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Cannabis facility grow rooms are dynamic environments requiring careful planning of heating, ventilation, air conditioning and dehumidification (HVACD) systems. For example, at a certain stage of the growth cycle, plants are transferred from the Veg room to the Flower room, where they continue to grow in height and size until they are harvested (see “Cannabis Grow Facility Design 101, Part 1,” https://bit.ly/3BxQoWv).
Along the way, light levels, irrigation schedules and environmental conditions will likely be varied and manipulated, depending on the stage of growth, to develop the desired product characteristics.
A detailed discussion with the owner’s lead grower or cultivation team is the first step in understanding how to properly size and select equipment. Basic grow room parameters impacting an HVACD system include quantity, type and wattage of grow lights; lighting schedule; quantity of plants and the watering schedule and rates, or the plant canopy area and associated water usage estimate; target temperature and relative humidity for day and night; and the harvesting method.
Most grow rooms are constructed as a “box inside a box” and, therefore, envelope thermal losses can be largely ignored. Grow lights, or horticultural light fixtures, account for the bulk of the sensible cooling load required for the space. Other internal equipment such as supplemental dehumidifiers and circulation fans also will reject sensible heat to the room.
Latent cooling load is mainly derived from the water delivered to the plants via irrigation. In simple terms, water in equals water out. Plants take in water through their roots and exhale, or transpire, water vapor through their leaves through a process known as transpiration. Light levels, airflow, temperature and relative humidity all have an impact on the rate of transpiration.
Vapor pressure deficit (VPD), a parameter commonly referenced by growers and designers, is the difference between the vapor pressure inside the leaf and the vapor pressure of the surrounding air. VPD is influenced by both the temperature and the relative humidity of the space and affects the transpiration rate of the plants.
HVACD equipment serving grow rooms should be sized and selected to maintain the target space temperature during both lights on (day) and lights off (night) periods, maintain the target relative humidity, and provide continuous airflow to the room for general circulation. And it must do all this in the face of ever-changing conditions, all day, every day, all year long.
It is typical for indoor grow room owners to size equipment and provide 20 to 40 air turns per hour, which is a large amount of air to move. Most grow rooms recirculate 100% of the air and do not regularly exhaust anything or introduce outside air. This helps to preserve expensive carbon dioxide that is used for plant enrichment and to prevent pathogens and insects from being introduced from the outdoors.
Furthermore, most facilities use a dedicated air handling unit for each grow room to prevent cross-contamination and to allow for maximum flexibility in room environmental parameters.
Early in the cannabis craze, it was not uncommon for facilities to try to use typical residential- or light-commercial-grade HVAC equipment (notice the “D” was dropped). Most learned the hard way that equipment not designed and built specifically for the demands of an indoor grow environment will struggle to manage humidity and maintain control through changing conditions, let alone handle the continuous duty cycle required.
Hence the evolution of purpose-built HVACD equipment specifically designed for environmental and operating challenges of indoor grow rooms (see Figure 1). This type of equipment can effectively manage the changing sensible heat ratios inherent to a grow room.
Several types of HVACD equipment and systems are typically employed for indoor grow environments. A basic system will consist of standard packaged air-handling units with direct expansion (DX) cooling, paired with supplemental dehumidifiers suspended from the ceiling inside the grow room. Water-cooled heat pumps can be used to reject heat to a geothermal loop or fluid cooler. Supplemental dehumidifiers are typically required with heat pumps.
A more purpose-built approach involves a packaged air-handling unit with DX cooling using refrigerant hot gas for reheat. Additional or supplemental post-heating also may be required. This type of equipment is able to generate a relatively low dewpoint required for dehumidification and, therefore, supplemental dehumidifiers are not generally necessary.
Typically, larger facilities will opt for a chilled water system with heating hot water or other means of reheat/post-heating, piped to air-handling or fan-coil units.
HVACD systems for indoor grow rooms do not have a one-size-fits-all solution. Rather, owners, designers and contractors must consider many factors, including first cost vs. operating cost, construction timeline and phasing/sequencing, lead times of equipment and components, geographic location, building structural and/or site restrictions, redundancy, maintenance and access requirements, flexibility and controllability, energy efficiency, and availability of utilities.
Determination of loads and equipment selection is the first step, but the distribution of air to the grow room is of equal importance in ensuring the success of the growing plants. Lack of airflow and poor circulation within a room can result in stagnant areas, which can promote the growth of mold and lead to plant stress.
