Cannabis Grow Operations and Design Setpoints

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Key Takeaways

- Dry bulb temperature, wet bulb temperature, humidity, and vapor pressure differential are critical design setpoint considerations for growers

- The right engineering team will work with growers to design systems based off of identified growing conditions and design setpoints

- Understanding how to adapt your growing process to use VPD to minimize energy consumption can be a powerful tool to reduce energy consumption and ultimately your bottom line

The key to any successful cannabis cultivation operation is to design systems around the correct conditions to maintain the optimal plant growing environment and to balance operational costs with upfront costs to meet budgets. In order to ensure cannabis grow rooms perform as desired, system performance requirements must be considered alongside upfront costs, operating costs, maintenance costs, expandability, and code compliance.

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Cannabis Grow Operations and Design Setpoints: An Important Conversation

Root Engineers, a division of ColeBreit Engineering, is a group of experienced professional engineers specializing in the design of systems that support cannabis cultivation and processing operations.

Cannabis cultivation requires a unique set of engineering considerations that can be vital to success, such as specialized heating, ventilation, and air conditioning (HVAC) systems that meet specific design setpoints. The key to any successful project is to make sure that we are designing around the correct conditions, not only to maintain the proper growing environment as required by the grower throughout all stages of the growing process, but also to minimize the operational cost to meet budgets. If designed incorrectly, HVAC systems can become overwhelmed and jeopardize product quantity and quality, or may consume more energy than necessary to obtain the desired harvest quantity and quality. In order to ensure that cannabis grow rooms perform as desired, and consume the least amount of energy to do so, engineers need to

balance system performance requirements with first cost, operating cost, maintenance cost, future expandability, owner-required redundancy, and code compliance.

In this paper, we want to discuss the issue of setpoints and how different setpoints may be able to obtain the same final quality and/or quantity of product. We will discuss the use of vapor pressure differential (VPD) as the potentially more important metric when compared to relative humidity (RH) and dry bulb temperature (Tdb). A design setpoint VPD can be met at varying temperatures and relative humidities, but selecting the wrong temperature and humidity can lead to significant increases in both first cost and long-term operational costs of HVAC equipment. Before diving deeper into VPD, we will look a little more closely at temperature and humidity.

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Dry bulb temperature (Tdb) is the temperature with which we are most familiar (i.e. what is shown on wall thermostats in your home). Wet bulb temperature (Twb) is the temperature that a thermometer reads when its bulb is wrapped in a moist cloth at room conditions. This reading is important for our purposes because it helps tell us how much moisture is in the air and the air’s potential to absorb more moisture.

The amount of water present in the air will dictate how fast the water in the cloth will evaporate, causing the wet bulb temperature

to be lower than the dry bulb temperature. In other words, as the water evaporates it will cool the thermometer, and the faster it evaporates the lower the wet bulb temperature will be relative to the dry bulb temperature. If the difference between the dry and wet bulb temperatures is small, the air is humid because there is so much moisture in the air it cannot absorb the water from the cloth very quickly. If there is a large difference between the dry and wet bulb temperature readings, the air is very dry and is able to absorb the water from the cloth quickly.

Two Types of Temperature: Dry Bulb vs Wet Bulb Temperature

Figure 1. DRY BULB WET BULB

MOIST CLOTH

LOW HUMIDITY

70°F

HIGH HUMIDITY70°F

50°F

65°F

WIND

WIND

MOIST CLOTH

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Absolute humidity is the total amount of water in the air regardless of the dry bulb temperature of the air, measured in mass of water per volume of air. Another more commonly used way of describing the humidity of the air is through relative humidity. Relative humidity (RH) is the amount of moisture in the air expressed as a percentage of the maximum possible moisture that there could be in the air at a given dry bulb temperature.

As the amount of water in the air increases at a constant dry bulb temperature, the air will eventually reach a state of “saturation.” This is when the air can no longer hold any more moisture (100% RH) and the dry bulb temperature will be the same as the wet bulb temperature, so water begins to “fall out” of the air in the form of clouds, dew, or condensation. This is especially apparent on cooler surfaces that are able to absorb the heat energy from the air molecules with which they are directly in contact, thus slowing the molecules, reducing the dry bulb temperature, and causing the water in the air to condense and liquefy. In warmer temperatures, the air is able to hold more moisture.

Two Types of Humidity: Absolute Humidity and Relative Humidity

When discussing building loads associated with

maintaining grow room set points, the terms

sensible and latent heat are used prevalently.

Sensible heat, as the name suggests, is heat that

you can sense - or feel - and is associated with the

dry bulb temperature. Latent heat is energy that

must be added or removed to cause a change from

solid, liquid, or gas and is associated with the wet

bulb temperature. In cannabis grow operations,

we often use the HVAC system to dehumidify

a flower room. In order to do this, energy must

be expended to cool the air from its ambient

temperature down to its dew point (temperature

of saturation); this is a sensible load. More energy

must then be expended to transform the water

molecules in the air from a gas to a liquid. This is

done so that the liquid condensate can be removed

from the space; the energy required to do this is

a latent load. Both of these loads must be known

in order to properly size the HVAC system that

will be necessary to condition the grow space.

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If the amount of moisture in a space were to remain constant, and the temperature was to increase, the relative humidity would decrease. This is because the total amount of moisture is the same, but it is now possible for the air to hold more moisture, thus the air is further from the maximum amount of moisture it could possibly hold (lower RH).

Conversely, if the moisture content were to remain constant while the temperature decreased, the relative humidity would increase. This is because the air is now able to hold less moisture, so the moisture in the air is closer to the maximum amount of moisture the air can hold (higher RH).

