Introduction

We aim to replicate and improve the natural environment in indoor microgreen farming. Instead of the sun, we use LED lights. Instead of the rain, we use various irrigation systems. Instead of the earth, we use specially formulated potting soil. Most of these variables are easy to measure, which makes them easier to optimize. Take lighting, for example. You can test lower wattage (22W) lights against higher wattage (42W) lights. You can change the shelf height to compare 10” spacing against 8” and 12” spacing. You can test 12 hour photoperiods against 14, 16, and 18 hour photoperiods. Each variable is discrete and easy to measure. You don’t need expensive tools or complex math. You can run trials to compare different variables. Measure the yield per tray of similar crops grown in various conditions. This helps determine their efficacy. I say all this because airflow, in my opinion, is the exact opposite. If it’s hard to measure, it’s hard to manage and hard to optimize. Airflow in a microgreen farm is hard to measure, which makes it a difficult variable to perfect. So, how do you know if you have enough airflow in your farm? We’ll share our advice for determining if you have enough airflow to keep your crops happy.

TLDR: Airflow is critical for keeping your plants happy and healthy. Airflow is measured in cubic feet per minute (CFM). To determine how much CFM to create in your farm, start by calculating your farm’s air volume (LxWxH). Multiply the cubic-foot volume of your farm by 0.5. You now know how much CFM to install in your farm using large circulation fans. I recommend this 18” fan from Hurricane, which produces 3,306 CFM. Avoid small computer fans for airflow because they’re expensive and ineffective.

Why Airflow is Important

Airflow is critical in an indoor farm because it's responsible for removing moisture. At the start of the process, it removes high-moisture air from the crop canopy. After the air leaves the crop canopy, it goes to either the dehumidifiers or an air exchange fan. This helps remove it from the farm completely. Either way, airflow is key to healthy plants. Why? Plants respire, which is just a fancy word for “breathe.” When a plant respires, it expels oxygen from its leaves, but critically, along with water vapor. Without airflow in the crop canopy, moisture-laden air will build up around the plants. If the relative humidity (RH) around the canopy gets too high, water vapor will condense on the leaves. This leads to a wet canopy. We want to avoid wet crops at all costs because it will lead to disease and, eventually, a shorter shelf life after harvesting.

If you’re unfamiliar with relative humidity (RH), let’s discuss it real quick. RH is a calculation of how much water the air is holding relative to how much it can hold. If you didn’t know, air can hold a certain volume of water as water vapor. Air can hold more moisture at higher temperatures and less moisture at colder temperatures. So, an RH of 70% means the air is holding 70% of the water vapor it could hold at that temperature before it would begin condensing. A good farm RH to aim for is 55-60%. What’s an easy way to remember? If you take a cold soda can from the fridge on a warm day, water droplets will form on the exterior of the can. Why? The cold surface is cooling down the warm air immediately in contact with the aluminum. As that air cools down, its ability to hold onto that water vapor decreases. At a certain point, the water vapor condenses into water droplets. The temperature at which water vapor turns into liquid water is the dew point.

Why did we explain RH and respiration? We shared these ideas to show why airflow matters for your farm. Airflow is important because (i) plants drink water from their roots and (ii) respire it out of their leaves. (iii) We must manage the moisture-laden air to prevent it from becoming a problem for our crops. (iv) We manage that moisture by introducing airflow to move it away from the canopy and toward an exit point.

Measuring Airflow

In the introduction, I explained that airflow is hard to measure, and it is. However, there are two metrics we can use. The metrics are CFM and ACH.

Cubic Feet per Minute (CFM):

CFM measures the volume of air a fan can move in 60 seconds. CFM depends on the fan’s size, speed, and blade pitch. Most websites will list the CFM on their product pages. For example, we exclusively use 18” wall-mounted fans from Hurricane, which have a listed CFM of 3,306.

Air Changes per Hour (ACH):

ACH measures how many times in an hour the whole volume of your space is replaced with fresh air. Your desired ACH depends on a few factors, but it’s best to aim for 20-30 air changes per hour. In other words, the whole volume of air in your farm is replaced every 2-3 minutes.

