Mechanical dehumidifiers remove moisture by passing air over a surface that has been cooled below the air’s dew point. This cold surface may be the exterior of a chilled-water coil or a direct-expansion refrigerant coil. To prevent overcooling the space (and avoid the need to add heat energy from another source), a mechanical dehumidifier also usually has means to reheat the air, normally using recovered and recycled energy (e.g., recovering heat from hot refrigerant vapor in the refrigeration circuit). Using external energy input for reheat is wasteful and is prohibited or limited in many countries (see ASHRAE Standard 90.1).

A mechanical dehumidifier differs from a typical off-the-shelf air conditioner in that the dehumidifier usually has a much lower sensible heat ratio (SHR). The dehumidifier starts the compressor on a call for dehumidification, whereas an air conditioner starts the compressor on a call for sensible cooling. Typically, a room dehumidifier has an SHR of 0.6 or less, compared to a standard air-conditioning system of 0.8 SHR. Dehumidifiers must also allow condensation from the cooling coil to drain easily from the coils. They may need air velocities over the cooling coil lower than those for a typical air conditioner, to improve moisture runoff and minimize carryover of condensed moisture.

In addition, the need to introduce code-mandated ventilation air may require that outdoor air be treated to avoid introducing excessive moisture. Basic strategies include precooling outdoor air entering the air-conditioning evaporator coil, or providing a separate system to provide properly conditioned outdoor air. For some low-dew-point (below 45°F) applications, mechanical dehumidification may be used as the first stage, with desiccant dehumidification for the final stage to maximize efficiency and minimize installed cost.

Although the main purpose of a mechanical dehumidifier is to remove moisture from the air, many features can be incorporated for various applications, such as

Often, mechanical dehumidifiers can be incorporated in a system to use waste heat from mechanical cooling (e.g., heat rejection to a swimming pool, whirlpool, domestic hot water, heat pump loop, chilled-water loop, or remote air-cooled condenser).

Outdoor dehumidifiers should be protected against internal moisture condensation when winter conditions are severe, because of the higher dew-point temperature of air circulating in the unit.

 Psychrometrics of Dehumidification

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.

Figure 1. Dehumidification Process Points

Figure 2. Psychrometric Diagram of Typical Dehumidification Process

Figure 3. Psychrometric Diagram of Typical Dehumidification Process with Bypass Air

Figure 4. Dehumidification Process Points with Bypass Air

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 50 and 95°F. 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 65 and 105°F. 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.

 Residential Dehumidifiers

Portable Dehumidifiers. These are smaller (usually less than 1 ton), 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.

Figure 5. Typical Portable Dehumidifier

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 125 to 250 cfm.

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 16 to 24 pints 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 80°F db room temperature and 60% rh. When the room is less than 65°F 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 11 to 60 pints 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.

Figure 6. Typical Whole-House Dehumidifier Installation

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.

Figure 7. Typical General-Purpose Dehumidifier

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.

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 920 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.

Figure 8. DX-DOAS Unit

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.

Figure 9. DX-DOAS Unit with Exhaust Air Heat/Energy Recovery

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 910 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.

Figure 10. Typical Single-Blower Pool Dehumidifier

Figure 11. Typical Double-Blower Pool Dehumidifier with DX Coil in Supply Air Section

Figure 12. Typical Double-Blower Pool Dehumidifier with DX Coil in Return Air Section

Figure 13. Supply Blower and Double Exhaust Blower Pool Dehumidifier

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 15°F. 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 2019 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 62°F. 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.

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, 12 to 15 ft 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.

Figure 14. Typical Installation of Ice Rink Dehumidifiers

The spectator area is typically maintained at 70°F. 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 50°F or less. The air temperature over the ice sheet in the dasher boards, however, is approximately 5°F lower than the air in the spectator area. With an air temperature of 65°F and a dew-point temperature of 50°F 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 15°F above the dew point. For additional fog and condensation prevention methods, see Chapter 44 of the 2018 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 2019 ASHRAE Handbook—HVAC Applications.

Dehumidifiers for controlled environment agriculture (CEA; also called indoor grow rooms) 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.

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 59 to 100°F db and 50 to 100°F dew point. An average air velocity of 500 fpm across the tunnel net free area is recommended for effective drying.

Tunnels should be no longer than 30 ft, 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.

Figure 15. Typical Tunnel Dryer Dehumidifier

Care is needed when locating the humidity-sensing device that triggers compressor operation. A poorly located sensor results in failure of the system to properly control the humidity levels. The sensor must be located in an area that gives an accurate reading and feedback of the conditions needing control. In larger-volume areas, multiple sensors with an averaging algorithm might provide the best result. Consider the following when selecting sensor locations:

Heat recovery devices require additional sensors for proper operation.

Equipment and sensors must be installed properly so that they function in accordance with manufacturers’ specifications. Interconnecting diagrams for the low-voltage control system should be documented for proper future servicing. Planning is important for installing large, roof-mounted equipment because special rigging is frequently required.

The refrigerant circuit must be clean, dry, and leak-free. An advantage of packaged equipment is that proper installation minimizes the risk of field contamination of the circuit. Take care to properly install split-system interconnecting tubing (e.g., proper cleanliness, brazing, evacuation to remove moisture). Charge split systems according to the manufacturer’s instructions.

