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Dehumidification in manufacturing
Controlling humidity levels in manufacturing and packaging operations can have a major impact on the bottom line. Whether it is candy coating, meat processing, battery storage or glass making, maintaining the optimum level of humidity reduces production costs through greater efficiency and fewer defects.
Dehumidification solves four common problems typically encountered in manufacturing:
Moisture regain or 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, which can lead to mold, fungus growth and contamination from overhead dripping, 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 dew point 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, thermo-set resins, industrial enzymes and most proteins. Using a dehumidifier to speed up drying time without damaging the product is most beneficial when the products temperature limit is around 120 F and the limit is 95 F or below.
These are the operating principles used in consumer-grade dehumidifiers which one might use to dehumidify a damp basement. Air passes across a cold coil, which cools and dries the air. Then the saturated air passes through a second coil, where the heat from the compressor and refrigerant is added back into the air stream, lowering the relative humidity before air is supplied to the space.
Conventional air conditioning systems also accomplish dehumidification the same way, but such equipment is usually configured for optimal heat removal, and dehumidification is a by-product of the primary function of cooling the air. For industrial purposes, cooling-based dehumidification is accomplished by custom-engineered air handling units optimized for removing moisture rather than removing heat. These units cool small amounts of air very deeply, as opposed to cooling large amounts of air only slightly. The deeper cooling condenses more moisture from the air.
Dessicant-based dehumidification
When the required dew point is low, or when very low relative humidities are needed, desiccant-based dehumidification is usually the most cost-effective alternative. 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 the desiccant must be dried (reactivated) by applying heat, before it is re-cycled to remove more moisture from the air stream.
There are many ways of presenting a desiccant to an air stream. In most modern, atmospheric pressure industrial dehumidifiers, the desiccant is held in a lightweight matrix in the shape of a wheel, which rotates between two separate air streams.
The desiccant is contained in the walls of thin air channels that extend through the depth of the wheel. The diameter of the channels varies, but is usually about two millimeters. The diameter of the wheel depends on how much air must pass through it. Larger airflow requires a larger-diameter wheel. The process air passes through the desiccant wheel giving off its water vapor to the desiccant contained in the walls of the air passages. The dry air leaves the wheel and is carried to the point of use by fans or blowers.
While that is happening, part of the wheel is rotating through a second, smaller air stream -- the reactivation air that has been heated. The hot reactivation air heats the wheel, driving water vapor out of the desiccant. As each section of the wheel rotates out of the reactivation air, its desiccant is dry, and can once again remove moisture from the process air.
As air is dried, the temperature of the process air rises in proportion to the amount of water removed. Drier air means warmer air. This is the reverse of the more familiar process of evaporative cooling. When water is evaporated into air, the heat needed for evaporation comes
from that same air, so its sensible temperature falls. Conversely, when air is dehumidified, the heat needed to evaporate the water originally is liberated, raising the temperature of the air stream.
Because a desiccant dehumidifier removes water from the air as a vapor rather than as a condensed liquid, there is no risk of freezing condensate. So this type of equipment is most often used for applications where dew points below 50 F are required.
An extreme example is lithium foil processing. Lithium metal is used for batteries. In its pure form, the metal surface is energetic enough to break water molecules apart forming gaseous hydrogen and oxygen, and liberating heat. With enough water vapor, that heat can ignite the hydrogen. Consequently, lithium foil processing takes place in rooms kept at dew points between -30 and -40 F. Plutonium and calcium metal processing require similar environments. Desiccant-type dehumidification systems maintain rooms at that level of dryness even with a moisture load of 30 to 60 people working in the room.
Desiccant vs. cooling-based dehumidification
Engineers new to dehumidification technology frequently question which method of cooling or desiccant is the best choice. In most manufacturing/processing applications, the simple answer is that both technologies are used together so they cooperate rather than compete. Cooling-based dehumidification handles the moisture load occurring at high dew points, and desiccant-based dehumidification removes the moisture load at lower dew points. The specific mix of the two technologies will depend on the characteristics of the application in question.