ACH & CFM:

How do these two variables come together? If we want to exchange our air every 2-3 minutes, we need to install enough collective CFM to move that volume of air. Start by calculating the volume of your farm. Multiply the total volume by an ACH of 30 to get the total volume of air moved in an hour. Divide by 60 to get the collective CFM needs across all fans in your farm.

Example 1:

My garage is 400 square feet (SF) with 10’ ceilings (20’x20’x10’)

That makes for a total volume of 4,000 cubic feet (CF)

If we want an ACH of 30, we must multiply the 4,000 CF by 30 to get 120,000 CF

Finally, we divide 120,000 CF by 60 minutes to get 2,000 CFM

We need to install enough fans to create 2,000 CFM in the farm

If we use the aforementioned Hurricane fans, which offer 3,306 CFM, we’d only need one fan

Example 2:

Our commercial space is much bigger than the garage

The current farm space measures 75’x25’x13’ or 24,375 CF

Multiply by an ACH of 30 to get 731,250 CF

Divide by 60 minutes to get a total CFM demand of 12,187 CFM

Divide by 3,306 CFM/fan to get a demand of 3.69 or 4 Hurricane fans

ACH calculations assume you have an air exchange fan. An air exchange fan is usually a very large fan integrated into an external wall of your space. The vents on the fan open and close periodically to let fresh air in and stale, humid air out. We don’t have an air exchange fan in our space, but the ACH calculations are still valuable. As an indoor farm, we still want to prevent dead spots and stratification. Dead spots and stratification happen when the air in your farm is not homogeneous. For example, hot air rises and cold air falls because of differences in density. Without airflow, your farm would have a warm, humid upper layer and a cold, dry lower layer of air. To hit our target of 60% RH and 75°F, the air from the HVAC system must mix completely as it enters the farm. We also need to move the moist air away from our crop canopy. Airflow is key to sending this air away from the crops and toward the dehumidifier or exchange fans.

In reality, we use one 18” Hurricane fan for every aisle in our farm. We have nine rack systems of six to eight racks each, which means eight Hurricane fans. If you do that math, you’ll see that means we have 26,448 CFM of airflow. If our space is 24,375 CF, we have a hypothetical ACH of >60. Why? It’s been difficult for us to manage our humidity. Our farm’s RH regularly sits around 70%, which is much higher than desired. To compensate for high humidity, we added more airflow to ensure our crops are never wet.

Sources of Airflow

Technically, airflow comes from more than just your fans. Airflow can also come from your dehumidifiers and HVAC system(s). The equipment controls the air by dehumidifying, warming, or cooling it. It pushes and pulls air from the farm. We use two Quest 155 commercial dehumidifiers to help control RH. They’re rated to move 500 CFM, which means 1,000 CFM total for our farm. For HVAC systems, the rule of thumb is 400 CFM per ton. A ton is 12,000 BTUs. HVAC websites will list either the tonnage or the BTU. If it lists BTUs, divide by 12,000 to get the tonnage. For example, we used a mini-split system in our garage farm, like this 24,000 BTU system from Pioneer. A standard single-family home has a 2-3 ton unit. Our farm has a 3 ton unit and a 2.5 ton unit, which means another 2,250 CFM. If we add up the airflow from our fans and major equipment, we get a CFM of 29,698.

My Suggestion

As much as I love Bootstrap Farmer, avoid these Grow Rack Fans. A single tier of three fans puts out 163 CFM and costs $96.00. If we get the five-tier version, it costs $280.00 for 815 CFM. Each CFM costs $0.34. Let’s compare that to an 18” Hurricane Fan. Each fan costs $78.00 but provides 3,306 CFM, which is $0.026 per CFM. In other words, the cost per CFM is 13.2x greater with the Grow Rack Fans. The Bootstrap Farmer fans appear to use less electricity, which could be a consideration, but I doubt it.

My suggestion is to avoid intra-rack fans or computer fans, like those from Bootstrap Farmer. Stick to large (>12”), wall-mounted oscillating fans like those from Hurricane.

As for calculating your farm’s CFM needs, I’d start with an ACH of 30, but only focus on your fans. If you get extra airflow from your dehumidifier(s) and HVAC system(s), that’s great. Airflow is cheap to buy, so buy enough fans to create an ACH of 30 when your farm is small. As you scale into having a few hundred trays in production, consider increasing your ACH to 60+.

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