Equipment must be located to avoid noise and vibration problems. Mount single-package equipment of over 20 tons capacity on concrete pads if vibration control is a concern. Large-capacity equipment should be roof mounted only after the roof’s structural adequacy has been evaluated. Additional installation guidelines include the following:

Equipment should be listed or certified by nationally recognized testing laboratories to ensure safe operation and compliance with government and utility regulations. Equipment should also be installed to comply with agency standards’ rating and application requirements to ensure that it performs according to industry criteria. Larger and more specialized equipment often does not carry agency labeling. However, power and control wiring practices should comply with the National Electrical Code^® (NFPA Standard 70). Consult local codes before design, and consult local inspectors before installation.

A clear, accurate wiring diagram and well-written service manual are essential to the installer and service personnel. Easy, safe service access must be provided for cleaning, lubrication, and periodic maintenance of filters and belts. In addition, access for replacement of major components must be provided and preserved.

Service personnel must be qualified to repair or replace mechanical and electrical components and to recover and properly recycle or dispose of any refrigerant removed from a system. They must also understand the importance of controlling moisture and other contaminants in the refrigerant circuit; they should know how to clean a hermetic system if it has been opened for service (see Chapter 7 of the 2018 ASHRAE Handbook—Refrigeration). Proper service procedures help ensure that the equipment continues operating efficiently for its expected life.

An air-to-air heat exchanger (heat pipe, coil runaround loop, fixed-plate heat exchanger, or rotary heat exchanger) in a series (or wraparound) configuration can be used to enhance moisture removal by a mechanical dehumidifier, improving efficiency, and possibly allowing reduced refrigeration capacity in new systems. Other uses of air-to-air heat exchangers are covered in Chapter 26.

Air-to-air heat exchangers are used with a mechanical dehumidification system to passively move heat from one place to another. The most common configuration used for dehumidification is the runaround (or wraparound) configuration (Figure 16), which removes sensible heat from the entering airstream and transfers it to the leaving airstream. (Points A to E correspond to points labeled in Figure 17.) This improves the cooling coil’s latent dehumidification capacity. This method can be applied if design calculations have accounted for the condition of air entering the evaporator coil.

Figure 16. Schematic of Dehumidification Enhancement with Wraparound Heat Pipe

Figure 17. Enhanced Dehumidification Process with Wraparound Heat Pipe

In the runaround or wraparound configuration (Figure 16), one section of the air-to-air heat exchanger is placed upstream of the cooling coil and the other section is placed downstream of the cooling coil. The air is precooled before entering the cooling coil. Heat absorbed by the upstream section of the air-to-air heat exchanger is then transferred to air leaving the cooling coil (or supply airstream) by the downstream section.

Sensible precooling by the air-to-air heat exchanger reduces the sensible load on the cooling coil, allowing an increase in its latent capacity (Figure 17). The combination of these two effects lowers the system SHR, much like the process described in the Mechanical Dehumidifiers section. Adding the air-to-air heat exchanger brings the condition of air entering the evaporator coil closer to the saturation line on the psychrometric chart (A to B). In new installations, this requires careful evaporator coil design that accounts for the actual range of air conditions after the air-to-air heat exchanger, which may differ significantly from the return air conditions.

In retrofits, the duct-to-duct (or slide-in) configuration (Figure 18) is sometimes used. One section of the air-to-air heat exchanger is placed in the return airstream, and the other section in the supply airstream. This configuration, however, does not provide as much benefit as the wraparound configuration because (1) the upstream side of the heat exchanger is located upstream of where outdoor air enters the system, (2) the higher velocity reduces the effectiveness and increases the air-side pressure drop of the heat exchanger, and (3) it requires an additional filter upstream of the air-to-air heat exchanger.

Figure 18. Slide-in Heat Pipe for Rooftop Air Conditioner Refit

(Kittler 1996)

In retrofits, the lower entering-air temperature at the evaporator coil lowers the temperature of air leaving the evaporator coil. Evaporator coil capacity is reduced because of the lower entering wet-bulb temperature, changing the system’s operating point. This must be analyzed to ensure that the mechanical refrigeration system still operates correctly. If evaporator coil freeze-up is possible, the system must include some means of deactivating the air-to-air heat exchanger or increasing airflow to prevent evaporator freezing. Some way to modulate the air-to-air heat exchanger’s capacity may be incorporated to better meet the load requirement of the mechanical dehumidifier.

Adding an air-to-air heat exchanger typically improves the moisture removal capacity of an existing mechanical dehumidification system by allowing a lower supply air dew point, while providing some reheat without additional energy use. Proper design practices are necessary to ensure that the unit’s mechanical refrigeration system still operates efficiently with the new entering air conditions and additional air-side pressure drop. Also, the added pressure drop of the air-to-air heat exchanger is likely to reduce the airflow delivered, unless fan speed in increased. If increasing fan speed is necessary, verify that the fan motor can handle the added load.

Figure 17 shows the dehumidification process when an air-to-air heat exchanger is added to an existing evaporator coil. Point A to C shows the cooling and dehumidification process of an existing DX evaporator coil, without the air-to-air heat exchanger. Point A to B shows precooling by the upstream section of heat exchanger. The process line from B to D (versus B to C, without the heat exchanger) shows how the evaporator coil’s dehumidification performance improves (lowering leaving air dew point, from C to D) if an air-to-air heat exchanger is added to the existing system, because the enthalpy of the air entering the evaporator is lowered. Point D to E illustrates that the heat removed from air upstream of the evaporator (A to B) is added back into air leaving the evaporator. The total amount of heat energy (enthalpy) removed in section A-B is equal to the amount of heat added in section D-E.