Figures 1 and 2 show the process of air moving over the dehumidifying coil and being reheated. Many manufacturers size the dehumidifying coil for only part of the unit total airflow. This allows them to introduce outdoor air after the dehumidifying coil and allows more unit airflow for the condenser reheat coil to maintain proper system refrigeration pressures when rejecting refrigerant heat to the airstream. Figures 3 and 4 show a bypass air process.

In Figures 1 and 2, air enters the dehumidifying coil at point A. The dehumidifying coil removes sensible heat (SH) and latent heat (LH) from the airstream. The dehumidified, cooled air leaves the coil at its saturation temperature at point B. The total heat removed (TH) is the net cooling capacity of the system.

In reheating, the refrigerant (hot gas) rejects heat it has obtained from multiple sources. Sensible heat absorbed in the air-cooling process is rejected to air leaving the cooling coil. This air is at point C, which is the same dry-bulb temperature as the entering air minus the moisture content. This heat is also rejected into the airstream, raising the air temperature to point D. Also, nearly all electric power required to drive the refrigeration cycle is converted to heat. This portion of heat rejection raises the air leaving temperature to point E.

This process assumes that all heat is rejected by the refrigerant reheat coil. Depending on refrigerant system complexity, any part of the total heat rejection can be diverted to other heat exchangers (condensers/desuperheaters).

Dehumidifier supply air temperatures can be controlled between 10 and 35°C. However, system design should not rely on a mechanical dehumidifier as a dependable heat source for space heating, because heat is only available when the unit is operating.

In Figures 3 and 4, air enters the dehumidifying coil at point A. The dehumidifying coil removes sensible heat (SH) and latent heat (LH) from the airstream. The dehumidified, cooled air leaves the coil at its saturation temperature at point B. The total heat removed (TH) is the net cooling capacity of the system.

In Figure 3, the air at B mixes with the bypass air to get to point C. In reheating, the refrigerant (hot gas) rejects heat it has obtained from three sources. First, sensible heat absorbed in the air-cooling process is rejected to air leaving the cooling coil. This air is at point D, which is the same dry-bulb temperature as the entering air minus the moisture content. Next, the latent heat removal that causes the moisture to condense also adds heat to the hot refrigerant gas. This heat is also rejected into the airstream, raising the air temperature to point E. Finally, nearly all electric power required to drive the refrigeration cycle is converted to heat. This portion of heat rejection raises the air’s leaving temperature to point F.

This process assumes that all heat is rejected by the refrigerant reheat coil. Depending on refrigerant system complexity, any part of the total heat rejection can be diverted to other heat exchangers (condensers/desuperheaters).

Dehumidifier supply air temperatures can be controlled between 18 and 40°C. However, system design should not rely on a mechanical dehumidifier as a dependable heat source for space heating, because heat is only available when the unit is operating.

Portable Dehumidifiers. These are smaller (usually less than 3.5 kW), simpler versions of commercial dehumidifiers. They are self-contained and easily movable. They are designed to be used in localized areas, such as basements or other high-moisture areas.

As shown in Figure 5, a single fan draws humid room air through the cold coil, removing moisture that either drains into the water receptacle or passes through the cabinet into some other means of disposal. The cooled air passes through the condenser, reheating the air.

Portable dehumidifiers ordinarily maintain satisfactory humidity levels in an enclosed space when the airflow rate and unit placement move the entire air volume of the space through the dehumidifier once an hour.

Design and Construction. Portable dehumidifiers use hermetic motor-compressors; the refrigerant condenser is usually conventional finned tube. Refrigerant flow is usually controlled by a capillary tube, although some high-capacity dehumidifiers use an expansion valve. A propeller fan moves air through the unit at typical airflows of 70 to 120 L/s.

