MILWAUKEE, WI, June 10, 2020: Desert Aire is pleased to announce its latest GrowAire Series product to the indoor cannabis cultivation market: the DriCure system, an all-in-one climate control system for cannabis drying rooms. DriCure is a single unit solution for a production level drying facility. Units are designed for a year round operation.

“Desert Aire designed GrowAire DriCure specifically for cannabis drying room applications,” said Paul Stewart, Vice President of Sales for Desert Aire. “It is an extension of the GrowAire Series product line that helps cultivators ensure they optimize the final step to their cultivation process.”

DriCure dehumidifiers combine numerous design features into a cost-competitive system that removes unwanted moisture in a wide range of entering air conditions. The typical drying room design conditions are 63°F to 75°F @ 40 to 60% RH.

The DriCure solution is a vapor-compression refrigeration cycle based unit designed to remove the moisture held by the plant after harvest. The DriCure unit will control the temperature and humidity during the drying process.

DriCure is designed for a typical drying room application where the harvested crop needs to dry in 5 to 10 days. Using DriCure, the plant will lose about 40% of its total weight in the first 24 hours and then the system slowly removes the remaining water over the remaining drying period. Sized for the average moisture load per hour that will occur over the first day of drying, the unit will run constantly for the first 12 hours of drying at full capacity and then cycle to maintain optimal conditions.

To help cannabis operation managers understand DriCure drying room control technology, Desert Aire provides DriCure details about unit sizing, features, design for low temperatures, an explanation of a 5- to 10-day drying cycle program and unit options. Learn more about DriCure.

Overview

Heat Sinks

Refrigerant type dehumidifiers are a closed loop system which effectively transfers both latent and sensible heat from an indoor environment to variety of alternate heat sinks. The key point to understand is that the refrigerant which gained the energy must dissipate the energy or the system will shut down because the internal pressure would be too high. The most common heat sinks are listed here, along with a short description about how they remove energy.

REHEAT – A reheat or condenser coil is present in all DESERT AIRE dehumidifiers and is the most common heat transfer method.  The actual heat sink is the air from the evaporator coil which has been cooled down when it released its energy to the refrigerant. This cool air now passes over the hot condenser coil and is reheated. Thus the air temperature leaving the dehumidifier is higher than the air entering. The exact temperature differential is a function of how much latent energy (moisture) was present in the
room and the run time of the compressor which also generates heat as a by-product. This method is also known as air to air exchange.

WATER HEATER COIL – In this method, an additional component is added between the compressor and the reheat coil. This component is generally a tube in tube heat exchanger which allows water to absorb the heat from the hot refrigerant. A diverting valve controls whether the refrigerant goes to the air reheat coil or the water heater coil. The application will dictate which heat sink has priority. There are several different water sources of heat sink. Examples are listed below:

• Pool water
• Spa water
• Potable water
• Cooling tower water
• Hydronic heat water
• Industrial process water

The actual water heater coil is selected to be compatible with the water used. For example: copper, cupronickel, or stainless steel.

REMOTE CONDENSER – In this situation, all other heat sinks for recovery of heat have been exhausted. In order for the dehumidifier to continue working, an additional heat sink must be added. This becomes similar to a standard air conditioner by adding a condenser outside of the conditioned space. When all other heat sinks are at a proper condition, then a valve diverts the hot refrigerant outside where the remote condenser dissipates the heat to the surrounding environment. This condenser must be sized to the dehumidifier to insure proper charging and operation.

When the remote condenser is utilized, the cool air from the evaporator coil does not get reheated. The air leaving the dehumidifier is cooler than the entering air. The total air conditioning capability is a function of the latent and sensible load in the room.

SYSTEM DESIGN – In designing a dehumidifier, there are several key specifications which must be considered.

First, how much moisture must be removed from the room? This is generally calculated in pounds per hour of water. Every application can have several sources of moisture such as open water vessels, infiltration or processes. All of these sources must be considered as well as what design condition is acceptable. With this information a dehumidifier size can be selected. Refer to other DESERT AIRE application notes for assistance in the sizing procedure.

Once a size is selected, then a decision on what heat sinks are available must be made. Will the room handle the extra heat load?  Is a water supply available? Answers to the heat sink question will then dictate whether an air cooled or water cooled unit is selected and if a remote condenser is required.

