Precision Dehumidification Control in the Modern Grow Facility

by | Sep 5, 2017

Written by Editorial Team

A diverse range of articles covering the latest advancements in the cannabis industry authored by writers who prefer anonymity, former contributors, or collaborative groups.

Precision air conditioners, originally designed for mission-critical environments (think data centers), are well suited for grow room applications. They have advanced controls for maintaining tight tolerances of temperature and humidity, typically plus or minus 2°F and plus or minus 5% RH (relative humidity) when properly applied|

Mold or mildew on cannabis plants can lead to the partial or total loss of a crop. The economic impact of this loss is not solely limited to the value of the crop itself but also includes the cost of the nutrients, water, energy, and labor expended to grow the crop.  Since mold and mildew are a direct result of uncontrolled excessive moisture in the grow space, the ability to remove this excess moisture in a rapid yet controlled manner is critical.  Cutting-edge heating ventilation and air conditioning (HVAC) technology is available to adjust and maintain moisture content at the correct level while maintaining optimal grow room temperatures.

Precision air conditioners, originally designed for mission-critical environments (think data centers), are well suited for grow room applications.  They have advanced controls for maintaining tight tolerances of temperature and humidity, typically plus or minus 2°F and plus or minus 5% RH (relative humidity) when properly applied. The controls can also provide alarms and trending data that is invaluable to a head grower analyzing why a specific crop was or was not a success.  Unfortunately, traditional precision air conditioners were designed to remove as little moisture from a data center or information technology space as possible to prevent the need for costly re-humidification of the space.  This high sensible heat removal design, while highly desired in a data center, fails in a grow room situation where significant amounts of water vapor that raise the grow room relative humidity (RH) must be removed in real time as the plants transpire.  If the RH is not controlled and becomes too high, white mildew or mold can form on the plants in a relatively short period of time.

The plants, when grown in soil, are watered somewhere between 0.5 and 0.75 gallons per day. This water contains the nutrients that the plants need to grow.  Approximately 90% of this water must be removed by the air conditioning system in the form of water vapor released by the plants as they transpire.   As the plants go through their lights-on and lights-off cycles, they “transpire” water into the environment, creating the dangerous moisture content that must be removed.  Vascular plants such as cannabis have specialized cells known as xylem which work to raise water from the roots up to the leaves to facilitate the photosynthesis process and cool the plant.  Small openings in the outer skin of the plants known as stomata allow the water stored in the plant to be evaporated and flow from the plant into the air.  There are many environmental control challenges associated with transpiration beyond simply removing moisture.  The transpiration rate is not constant and must be addressed rapidly when the RH in the room begins to climb.  In certain humid locations moisture entering the grow space through infiltration must also be removed.

Plant transpiration rises while grow lights are on, but this moisture is naturally stripped on the fins and tubes of the cool air conditioner evaporator coil that is primarily removing the heat of those lights. The challenge comes when the plants enter the “lights out” portion of their day. As the high heat load in the grow room is turned off, temperatures drop and even though the absolute moisture content has not necessarily increased, the relative humidity of the air has. At lower temperatures the air cannot hold as much water vapor as it can when it is warmer and as a result the water vapor begins to condense, the grow space becomes damper and  water droplets begin to form on the leaves.  This is the series of events that what we need to prevent of risk the formation of mold or mildew on the plants.

To address this challenge, STULZ developed a high tech enhanced dehumidification feature for its cannabis units that is integrated into the air conditioner to rapidly reduce moisture content in the grow space while making minimal temperature changes. The unit’s programmable logic controller utilizes calibrated air sensors to monitor RH in the space and when it identifies a moisture spike due to increased plant transpiration or decreasing temperatures, it initiates a sequence of operations to reduce the RH.  The rotational speed of the air conditioner’s variable speed fan(s) is slowed, decreasing the air flow rate over the unit’s evaporator coil.  The lessened air flow decreases the pressure, and thus temperature, of the refrigerant flowing though the coil due to reduced energy transfer. This decreases the average coil temperature to the low temperature set point (a programmable point that should never be set below 33°F).  The colder coil rapidly cools air below its dew point and quickly strips the unwanted moisture. This sequence of operations is governed by a sophisticated PID (proportional-integral-derivative) control loop in the unit’s controller software that anticipates these moisture spikes and reacts to the dynamic environment of the grow space.  As excess moisture is removed, the fan’s speed is continuously modulated to maintain the correct coil temperature.

A potential side effect of stripping moisture is overcooling the air in the space.  Since maintaining temperature is just as important as maintaining the proper moisture content, some form of reheat must be available and it must be closely controlled to only offset the overcooling that results during the condensing of water vapor on the evaporator coil.   STULZ cannabis units have options to provide reheat using hot gas, electric, steam, or hot water that precisely maintain the leaving air temperature during dehumidification to stabilize the grow room and limit stresses on the plants.   Multi stage reheat is available and the hot gas reheat feature should always be chosen as the first stage due to it inherent energy efficiency. The second stage (if required) can be any of the other reheat mediums available.

In some locations where the air’s moisture content is natural high due to geographical location additional methods of dehumidification may be required to offset the dehumidification load. In these situations a desiccant dehumidifier acting as a “helper” unit with it’s controls synchronized to those of the HVAC unit can be used to tightly control moisture content in the grow space.

The environment in a modern cannabis grow facility is dynamic, fortunately there is equipment out there when properly applied that can protect the crops from mold and mildew while maximizing crop yield and the growers return on investment.