The ventilation and circulation of air in horticultural greenhouses are of great importance for the growth and well-being of plants. Depending on the situation in and around the greenhouse, various issues can arise in the indoor climate, which can be resolved by, among others, the correct use of fans.
In this article, common climate problems in horticultural greenhouses are discussed, plus advice is given on what can be done to counter these problems.
Ventilation or circulation issues?
First, a distinction is made between ventilation and circulation. When it comes to ventilation, ventilating the air in the greenhouse is meant. Circulation, on the other hand, ensures uniformity of the air in the greenhouse. These two are closely linked with regards to maintaining a healthy greenhouse climate.
Basically, climate problems in horticultural greenhouses can be traced back to the following causes:
- Insufficient or excessive ventilation
- Insufficient or excessive circulation
- Fan installation errors
- Greenhouse construction errors
The ideal moisture content in a greenhouse varies from situation to situation and depends on the type of cultivation. If the moisture content is too high, fungal problems or plant diseases can arise, for example.
A grower can use various solutions to maintain the desired moisture content. For example, moisture can be discharged through windows in the roof of the greenhouse using dehumidifying equipment or by means of fans.
In addition to moisture, too low or high temperatures can also have negative effects on the crop. In warmer weather conditions, the grower can choose to exhaust warm air by means of ventilation. In addition, fans are sometimes used in combination with pad cooling to cool the incoming air. In this way, the fans draw in cooled outside air.
When installing fans for ventilation, the volume of the greenhouse and how often the greenhouse air must be exchanged in order to maintain a healthy climate have to be calculated first. This is referred to as the ventilation rate and expressed in m3 air exchange per hour. When these ratios are incorrect, it can have a negative effect on the climate and thus on the cultivation. A customized solution per location and plant type is therefore required.
Uneven distribution of temperature and moisture
For good cultivation, it is necessary that a greenhouse's indoor climate is uniform. Circulation fans are often used for this purpose. If there are large differences in temperature and moisture content within the greenhouse, this can affect both the crop and the grower's energy costs. The uniformity of the greenhouse air can be negatively affected by various problems.
Below several examples are given:
-Leaks in the greenhouse windows
Glass is, of course, a vulnerable part of horticultural greenhouses. In colder climates, a small leak in the glass can already cause draft or cold spots in the greenhouse. This is usually checked by means of temperature measurements.
-Location of the greenhouse
When a greenhouse is located in such a way that one side is exposed to a lot of sunlight or a lot of cold wind, this can cause cold or warm spots even when the glass is intact. Growers can counter such influences by adjusting the installation and settings of circulation fans to the greenhouse situation.
-Heat accumulating at the top of the greenhouse
Heat caused by heating, sunlight or lighting can accumulate near the top of the greenhouse. Vertical circulation is used to take advantage of this warm air and to evenly distribute it at plant level. By better mixing warm air with cold air and moist air with dry air, growers save on heating costs and improve the uniformity of the greenhouse air.
-Obstacles in the greenhouse
Structures in the greenhouse can also influence the uniformity of the climate. For example, a truss can have a cooling effect on the local air and thus the plants.
Fan installation errors
In some cases, fans are connected incorrectly, causing circulation problems in the greenhouse. For example, an incorrect installation can cause fans to rotate in the wrong direction or hang too far apart. This obviously has a negative effect on both the climate and the airspeed. Due to limited suspension points, customization is needed in order to nicely distribute the fans over the greenhouse. This is often constructively taken into account during the construction of the greenhouse.
In addition to uniformity, airspeed is also important. The ideal airspeed differs per crop. With the correct air circulation and speed, moisture is removed from the crop, among others, making the climate at plant level drier. This is indicated by the circulation rate: The number of times per hour that air is circulated in the greenhouse.
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