Factors include the following:
Evaluating the dew-point level
When the required moisture control level is comparatively high (above a 50 F dew point), cooling-based dehumidification is very economical in terms of both operating cost and initial equipment cost. Low-cost, high-volume, standard equipment is available for this control level and above. Below that control level, the cooling approach begins to be less economical, primarily because of the precautions needed to avoid freezing the condensed water on the cooling coil.
Although water does not freeze until temperature falls below 32 F, a dehumidification system may well have to deliver air below that dew point in order to maintain a room below 50 F dew point. (This is analogous to home heating, where air must be supplied at 120 F in order to maintain a cold house at 70 F.) So, a cooling-based dehumidifier providing air at low dew points can freeze unless special precautions are taken in the design of the unit. Such features result in higher-cost, custom equipment, and equipment that has a higher operating cost per kilogram of water removed, so desiccants become more economical than cooling-based systems at low dew points.
Gauging relative humidity sensitivity
When a process needs low moisture level in absolute terms, but can tolerate a high relative humidity, a cooling-based dehumidification can be very cost-effective without the need for desiccants. An example is fruit and most vegetable storage. The ideal temperature might be 40 F, so of course the dew point must be below that level. But if relative humidity is below 90%, the fruit can dry-out in storage and lose value. Since the product needs both low temperature and high humidity, cooling-based systems are ideal for the application. In contrast, other processes might demand a low relative humidity in addition to a low dew point.
Narrow or wide temperature tolerance
If an application has a narrow temperature tolerance, then cooling and heating will be essential in addition to dehumidification. If the application can tolerate wide temperature variations, such as occur in un-heated storage, then dehumidification equipment alone may be sufficient.
Designing The Ideal Dehumidification System
Industrial dehumidification systems are custom-engineered for each particular project. Consequently, manufacturers have developed a near-infinite variety of possible components to serve the near-infinite variety of possible applications. These components make it easy to optimize a system design, but the variety also presents the project engineer with many decisions at a early stages of a project usually before the cost/benefit implications of those decisions are completely clear.
Define the purpose of the project
The designer must clearly understand and document the purpose of the project. This understanding sorts all other design decisions in order of their true importance. For example, if the purpose of the project is to prevent the growth of mold on starch in a storage silo, there is no need to maintain a strict tolerance of 1% relative humidity (RH). The only real concern is that the humidity does not exceed 60% and that condensation does not occur. The system can be simple and inexpensive.
Conversely, if the purpose of the project is to prevent the corrosion of lithium, there is no point to try to save money by using a control which has a tolerance of 5% RH. Above 2% RH, lithium corrodes, giving off hydrogen, which eventually explodes. A sensor with a tolerance greater than the critical control level itself could not start the system in time to prevent that explosion. Understanding the purpose of the project in these terms helps the system designer avoid both unnecessary expense and false economy.
Establish control levels and tolerances
After the purpose or purposes of the project are clearly defined, the designer must decide what humidity and temperature control levels and tolerances will achieve those purposes. These decisions may require research, but in many cases, the relationship between a process and moisture is understood clearly enough to allow the project to proceed. For example, if a process bogs down during summer, but not during spring, fall or winter, one can assume that the humidity tolerance is quite wide, and that only summer extremes of humidity must be removed by the dehumidification system. In other cases, the supplier of a problem material may be able to recommend optimal environmental conditions for processing the product.
The control set point must be established to allow calculation of the peak heat and moisture loads, and without loads, there is no way to estimate equipment sizes and costs. Loads are relative to the temperature and moisture levels maintained. All other variables being equal, a system to hold humidity at 72 F, 35% RH will be much smaller than one which must hold 72 F, 25% RH. The lower the humidity level, the more costly the system will be. Higher moisture loads also increase system cost. Therefore, calculating these loads is the next critical step in designing a system.