The refrigerated surface (evaporator) is usually a bare-tube coil, although finned-tube coils can be used if they are spaced to allow rapid runoff of water droplets. Vertically disposed bare-tube coils tend to collect smaller drops of water, promote quicker runoff, and result in less condensate reevaporation compared to finned-tube or horizontally arranged bare-tube coils. Continuous bare-tube coils, wound in a flat circular spiral (sometimes with two coil layers) and mounted with the flat dimension of the coil in the vertical plane, are a good design compromise because they have most of the advantages of the vertical bare-tube coil.

Evaporators are protected against corrosion by finishes such as waxing, painting, or anodizing (on aluminum). Waxing reduces the wetting effect that promotes condensate formation; however, tests on waxed versus nonwaxed evaporator surfaces show negligible loss of capacity. Thin paint films do not have an appreciable effect on capacity.

Removable water receptacles, provided with most dehumidifiers, hold 7.5 to 11 L and are usually made of plastic to withstand corrosion. Easy removal and handling without spillage are important. Most dehumidifiers also provide either a means of attaching a flexible hose to the water receptacle or a fitting provided specially for that purpose, allowing direct gravity drainage to another means of disposal external to the cabinet.

An adjustable humidistat (30 to 80%) automatically cycles the unit to maintain a preselected relative humidity. The humidistat may also provide a detent setting for continuous operation. Some models also include a sensing and switching device that automatically turns the unit off when the water receptacle is full.

Dehumidifiers are designed to provide optimum performance at standard rating conditions of 27°C db room temperature and 60% rh. When the room is less than 18°C db and 60% rh, the evaporator may freeze. This effect is especially noticeable on units with a capillary tube.

Some dehumidifiers are equipped with defrost controls that cycle the compressor off under frosting conditions. This control is generally a bimetal thermostat attached to the evaporator tubing, allowing dehumidification to continue at a reduced rate when frosting conditions exist. The humidistat can sometimes be adjusted to a higher relative humidity setting, which reduces the number and duration of running cycles and allows satisfactory operation at low-load conditions. Often, especially in the late fall and early spring, supplemental heat must be provided from other sources to maintain above-frosting temperatures in the space.

Capacity and Performance Rating. Portable dehumidifiers are available with moisture removal capacities of 5 to 30 L per 24 h, and are operable from ordinary household electrical outlets (115 or 230 V, single-phase, 60 Hz). Input varies from 200 to 800 W, depending on the output capacity rating.

AHAM Standard DH-1 establishes a uniform procedure for determining the rated capacity of dehumidifiers under specified test conditions and establishes other recommended performance characteristics. An industry certification program sponsored by AHAM covers the great majority of portable dehumidifiers and certifies dehumidification capacity.

The U.S. Environmental Protection Agency (EPA) qualifies dehumidifiers to carry its ENERGY STAR^® label if they remove the same amount of moisture as similarly sized standard units, but use at least 10% less energy. The EPA’s ENERGY STAR website provides additional information on qualifying products (EPA 2020).

Whole-House Dehumidifiers. Whole-house dehumidifiers have higher moisture removal capacity than portable dehumidifiers. Blowers in whole-house dehumidifiers are typically more powerful because they draw air through the unit at higher external static pressures as compared to portable dehumidifiers. The design allows for connection of ductwork to the unit’s inlet and outlet. Moist air is typically drawn from either the return plenum of the HVAC system ductwork (Figure 6) or from a centrally located register. The air is dehumidified and then discharged into the supply ductwork of the HVAC system for distribution. To prevent reevaporation of moisture from the HVAC system evaporator coil, the dry air enters the system downstream of the coil.

Whole-house dehumidifiers typically integrate controls to allow operation of the HVAC system blower while dehumidifying the air; this allows the dry air to be distributed throughout the home. Some dehumidifiers are equipped with controls to bring in outdoor air, which can then be run through the dehumidifier before being distributed to the rest of the home. Sensors are typically located inside the dehumidifier to measure the temperature and relative humidity of air passing through the unit as opposed to the surrounding air. Controls on some models may provide for installation of a remote humidistat or humidity sensor. Drain tubing must typically be installed on the dehumidifier and routed to the nearest floor drain. Most models include an internal condensate overflow switch or provide for the installation of a field-installed overflow switch that will turn the unit off if it overflows.

