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Manufacturing processes and environmental control requirements in production plants often lead to the use of dehumidification as part of the overall system to remove excess water contained in products or from the ambient air. The dehumidification process, however, releases energy. This energy does not come from the electrical energy needed to operate the equipment; instead, it is latent heat, which often is referred to as hidden heat.

To understand this hidden heat, one must remember the phase transitions of water, and the energy inputs and outputs involved. Water can exist in three states:

  • As a solid (ice).
  • As a liquid (water).
  • As a gas (water vapor or moisture).

Figure 1 illustrates the energy inputs and outputs that take place when a phase change occurs. As figure 1 shows, the transformation of ice to water vapor consumes energy. During the transition from water vapor to liquid, and from liquid to solid, however, considerable amounts of thermal energy are released.

This explains why the air leaving a dehumidifier usually is at a much higher temperature than the air at the beginning of the dehumidification process. It absorbs this heat. The more moisture that is removed from the air, the warmer the air will be when it leaves the dehumidifier. For example, suppose a dehumidifier removes 50 pounds of water per hour. That unit will add approximately 48,500 BTUs to the airstream temperature. This results in a requirement for subsequent cooling of the air as part of the total package.

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FIGURE 1. This figure illustrates the energy inputs and outputs that take place when a phase change occurs as water changes state. Image provided by Condair

Moisture. Sources in the Air

Three sources of moisture in production environments are:

  • Ventilation systems. They introduce moisture from outside air, particularly in the summer when it is hot and humid.
  • Moisture released into the facility. Typically, such moisture is released from raw materials and products in the space, but it can be from personnel working on-site as well.
  • Moisture that penetrates the building structure. This includes moisture that infiltrates through the structure as well as moisture introduced into the space through doorways.

All sources must be considered when designing the dehumidification system.

Dehumidification solutions — and, in particular, desiccant dryers — provide the precise moisture control necessary to ensure that the final product achieves the required quality. Desiccant dryers also help increase the efficiency of the production line.

How Desiccant Dryers Work

Desiccant dryers, as the name implies, are based on sorption by desiccant technology. Moist air is drawn into the unit by a fan. The air is dried (dehumidified) continuously as it passes through a slowly revolving desiccant rotor consisting of a highly hygroscopic material. The air exiting the dehumidifier is dry and hot due to the latent heat released as the water molecules are captured, plus some residual heat from the rotor as a result of desiccant regeneration.

A post-cooling system installed downstream of the dehumidifier is employed to remove the sensible heat added by the dehumidification process. The post-cooling system also can be used to lower the air temperature to that required for a specific process.

To ensure the continuity of the drying process, the moisture absorbed by the desiccant must be removed. This is achieved with a second airstream — a heated reactivation air — that removes the moisture from the desiccant rotor as the hot moist air passes through it. The desiccant drying process is followed by sensible cooling (figure 2).

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FIGURE 2. A sensible post-cooling system installed downstream of the dehumidifier is employed to remove the heat added by the dehumidification process and reach the temperature required for a specific process. This schematic represents the air-drying process followed by post-cooling. Image provided by Condair

Increasing the Moisture-Removal Capacity

As with all systems, dehumidifiers are limited by the maximum dehumidification duty they can handle. Indeed, when considering all potential sources of moisture, the amount of water to be removed can, in some cases, exceed the dehumidifier capacity. When this situation occurs, a pre-cooling coil can be placed upstream of the dehumidifier. Often, this is a chilled water-cooling coil with supply and return temperatures of 42 and 55°F (5.5 and 13°C), respectively. The high latent load-removal capacity of the pre-cooling coil reduces the absolute humidity of the air to a level that becomes within the processing capacity of the dehumidifier (figure 3).

For this reason, the choice of pre-cooling coil becomes critical: a chilled-water type is recommended. The coil must be highly latent, but it must never be cold enough for ice to form within it. If a defrost cycle was required, preconditioning of the incoming airflow would be interrupted. During defrost, the humidity resulting from the dehumidification process would no longer be controlled accurately.

Unlike the pre-cooling system, the post-cooling system can be either the direct-expansion type or the chilled-water type. The air is dry enough that post-cooling below the dewpoint temperature of the air would be unusual. Therefore, post-cooling coils are only “dry” sensible coils.

The choice between direct expansion or chilled-water coils for post cooling should be based on the accuracy of the required target conditions. The greater the degree of control required, the more the decision leans towards chilled-water coils. For instance, a pumped-coil design with three-way valves allows for temperature control accuracy greater than ±0.4°F (±0.22°C).

