Improving Plant Processes With Dehumidification

Dehumidification solves four common problems typically encountered in manufacturing.

Moisture regain (clogging and sticking) . Dehumidification prevents moisture regain from occurring in powder processing and product handling activities such as granulated sugar storage silos, packaging rooms, ammonium nitrate fertilizer storage buildings, and candy wrapping.
Condensation or sweating. Condensation can lead to mold, fungus growth, and contamination from overhead dripping. It occurs when cold surfaces such as pipes, silos, and ceilings in manufacturing plants are surrounded by moist air. Dehumidification systems prevent condensation by keeping the air surrounding the cold surface at a constant dewpoint set just below the temperature of the coldest surface.
Corrosion. Removing moisture from the air prevents rust from developing on metal surfaces and organic material from rotting.
Drying heat-sensitive products. Many types of products must be dried to low moisture levels but cannot stand excessive heat, including pharmaceutical diagnostics, thermoset resins, industrial enzymes, and most proteins. Using a dehumidifier to speed up drying time without damaging the product is most beneficial when the product's temperature limit is around 120 F and the humidity limit is 95 F or below.
Methods of dehumidification
Moisture can be removed from air in two ways:
Condense out the water by chilling the air
Pull out the water by passing the air across a desiccant surface.
Dehumidification by condensation. Dehumidification by cooling (Fig. 1) can be especially effective when air is warm and humidity is high. Under those circumstances, a cooling system can remove 2-4 times as much energy (temperature and moisture) from an airstream as the machine consumes in electrical power to accomplish this removal.

Air to be dried passes through a cooling coil. As air cools, it loses its capacity to hold water vapor. Water condenses on the cooling coil surface, and falls to the drain pan as liquid. The air is drier in absolute terms, but is now saturated. (Its relative humidity [RH] is close to 100%.) If a low RH is needed in addition to a low absolute moisture level, the air can be heated after it leaves the cooling coil.

To dehumidify a damp area, a consumer-grade dehumidifier might be used. In this situation, air passes across a cold coil, which cools and dries the air. Saturated air passes through a second coil, where heat from the compressor and refrigerant is added back into the airstream, lowering the RH before the air is supplied to the space.

Conventional air conditioning systems accomplish dehumidification in the same way. Such equipment is usually configured for optimal heat removal. Dehumidification is the byproduct of the primary air cooling function. In an industrial setting, cooling-based dehumidification typically is accomplished by custom-engineered air handling units optimized for removing moisture rather than removing heat. Such units cool small amounts of air significantly instead of large amounts of air slightly. Such deep cooling condenses more moisture from the air.

Dehumidification using desiccant. When the required dewpoint is low, or when very low RH levels are needed, desiccant-based dehumidification is often a more cost-effective alternative (Fig. 2). This equipment uses differences in vapor pressure to remove moisture from air by chemical attraction. The surface of dry desiccant has a very low vapor pressure compared to the much higher vapor pressure of humid air.

Water vapor moves out of the humid air onto the desiccant surface to eliminate the vapor pressure difference. Eventually, the desiccant surface collects enough water vapor to equal the vapor pressure of the humid air. Then, it must be dried (reactivated) by applying heat before it can be used to remove more moisture from the airstream.

There are many ways to present a desiccant to an airstream. In most modern, atmospheric-pressure industrial dehumidifiers, the desiccant is held in a lightweight, wheel-shaped matrix that rotates between two separate airstreams (Fig. 3). The desiccant is contained in the walls of thin air channels that extend through the depth of the wheel. Channel diameters vary, but typically are about 2 mm. Wheel diameter depends on how much air must pass through it. Large airflow volumes require a larger diameter wheel.

Process air passes through a portion of the rotating desiccant wheel, giving off its water vapor to the desiccant in the air passage walls. Dry air leaves the wheel and is carried to the point of use by fans or blowers.

Simultaneously, the rotating wheel passes through a second, smaller airstream carrying heated reactivation air. The hot air drives the water vapor from the desiccant. As the wheel rotates from the reactivation air into the process airstream, it can once again remove moisture.