Units are typically sheltered from the elements and should not be installed in locations that can experience freezing temperatures. Whole-house dehumidifiers are typically located near the HVAC system for convenient access to the supply and return plenums and to drains. Systems may be installed in basements, closets, crawlspaces, or attics, so dehumidifier cabinets are typically insulated. When units are installed over a finished area, drain pans are commonly installed under the unit.

Codes. Domestic dehumidifiers are designed to meet the safety requirements of UL Standard 474, Canadian Electrical Code, and ASHRAE Standard 15. UL-listed and CSA-approved equipment have a label or data plate indicating approval. UL also publishes the Electrical Appliance and Utilization Equipment Directory, which covers this type of appliance.

Basic components of general-purpose dehumidifiers are shown in Figure 7. An air filter is required to protect the evaporator. Dehumidifying coils, because of their depth and thoroughly wetted surfaces, are excellent dust collectors and not as easily cleanable as much thinner air-conditioning evaporator coils. However, the large amount of condensate has a self-cleaning effect. A bypass damper at the evaporator coil allows airflow adjustments for the evaporator without decreasing airflow for the reheat coil. Dehumidifying and reheat coils may operate at different airflows.

The compressor may be isolated from the airstream or located in it. Locating the compressor in the airstream may make service more difficult, but this arrangement allows heat lost through the compressor casing to be provided to the conditioned space while reducing the size of the enclosure. During the cooling season, this compressor location reduces the unit’s sensible cooling capacity.

Code-required outdoor air may be introduced between the evaporator and reheat coil. The amount of outdoor air should be controlled to not adversely affect the refrigeration system’s operation. Preheating outdoor air may be required in colder climates. Use caution when introducing outdoor air before the evaporator coil. Some applications (e.g., indoor swimming pools) have a constant moisture load year round. The dehumidifier’s performance is based on the conditions of the air moving across the evaporator coil. Mixing outdoor air with space return air before the evaporator coil changes the dehumidifier’s performance to latent removal, which may be less than the space load and result in the inability to maintain the original design conditions. The unit’s latent capacity can be significantly reduced when outdoor air is introduced before the evaporator coil.

Computerized controls can sense return air temperature and relative humidity. Remote wall-mounted sensors are also available. More sophisticated controls are desirable to regulate dew-point temperature and maintain the desired relative humidity in the space.

General Considerations. Before considering installation of any type of dehumidification equipment, all latent loads should be identified and quantified. In many cases, this might lead to decisions that reduce the latent load. For example, a storage facility that does not have an adequate vapor retarder in the building envelope should be retrofitted before attempting to calculate the amount of moisture migration through the structure. The same approach should be taken to reduce the amount of uncontrolled air infiltration.

Consider covering large water surfaces, such as vats, and/or providing a local exhaust hood to evacuate concentrated water vapor from where it is generated. Although these corrections seem to add cost to a project, the resulting reduced size of the dehumidifier and its lower operating cost often result in an attractive financial payback.

Other special considerations include the following:

A DX dedicated outdoor air system (DX-DOAS) unit is used to separately condition outdoor air brought into the building for ventilation or to replace air that is being exhausted. (As such, select a DX-DOAS unit based on its latent dehumidification capacity, not necessarily on its total air-conditioning capacity.) This conditioned outdoor air is then delivered either directly to each occupied space, to small HVAC units located in or near the space, or to a central air handler serving the spaces. Meanwhile, the local or central HVAC equipment is used to maintain space temperature. Treating outdoor air separately from recirculated return air makes it more straightforward to verify sufficient ventilation airflow and enables humidity control in the occupied spaces. Decoupling the latent load from the sensible using a DX-DOAS can make both the dehumidifier and sensible cooling equipment more efficient.

DX-DOAS units use a vapor compression refrigeration cycle as part of the system to cool the air below its dew point and condense the moisture on a dehumidifying coil.