There are many benefits to integrating desiccant dehumidifiers as well as pre- and post-cooling coils into industrial processes. Such an approach can help ensure precise control of climatic conditions, for instance, which will help to reduce financial losses and avoid unplanned production downtime.

The following sections highlight some of the key benefits for industrial processes in greater detail.

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FIGURE 3. The high latent load-removal capacity of this pre-cooling coil reduces the absolute humidity of the air to a level that becomes within the processing capacity of the dehumidifier. Image provided by Condair

Accurate Humidity Control for Consistent Product Quality. The main advantage of using a desiccant dryer in industrial processes is that it allows precise control of the air humidity down to ±2 percent relative humidity. Thus, consistent air conditions are available for manufacturing processes when the temperature is also precisely controlled.

By ensuring that quality air is available for all production processes as well as during storage and packaging, manufacturers can ensure that their products meet the highest quality and are consistent. Because desiccant dryers are effective in applications with a higher latent load component, they can successfully remove unwanted moisture from a space even at low temperatures.

To achieve the climate conditions required for processes that can only be carried out at a relative humidity level below 40 percent relative humidity (table 1), a designed dehumidification solution comprising a desiccant dryer and a sensible post-cooling coil is required.

Prevention of Hygroscopic Materials Moisture Absorption. One moisture-related challenge common to production processes in the food, beverage and pharmaceutical industries is the moisture regain prevention of hygroscopic materials. This will occur whenever a high relative humidity is present. Some hygroscopic materials may require a relative humidity as low as 5 percent to be processed and maintain their properties.

Desiccant dryers are suited to supply dry air at low humidity levels, and any undesired heat can be removed by passing the dried air through a post-cooling system before it is supplied to the processing area. For example, the packaging area in a confectionary manufacturing plant requires air sufficiently dry and cool enough to prevent the sugars in the candy from absorbing the moisture in the surrounding air and sticking to packaging machines and wrapping material.

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FIGURE 4. Moisture regain of sugars contained in candies during packaging can be prevented by supplying dry air with a desiccant dryer. Source: sergeyryzhov/iStock/Getty Images Plus

Corrosion Prevention. Corrosion and air moisture are intrinsically related. A sudden increase in the ambient air temperature is often accompanied by increased moisture in the air. This thereby increases the risk of condensation occurring on any metal surface that is cooler than the dewpoint temperature.

Desiccant dehumidifiers will dry the surrounding air before moisture condenses on metal surfaces. This application of desiccant dryers is especially beneficial in the power plant industry to protect the plant equipment during maintenance. Similarly, desiccant dryers paired with a cooling system will provide the optimal conditions — typically a temperature of 68°F (20°C) and relative humidity lower than 5 percent relative humidity — required to prevent surface oxidation during the storage of electronic components.

Ensuring a Hygienic Production Environment. When favorable conditions are present — for example, high air temperature and relative humidity — microorganisms such as mold, mildew and fungi can proliferate. The spread of microorganisms is a significant challenge in the beverages, food and pharmaceutical industries. These industries are required to maintain high hygiene standards. Every surface with condensation (liquid water) is a breeding ground for microorganisms, however. This can occur on steel tanks and silos, walls and chilled-water distribution piping.

Desiccant dryers are useful in maintaining lower humidity levels to prevent the proliferation of microorganisms on surfaces. As well, they can provide sufficiently dry air required to preserve raw materials or pharmaceutical products from microbial growth during storage. However, most storage areas in the pharmaceutical industry must be kept at temperatures between 68 and 72°F (20 and 22°C); thus, the dry air provided by the dehumidifier must be cooled before it is supplied to the storage areas.

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FIGURE 5. Corrosion damage on piping due to condensation can be prevented with the use of desiccant dryers. Source: fotoslaz/iStock/Getty Images Plus

In conclusion, desiccant dehumidifiers can improve industrial processes by providing precise or very low humidity levels. They are particularly suitable when they can be installed upstream of a cooling system to provide the desired year-round climate control in production environments.

Because the moisture-removal process releases heat energy into the air, post-cooling often is required to provide air at a temperature more suitable for the application. Additionally, some applications require significant moisture removal to achieve the target level of dryness. In these applications, a pre-cooling coil may be applied to remove some moisture and optimize the conditions entering the dehumidifier to achieve the target condition.

As each application has its own requirements, dehumidification and cooling coils solutions must be carefully designed to achieve the desired performance. With some careful planning and attention to detail, humidity control systems can provide a range of benefits for industrial processes.