It's the Humidity, Not the Heat
Dewpoint Control Level. When the required moisture control level is comparatively high (above a 50oF [10oC] dewpoint), cooling-based dehumidification is economical in both operating and initial equipment costs. Low-cost, high-volume standard equipment is available for this control level and above. Below the 50oF (10oC) dewpoint 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 32oF (0oC), a dehumidification system may well have to deliver air below that dewpoint in order to maintain a room below 50oF (10oC) dewpoint. (This is analogous to home heating, where air must be supplied at 120oF (49oC) to maintain a cold house at 70oF [21oC].) So, a cooling-based dehumidifier providing air at low dewpoints can freeze unless the unit is specially designed. Such features result in higher cost custom equipment with a higher operating cost per kilogram of water removed. In these applications, desiccants become more economical than cooling-based systems.
Relative Humidity Sensitivity. When a process needs a low moisture level in absolute terms but can tolerate a high relative humidity, a cooling-based dehumidification can be cost effective without the need for desiccants. An example is fruit and most vegetable storage, where the ideal temperature might be 40oF (4oC). Of course, the dewpoint must be below that level. But, if relative humidity is below 90 percent, the fruit itself can dry out in storage, losing value. Because the product needs both low temperature and high humidity, cooling-based systems are well suited for the application. Desiccant-based dehumidification systems are best suited for processes that require both low relative humidity and low dewpoint.
Application 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 those that occur in unheated storage areas, then dehumidification equipment alone may be sufficient.
3 Steps to Effective DesignIndustrial dehumidification systems require custom engineering. Consequently, manufacturers have developed a near-infinite variety of possible components to serve the near-infinite range of uses. The components make it easy to optimize system design, but they also require the project engineer to make many decisions at the early stages of a project, usually before the cost/benefit implications are completely clear.
1. Define the Project
The engineer first must clearly understand and document the purpose of the project, which then will sort design decisions in order of their true importance. For example, if the purpose is to prevent the growth of mold on starch in a storage silo, there is no need to maintain a strict tolerance of 1 percent relative humidity. The only real concern is that the humidity does not exceed 60 percent and that condensation does not occur. The system can be simple and inexpensive. Conversely, if the purpose is to prevent the corrosion of lithium, there is no point to try to save money by using a control with a tolerance of 5 percent relative humidity. Above 2 percent relative humidity, lithium corrodes, giving off hydrogen, which eventually explodes. A sensor with a tolerance greater than the critical control level itself would 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.
2. Establish Control Levels and Tolerances
After the project's purpose is clearly defined, the designer must decide which humidity and temperature control levels and tolerances will achieve that purpose. The decisions may require research, but often the relationship between a process and moisture is understood clearly enough to allow the project to proceed. For instance, if a process bogs down during summer but not during spring, fall or winter, the engineer can assume the humidity tolerance is 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 setpoint must be established to allow calculation of peak heat and moisture loads. Without loads, there is no way to estimate equipment sizes and costs, because loads are relative to the temperature and moisture levels maintained. All other variables being equal, a system designed to hold humidity at 72oF (22oC), 35 percent relative humidity will be much smaller than one designed to hold 72oF (22oC), 25 percent relative humidity. 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.
3. 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 exhaust air to the minimum, which minimizes 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 modest investment in caulking material.
Fresh or ventilation air is essential in most controlled spaces. Usually, municipal 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, the engineers who designed the facility may not be fully aware of the effect of insufficient makeup air on humidity-controlled spaces.
Miscellaneous openings also are load sources. 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 humidity control levels go lower, air lock doors become more economically advantageous. With an air lock, it is assumed that equilibrium is reached halfway between the inside and outside conditions and that all the air enters the room each time the lock opens. The air lock provides a buffer zone on either side of a controlled area.
Often, product must enter or leave a humidity-controlled room on a conveyor. The conveyor opening should not be overlooked as a possible air-infiltration source. To reduce moist-air infiltration through large openings such as ducts, engineers often supply a slight overpressure of makeup air so dry air leaks out of cracks rather than let moist air 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.
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 a product's initial wet weight and its weight when at equilibrium with the lower humidity.
Vapor permeation through building components typically is the smallest portion of the load, accounting for less than the 2 percent 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 (moisture permeates across a large surface area) or if the control condition is very low. Below 5 percent relative humidity, every leak, no matter how small, becomes critical.
Peak design weather conditions are an important element in load calculations. The process engineer must decide how conservatively the system should be sized. If extreme weather data are used, the system will control humidity throughout all 8,760 hours in a typical year. Such a system also will be very costly. If some out-of-spec hours can be risked, the system may cost 20 percent to 30 percent less. But, if all moisture loads peak at the same moment during extreme weather, the humidity may rise above setpoint.
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 percent 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 percent or 2.5 percent values, which may be exceeded for 70 and 219 hours, respectively.
Which data to use is a decision 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 TechnologyThe project engineer investigating the use of dehumidification systems likely will be working closely with equipment suppliers to determine costs and benefits of dehumidification vs. 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 problem and its consequences.
- Defining the purpose of the project in a simple sentence that describes measurable results.
- Researching available utilities and physical characteristics of the site.
Dehumidification systems are widely used throughout manufacturing and processing industries, but abundant opportunities remain for further use of the technology. Where weather variations affect production rate or product quality, when corrosion or condensation causes problems, or when product must be dried at low temperatures, dehumidification systems are viable solutions.