What kinds of cooling methods can be divided into according to physical conditions? What are their characteristics?

In summer, many vegetable farmers use plastic film for a year to cover the greenhouse for shading. Although this method is simple and convenient, it also has some shortcomings. First, the shading effect is not very good. Although the light transmittance of greenhouse film used for one year has decreased, it is still relatively high, especially at noon on consecutive sunny days. Although some sunlight is blocked by covering with old plastic film, the temperature in the greenhouse can still reach 40℃, so it is difficult to obtain ideal shading and cooling effect. Secondly, sometimes when the wind is strong, it is easy to scrape off the shed film. Thirdly, if covered with a film, it will hinder the ventilation in the shed and affect the cooling in the shed. Therefore, covering the old shed film on the greenhouse is only suitable for use when the light is not very strong, and cannot be used as a long-term cooling method.

Step 2 cover the sunshade net

Sunshade nets are familiar to vegetable farmers. However, if the selection is improper or the covering time is too long, a weak light environment will be formed in the shed for a long time, which will not only cause the plants to grow white, but also be detrimental to the flowering and fruit setting of vegetables. So how to choose a sunshade net?

First, choose the sunshade net according to the vegetable types. At present, the shading nets on the market are mainly black and silver gray. Black shading net has high shading rate and rapid cooling, which is suitable for short-term field coverage that needs careful management in summer. Silver-gray shading net has low shading rate and is suitable for light-loving vegetables and long-term coverage. For example, tomato is a light-loving vegetable. As long as the illumination time is 1 1~ 13 hours, plants can grow healthily. Although the influence of light time on tomatoes is not so serious, the light intensity is directly related to the yield and quality, and insufficient light is easy to cause malnutrition. The light saturation point of tomato is 70000 lux, and the strongest light intensity at noon is 90000 lux. The black shading net has high shading rate, up to 70%. However, using such a sunshade net, the light intensity can not meet the normal growth needs of tomatoes, and it is easy to cause tomatoes to grow white. Therefore, it is best to choose a silver-gray sunshade net. Most silver-gray shading nets have a shading rate of 40%~45% and a light transmittance of 40,000 ~ 50,000 lux, which can meet the normal growth needs of tomatoes.

Secondly, the time period of coverage should be well grasped. Generally speaking, it is necessary to cover the sunshade net on sunny days and not cover the sunshade net on cloudy days; Cover at noon and uncover in the morning and evening; Cover in the early stage and uncover in the later stage of growth. If there are many rainy days and the temperature is not very high, after planting vegetables, the sunshade net can only be used in the slow seedling stage of 3~5 days. If you use a black sunshade net, it will only be covered at noon on a sunny day. The specific use time of the sunshade net is 10: 30 am to 2: 30 pm. The sunshade net should not cling to the shed film when covering, otherwise the absorbed heat can be easily conducted into the shed through the shed film, and the sunshade net can not play a good role in reducing the shed temperature.