Proper installation of the dehumidifier into the total HVAC/R system takes careful planning. All of the heat available from the dehumidification process is derived from the compressor and the conversion of latent energy through refrigeration technology. The  design engineer must make the decision whether the latent load will always be present.  With a seasonally fluctuating load or maintenance condition, such as drainage of an indoor pool, supplemental heaters must be added to compensate for the lack of heat from dehumidification. If room temperature is critical the supplemental heat source must be sized to handle the total heating requirements.

Another factor requiring attention is condensate from the dehumidifier.  Some local codes state that condensate be plumbed to a drain. The dehumidifier employs a gravity drainage system. An unpressurized drain connection or a condensate pump must then be added.

Overview

The Chart

Figure 1 shows a typical psychrometric chart. Dry Bulb temperatures are shown on the chart as vertical lines. The horizontal lines represent Dew Point temperatures. Lines representing Wet Bulb temperatures are the straight diagonal lines sloping downward from left to right.

The curve forming the top edge of the chart is called the “saturation curve”. Air in a condition that falls on any point along this curve is totally saturated with moisture. Any additional moisture added could not be absorbed and would remain in a
liquid state as condensation.

The sweeping curved lines that follow the saturation curve are relative humidity lines expressed as percentages. These lines represent the degree of volume displaced by moisture with Respect to the total air volume.

Relative Humidity

Relative humidity is a misapplied term. It is often used in place of absolute humidity. The key is the word “relative.” To understand this concept, a law of nature must be Reviewed. Air is a compressible fluid and its volume is represented by the following equation:

v=K(T/P)

V = Volume
T = Temperature
P = Pressure
K = Constant

As the air temperature increases, its total volume increases and decreases on reduction of temperature. Pressure has the opposite effect. As pressure increases volume decreases.

Water, however, is not compressible. Therefore given a specific amount, it will always occupy the same amount of volume.

Figure 2 illustrates how this applies to the psychrometric chart. As moisture laden air is heated or cooled the air volume changes but the moisture does not. Thus there is a change in relative humidity, without a change in actual water content.

This is important to understand because water damage occurs at an absolute humidity concentration regardless of its relative humidity. This is known as the constant Dew Point Temperature.

Sensible and Latent Heating and Cooling

There are four types of energy changes when heat of moisture is added or removed. Sensible heat occurs when heat is added without the addition or reduction of moisture. Sensible cooling is the reverse. Latent heat, also known as humidification, is the addition of moisture without changing the dry bulb temperature.  Latent cooling or dehumidification is the removal of  moisture. Figure 3 shows how these are displayed on the chart.

Rarely will these occur as shown but will rather be a mixture of them. A refrigerant dehumidification system is a combination of sensible and latent cooling and sensible heating. First the system cools the air to reduce the dry bulb temperature to the dew point. Then latent cooling reduces the absolute humidity and finally the air is reheated increasing its dry bulb temperature.  Figure 4 graphs this process.

Dehumidifier Sizing

To properly apply a dehumidification system, the amount of moisture to be removed must be calculated. For most applications the only information available is the dry bulb and relative humidity or dry bulb and wet bulb temperatures. The psychrometric chart is used to plot these two values by finding their intersection and then following the horizontal line to the right to determine the moisture content in grains per pound.

By obtaining the starting and finishing grains per pound, the amount of moisture to be removed can be calculated. The amount of moisture to be removed is the difference between these two values known as GR.

Figure 5 shows how a dehumidification system was sized. The ambient design was 91°F dry bulb and 78°F wet bulb. The desired indoor value was 80°F dry bulb and 50% relative humidity. The outside ambient has a moisture content of 124 grains and the indoor design has 78 grains. Thus the required moisture removal rate is 124-78 = 46 grains per pound of dry air.

Overview

For most DX refrigeration-based dehumidification systems the traditional method to provide air conditioning to the space was to add an outdoor air-cooled condenser and reject the absorbed heat to the outdoors via the refrigerant.
When the application installation requires a very large distance between the unit and the condenser, it becomes challenging to design and execute a proper refrigeration line set for proper pressure drop and oil return. When this is the case, a fluid cooler would be considered as an alternative method of rejecting the heat. Lately a design fad has been to use a fluid cooler even for short line set designs. The main reason provided is to reduce the refrigerant charge.

