Dehumidification Alternatives for Commercial Buildings
By Consulting Specifying Engineer Staff
Controlling humidity in commercial buildings used to be a difficult undertaking that was best accomplished by overcooling the supply air to increase moisture removal, then reheating the overcooled air to the desired temperature. This was a huge waste of energy. While gas desiccant systems were a viable alternative, they were relatively costly and more often used for industrial applications where very low humidity levels were required. The advent in recent years of alternative dehumidification technologies has made the dehumidification process far more energy-efficient and affordable.
The commercial building market for dehumidification systems has been largely driven by concerns over indoor air quality (IAQ) and energy usage. Ironically, one of the major causes of IAQ problems has been revised code requirements that have necessitated that more outdoor air be introduced into buildings. As a result, outdoor air today can account for up to 80% of the dehumidification load in a building and can tax the ability of existing air-conditioning systems to handle this increased latent load.
Schools require a comfortable, healthy environment for proper teaching and learning to take place. Concerns over illnesses caused by air-borne bacteria are another motivating factor. To satisfy these concerns outside air must be continuously introduced during occupied periods. Libraries require moisture control to prevent the growth of mold. Gymnasiums require floor protection. Locker rooms, showers and swimming pools need protection from mold, corrosion and condensation.
Restaurants, as a result of kitchen exhausts and latent loads created by cooking and customers, require the introduction of large amounts of outdoor air. Without proper dehumidification, mold and mildew can contaminate kitchen areas, dining areas can become cold and clammy, condensation can drip from supply diffusers onto customers and tables, and floors can become slippery. All of this could add up to health concerns, customer dissatisfaction and potential lawsuits.
In hotels, mold and mildew caused by excess humidity is a constant concern, particularly those in warm, moist climates. In addition to customer dissatisfaction, mold and mildew can increase maintenance costs by shortening the time between room remodels.
In the supermarket, high humidity can create havoc. As reach-in glass doors fog up, the duration and frequency of defrost cycles is increased, and cooling-related energy use goes up. Defrost issues create additional problems as product temperatures can rise above the recommended U.S. Food and Drug Administration levels and product life can be shortened.
Proper dehumidification can provide economic benefits through reduced operating costs, better IAQ, increased comfort levels and better product quality. For example, in manufacturing facilities dependent on maintaining proper humidity levels, better humidity control means increased productivity through less downtime and product loss. In hotels, the elimination of mold and mildew can prolong the life of room furnishings, reducing the need for capital expenditures. Controlling humidity can also extend the life of the physical plant equipment by eliminating corrosion.
Types of dehumidification systems and their benefits include the following:
Desiccant Systems. These have become increasingly popular in recent years because of lower equipment costs achieved through economies of scale, modular designs and the use of silica gel wheels. In desiccant systems, a desiccant wheel is used to remove moisture from the air. The dry air is then cooled, and the moisture that has been absorbed in the wheel is removed by adding heat, thereby regenerating the desiccant and making it available to absorb more moisture.
It should be noted that, whereas the reactivation heat in the original desiccant systems was produced by burning natural gas, all-electric desiccant systems are now available. These new desiccant systems are efficient and simple in that they use rejected heat from the condenser to regenerate the desiccant wheel. The all-electric desiccant systems can be applied in two ways. First, they can be used as a dehumidifier to treat 100% makeup air and work in conjunction with existing air-conditioning systems to provide operators with the ability to control temperature and humidity independently. Second, they can be applied as a standalone air conditioner with a built in dehumidifier. In these applications the systems handle the cooling loads and dehumidify with the desiccant wheel to handle the latent load. When there is a call for cooling only, they can help lower energy costs by providing a coefficient of performance (COP) that is higher than a typical air-conditioning system. The industry is quickly learning that by controlling the humidity to 50% RH or less, and independent of temperature, thermostats can be raised to 77-78
Enthalpy wheels. These use a desiccant-coated honeycomb to absorb moisture. They differ from desiccant systems in that the wheels contain very little desiccant, turn much faster and use exhaust air rather than heated air to reactivate the desiccant. Enthalpy wheels extract moisture from incoming outside air and transfer it to exhaust air. But there’s a problem: This only works when the exhaust air is drier than the outside air. Furthermore, moisture removal is usually less than 70% of the difference between the two airstreams, and the exhaust air also must be free of oil and grease and brought to the same location as the incoming air. And since enthalpy wheels rely on the moisture differential between the opposing airstreams to work, when the differential is low, their performance drops off.
DX Systems. The traditional DX approach to dehumidification is to overcool the supply air and then re-heat it. In doing so, additional moisture is removed and the air is delivered to the space at the desired temperature. While many facilities still use this method, in these days of increased energy awareness, it is falling out of favor. Furthermore, in some instances the overcool/re-heat strategy still may not provide sufficient dehumidification to satisfy the requirements of the application.
Heat Pipes. Heat pipes, which are sealed tubes containing refrigerant, are added to A/C units to increase their dehumidification capability. Heat is transferred through the evaporation of the refrigerant at one end of the tube as it absorbs heat, and the condensing of the refrigerant at the other end of the tube as the heat is dissipated. In the dehumidification application, heat pipes are arranged in a coil configuration. A pre-cooling coil is installed in the air stream prior to the evaporator, and piped to a reheat coil installed in the air stream after the evaporator. By passively removing sensible heat from the incoming air stream with the pre-cooling coil, the evaporator is able to extract more humidity without increasing energy use. The reheat coil then adds roughly the same amount of heat as the pre-cooling coil removed, so the air is delivered to the space at the desired temperature. The application of heat pipes is limited by the availability of space within the A/C units for the new pre-cooling and reheat coils. In addition, there is a finite limit to which they can increase the dehumidification capability of an existing system.
Dual Path Systems. These systems work by separately dehumidifying and cooling outdoor air, then mixing it with conditioned return air before ducting it into the retail space. Part load operation also is very efficient as the system can be controlled CO 2 or occupancy sensors. A predetermined schedule can vary the ventilation airflow rate to meet changing ventilation air requirements while maintaining desired temperature and humidity levels under part load conditions. In most instances through, dual path systems must be custom-manufactured so they tend to be relatively expensive and require a long wait.
Once a difficult and inefficient process, today’s dehumidification technologies are highly reliable, affordable and efficient. While end users have a number of alternative solutions to choose from, not every technology will be appropriate for every application.
(From Munters Corporation. For more information go to www.munters.com .)
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