movement between the two layers of the double-layer insulatedarch greenhouse is intense. Moreover, Arch greenhouse the force and pressure acting on the windward side cause the inflatable interlayer to decrease or even disappear, reducing the overall insulation performance of the covering material. However, the double-arch double-film arch greenhouse, since both layers of film can be the outer layer film (when the inner layer film is rolled up, it acts as the outer layer film), in northern regions with high air dust content, the film pollution is relatively large, and the overall light transmittance of the arch greenhouse is relatively low. Therefore, when choosing to use the double-arch double-film structure Arch greenhouse, higher requirements are placed on the selection of plastic films.
In the calculation of snow load for double-arch double-membrane greenhouses, the insulation thermal resistance of the greenhouses should be fully taken into account. Due to the increased thermal resistance of the roof of the Arch greenhouse, the snow on the outer roof will not melt as quickly as the snow on the roof of double-layer inflatable membrane greenhouses. The retention time of snow load will be relatively prolonged. For areas with frequent snowfall, the snow-clearing load of the roof should also be considered, especially the uneven load on the roof. To some extent, this also increases the material consumption of the Arch greenhouse structure.
