Nearly everyone in the food processing industry is familiar with the risk and inherent food safety danger associated with having liquid-moisture droplets form over products that are being processed for human consumption. Few would question the need to prevent such condensation from occurring, though a great deal of debate exists over how to accomplish this goal effectively.
Food processing environments often are wet and highly moisture-laden. The nature of the process and the need for constant sanitation (which often is accomplished with hot water) means that copious amounts of moisture are introduced to the environment. Additionally, food processing rooms generally are held at low temperatures to aid in contamination control and product enhancement. Cold space temperatures exacerbate condensation problems.
To understand how to control condensation, one must first understand how and why condensation forms. The key is a simple principle of psychrometrics known as dewpoint. By definition, dewpoint is the temperature at which existing moisture vapor in a given air sample will condense from the air, forming dew. For example, if a room contains air at a dry bulb (db) temperature of 40°F (4°C) and a dewpoint temperature (wet bulb, or wb) of 35°F (2°C), then any surface in the room that has a temperature of 35°F or lower will generate condensate. This process occurs as the air that is in contact with the cool surface is chilled and reaches its dewpoint, releasing moisture to the surface as liquid droplets.
Dewpoint control can be accomplished either by heating the cool surface to a temperature that is greater than the dewpoint of the air or by reducing the dewpoint of the air to a temperature that is lower than the sensible temperature of the surface. Either of these solutions is acceptable, but the latter often is the most practical and is therefore the one that this article will explore in more depth.
Reducing the dewpoint of the air within a space can be accomplished by introducing and treating outdoor air, dehumidifying the air with refrigeration, or dehumidifying the air with dry desiccant technology.
Using Outdoor AirIntroducing outdoor air to the space can be an effective dewpoint control approach, but, of course, it works best when the dewpoint of the ambient air, through Mother Nature’s graces, is lower than the dewpoint that must be controlled. This method generally requires that the outdoor temperatures are low, and often some heating will be needed to prevent the space from being overcooled. The energy required for heating the air can be large during the winter months, especially in northern climates.
When the dewpoint of the makeup air is higher than the dewpoint that must be controlled, using outdoor air will only exacerbate the problem - by introducing more moisture to the environment - rather than solve it. To battle this effect, many outdoor-air systems use refrigeration to induce mechanically what nature does naturally during cooler seasons. However, this method also can consume a large amount of energy.
For instance, returning to the example of the 40°F db/35°F wb room, supposed that the ambient outdoor conditions are a lovely 85°F (29°C) db/78°F (26°C) wb. In those conditions, 11 tons of refrigeration would be required for every 1,000 cfm of outdoor air introduced simply to achieve a neutral dewpoint of 35°F. Delivering air at a dewpoint less than 35°F would be impractical in this case because the coil surface temperature would need to be less than 32°F (0°C), generating ice and reducing effective cooling.
Using RefrigerationDehumidification through refrigeration is a method worth exploring. Facilities with 40°F db/35°F wb food processing rooms already will be using refrigeration for the sensible cooling of the space. Therefore, it is often a natural tendency to gravitate toward this approach to achieve dewpoint control.
Refrigeration can remove large quantities of vaporized moisture from air efficiently. This method is limited by the ice-over threshold (32°F/0°C) though. Because a coil will begin generating frost whenever the coil surface temperature is less than 32°F, the practical limit to which the air can be cooled and effectively dehumidified using refrigeration is 35 to 40°F.
An argument can be made that the refrigeration routinely takes place at temperatures below 35°F. For lower-temperature refrigeration to occur, though, some modifications need to be employed, primarily the implementation of a defrost cycle for the coil. Such cycles can make the delivered temperature and dewpoint difficult to predict, and multiple evaporators may be required for some applications.
For example, an evaluation of the moisture-removal performance of a given evaporator would show a bell-shaped curve over time. The coil would perform well immediately after defrost, but it would deteriorate in capacity as frost and ice formed, insulating the coils’ cooling fins from the air to be refrigerated. The leaving-air temperature would climb until it again became necessary to defrost the coil. This cyclical performance curve, especially when employed in an air-handler, would complicate efforts to achieve predictable, reliable dewpoint control.
Given the problems associated with the freeze threshold, the difference between the delivered air dewpoint and the desired room dewpoint is minimal. The best-case scenario is limited to a 4 to 8°F (2 to 4°C) difference between the delivered air and room design dewpoint. As a result, a system designed to control dewpoint using refrigeration may require high air-exchange rates to counterbalance the space-moisture load. This approach adds more associated mechanical and operating costs to the room.
Using Dry Desiccant SystemsA third way to control humidity in a food-processing space is to use dry desiccant technology (figure 1). These systems generally use a desiccant rotor to accomplish the drying process. The rotor is composed of a substrate that has a desiccant material bonded to it. The substrate most commonly is a high-surface-area fluted mass through which air can pass. As the air passes through the rotor, the desiccant removes moisture from the air in a vapor phase. The air then exits the rotor significantly drier than it entered (figure 2).
Because moisture removed by the desiccant eventually will saturate the rotor material, the rotor must be regenerated continuously. A secondary airstream, normally designated as “regeneration” or “reactivation,” is heated and passed across a segment of the rotor. The rotor revolves at a constant speed between these airstreams, generating predictable, reliable dehumidification.
Desiccant technology can achieve delivered supply dewpoints of 0°F (-18°C) or lower, which provide a significant difference between delivered-air conditions and design room dewpoint control levels. Humidity can be controlled effectively with lower air-exchange rates than with other technologies. Additionally, system air volumes necessary to achieve adequate control will be minimized compared to a cooling-based system.
Because the drying occurs in a vapor state, moisture is rejected easily by the regeneration airstream. This rejection eliminates the need for consequent removal of condensed liquids that compromise the hygiene of the conditioned space. Most applications allow the desiccant system to be located on a roof or in another space that is separate from the control environment.
Desiccant technology also allows the room’s dewpoint (source of condensation) to be controlled independently from the room’s sensible temperature. This means that the space can be controlled easily at a specified temperature and dewpoint.
The desiccant approach works symbiotically with the refrigeration system in the room. Refrigeration capacity and operation are enhanced by the reduction in the latent load in the space that the desiccant system provides. The reduced latent loading on the evaporator coils serves to allow the refrigeration to accomplish its job with less effort, thereby reducing the space temperature. Likewise, the desiccant system can focus on achieving its duty, to control the dewpoint so that condensation will not occur.
In conclusion, dehumidification using dry desiccants provides reliable, consistent and tangible condensation control within food processing environments without requiring makeup or frost control options. Desiccant dehumidifiers provide benefits to the hygienic integrity of the space. Many facilities within the food processing industry have had the opportunity to experience these benefits firsthand.