AHRI Standard 921 establishes requirements for the testing and rating of moisture removal capacity and moisture removal efficiency of DX-DOAS units. ASHRAE Standard 198 prescribes test methods for rating DX-DOAS units.

DX-DOAS units may require simultaneous heat rejection to the reheat coil and/or another condenser (air- or water-cooled), because it may not always be possible or warranted in the application to reject the total heat of rejection from the dehumidifying coil to the conditioned airstream. A rainproof air intake and cooling capacity modulation (or staging) are important. With constantly changing weather conditions, even throughout the day, compressor capacity must be adjusted to prevent coil freeze-ups. Basic components are shown in Figure 8.

Auxiliary heating may be required for year-round operation in some climates. Water- and steam-heating coils should have freeze protection features. When using indirect-fired gas heaters, the combustion chamber should be resistant to condensation.

DX-DOAS units may be interfaced with a building automation system (BAS) to control the unit’s on/off status and operating mode, because most spaces do not require continuous ventilation or replacement air. Air exhaust systems must also be synchronized with the DX-DOAS unit to maintain proper building pressurization.

The inclusion of exhaust air energy recovery within DX-DOAS units provides the opportunity to transfer energy between the two airstreams. A typical arrangement is shown in Figure 9.

For more information on DOAS, see Chapter 51.

Indoor pools (natatoriums) are an efficient application for mechanical dehumidifiers. Humidity control is required 24 h a day, year-round. Dehumidifiers are available as single- and double-blower units (see Figures 10 to 13). AHRI Standard 911 establishes requirements for testing and rating moisture removal capacity and moisture removal efficiency of indoor pool dehumidifiers. ASHRAE Standard 190 prescribes test methods for rating these units.

The latent heat (LH) from dehumidification (see Figure 2) comes nearly exclusively from pool water (excluding humidity from makeup air and latent heat from large spectator areas). Loss of evaporation heat cools the pool water. By returning evaporation heat losses to the pool water, the sensible heat between points C and D of Figure 2 is not rejected into the supply air, which can reduce supply air temperature by approximately 8 K. ASHRAE Standard 90.1 requires that heated pools be provided with a vapor-retardant pool cover unless 60% of their energy for heating is site recovered. Use of compressor waste heat for pool water heating may satisfy this requirement.

Energy Considerations. The pool water temperature, space temperature, and relative humidity conditions maintained at a facility directly affect occupant comfort, dehumidifier size, and operating costs. It is important that the design engineer understand the effects of changes to these conditions to properly establish realistic operating conditions for a given project. Operating temperatures can change dramatically, depending on the type of pool being designed. See Table 2 in Chapter 6 of the 2023 ASHRAE Handbook—HVAC Applications for additional information.

The peak dehumidification load and peak energy conditions may not be concurrent in a natatorium. The peak dehumidification load generally occurs on a summer design day where the latent load from ventilation air is at its peak. The peak energy condition often is in the winter, when cold, dry ventilation air is a significant heating load and dehumidification credit to the space. In addition to extra heating costs, introducing more outdoor air than required by code in winter increases operating costs by driving space relative humidity levels down. This increases pool water evaporation and, consequently, pool water heating, makeup water, and pool chemical costs. Low humidity levels also increase bather discomfort through the evaporation effect across the surface of bathers’ skin, causing a chilling effect on leaving the pool.

Fan energy can be a significant portion of the total energy consumed by a natatorium dehumidifier because the supply air fan operates constantly and delivers relatively high amounts of air.

Many dehumidifiers have exhaust fans to maintain the natatorium at a negative pressure. This exhaust air is energy rich, and heat recovery should be considered. Dehumidifiers are available with heat recovery such as heat pipes, glycol-runaround loops, compressorized heat pumps, and plate heat exchangers that transfer heat from the energy-rich exhaust air to cold incoming ventilation air (see Chapter 26).

Condensation. Note that the space dew point in most pools is above 17°C. Even when space conditions are properly maintained, there is still a significantly higher chance, compared to traditional conditioned spaces, that condensation might occur in the space or within the HVAC system. Condensation is possible in winter or summer.

