Desiccant technology has advanced significantly in the last few years. Where humidity control is needed, the use of reheat (hot gas from the refrigeration system) can prolong periods of controllable humidity, and desiccant units can provide continuous humidity control year round.
Desiccant use has advanced more rapidly in commercial areas such as in large coliseum complexes, particularly where special conditions such as ice rinks provide a challenge for the refrigeration and HVAC designer. At the same time, industrial refrigeration designers need to climb the learning curve rapidly to stay in tune with the sanitary requirements of the food industry and regulatory agents such as the U.S. Department of Agriculture (USDA) and Food and Drug Administration (FDA).Traditionally, industry considered the “mold line” at 80 percent humidity for process areas and coolers. Although there are sporadic “idealists” who want high humidities for fruits and vegetables, high product turnover rates and the wisdom of past experience generally have prevailed in maintaining clean, perishable food environments at less than 80 percent humidity. Nonetheless, in many cases, low ceiling areas in older process facilities necessitate use of desiccant technology and/or reheat to minimize ceiling condensation. Although many other parameters may affect the processing facility’s ability to maintain humidity in a sterile environment, desiccant air systems can provide the answer when no other is available. Areas conducive to desiccant and reheat use include the following:
- Automated blast freezers such as spiral, tray and belt freezers can benefit tremendously from desiccant technology to provide ice-free entry and exit conveyor openings (figure 1).
- Low ceiling process areas.
- Areas that house wet and damp automated machinery such as steam de-casers, water-jet portion cutters and various other steam and sanitary water and moisture-laden equipment.
- Truck receiving and shipping dock areas.
- Fork truck service and parking areas.
- Entry vestibules.
Although this is not intended to be a technical article on desiccant applications, it is important to provide some layman’s information by describing ways desiccant units can be configured. They are used in various ways with pre-cooling coils (mechanical), post-cooling coils or a combination. Many times, an area that would require heat to reduce condensation can use a standalone desiccant unit to achieve the same humidity control. These applications are a function of the environment in which the desiccant unit is operating and influenced by conditions such as the number of air changes, refrigeration requirements, and humidity level and temperatures desired.
Where processing is involved with washdown and cleanup, coordination and cycling is needed. Therefore, normal humidity control would be bypassed depending on the type of cleaning cycle required. There are several manufacturers specializing in industrial process refrigeration that are familiar with ammonia refrigeration. These companies can provide industrial units for the perishable food industry.
Environmental weather conditions vary on a daily basis as well as a seasonal basis. The only way to compensate for weather fluctuation is to reheat and dehumidify with desiccant units to maintain perishable sanitary conditions.
Be mindful of the energy cost. The mechanical refrigeration cycle, through its leveraged energy or coefficient of performance (COP), has the ability to extract more energy than the refrigeration system requires to operate. In other words, the heart and soul of a mechanical refrigeration cycle is its ability to transfer heat in greater quantities than the compressor requires in equivalent energy.
Desiccant dehumidification, by contrast, consumes more energy than the equivalent energy it extracts. The moisture absorbed by a desiccant unit must in turn be evaporated, or ejected, using a greater quantity of energy than was equivalently removed in space (or room) conditioning. Knowing this, you need to be mindful that desiccant cooling is not a substitute for mechanical refrigeration but is an enhancement of the process of dehumidification. In some cases, it can provide dehumidification and reheat in a manner unavailable to a mechanical refrigeration system.
All this is to say that mechanical refrigeration can dehumidify as long as there is a need for refrigeration and as long as the coil temperature is below the dewpoint of the air passing over it. For example, if refrigeration is needed on a dock to keep it cool, dehumidification will occur so long as the humidity is high enough to give the air-unit coils a low enough dewpoint (coil temperature) with which it can extract moisture. If the dock temperature is too cold or if winter conditions exist and refrigeration (cooling of the air) is not required, then the dehumidification process stops when the refrigeration stops. Then, the moisture passes on to the freezer or cooler storage rooms, where even greater energy is required to remove the moisture. This is where reheat can be effective dehumidification for warm rooms (i.e., docks, coolers and process areas) well into the winter months.
For instance, if outside air is required for process areas, it will no doubt be cost-effective to extract most of the moisture with the mechanical refrigeration system whenever possible. However, when air temperatures fall and humidity stays high, it would become less effective. A desiccant system working in series or, in some cases, parallel with a mechanical refrigeration system can provide year-round dehumidification.
If specific room temperatures are required, then post-cooling after the desiccant dehumidification can provide the final air temperature change needed for the controlled space. By having desiccant systems working with pre-cooling and post-cooling coils, air systems can provide continual control of space cooling. There are many designs where desiccant dehumidification will permit a post-cooling coil to operate below 32oF (0oC) continually without experiencing frost. The actual requirements for each room need to be considered as well as intermittent needs such as cleanup and product chilling.