Refrigerant Condenser System Components

A dehumidifier reheats the air when there is a call for dehumidification but not a call for cooling. The typical system uses a hot gas reheat coil that captures all of the energy (sensible, latent and heat of compression) and returns it to the space. The example system below also has a three-way diverting valve to send the hot gas to the remote condenser.

This system uses a single heat exchange method to transfer the refrigerant’s energy to the air (via the hot gas reheat or remote condenser coil). With the large temperature differential, these two coils can be easily sized to maintain the system’s condensing temperature constant in all modes. The lift (difference between condensing and suction temperatures) is a key element in determining the power consumption of the compressor.

The heat exchangers that utilize refrigerant directly are very effective at keeping condensing temperature low as possible since the temperature of the refrigerant throughout the coil is substantially the same. This is because the  majority of the coil is used for two-phase heat exchange. This means that the refrigerant inside of the coil is releasing its energy by condensing rather than strictly dropping temperature. Keeping all the surfaces at a higher temperature means that the full surface of the coil is effectively used and even a small exchanger can cool effectively. The heat exchanger can easily be designed for a close approach temperature (the difference between the leaving air temperature and the refrigerant condensing temperature). This minimizes the condensing temperature and reduces the lift. The system moves the refrigerant directly through the compressor. A minimum number of moving parts are needed since the compressor creates the pressure differences required for the heat transfer and also the flow of the refrigerant to move the energy from one place to another.

Fluid Cooler System Components

A dehumidifier that utilizes a water loop for the heat rejection must first reject the total heat of rejection to a water condenser. A fluid pump circulates the water through the condenser and transfers the heat to the air via a hot water coil (for reheating the air) or a fluid cooler for rejection to the outdoors. This process requires two heat transfer processes versus the outdoor air cooled condenser method described above.

Relative to the direct exchange of the air-side refrigerant condenser coils, a system with a fluid-cooler glycol loop has the following disadvantages:

• More components may increase initial costs.
• The pump is an additional moving part in the system that requires more energy and may require additional maintenance and service.
• Glycol must be added to the system and additive packages must be maintained to avoid corrosion.
• The additional heat exchanger in the chain requires another approach (refrigerant-to-water then water-to-air). This increases condensing pressure and lift.
• The single-phase heat exchange in the fluid cooler and hot water coil is less effective than more direct methods. As soon as the fluid starts to exchange energy with the air, the fluid starts to cool. This makes the subsequent portions of the heat exchanger less effective than with two-phase heat exchange where the temperature does not change. Either the heat exchanger must be designed to be larger or the lift will increase even further.

Moisture Removal Efficiency

AHRI Standard 910 defines Moisture Removal Efficiency (MRE) as the pounds per hour of condensate removed by the dehumidifier divided by the total kilowatts of energy needed to run the system. The power to run the system includes the compressor, blower/motor, transformers and pumps.

When comparing a basic refrigerant based system to one utilizing a fluid condenser, there are several additional power consuming elements to be considered:

• Water pump electrical power to move the fluid around the water loop
• A hot water coil with low temperature water is significantly larger than a D/X hot gas reheat coil. This increases the system static pressure and the dehumidifier blower must work harder.
• An outdoor fluid cooler’s air volume across the coil is higher than a refrigerant condenser’s air volume, requiring a larger size fan motors.
• An outdoor fluid cooler’s approach temperature is higher, increasing compressor energy consumption.
• Glycol in the water loop must be maintained and decreases the energy transfer efficiency of the fluid. This adds to the approach temperature.

Conclusions

Although a fluid cooler loop can help to address issues with difficult installations where refrigeration line length or large liquid line risers are unavoidable, extreme care must be taken in selecting a fluid cooler system. A reduction in refrigerant charge may be possible, but this could come at the expense of increased energy consumption and/or reduced system capacity.

A fluid cooler system’s additional approach temperature of 6°F to 10°F generally increases lift by the same amount and this roughly translates to 12% to 20% efficiency decrease when compared to the conventional refrigeration condenser systems. Additional components such as higher horsepower blowers and fans plus a recirculating pump will increase the efficiency losses even more.

A fluid cooler system would significantly increase the operational costs of an indoor pool dehumidification system which runs 24 hours per day, 365 days per year.

Using a refrigerant to air exchanger provides the most effective method for optimizing the compressor power consumption and thus the unit’s efficiency and should be considered first.