Most codes have adopted ASHRAE Standard 62.1 to determine the amount of outdoor air required for acceptable indoor air quality. Introducing outdoor air that is cooler than the room dew-point temperature could lead to condensation. This is especially important in areas that experience cold winters. If condensation might occur, preheating the outdoor air is required.

Most dehumidifiers have a cooling mode. The units are generally designed to ensure supply air to the space is warmer than the space dew point. When adding additional cooling capacity to a dehumidifier, summer condensation is a concern if supply air could be cooled below the dew point of either the outdoor air or the space.

Designers are encouraged to verify that all necessary measures to avoid condensation have been taken, especially when modifying or customizing standard products from manufacturers. Discharge air must always be supplied at a temperature higher than the room dew-point temperature. Similarly, blended airstreams must not be allowed to result in temperatures cooler than the space dew point.

Single-blower pool dehumidifiers (Figure 10) are similar to general-purpose dehumidifiers (see Figure 4), with the following exceptions:

Figure 11 shows a double-blower pool dehumidifier with economizer dampers and a full-sized return fan located upstream of the evaporator coil. This configuration can provide up to 100% makeup air to maintain humidity levels, which can be attractive when the outdoor dew point is below the required indoor dew point during mild weather, or in climates when enough hours of dry- and wet-bulb conditions are below the level to be maintained. Preheating outdoor air may be required to prevent condensation inside the mixing box. Also, this configuration does not recover energy from the warm, moist exhaust air. During dehumidifying coil operation, the amount of makeup and exhaust air is limited by outdoor conditions, especially during the heating season. Cold makeup air may lower the mixed-air temperature to the point where the dehumidifying coil cannot extract any moisture. Be careful during economizer operation to not introduce more outdoor air than required by code, so as not to lower the space humidity below design levels, as discussed in the section on Energy Considerations.

In some regions, it is economically attractive to remove moisture from the exhaust air to recover its latent heat. In this case, the dehumidifying coil is installed in the return air section. Figure 12 shows a double-blower pool dehumidifier with economizer dampers and return fan located downstream of the evaporator coil. A damper system can also be incorporated to exhaust before the evaporator coil during colder conditions and after the evaporator during warmer conditions. This configuration recovers energy from the warm, moist exhaust air; however, exhausting air from downstream of the evaporator coil also reduces the unit’s sensible capacity by the amount of the exhaust air. The ratio of return air to exhaust air must be considered to determine the unit’s capacity to remove moisture from the conditioned space.

Figure 13 shows a different unit configuration that addresses concerns related to blower energy use during the various operating modes. This unit can operate with the supply blower only, or with the addition of one or two exhaust blowers.

Most manufacturers also offer some means of air-to-air heat recovery between the exhaust and makeup airstreams (see Chapter 26). During cold weather, this arrangement preheats entering makeup air with heat recovered from the exhaust airstream. Latent heat recovery may not be practical, however, because it transfers moisture to the entering air, thus possibly increasing dehumidification requirements.

Control systems should be compatible with building automation systems; however, the BAS must not disable dehumidifier operation because indoor pools always need some dehumidification, regardless of occupancy, and require specialized control sequences.

General Considerations. The primary function of an indoor swimming pool dehumidifier is to maintain the space dew-point temperature at the design level year round and to provide adequate air circulation to comply with minimum air change rates.

For more information on indoor swimming pool (natatorium) applications, see Chapter 6 of the 2023 ASHRAE Handbook—HVAC Applications.

Types of Equipment. Indoor swimming pool dehumidifiers are available in single- and double-blower configurations (see Figures 10 to 13). Heat from the refrigeration circuit can be (1) used to reheat supply air, (2) used to heat pool water, or (3) rejected to the outdoors by an optional air- or water-cooled condenser. Equipment configurations are available to use the heat for any combination of these three purposes.

