School's in Session - Dehumidification Basics

May 29, 2000
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Moisture can be removed from the air by cooling it to condense the water vapor or by presenting a desiccant to the air, which pulls moisture from the air through differences in vapor pressures. This article will discuss cooling and desiccant dehumidification techniques — how they work, how they differ from one another, and where each can be applied with advantage.

Cooling-based Dehumidification

Most people are familiar with the principle of condensation. When air is chilled below its dewpoint temperature, moisture condenses on the nearest surface. The process of cooling and condensation dehumidifies the air. The amount of moisture removed depends on how cold the air can be chilled — the lower the temperature, the drier the air.

This is the operating principle behind most commercial and residential air conditioning systems. A refrigeration system cools air, drains away some of its moisture as condensate, and sends the cooler, drier air back to the space. The system basically pumps the heat from the dehumidified air to a different airstream in another location, using the refrigerant gas to carry the heat.

Heat is removed from the dehumidified air by first transferring its thermal energy to an expanding gas — refrigerant — which is inside the cooling coil that chilled the air. This coil is called the evaporator, because inside the coil, the refrigerant is evaporating and expanding from a liquid to a gas. For that gas to expand inside the coil, it needs heat, which it gets by cooling the air passing through the coil.

From the cooling coil, the refrigerant gas is sent to a compressor, where its pressure is increased substantially — 5 to 10 times greater than when it left the evaporator coil. The gas has therefore a much smaller volume, but compression has raised its temperature. For instance, the gas may have been at 60°F after it absorbed the heat from the air on the other side of the evaporator coil, but after compression, the refrigerant gas may be 200°F or higher.

That heat — and the heat from the process of compression itself — must now be removed from the refrigerant. This is accomplished by running the gas through a second coil.

This coil, called the condenser, is located outside the conditioned space, in a place where the heat can be rejected to the air without causing problems. These units are often located outside a building or on a rooftop.

The compressed, hot refrigerant condenses back to a liquid inside the coil, and its heat — which started out in the air being dehumidified — is transferred to the air on the other side of the condenser coil. The cooled refrigerant liquid can now return to the coil cooling the original airstream. As the liquid expands again and changes back to a gas inside the evaporator coil, it gathers more heat from that airstream and the cycle repeats.

Measuring Dryness

The process can be very efficient. The common measure of efficiency is the coefficient of performance, which is the energy removed from the dehumidified airstream divided by the energy invested to accomplish the transfer to the condenser airstream. This transfer energy consists of the energy needed to run the compressor plus the energy needed to run fans pushing air through the two coils. Many electrically-driven refrigeration systems enjoy coefficients of performance of from 2.0 to 4.5, which is to say the system moves two to four-and-a-half times as much thermal energy as it consumes in electrical energy — a very favorable ratio.

The actual hardware that accomplishes cooling dehumidification is exceptionally diverse. Literally thousands of different combinations of compressors, evaporators, and condensers are in use throughout the world. But there are three basic equipment configurations of interest to designers of humidity control systems, which include:

  • Direct expansion cooling;
  • Chilled liquid cooling; and
  • Dehumidification-reheat systems.

Direct Expansion: These systems use the system configuration outlined in the previous example. The refrigerant gas expands directly into the air cooling coil, removing heat from the airstream. Residential air conditioners and commercial rooftop cooling packages are generally direct expansion — sometimes called “DX” — units.

Chilled Liquid: Chilled liquid systems use the refrigerant gas to cool a liquid, which is then circulated through a cooling coil to cool the air being dehumidified. Such machinery is often called chilled water, glycol chiller, or brine chiller systems, according to the fluid cooled by the refrigerant gas. This is the same basic configuration that operates the water coolers so common in commercial and institutional buildings.

Although there are hundreds of thousands of small chiller systems — like water coolers — in air conditioning applications, chiller systems tend to be more complex and expensive than alternatives. As a result, chilled liquid systems are more often used in large installations where they have many advantages over DX systems, such as a lower installation cost and greater operating efficiency.

Dehumidification-Reheat: These systems can use either direct expansion or chilled liquid for cooling the air, but following cooling, the air is reheated before it is returned to the space. Most common residential dehumidifiers use this configuration. They are sold in appliance stores for use in basements and moist houses.

Commercial and industrial versions of the dehumidification-reheat system are used in swimming pools, lumber kilns and locker rooms —high-temperature, high-moisture environments.

Desiccant Dehumidifiers

Desiccant dehumidifiers are quite different from cooling-based dehumidifiers. Instead of cooling the air to condense its moisture, desiccants attract moisture from the air by creating an area of low vapor pressure at the surface of the desiccant. The pressure exerted by the water in the air is higher, so the water molecules move from the air to the desiccant and the air is dehumidified.

Actually, most solid materials can attract moisture. For instance, plastics like nylon can absorb up to 6% of their dry weight in water vapor. Gypsum building board can also store a great deal of water vapor, and the oxide layer on metals attracts and holds a small amount of water vapor under the right conditions.

The difference between these materials and commercial desiccants is capacity. Desiccants designed for water vapor collection attract and hold from 10% to over 10,000% of their dry weight in water vapor, where other materials have much lower moisture capacity.

