In most areas of our country, the climate is hot in summer, and the outdoor temperature is above 30°C. Due to the greenhouse effect, the temperature inside the greenhouse can easily exceed 40°C. Therefore, cooling measures must be taken to ensure that the greenhouse can carry out Normal production.
Currently, the commonly used cooling methods in greenhouses include: ventilation cooling, shading cooling, wet curtain-fan cooling, mist cooling, roof spray cooling, roof spraying, etc. Among them, ventilation cooling can be divided into natural ventilation cooling and forced ventilation cooling, and shading cooling can be divided into internal shading cooling and external shading cooling.
1. Ventilation and cooling In the design and use of greenhouses, ventilation and cooling are always inseparable. Generally speaking, greenhouse ventilation has three main purposes: 1. To remove waste heat in the greenhouse and reduce the temperature; 2. To remove excess water and reduce humidity; 3. To adjust the gas composition in the indoor air, eliminate harmful gases, and increase the CO2 content in the air. In summer, the main purpose of ventilation is to cool down the room, requiring sufficient ventilation; while in winter, the main purpose of ventilation is to adjust the composition of indoor air. In order to preserve heat and save energy, cold wind penetration is required to maintain the lowest air exchange rate. The ventilation volume can meet the ventilation requirements.
1. Natural ventilation Natural ventilation generally involves setting up windows on the top or side walls of the greenhouse, relying on heat pressure or wind pressure for ventilation, and the amount of ventilation can be adjusted by adjusting the width of the window opening.
The main factors that determine the amount of natural ventilation generally include: indoor and outdoor temperature difference, greenhouse vent height difference, vent area, vent orifice resistance, outdoor wind speed and direction, etc. The indoor and outdoor temperature difference and outdoor wind speed and direction are generally uncontrollable factors. Therefore, evaluating the performance of natural ventilation in a greenhouse mainly depends on the height difference and vent area of ??the vents.
Whether to use side wall ventilation, roof ventilation, or combined roof and side wall ventilation depends on the size and type of the greenhouse and the specific local climate environment. In general, the ventilation volume of the roof and side wall combined ventilation is the largest. However, for greenhouses with a total width of less than 30 meters, side wall ventilation accounts for a larger proportion of the entire greenhouse ventilation. For large-area multi-span greenhouses, the total area of ??roof vents is generally much larger than the area of ??side wall vents, so roof ventilation generally dominates.
The setting of vents should comply with the flow of air. For roof vents, they should be located at the highest point of the roof as much as possible to create a large height difference. For side wall vents, they should be located as low as possible, and try to make the side windows perpendicular to the dominant wind direction from late spring to mid-autumn.
An important indicator of natural ventilation performance is the actual vent area as a percentage of the greenhouse floor area. The larger the ratio, the better the ventilation performance. Therefore, generally the opening angle of greenhouse windows should be as large as possible to increase the actual air inlet area.
Natural ventilation is greatly affected by the external climate, and the cooling effect is unstable. Generally, the indoor temperature is 5~10℃ higher than the outdoor temperature.
2. Forced ventilation Forced ventilation is to set up side windows at one end of the greenhouse and set up a fan at the other end. The fan is used to exhaust air from the indoor to the outdoor, so that a negative pressure is formed indoors, and the air is forced into the greenhouse through the side windows. The air passing through the greenhouse is discharged to the outside by a fan.
The theoretical cooling limit of forced ventilation is that the indoor temperature is equal to the outdoor temperature, but it is impossible to achieve it in practical applications. Due to mechanical equipment and plant physiological reasons, the ventilation intensity of the general greenhouse is set at 0.75 to 1.5 air changes per minute, which can control the indoor and outdoor temperature difference within 5°C.
The advantage of forced ventilation is that the ventilation volume of the greenhouse is very little affected by the external climate. 3. Currently commonly used ventilation equipment accessories ① Roll film ventilation Roll film ventilation is used in plastic greenhouses. The main accessory is the film rolling machine, which can be divided into manual type and electric type.
For side wall ventilation, Φ19 or Φ22 steel pipes are generally used as film rolling shafts, and film clamps are used to fix the shaft and film together. A film rolling machine is installed at one end of the greenhouse, and Φ19 or Φ22 steel pipes are used as film rolling machines. The vertical guide rail, just connect the film rolling machine and the film rolling shaft with the shaft joint. It should be noted that the electric film roll also requires a travel switch.
The method of roof ventilation is similar to that of side wall ventilation, except that the manual film roller needs to extend the cranking handle, and the electric film roller needs to add a telescopic sleeve. When choosing a film reel, you should understand the maximum roll width and maximum film length of the film roll.
There are many types of side window roll film or roof roll film ventilation windows, but according to the transmission type, they can be divided into: gear-rack transmission system, punched rack and gear transmission system, crank connection Rod window opening system, VENLO greenhouse push rod window opening system, etc.
The main accessories of the gear-rack transmission system are: reduction motor, gear row, gear, bearing seat, transmission shaft, etc. The reduction motor is fixed on the greenhouse frame, and the output end is connected to the transmission shaft. The drive shaft passes through the shaft support and the shaft support is fixed to the greenhouse frame. The gear is fixed on the transmission shaft, and the teeth mesh with the gear. One end of the row of teeth is connected to the ventilation window. When the reduction motor rotates, it drives the transmission shaft to rotate, the transmission shaft drives the gear to rotate, and the gear drives the row of teeth to move up and down to open and close the ventilation window. The rack and pinion transmission system has stable operating performance and strong load-bearing capacity. It is currently the best window opening system.
Replacing the gear-rack in the gear-rack transmission system with a punched rack and gear is a punched rack-and-pinion transmission system. The disadvantage of the punched rack and gear transmission system is that the rack has weak load-bearing capacity and is prone to accumulated errors during operation.
If the gear-rack in the gear-rack transmission system is replaced by a crank-connecting rod, it becomes a crank-connecting rod windowing system. Due to structural reasons, the crank connecting rod window opening system is prone to excessive stress on the transmission shaft and reducer, and the reducer motor generally only rotates 1/3 turn during operation, which can easily cause wear and operating errors of the reducer motor.
The VENLO greenhouse push rod window opening system is mainly used for VENLO greenhouse cross-opening windows. The main accessories include swing arm transmission gears, reduction motors, transmission shafts, skylight support arms, push-pull rods, etc. When the reducer is running, the transmission shaft drives the swing arm transmission teeth to reciprocate in the sunroof opening direction, and the swing arm transmission teeth drive the push-pull rod and the skylight fixed on the push-pull rod.