The role dehumidifiers play in business and industry today is changing. Once limited to specialized applications where process requirements call for the removal of moisture from the ambient air, dehumidifiers are making their way into a wide range of commercial and institutional HVAC applications. In addition to the historically identified benefits of reducing corrosion of equipment and materials, preventing condensation on cold surfaces, and controlling the growth of microscopic organisms, construction engineers, architects, and maintenance managers are discovering that the systems are suitable for use in a wide range of applications. .
Dehumidification applications
One of the most common commercial and institutional applications for dehumidifiers is in computer rooms and areas with high-tech electronic equipment, which are sensitive to atmospheric humidity. In the heavily air-conditioned atmosphere where much of the equipment operates, relative humidity readings often exceed 75 percent. At these levels, atmospheric moisture promotes corrosion of metal contacts and components within equipment, leading to deterioration of system performance.
High-speed paper-feed printers absorb moisture, causing frequent paper jams. Dehumidifier systems help provide precise humidity control, keeping the moisture content of the air low enough to avoid these problems, but high enough that static electricity does not become a problem.
Another institutional application where dehumidifiers have been used extensively is for healthcare facilities, particularly those with operating rooms and intensive care units. They have strict requirements governing applied air quality. The supplied air is normally 100 percent outside air and must be controlled within a fairly narrow temperature and humidity band. In many cases, dehumidification systems are necessary to meet these requirements.
Another commercial application that has made extensive use of dehumidification systems is the grocery industry. Grocery stores present unique humidity control problems for the HVAC system engineer. High humidity levels lead to frost formation on the cooling coils of frozen food boxes, as well as on display products. Frost on cooling coils increases cooling energy requirements. Frost on the product decreases customer satisfaction and increases labor requirements. Dehumidification systems, by reducing relative humidity from the range of 60 percent to 65 percent to 50 percent or less, greatly reduce the rate at which frost forms.
However, the use of dehumidification systems in commercial and institutional facilities is not limited to these specialized applications. The relative humidity of the air supplied to an area is an important component of the overall indoor air quality of the space. Building managers are discovering that dehumidification systems can help many other facilities improve indoor air quality while reducing energy requirements.
System Advances
There are two main ways to remove moisture from a building’s air supply: cooling the air below its dew point to condense water vapor, or by passing the air over a material that absorbs water easily.
Systems that cool the air below its dew point use mechanical cooling. Air passes through a cooling coil, causing some of the moisture in the air to condense on the surface of the coils and out of the airflow.
The relative humidity of the air leaving the coil is almost 100 percent, so before it can enter the conditioned space, it must be mixed with warmer air or reheated. Varying the temperature of the cooling coils controls the amount of moisture remaining in the supply air and the resulting moisture.
Although mechanical refrigeration dehumidification systems were popular in the past, their high energy costs have forced building operators to seek other options. The option that has been most accepted for use in commercial, institutional and industrial facilities is the desiccant system.
Desiccant dehumidification systems consist of a slowly rotating disc, drum or wheel that is coated or filled with an absorbent. The air that enters the installation passes through a part of the wheel, where the desiccant absorbs the humidity from the air.
As the desiccant structure slowly rotates, it passes through a second stream of hot air. Then the humid air is expelled from the installation. By continuously rotating, the desiccant wheel always has a freshly regenerated area available for dehumidification.
The first generation systems used lithium chloride gas as a desiccant. A problem with these systems occurred when the reactivation heater malfunctioned or the wheel did not turn. The lithium chloride continued to absorb water from the air stream, but without reactivation, the wheel became saturated with moisture. Once saturated, the lithium chloride separated from the wheel and was carried away by the air flow.
Costing several thousand dollars to replace the lithium chloride, maintenance managers soon modified the systems to include automatic alarms in case the reactivation cycle was interrupted. New generation systems mainly used silica gel as a desiccant. Silica gel does not lose its adherence to the spinning wheel when saturated.
By following a preventive and scheduled maintenance program that includes inspection of key system components, building maintenance managers can help ensure that they get the most benefit from the dehumidification system’s operation.
Desiccant system maintenance
The most important maintenance activity for desiccant dehumidification systems is periodic inspection. Unfortunately for many system operators, the first indication of system problems is increased humidity levels in the occupied space. This is particularly important for lithium chloride-based systems, as the cost of replacing the desiccant material can be as high as $ 9,000-11,000. Consider these maintenance points:
• Air filters. To protect the desiccant from dirt build-up from the air flow through the wheel, air filters are installed on the intake side. To avoid reduced airflow from clogged filters, inspect the filters periodically. The frequency of the inspection depends on the level of materials present in the air.
• Transmission belts. Due to the low rotational speed of desiccant wheels, one or more drive belts transfer power from the motor to the wheel. Loose, damaged or misaligned belts can slow or stop wheel operation, thus reducing the effectiveness of the dehumidification system. In the case of lithium chloride desiccant, incorrectly turning wheels can cause loss of desiccant. Inspect the entire drive system weekly, checking the belts for signs of misalignment, wear, and slippage.
• Regenerative. It is one of the most critical components in the operation of a desiccant-based dehumidification system. Without the regenerator, there would be no way to remove moisture from the desiccant and the system would no longer be able to remove moisture from the building. Inspect the regenerator operation weekly. Security controls and system trouble alarms must be manually tested at least once a month.
• Seals. The absorption and regeneration portions of the wheel are separated by a system of partitions and seals. Over time, the seals can wear out, allowing air to flow between the two sections of the wheel. This mixing of airflows reduces the overall efficiency of the wheel. Under normal operating conditions, the seals can be expected to last approximately five years. However, they should be closely inspected at least once a month for wear or damage.
• Dirt contamination of the drum. The effectiveness of the desiccant is highly dependent on the amount of surface that comes into contact with the air flow. The accumulated dirt in the desiccant can reduce the effective surface area available for moisture absorption, thus reducing the overall effectiveness of the dehumidification system.
• Although the air filter system is designed to remove dirt from the airflow, it is not 100 percent effective. Also, the process of water absorption and desorption by regeneration deposits a significant amount of dirt on the surface of the desiccant. The desiccant surface should be closely inspected at least once a month for dirt build-up. If a significant amount of dirt is found, the surface should be cleaned according to the manufacturer’s recommendations.
• Desiccant. Although the loss of desiccant is a greater problem in lithium chloride-based systems than in those that use silica gel, the desiccant can be damaged. Inspect the desiccant at least once a month for signs of loss or damage. If the system is shut down for maintenance, inspect the desiccant before it is turned back on.
• Alarm and control system. Most desiccant systems are equipped with alarms that monitor the operation of key elements of the system, such as wheel rotation and regenerator. If the system uses a lithium chloride based desiccant, those controls and alarms should be tested at least monthly. Systems using a silica gel desiccant do not require as frequent testing as the desiccant is not easily damaged by moisture saturations. However, the controls and alarms of these systems must also be tested periodically.