The essential characteristic of desiccants is their low surface vapor pressure. If the desiccant is cool and dry, its surface vapor pressure is low, and it can attract moisture from the air, which has a high vapor pressure when it is moist.

After the desiccant becomes wet and hot, its surface vapor pressure is high, and it will give off water vapor to the surrounding air. Vapor moves from the air to the desiccant and back again depending on vapor pressure differences.

Desiccant dehumidifiers make use of changing vapor pressures to dry air continuously in a repeating cycle. The desiccant begins the cycle with a low surface vapor pressure because it is dry and cool. As the desiccant picks up moisture from the surrounding air, its surface changes. Its vapor pressure is now equal to that of the surrounding air because the desiccant is moist and warm. At this point, the desiccant cannot collect more moisture because there is no pressure difference between the surface and the vapor in the air.

At that point the desiccant is taken out of the moist air, heated, and placed into a different airstream. The desiccant surface vapor pressure is now very high — higher than the surrounding air — so moisture moves off the surface to the air to equalize the pressure differential. Now the desiccant is dry, but since it is hot, its vapor pressure is still too high to collect moisture from the air. To restore its low vapor pressure, the desiccant is cooled — returning it to the beginning condition and completing the cycle so it can collect moisture once again.

Desiccant Design

Thermal energy drives the cycle. The desiccant is heated to drive moisture off its surface. Then the desiccant is cooled to restore vapor pressure. The efficiency of the process improves when the desiccant has a high moisture capacity and a low mass. The ideal desiccant dehumidifier would have an infinitely high surface area for collecting moisture, and an infinitely low mass, since the required heating and cooling energy is directly proportional to the mass of the desiccant and the mass of the machinery which presents the desiccant to the airstream. The heavier the desiccant assembly compared to its capacity, the more energy it will take to change its temperature — which accomplishes dehumidification.

Desiccants can be either solids or liquids; both can collect moisture. For example, the small packets inside camera cases and consumer electronics boxes often contain silica gel, a solid desiccant. Also, triethylene glycol — a liquid similar to auto antifreeze — is a powerful desiccant that can absorb moisture. Liquid and solid desiccants both behave the same way — their surface vapor is a function of their temperature and moisture content.

One subtle distinction between desiccants is their reaction to moisture. Some simply collect it like a sponge collects water — the water is held on the surface of the material and in the narrow passages through the sponge. These desiccants are called adsorbents, and are mostly solid materials. Silica gel is an example of an adsorbent.

Other desiccants undergo a chemical or physical change as they collect moisture. These are called absorbents, and are usually liquids, or solids that become liquid as they absorb moisture. Lithium chloride is a hygroscopic salt that collects water vapor by absorption. Sodium chloride — common table salt — is another.

There are five typical equipment configurations for desiccant dehumidifiers, including:

  • Liquid spray-tower;
  • Solid packed tower;
  • Rotating horizontal bed;
  • Multiple vertical bed; and
  • Rotating Honeycombe®.

Each configuration has advantages and disadvantages, but all types of desiccant dehumidifiers have been widely applied.

Liquid Spray-Tower: Spray-tower dehumidifiers function much like an air washer, except they spray liquid desiccant into the process air instead of simply water. The heat and moisture from the dehumidification process is transferred to the desiccant. Heat is rejected through an external cooling system and moisture is rejected in the desiccant regenerator, which re-concentrates the diluted desiccant solution.

Solid Packed Tower: With this system, air flows through large containers of granulated solid desiccant. The desiccant is dried by a different hot airstream that purges the container after the desiccant has been saturated.

The system is used frequently for compressed air, pressurized process gases, and sometimes even liquids that need dehumidification. It is less common in ambient-pressure applications.

Rotating Horizontal Bed: In this device, dry, granular desiccant is held in a series of shallow, perforated trays that rotate continuously between the process and reactivation airstreams. As the trays rotate through the process air, the desiccant adsorbs moisture. Then the trays rotate into the reactivation airstream, which heats the desiccant, raising its vapor pressure and releasing the moisture into the air.

Although care must be taken to avoid leakage between moist and dry airstreams, the design is inexpensive to produce.

Multiple Vertical Bed: With this design, manufacturers have combined the better features of packed tower and rotating horizontal bed designs in an arrangement that is well-suited to atmospheric pressure dehumidification applications, yet can achieve low dewpoints.

The single or double tower is replaced by a circular carousel with eight or more towers that rotate by means of a ratcheting drive system between the process and reactivation airstreams. Granular desiccant beds are arranged vertically rather than in flat trays.

While this design includes more complex parts, the increased initial cost is offset by a lower operating cost than either packed tower or rotating horizontal bed type units.

Rotating Honeycombe: Another dehumidifier design uses a rotating Honeycombe wheel to present the desiccant to the process and reactivation airstreams. The finely divided desiccant is impregnated into the semi-ceramic structure, which in appearance resembles corrugated cardboard that had been rolled up into the shape of a wheel.

The wheel rotates slowly between the process and reactivation streams. The design combines high surface area with low total desiccant mass, making these units especially efficient. The small number of parts reduces maintenance to a minimum.

This information is excerpted from "The Dehumidification Handbook," 2nd edition, published by Munters Cargocaire. For more information, contact Munters Corp., P.O. Box 640, 79 Monroe St., Amesbury, MA 01913.

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