The equipment can be located indoors or outdoors, and may be manufactured as a single package or as a split system with a remote condenser. Avoiding remote condensers with long refrigerant lines reduces refrigerant charge. Indoor, air-cooled condensers are typically equipped with a blower-type fan suitable for duct connections. An optional economizer allows for introduction of up to 100% of outdoor air (turning off the compressors) when conditions are appropriate.

When selecting a dehumidifier for an indoor swimming pool application, several questions need to be addressed to ensure that the dehumidifier can maintain the space conditions:

Design for ice rink dehumidifiers is similar to that of general-purpose dehumidifiers (see Figure 4). However, because of the lower temperatures, airflow and dehumidifying and reheat coils are selected in accordance with the following conditions:

For large spectator areas, special makeup air dehumidifiers may be required.

General Considerations. For community ice rinks with small spectator areas (or none), it is customary to install two small dehumidifiers over the dasher boards in a diagonal arrangement, 3.7 to 4.6 m above the ice surface (Figure 14). Take care that discharge air from dehumidifiers is not directed toward the ice surface. Forced airflow at any temperature may damage the ice surface. Ice rinks with large spectator areas have different requirements.

The spectator area is typically maintained at 21°C. To limit moisture migration to the ice sheet, space conditions must be maintained at 50% rh or less. The resulting dew-point temperature is then 10°C or less. The air temperature over the ice sheet in the dasher boards, however, is approximately 3 K lower than the air in the spectator area. With an air temperature of 18°C and a dew-point temperature of 10°C or less, fog over the ice sheet cannot develop. As a general rule, mechanical ice rink dehumidifiers are most effective for condensation and fog control when dry-bulb space temperature is at least 8 K above the dew point. For additional fog and condensation prevention methods, see Chapter 44 of the 2022 ASHRAE Handbook—Refrigeration.

Industrial products and surfaces can sweat, causing corrosion, dimensional distortions, and other deterioration from excess humidity in the air. In some cases, this humidity is caused by leakage or infiltration; in other cases, it may be from indoor storage or processes. Normal air conditioning is not always adequate to control excess humidity, leading to material or structural damages and cool but damp working conditions. Industrial dehumidifiers use direct-expansion refrigeration coils to remove moisture and thus lower the supply air dew point to

Some mechanical dehumidifiers may also contribute to space heating and/or cooling, recover heat energy for processes, and/or recover water for allowable purposes.

Sources of Humidity. Indoor air quality is affected by several key factors, which vary in importance depending on the location of the building and on the activity for which a building is designed. Relative humidity is usually a critical air quality factor, with high indoor relative humidity resulting from the following sources of moisture:

Moisture migrates from areas of high vapor pressure to areas of low vapor pressure. In the summer, when the outdoor air is warm and humid, moisture can find a path to the interior of a cooler or drier structure. This could be from openings in the building such as doors, infiltration through cracks and poorly sealed joints, or permeation in the case of low-quality or nonexistent vapor retarders. In many instances, the primary source of humidity is from outdoor air purposely brought into the structure to meet air quality standards, or to replace air being exhausted because of contamination.

Occupants contribute to the moisture load through respiration and perspiration; amounts depend on the number of people and their activities. A worker involved in heavy lifting can generate seven times the moisture of a co-worker seated at rest. In agricultural or laboratory structures, animals also produce a moisture load.

Indoor materials or processes may give off moisture. Storing or cooking fruits, vegetables, or other foodstuffs may release moisture indoors. The presence of open tanks of water or the storing or handling of wet materials, such as wood, may also release moisture indoors.

Moisture dissolved in indoor air will condense onto any surface that is at a temperature lower than the room air’s dew-point temperature. This can lead to quality and productivity problems or even to damage to the building and plant equipment. Rust and corrosion can affect metal surfaces, electrical controls and contacts, etc., which can lead to increased costs and even to potentially hazardous conditions.

For typical conditions (temperatures and relative humidities) of industrial applications, see Table 1 of Chapter 15 of the 2023 ASHRAE Handbook—HVAC Applications.