As the air dries, its temperature rises in proportion to the amount of water removed. Drier air means warmer air. This process is the reverse of evaporative cooling. When water is evaporated into air, the heat needed for evaporation comes from that same air and the sensible temperature falls. Conversely, when air is dehumidified, the heat needed to evaporate the water originally is liberated and the temperature of the airstream rises.

Because a desiccant dehumidifier removes water vapor rather than condensed liquid from the air, there is no risk of freezing. This type of equipment is most often used for applications requiring dewpoints below 50 F.

Which method?
In most manufacturing/processing applications, desiccant and cooling-based dehumidification technologies are used cooperatively. Cooling-based dehumidification handles the moisture load occurring at high dewpoints; desiccant-based dehumidification removes the moisture load at lower dewpoints. The mix of the two technologies depends on the characteristics of the specific application. The following factors should be considered.
Evaluating the dewpoint control level. When the required moisture control level is comparatively high (above 50-F dewpoint), cooling-based dehumidification is economical, in terms of both operating costs and initial equipment cost. Low-cost, high-volume, standard equipment is available for this control level. Below this level, costs increase because precautions need to be taken to prevent condensed water on the cooling coil from freezing.

Although water does not freeze until temperatures fall below 32 F, a dehumidification system may have to deliver air below that level to maintain a room below 50-F dewpoint. Without protective measures, a cooling-based dehumidifier providing air at low dewpoints can freeze. Equipment fitted with freeze-protection devices costs more and has higher operating costs/kg of water removed. Under these circumstances, desiccants become more economical than cooling-based systems at low dewpoints.

Gauging relative humidity sensitivity. When a process needs a low moisture level in absolute terms, but can tolerate a high RH, a cooling-based dehumidification system can be cost effective without the need for desiccants. An ideal temperature for fruit and vegetable storage might be 40 F. Of course, the dewpoint must be lower than that. If the RH is below 90%, the fruit can dry out and lose value. Because the product needs both low temperature and high humidity, cooling-based systems are ideal. In contrast, other processes might demand a low RH and a low dewpoint.
Determining temperature tolerance. An application with a narrow temperature tolerance needs cooling and heating in addition to dehumidification. If the application can tolerate wide temperature variations (such as occur in unheated storage, for example), the dehumidification equipment alone may suffice.

Designing the system
Industrial dehumidification systems are tailored for each project. Therefore, a near-infinite variety of possible components are available to serve a near-infinite variety of possible applications. These components make it easy to optimize a system design. However, the variety also presents the need to make many decisions early in the design process, often before cost/benefit implications are clear.

Define the purpose of the project. The purpose of the project must be clearly understood and documented so that decisions can be properly ordered according to their importance. If the system must prevent mold growth on starch in a storage silo, a strict tolerance of ±1% RH is not needed. The only concerns are that humidity does not exceed 60% and that condensation does not occur. A simple, inexpensive configuration will do the job.

Conversely, if dehumidification is needed to prevent the corrosion of lithium, a control with a tolerance of ±5% RH is useless. Above 2% RH, lithium corrodes and gives off hydrogen which eventually explodes. A sensor with a tolerance greater than the critical control level could not start the system in time to prevent the explosion. Understanding the purpose of the project helps prevent both unnecessary expense and false economy.

Establish control levels and tolerances. Next, the humidity and temperature control levels and tolerances needed to achieve the purposes of the project must be determined. Sometimes, these decisions require research. Other times the relationship between the process and moisture levels is well understood. For example, if a process bogs down in the summer, but not the rest of the year, humidity tolerance is likely quite wide. The dehumidification system need only remove summer humidity extremes. Or perhaps the supplier of the problem material can recommend optimal environmental conditions for processing the product.
The control setpoint must be established to allow the peak heat and moisture loads to be calculated. Without loads, equipment sizes and costs cannot be estimated. For example, a system that holds humidity at 72 F and 35% RH is much smaller than one that holds humidity at 72 F and 25% RH, all other variables being equal. The lower the humidity level, the more costly the system. Higher moisture loads also raise costs. For these reasons, calculating loads is a critical step in system design.

