5 Things to Know About Airflow in Your Greenhouse
April 5, 2024 Airflow in the greenhouse is essential for creating a productive and healthy environment for your plants. Airflow is responsible for moving air in and out of the production space, for exchanging gases and heat that affect plant growth and quality, and for delivering the air to where we want it.
How much air should you deliver to the room?
How fast should you blow air across the plant?
Does the air delivery method matter?
Here are five things you need to know about airflow in the greenhouse.
1. Airflow vs. Air Velocity vs. Air Change Rates
First, let’s talk terminology. The word “airflow” is generally used to describe the movement of air from one location to another. In engineering terms, airflow is the volume of air that is being moved from one location to another. When you see airflow described in units of “CFM” (cubic feet per minute) or “m3/s” (cubic meters per second) that’s the cue we’re talking about the volume of air we’re delivering from an HVAC unit, through a duct, or by a fan.
Air velocity is the speed at which air is moved across a distance. Air velocity is described in “meters per second” (m/s), “feet per minute” (fpm), or “miles per hour” (mph). Note: if you’re measuring air speed across your plants in units of mph, you’re probably blowing too hard!
Air change rate is the number of times the total volume of air inside a room is removed and replaced with new air over a period of time. There are specific two terms to describe air change rates. Air exchange rate (AER) is the number of times the air inside a room is exchanged with outside air and is another way to describe ventilation. You will most often hear this term associated with greenhouses. The air turnover rate (ATR) is the number of times the air inside a room is processed by the air conditioning system.
2. Why is Air Movement Important?
Moving air to and within greenhouses and indoor farms is important for several reasons.
- Air movement facilitates gas exchange. The air delivers carbon dioxide (CO2) to plants so they can photosynthesize. The air also removes the waste product of photosynthesis, namely oxygen (O2). The air also removes water vapor generated by plant evapotranspiration.
- Air movement helps with heat exchange. When there’s a lot of light radiation, the air can help dissipate heat and cool the plant down. When it’s dark or low-radiant energy lights are used, the air can deliver heat to the plants to maintain desired leaf temperatures.
- Air movement helps build strong plants. When gas exchange and heat exchange are good, the plant can freely photosynthesize and transpire. Air movement can also help the plant build thicker cell walls and deter flying insects from landing on plants.
3. What Air Speed do Plants Like?
If you can deliver an air speed between 0.35 and 1.0 m/s (80-200 fpm), you are right on target! Horticultural researchers around the world have demonstrated that this is the ideal range. Plants exposed to air speeds lower than 0.35 m/s and greater than 1.0 m/s tend to have higher incidence of leaf tip burn and blossom end rot caused, respectively, by too low or too high evapotranspiration rates. Air speeds near the top of the range result in more vegetative growth, as plants build their cell walls to resist air pushing on them. You may want to facilitate this type of growth if you are trying to grow crunchy lettuce or thick stalks, branches, and stems to support fruit and flower production during the reproductive stage. When your plants do start to flower and fruit, finding a velocity in the middle of the range will promote good evapotranspiration and nutrient delivery to the parts that matter most.
4. How is Air Delivered?
Air can be delivered to the grow space and your plants several ways. When air conditioning is used, ducts are used to transport conditioned air from the HVAC unit to the room and often distributed to several locations using diffusers. Return ducts are used to remove the room air and send it back to the HVAC unit. You may also see in-room ducts that are not attached to the HVAC unit and instead pull air directly from the room and deliver it to plants from above or below.
Greenhouses typically rely on exhaust fans located in the wall at one end that pull air in through inlets or a wet wall at the opposite end. Greenhouses may also use the natural forces of wind and buoyancy to passively move air in and out of production space through openable roof and sidewall vents.
In both greenhouses and indoor production spaces, horizontal airflow (HAF) fans are often used to help circulate air and direct the air toward the plants. Vertical airflow (VAF) fans can also help destratify the air or direct air down into the plant canopy, depending on which VAF fan type is used.
5. What Air Change Rate?
How much air should you move through the production space? It depends on the crop density, whether you have a single or multi-level growing system, and the condition of the air being removed and delivered to the space. For greenhouses, we typically design for a range of 45-60 air exchanges per hour when peak cooling is required. When we are using ventilation to dehumidify the greenhouse at night or in the winter, the AER is much lower and something we will calculate to limit the need for heating and to avoid the environmental fluctuations that can occur when over-ventilating.
In an indoor facility that is recirculating the majority of the room air, we often design for a range of 15-30 air turnover rates. The volume depends on several factors, including how many plants we are trying to deliver air to.
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