To calculate the number of fans in a greenhouse, it is important to have three basic factors available. With these basic factors, we can calculate the number of greenhouse fans step by step.
Basic Factors
- Greenhouse dimensions: Start by calculating the volume of the entire greenhouse and make a note of the dimensions.
- Circulation rate: The circulation rate is equal to how many times per hour the air is circulated in a greenhouse. In practice, we often see that a circulation rate of 2 or 3 is chosen. In some situations, a circulation rate of 12 or higher is chosen. The ideal circulation rate depends on the situation on-site, and the crop.
- Fan specification: Finally, it is important to know how many m3 (cfm) of air the fan moves per hour, and what the maximum recommended usable throw is. Our Greenhouse Fans are divided into 3 lines: the 5,000, 7,000 and 8,500 m3/h (3,000, 4,000 and 4,750 cfm) versions. You can find this information on the lid of the fan motor, in the brochure, or the data sheet.
Current situation
In the current situation, our assumptions are based on a greenhouse of 160,000 m3 (5,650,000 cu ft). If we multiply this by the circulation rate of 3, we know that the circulation fans have to move 480,000 m3 (16,950,000 cu ft) of air per hour.
Step 1: Number of greenhouse fans to achieve the circulation rate
To know how many m3 (cu ft) of air must be circulated each hour, we can simply divide this by the number of m3 (cu ft) that a fan moves per hour. Depending on your wishes, you can choose a fan from the 5,000, 7,000 or 8,500 m3/h (3,000, 4,000 and 4,750 cfm) line. In this case, we have assumed an ideal situation with ideal greenhouse sizes and suspension points at any desired location. We have included these variables in steps 2 and 3. We then arrive at the following number of fans:
- 4,850 m3/h (2850 cfm): 99 fans
- 6,500 m3/h (3850 cfm): 74 fans
- 8,700 m3/h (5100 cfm): 55 fans
Step 2: Distance between greenhouse fans
To prevent fans being located outside of each other's airflow, we recommend that you do not exceed the recommended throw. We have verified this recommended maximum usable throw in practice, with smoke tests. In practical terms, this means that with hanging options every 4 meters (13 ft), a fan with a recommended throw of 35 meters (115 ft) will be suspended at a distance of 32 meters (105 ft).
Step 3: Greenhouse dimensions
Finally, the fans should be evenly distributed along the length of the greenhouse. In practice, the length of the greenhouse is often not divisible by the fan distance calculated in step 2. In this case, the fans are hung even closer together to obtain the right distribution in the greenhouse, and to avoid exceeding the maximum recommended throw.
Specialist advice
Although you can easily calculate the number of fans yourself, we always recommend an on-site visit from a specialist. Please contact one of our specialists for advice.
This article is part of our 5 page white paper on greenhouse ventilation and circulation. Download it here to receive a collection of insights to consider when installing fans in greenhouses.
