Indoor Cannabis Growing: Optimizing Air Circulation

In this interview, Zacariah Hildenbrand, PhD, partner of Medusa Analytical, LLC discusses the importance of configuring the HVAC system in an indoor cannabis grow in order to maintaining the vapor pressure differential (VPD).

Additionally, mechanical and agricultural engineer, Nadia Sabeh, PhD, PE, LEED, AP founder of Dr. Greenhouse, provides a detailed explanation on how horizontal air flow circulation can optimize air flow compared with other configurations.

Read more in our feature article, Engineering Efficiency: Optimizing Energy Systems in Indoor Cannabis Cultivation Facilities.

Transcription

Zacariah Hildenbrand: Energy efficient-wise in terms of air circulation. You're going to have basically how you configure the entire HVAC system, you're going to have fans in there, the airflow. I mean, you're going to have to do, there's whole expertise and guides out there that all they do is look at airflow, because that has a huge impact on maintaining that vapor pressure differential, VPD, which, again, you're trying to be in an optimized range of temperature and humidity, where the plants are happy: not too hot, not too humid, not too dry, not too cold. So there's a lot of factors. I've seen some fantastic presentations, actually, the Cannabis Science Conference, where guys are in there with thermal imaging through a whole facility, and you literally see in real time, differences in temperature and air flow, and then they just make one kind of structural change to a building, maybe even closing off a door, things like that, and it can have a profound impact. That's more on the structural engineering side of things, but fortunately, there are people that specialize that, and they're definitely worth their weight in gold. If you can bring them in to help you set up your growth.

Nadia Sabeh: The two requests for consulting that we get the most often are dehumidification and air circulation. Every grower, it seems, is challenged by having really good air distribution and movement in their rooms. And it could be because they don't have a lot of air movement from their HVAC system, or maybe they're using mini splits with those little wall mounted units, and they're just circulating air right in front of their faces. And so how do we get air from here to there, right, so that we don't get these hot spots?

We are big believers on the traditional HAF [horizontal air flow] fan air circulation strategy. It has been proven in greenhouses for decades, and we have helped a lot of growers to restructure their air circulation design to an HAF pattern, and every time we do, we get really positive results and responses from our clients. So you know, if it ain't broke, why fix it? And you know, there are nuances to having a good HAF design. I mean, we've gone into facilities that have horizontal airflow fans, but they have like, 40 of them in a 500 square foot space, and they're all pointing in different directions. That is beside the point. That is not what we're trying to do. We are trying to create basically a racetrack pattern, a circular pattern over the top of the plant. And we do that for a couple of reasons. For a greenhouse or for heating, we do that because that that hot air gets trapped above the canopy, and so by having that HAF fan, or fans, rotate the air, then we make sure that that hot air gets down to the tops of the plant, and within the plant canopy.

It also stirs the air. So by having this circular pattern, just imagine a whirlpool, right? And so now we have this circular pattern. And when you have a whirlpool, it doesn't just stay at the surface of the water, right? It actually creates a funnel down in the water. So it's the same exact idea, if we have this circular pattern, it is going to create this funnel which is going to drop that air down into the canopy. Aerodynamics are, you know, probably one of my favorite topics as a mechanical engineer. And so when we see growers try vertical airflow fans, we get it. You're trying to get air down into the canopy or pull it up from the canopy. It's almost like a reverse cyclone or funnel kind of an idea. But the problem with cannabis is that we grow it so densely, and they're really tall, and they have different morphologies a lot of times, the different strains grown in different plants, and that we actually end up seeing less uniformity from a vertical airflow fan a lot of times.

Vertical airflow fans blow air up and out and on top of the plants and then suck it back up to it. They're not like a ceiling fan that blow air down. So it's very light flow, it's low velocity, and it might actually end up getting trapped on top of the canopy and do less underneath. And if it does something underneath, it's right under it, it's not 10 feet away from it. So, be cautious if you want to look at vertical airflow.

Under bench airflow we see quite frequently, and I understand why people do it, especially in a greenhouse, because we don't want to block, say, sunlight coming down onto the canopy. But the challenge with that is that those ducts tend to not, especially they're usually plastic tubes, and they're not really engineered, so when you take measurements, you see really high airflow and velocity at the beginning of that duct, and you can see zero airflow at the end of that duct. And so it's not really doing what you thought it was supposed to do, right? And the other thing is that if we blow air from under the canopy, we have the potential to increase the transpiration rate of the plants, because now we're blowing air on those bottom leaves that otherwise would have been growing pretty slowly, but now we're going to be pushing them, and so now they're going to use more water. So if we're talking also about water conservation, under bench air flow is not necessarily a solution to water conservation. You're probably going to end up using more water as your plants breathe harder to manage their own leaf temperature.

So just to answer your question, I think the most effective and energy efficient means of air distribution is just our traditional HAF fan in a circular pattern.