If the main conditioning system is not selected to handle the high dehumidification load of a grow or drying space, the engineer may use a combination of a primary cooling air-handling system combined with in-room or ducted dehumidifiers to support the main cooling system for the water load in these rooms. Dehumidifiers for controlled environment agriculture share many of the characteristics of general-purpose dehumidifiers (see Figures 4 and 7), but may have a few additional design considerations because of the specialized nature of their application:

General Considerations. Because most grow rooms are sensible cooling primary load facilities, care is needed to ensure that heat gain from the dehumidification process can be managed by the existing or designed sensible cooling devices. A CEA dehumidifier can be selected with remote heat rejection to provide some or all of the sensible cooling required. This approach may achieve better energy management by ensuring that the facility is not forced to oversize or overuse sensible cooling devices to counter heat gain from dehumidification equipment in the space.

Grow rooms can require washdown cycles. Ensure that selected equipment has washdown capabilities to reduce pathogen and mold growth, and that filters are easily accessible for regular replacement.

CEA rooms should be considered process air and handled appropriately as such. The typical relative humidity excursions from set point that are typically acceptable in a comfort cooling environment are not acceptable for grow rooms.

Dehumidification of grow rooms is a wildly variable application of HVAC equipment. The room calculations should be made for selections of the equipment in an all-sensible environment: lights and fans on, but no plants. They should also be made for the heaviest day condition: lights on and loaded with plants transpiring at the maximum expected amount. What is commonly missed is calculating the day-night transition and the steady-state night conditions. All four of these major load cases must be satisfied successfully by the combination of central cooling and unitary dehumidification units. Further, these load cases should be calculated for young plants entering rooms that can be over-dried with systems that cannot be turned down or bypassed to reduce the water removal rate.

When looking at the calculations for the sensible and latent heat loads within the room, the engineer should be mindful that the plants themselves do not constitute a latent load as they are not a heat source. The plants will take the energy from the lights, and as long as the vapor pressure deficit is correct for them to transpire, the transpiration will convert the sensible load from the room into a latent load through the latent heat of evaporation. With this in mind, the calculations can be made for the maximum sensible load (lights on, no plants) and the maximum water removal rate, which can actually require more energy than the sensible case, making the SHR of the total system effectively 0 or negative.

The calculations should not add the calculated sensible and latent loads. The calculation should start with the total sensible energy gain into the room as case 1. This is very easily addressed by the main cooling system. Then, the watering recipes should be examined to find the steady state transpiration load into the room. This transpiration converts sensible heat into latent heat through adiabatic cooling. When looking at case 2 (lights on/heavy plants), the engineer may find that the water load going into the room creates a latent load that exceeds the lighting power. This is often the case in nurseries where there can be heavy young, non-flowering plants, within a space with nursery-level lighting. The engineer should then select dehumidification equipment to address the water load, taking into consideration the contribution from the main cooling equipment, and then recalculating the main cooling equipment with the dehumidification heat rejection taken into consideration. In many cases, the reheat provided by the dehumidification equipment can make up for the sensible shortfall, allowing the main cooling equipment to be roughly equivalent size as the sensible case. In many existing installations, Engineers have made the mistake of adding the sensible and latent loads together.

Then, the selected equipment should be calculated against the two final operating conditions: case 3 (lights off/plants transpiring) and case 4 (lights off/plants asleep). Case 3 represents a pure dehumidification operation case. The sensible heating elements are all shut down, while the plants continue to transpire at the same rate they had during the day for a period of about 40 minutes (±15 min). However, the systems should not immediately decrease the temperature set point at the same time, or there will be an over-cooling (further pressure on relative humidity) followed by an over-drying condition when the plants go to sleep. Case 4 represents the lights off hours where the plants have gone to sleep. The transpiration load typically decreases to about 5 to 10% of the daytime rate and there is no real sensible loads aside from circulation fans and the building envelope.