Calculate moisture loads. In most cases, the system supplier assists the plant engineer in calculating moisture loads. Typical loads come from ventilation air, air infiltration, miscellaneous openings, people, products and packaging, and vapor permeation. Lower loads mean less expensive equipment.

Ventilation air/air filtration. The most cost-effective adjustment to building operation is to minimize exhaust air, reducing the cost of dehumidifying the makeup air. Sealing building cracks also reduces dehumidification costs for a modest investment in caulk.
Fresh air is required in most controlled spaces. Most building codes specify a specific amount of air per person or per sq ft of occupied space. Often, insufficient attention is paid to making sure all exhaust air is replaced. The problem is especially acute in large areas where exhausts may not be obvious. Engineers need to be fully aware of the effect of insufficient makeup air on humidity-controlled spaces.

Miscellaneous openings. The next largest load source results from miscellaneous openings. Opening a door pulls in moist air. Observe and note the number of times a door is opened at busiest times.

Air locks greatly reduce moist air infiltration. As humidity control levels go lower, air lock doors become more economically advantageous. With an air lock, it is assumed that equilibrium is reached half way between the inside and outside conditions and that all the air enters the room each time the lock opens.

When product must enter or leave a humidity-controlled room on a conveyor, the opening must be considered as a possible infiltration source. The infiltration of moist air through large openings such as ducts can be reduced by supplying a slight overpressure of makeup air to force dry air out of cracks rather than letting moist air leak in.

People. When workers exhale or perspire, moisture is given off, creating another load source. The rate depends on the exertion level. When room loads are calculated, allowances must be made for "visitors" entering and leaving the room. Doubling "people" estimates to allow for changes in room use is often recommended.

Products/packaging. The load from products and packaging varies greatly by application. In large storage applications, moisture released from product may be the single largest load component. The load equals the difference between a product's initial wet weight and the weight when at equilibrium with the lower humidity.

Vapor permeation. Vapor permeation through building components is typically the smallest load component. It accounts for less than 2% of the total. This load should receive more attention if the building is very large and moisture permeates across a large surface area or if the control condition is very low. Below 5% RH, every leak is critical.

Peak design weather conditions are another important element in the load calculations. The end user must decide how conservatively the system should be sized. If extreme weather data are used, the system will control humidity throughout the 8760 hr in a typical year. The system will also cost a lot. If some off-hours can be risked, costs may be reduced 20-30%. However, if all moisture loads peak at the same time in extreme weather, the humidity level may rise above the setpoint.

These options are quantified in the Fundamentals Handbook published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers according to the percent of annual hours that weather conditions can be expected to be above certain values. For example, the 0.4% values are likely to be exceeded only 35 hr/yr. A less conservative design point would be 1% or 2.5% values, which may be exceeded for 70 hr and 219 hr respectively.

The end user must decide which data to use because he is in the best position to assess the economic and safety consequences of being above specification for short periods.

The engineer investigating the system should work closely with potential suppliers to compare costs and benefits of dehumidification with alternative solutions. Suppliers can be most helpful when key aspects of the project are well defined. The engineer should be able to provide or have on hand the following information.

Nature of the problem and its consequences. Be ready to communicate these aspects clearly.
Purpose of the project. Be able to define goals in simple declarative sentences and to describe measurable results.
Available utilities and physical characteristics of the site. Research these factors and have the data at hand.
Dehumidification systems find wide application in manufacturing operations and processes. An end user should consider its use whenever weather variations affect the rate or quality of a process or operation, when corrosion or condensation may cause problems, or if the product must be dried at low temperatures.

-Edited by Jeanine Katzel, Senior Editor, 630-320-7142, jkatzel@cahners.com

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More Information
Technical questions about this article may be directed to the author by phone at 813-269-9319 or by e-mail at speltz@munters.com. The company web site is located at www.muntersamerica.com.

For additional articles on this and related topics, see the Air conditioning, ventilation, and refrigeration channel at www.plantengineering.com.

The Fundamentals Handbook mentioned in this article is available from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, NE, Atlanta, GA 30329; 404-636-8400; www.ashrae.org.