Figure 2.

LOW MOISTURE CONTENT HIGH MOISTURE CONTENT

COOLTEMPERATURE

RH = 50%

RH = 10%

RH = 90%

RH = 50%

WARMTEMPERATURE

Grow room

Maximum allowablemoisture content in air

Actual moisture content in air

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First, let’s look at what a vapor pressure differential means. The vapor pressure of the water in the air can be calculated at any given dry bulb temperature, wet bulb temperature or humidity (absolute or RH). If there is a difference in vapor pressure between any two points, (i.e. the air of the room and at the surface of the plant leaf), this is known as a vapor pressure differential or deficit. This is important for two reasons: first, we know that gases will always move from low pressure to high pressure (think of a balloon at pressure - once opened it will release air into the room; air from the room will not move into the balloon). If we know the vapor pressure of two points, we know which direction the moisture will move,

and more importantly, we can quantify the rate of evaporation between two points. Second, the rate of evaporation will theoretically be constant at any given VPD, regardless of what the temperature or relative humidity is.

This means that the rate at which water moves through the plant and is transpired will stay the same regardless of room temperature and humidity so long as the VPD stays constant. It also means that the cost of moisture removal is NOT the same at all temperatures and humidities even though the VPD is the same, which is a key piece of information discussed in detail below.

How Dry Bulb Temperature, Wet Bulb Temperature and Humidity Relate to Vapor Pressure Differential

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Because master growers are the experts in their particular growing process and in plant growth behavior, engineers rely heavily on them to provide the desired growing conditions, often in the form of dry bulb temperature and relative humidity.

The growth and overall health of a plant are the direct result of a process known as transpiration. Transpiration is the process in which the plant moves water and essential nutrients throughout its cells. The movement of water is the result of a lower vapor pressure in the surrounding air than the pressure inside of the plant. This pressure differential causes water to be drawn out of the plant leaves where it evaporates, and causes the plant to draw more water up through its

roots. Transpiration is directly related to VPD, which is the difference between the pressure exerted by the moisture in the ambient air and the pressure of saturation in that air. When the air surrounding a plant is at saturation, meaning 100% relative humidity, the air cannot hold any more moisture, and therefore the plant is not able to evaporate any water. Without water evaporating, the plant is not able to transpire, and growth and nutrient movement is hindered. If the relative humidity of the air is low, the pressure exerted by the moisture in the air is also low, meaning that the vapor pressure is far from the pressure of saturation, and the VPD is high. Higher VPD results in the plant transpiring more.

Why VPD Should Be a Critical Design Consideration for Master Growers

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At Root Engineers, we work with growers to identify which growing conditions they desire, the corresponding VPD, and how we can design an HVAC system to meet that VPD. Most importantly, because relative humidity and saturation pressure vary with temperature, the same VPD and - therefore the same plant

growth - can occur at different room conditions. For example, a room at 78°F and 60% RH will have the same VPD as a room at 76°F and 55% RH. This is important to growers because they can yield the same results, but save money by conditioning the grow space to 78°F/60% RH instead of 76°F/55% RH.

Figure 3.

LEAF GROW ROOMVapor Pressure = High Vapor Pressure = Low

Water Vapor Transpiring

Stomatal PoreLeaf Epidermis Guard Cells

TEMPERATURE = GROW ROOM TEMP.

TEMPERATUREDEFINED BY GROWER

RELATIVE HUMIDITY= 100%

DESIRED RELATIVE HUMIDITY

OR VPD TO BE PROVIDED TO ENGINEERING

TEAM BY GROWER.

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As an example, let’s take a look at the amount of energy it takes to remove one gallon per hour of moisture from the air while maintaining a constant VPD but changing the room set points.

The table below shows that it takes more than 10 times more energy to remove the same amount of water at 65°F/50% RH than at 85°F/75% RH.

Table 1. Dehumidification Energy at Different Room Grow Room Temperature/RH Setpoints with Constant VPD

Approximate VPD [kPa] 1 1 1 1 1

Dry Bulb Temperature [F/C] 85/29.4 80/26.6 75/23.8 70/21.1 65/18.3

Relative Humidity 75 70 65 60 50

Moisture Removal Rate [gal/hr] 1 1 1 1 1

Dehumidification Energy Required [Btu/hr] 15,309 18,136 22,912 31,996 182,097

NOTE: Dehumidification energy shown does not account for any inefficiencies in the dehumidification which would further increase energy consumption.

The above setpoints are examples only to show how setpoints can affect energy usage. The setpoints are not necessarily appropriate for a cultivation facility; Engineers should always work closely with Cultivation experts to determine setpoints.

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As you can see, understanding how transpiration, dry bulb temperature and relative humidity setpoints in your grow room are related is important. Understanding how to adapt your growing process to use VPD to minimize energy consumption can be a powerful tool to reduce energy consumption and ultimately your bottom line. Utilizing the definitions above will help to ensure that all parties involved in the development of a cannabis grow operation are on the same page at the beginning and at the end of a project. Understanding the HVAC considerations

that impact grow operations will diminish time and money spent going back and forth between parties for clarifications and changes to design. The goal of the Root Engineers team is to be a partner that ensures a smooth project that meets both the budget and timeline, and also fosters a harmonious relationship with our client. Next time you find yourself in need of cannabis engineering service, reach out to Root; let’s nail down the right HVAC setpoints for your operation, and help you cultivate the best cannabis around.

Energy Consumption

Want to know more? Contact the Root Engineers Team at

541 728 3293

info@rootengineers.com