The issue here is that the equipment will not be sized appropriately (oversized) and cause short cycling. This short cycling not only leads to shorter equipment life, but it causes severe control excursions with respect to room humidity. The classic example is a dry room served by a two stage direct-expansion packaged HVAC unit when the DX coils both cycle off. The wet coil allows the discharge air humidity to spike and it takes some time for the system to cycle back on. This can cause excessive condensation within the supply ducts, water intrusion into the rooms, etc.

Dehumidification for CEA rooms is crucial to maintaining the water vapor levels, and what growers call vapor pressure deficit (VPD). This is simply the concept of relative humidity (rh) with a specific bent towards the vapor pressure of water rather than just a percent of total carrying capacity. The plants are able to transpire when the vapor pressure deficit is ideal across the stomata. Growers target different VPDs for different parts of the plant’s life, the type or strain of plant, and what they are trying to achieve with that plant.

Maintenance of VPD is more important than maintaining temperature. If the plant can continue to transpire, even in over- or under-temperature excursions, there will not likely be damage to the plants. In this way, dehumidification equipment becomes the driving factor in the system design. Over-temperature conditions of a space is OK, as long as it is not too dry. However, allowing a space to get overly wet in order to maintain the space temperature is a recipe for fungus and poor plant health, even for short periods of time. Very great care should be taken when considering equipment to eliminate short cycling, maintenance of off-cycle conditions, etc.

VPD, like relative humidity, is based on temperature. So, it is possible to achieve the proper VPD at any given operating temperature. Because of this, it should be assumed that the plant can and will transpire water into the space at the given watering rates, no matter the temperature within the room. It also means, that great care should be taken when transitioning set points and operating conditions.

The largest example of this is the change from day mode to night mode. Day mode set points can generally be in the Fsupper 20s or over 30°C with relative humidity in the 55 to 65% range. By the end of the day, the plants are transpiring at their steady day awake levels, and then the lights are either dimmed and turned off or just shut off quickly. In this event, it is critical to keep the night set point from being a double-step-change on the control systems because the lights have gone out, the sensible load in the room has all but been eliminated and the plants continue to transpire. It is very typical to see relative humidity spike out after the lights shut off in conventionally constructed grow rooms. When this happens, damage can occur to the plants as they suffocate in this condition. The propagation of fungus is also typically associated with this transition period.

In addition to the above conditions, it should be noted that grow rooms are to be considered harsh environments. There is dust and intense UV light, and the space is typically cleaned with strong chemicals between harvests to eliminate pests, fungus, bacteria, etc. Ensure that the equipment, ductwork, filters, etc., have been selected to give long service life even if treated with chemicals such as chlorine dioxide, hydrogen peroxide, etc. Further, the operators may use a wash-down approach to these spaces. Investigate the intended cleaning standard operating procedures or consider signage on the equipment if it might be susceptible to certain chemicals or conditions.

A general-purpose dehumidifier system removes moisture from a product inside the drying tunnel. Air at the desired temperature and humidity constantly recirculates around the product, continuously removing moisture. Tunnel dryer dehumidifier design is similar to that of a general-purpose dehumidifier (see Figure 4). Because of the closely controlled space humidity, temperature, and airflow, select the dehumidifying and reheat coils in accordance with the following conditions:

Process Considerations. A precision control system is required to prevent overdrying by automatically shutting off or by reducing capacity when the moisture content in the product has reached the desired level. Continuous control of temperature and relative humidity improves the drying process and maintains product quality, and potentially can reduce operating cost compared to ventilation only. Users should be able to specify drying temperature to best preserve initial product quality.

Design Considerations. The usual operating range for tunnel dryer dehumidifiers is 15 to 38°C db and 10 to 38°C dew point. An average air velocity of 2.54 m/s across the tunnel net free area is recommended for effective drying.

Tunnels should be no longer than 9 m, to prevent saturated air at the tunnel’s end. Construction must be air- and vaportight to prevent moisture infiltration from surrounding spaces.

Some products may produce corrosive contaminants, so tunnel dryer dehumidifier components should be corrosion resistant.

For a typical installation of a tunnel dryer dehumidifier, see Figure 15.