Dehumidification Technology To Keep Moisture From Wreaking Havoc

The role that dehumidifiers play in business and industry today is changing. Once limited to specialized applications where process requirements dictate 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 equipment and material corrosion, preventing condensation on cool surfaces and checking the growth of microscopic organisms, building engineers, architects and maintenance managers are discovering that the systems are well suited for use in a wide range of applications.

Dehumidification applications

One of the more common commercial and institutional applications for dehumidifiers is in computer rooms and areas with high-tech electronic equipment, which are sensitive to atmospheric moisture. In the highly air-conditioned atmosphere where much of the equipment operates, relative humidity readings frequently exceed 75 percent. At these levels, atmospheric moisture promotes corrosion of metal contacts and components within the equipment, leading to deterioration of the system operation.

Paper feeding high-speed printers absorb moisture, resulting in frequent paper jams. Dehumidifier systems help provide precise humidity control, keeping the moisture content of the air low enough to prevent these problems, but high enough so that static electricity does not become a problem.

Another institutional application where dehumidifiers have been widely used is for health care facilities, particularly those with operating rooms and intensive care units. They have strict requirements that govern the quality of applied air. The air supplied typically is 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 those requirements.

Another commercial application that has made wide use of dehumidification systems is the grocery market 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 cases, as well as on the products being displayed. Frost on the cooling coils increases cooling energy requirements. Frost on the product decreases customer satisfaction and increases labor requirements. Dehumidification systems, by reducing the relative humidity from the 60 percent to 65 percent range to 50 percent or less range, greatly reduce the rate at which frost forms.

The use of dehumidification systems in commercial and institutional facilities, however, is not limited to these specialized applications. The relative humidity of the air supplied to an area is an important component of the space’s overall indoor air quality. Building managers are finding that dehumidification systems can help many other facilities improve their indoor air quality while reducing energy requirements.

System advances

There are two primary ways to remove moisture from a building’s air supply: cool the air below its dew point to condense the water vapor, or pass the air over a material that readily absorbs water.

Systems that cool the air below its dew point use mechanical refrigeration. Air is passed over a cooling coil, causing a portion of the moisture in the air to condense on the coils’ surface and drop out of the air flow.

The relative humidity of the air leaving the coil is nearly 100 percent, so before it can be introduced into the conditioned space, it must be mixed with warmer air or reheated. Varying the temperature of the cooling coils controls the amount of moisture left in the supply air and the resultant humidity.

Although mechanical refrigeration dehumidification systems were popular in the past, their high energy costs have forced building operators to look for other options. The option that has been most widely accepted for use in commercial, institutional and industrial facilities is the desiccant system.

Desiccant dehumidification systems consist of a slowly rotating disk, drum or wheel that is coated or filled with an absorbent. Air being drawn into the facility is passed across one portion of the wheel, where the desiccant absorbs moisture from the air.

As the desiccant structure slowly rotates, it passes through a second heated air stream. The moist air is then exhausted from the facility. By rotating continuously, the desiccant wheel always has a freshly regenerated area available for dehumidification.

Early generation systems used lithium chloride gas as the desiccant. One problem with these systems occurred when the reactivation heater failed to operate properly or the wheel failed to rotate. The lithium chloride continued to absorb water from the airstream, but without reactivation, the wheel became saturated with moisture. Once saturated, the lithium chloride separated from the wheel and was carried off by the airflow.

With a cost of several thousand dollars to replace the lithium chloride, maintenance managers soon modified the systems to include automatic alarms should the reactivation cycle be interrupted. Newer generation systems primarily use a silica gel as the desiccant. Silica gel does not lose its adhesion to the rotating wheel when saturated.

By following a preventive and scheduled maintenance program that includes inspection of key system components, building maintenance managers can help insure that they get the biggest possible benefit from dehumidification system operation.

Dessicant System Maintenance

The most important maintenance activity for desiccant dehumidification systems is regular inspection. Unfortunately for many system operators, the first indication of system trouble is rising humidity levels in the occupied space. This is particularly important for lithium chloride based systems, as the cost to replace the desiccant material can be as high as $9,000-11,000. Consider these maintenance points:

• Air filters. To protect the desiccant from accumulating dirt from the air flow across the wheel, air filters are installed on the intake side. To prevent reduced air flow from clogged filters, inspect filters periodically. Inspection frequency depends on the level of airborne materials present.

• Drive belts. Due to the low rotation 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 the operation of the wheel, thus decreasing dehumidification system effectiveness. In the case of lithium chloride desiccant, improperly turning wheels can lead to the loss of the desiccant. Inspect the entire drive system weekly, checking the belts for signs of misalignment, wear and slippage.

• Regenerator. It’s 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 operation of the regenerator on a weekly basis. Safety controls and system malfunction alarms should 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, allowing air to flow between the two sections of the wheel. This mixing of air flows reduces the overall efficiency of the wheel. Under normal operating conditions, the seals can be expected to last approximately five years. They should be closely inspected at least once a month, however, for wear or damage.

• Dirt contamination of the drum. The efficiency of the desiccant largely depends on the amount of surface area that comes in contact with the air flow. Accumulated dirt on 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 air flow, it isn’t 100 percent effective. Also, the process of absorption of water and desorption by regeneration deposits a significant quantity of dirt on the surface of the desiccant. The desiccant surface should be inspected closely at least once a month for dirt build-up. If a significant amount of dirt is found, the surface will have to be cleaned in accordance with the manufacturer’s recommendations.

• Desiccant. Although desiccant loss is more of a problem in lithium-chloride-based systems than in those that use a silica gel, the desiccant can become 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 started up again.

• System alarms and controls. Most desiccant systems are equipped with alarms that monitor the operation of key elements of the system, such as wheel rotation and the regenerator. If the system uses a lithium chloride-based desiccant, those controls and alarms should be tested at least once a month. Systems that use a silica gel desiccant do not require as frequent testing, as the desiccant is not readily damaged by moisture saturations. The controls and alarms of these systems, however also should be tested on a regular basis.