Ideally, a constant air velocity is maintained at the plant canopy level, using a combination of throw from ductwork connected to HVACD equipment and supplemental circulation fans within the space. A computational fluid dynamics analysis can help ensure adequate airflow will be achieved in all areas within a room (see Figure 2).
Circulation becomes more complicated with multiple tiers or levels of plants, which is common in Veg rooms. In those instances, in-rack airflow systems can be used to ensure air movement between tiers. Sometimes small destratification fans, or vertical airflow fans, are beneficial.
Air distribution components must be carefully coordinated with other systems inside the grow room, such as lights, fire protection sprinklers, racks/benches, fertigation piping and plant growing clearance. Due to the large airflows required, ductwork can take up a fair amount of space; therefore, early planning is critical to ensure adequate building or ceiling height for all systems and equipment.
Many systems include an air-handling unit located outside of the grow room and use ductwork to convey supply and return air to and from the room. Sheet metal ductwork should not be constructed with internal insulation (liner) as that material can harbor harmful pathogens and insects and is difficult to clean. Fabric ductwork is commonly used to deliver air inside a grow room (see Figure 3).
Fabric duct can be customized with nozzles or orifices to optimize airflow based on the room configuration and can be removed for cleaning if necessary. It also generally weighs less than sheet metal ductwork, which could be an important factor when retrofitting an existing building.
Energy Usage and Efficiency Opportunities
Part 1 of this series discussed the energy intensity of indoor cannabis cultivation operations. It is not uncommon for grow rooms to require 80 watts per square foot, with about half of the load attributed to HVACD equipment serving the space. Therefore, it is important to consider any opportunity to improve the energy efficiency of these systems.
Some air-handling manufacturers catering to the industry design their equipment with integral energy recovery devices (see Figure 4). Larger systems using chilled water can take advantage of heat recovery chillers. Geothermal systems can be an option, depending on site-specific conditions. Controls and automation can greatly improve the energy efficiency of HVACD systems.
Benchmarking the energy usage of a facility can help an owner develop a baseline and then determine steps to make incremental improvements over time.
The Resource Innovation Institute (www.resourceinnovation.org) has published several best practices guides related to energy and water efficiency for controlled environment agriculture producers to use as reference.
Getting into the Weed(s)
There are many other details to consider when designing and specifying a complete HVACD system for cannabis cultivation facilities, including:
• Filtration. This should be incorporated into air-handling equipment to help scrub the room air and to protect the equipment. MERV 13 filters are commonly used, but some owners opt for even higher separation efficiency up to and including HEPA.
• Disinfection. Many options are available for consideration, such as ultraviolet lamps and photocatalytic oxidation, and can be applied in a central air-handling system or within the room. Between harvests, some growers will disable the HVACD system and turn on all the grow lights at full power, allowing the room temperature to climb to well over 125 F for a certain period to “cook” the space and help control contaminants.
• Odor mitigation. This typically is required for any exhaust source on a cannabis facility. Carbon filters can generally be used to control odor, although many other technologies also may work for this purpose.
• CO2 enrichment. This requires continuous monitoring of gas concentrations and purge exhaust ventilation if a high concentration is detected. This topic will be discussed in more detail in the final article in this series.
• Condensate. Due to the high latent heat load in grow rooms, a large amount of condensate will be generated from cooling and dehumidifier coils. This water can be collected and treated for reuse as irrigation water, greatly reducing the amount of raw water required to operate the facility (see “Cannabis Grow Facility Design 101, Part 2: Water Usage,” https://bit.ly/3vwovdX).
• Humidification. It may seem counterintuitive, but some grow rooms, especially Veg rooms, may require humidity to be added back to the space. In Veg rooms where plants are younger and smaller, less water will be applied via irrigation, and the heat load in the space will be sensible-dominant. To cool the space, the HVACD system will have a tendency to over-dry the room; thus, humidification may be necessary.
• Controls. Controls for HVACD systems serving indoor grow rooms are critically important and should not be overlooked. Ideally, HVACD equipment and sensors are integrated with controls for grow lights and the fertigation system for optimum coordination of the plant environment.
Part four in this series will explore fire protection and other life safety considerations for indoor cannabis cultivation facilities.
Luke Streit, PE, is a project executive and mechanical/process engineer for IMEG Corp. He also has an agricultural engineering degree from Iowa State University, was named an ENR Midwest 2020 Top Young Professional, and belongs to several industry organizations, including ASABE, ASHRAE, NFPA and RII.
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