Calculate moisture loads
In most cases, the application engineer employed by the dehumidification supplier will assist the project engineer in calculating moisture loads. In order from largest to smallest, typical loads come from ventilation air, air infiltration, miscellaneous openings, people, products/packaging and vapor permeation. Lower loads mean less expensive equipment. Consequently, the most cost-effective adjustment to building operation is to reduce the exhaust air to the minimum, reducing the cost of dehumidifying the air brought into the space to replace the exhaust. After that, sealing up cracks in the building greatly reduces the cost of dehumidification for a very modest investment in caulking material.
Fresh/ventilation air is essential in most controlled spaces. In most cases, codes require a certain amount of air per person or per square foot of occupied space. Often, less attention is paid to making sure all exhaust air is made up by the ventilation system. This is especially a problem in large spaces, where the exhausts may not be obvious. Also, engineers used to designing commercial buildings may not be fully aware of the effect of insufficient make-up air on humidity-controlled spaces.
The next load source is miscellaneous openings. Each time a door is opened, moist air is pulled into the room. When possible, spend time observing the number of times a door is opened during the busiest production period.
Air locks greatly reduce moist air infiltration (as long as one door is not propped open by the occupants). As the humidity control level goes lower, air lock doors become very advantageous economically. The assumption behind an air lock is that equilibrium is reached half way between the inside and outside conditions and all the air enters the room each time the lock opens.
Often, product must enter or leave a humidity-controlled room on a conveyor. This type of conveyor opening should not be overlooked as a possible infiltration source. To reduce infiltration of moist air through large openings such as ducts, engineers often supply a slight overpressure of make up air so that dry air leaks out of cracks rather than moist air leaking in.
When people exhale or perspire moisture is given off, creating another load source. The rate depends on the level of exertion -- more metabolism equals more moisture. When calculating loads in a room, be sure to allow for "visitors" flowing in and out of the room. Experienced engineers often double their "people" estimates to allow for changes in room use and "visitors".
The load from products and packaging varies greatly by application. In large storage applications, moisture released from product can represent the single largest load component. The load is the difference between the products initial wet weight, and its weight when at equilibrium with the lower humidity.
Vapor permeation through building components is typically the smallest portion of the load, accounting for less than the 2% of the total (as long as the walls, floor and ceiling are solid surfaces without air leaks). The permeation load becomes more worthy of attention when the building is extremely large, so that moisture permeates across a large surface area, or if the control condition is very low. Below 5% RH, every leak, no matter how small becomes critical.
The peak design weather conditions are a very important element in the load calculations. The owner must decide how conservatively the system should be sized. If extreme weather data is used, the system will control humidity throughout all 8,760 hours in a typical year. Such a system will also be very costly. If some out-of-spec hours can be risked, the system may cost 20 to 30% less, but if all moisture loads peak at the same moment during extreme weather, the humidity may rise above set point.
Air conditioning engineers quantify these choices in the ASHRAE Handbook of Fundamentals according to the percent of annual hours that weather conditions will be above certain values. For example, the 0.4% values are likely to be exceeded for only 35 hours per year (8,760 x 0.004). A less conservative design point would be the 1% or 2.5% values, which may be exceeded for 70 and 219 hours respectively.
The decision as to which data to use is made by the end user, who is in the best position to assess the economic consequences of being slightly above specification for short periods. Lithium processing, for example, usually demands a more conservative design than starch silos, because the consequences of high humidity with lithium are hazards not just expenses.
Evaluating dehumidification technology
The project engineer investigating the use of dehumidification systems will likely be working closely with equipment suppliers to determine costs and benefits of dehumidification versus alternate means of solving problems. Dehumidification suppliers can be most helpful and respond quickly when key aspects of the potential project are well-defined. These include: clearly communicating the nature of the problem and its consequences; defining the purpose of the project in a simple, declarative sentence describing measurable results; and researching available utilities and physical characteristics of the site.
A Host of Applications
Dehumidification systems are widely used throughout the manufacturing and processing industries, but abundant opportunities remain for further use of the technology. A project engineer would do well to consider its use whenever weather variations affect production rate or product quality, when corrosion or condensation cause problems, or whenever product must be dried at low temperatures.
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