Instead of cooling the air to condense its moisture, desiccants attract moisture from the air by creating an area of low vapour 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.
Most solid materials can attract moisture. For instance, plastics like nylon can absorb up to 6% of their dry weight in water vapour. Gypsum building board can also store a great deal of water vapour. The difference between these materials and commercial desiccants is capacity. Desiccants designed for water vapour collection can attract and hold over 10,000% of their dry weight in water vapour making them an exceptionally efficient vehicle for dehumidification.
The essential characteristic of desiccants is their low surface vapour pressure. If the desiccant is cool and dry, its surface vapour pressure is low and can easily attract moisture from the air. When a desiccant is wet and hot its surface vapour pressure is high and it will give off water vapour to the surrounding air. Vapour moves from the air to the desiccant and back again depending on vapour pressure differences.
Desiccants can be either solids or liquids - both can collect moisture. For example, the common small packet found in consumer goods is usually silica gel, a solid desiccant. Also, triethylene glycol, a liquid similar to antifreeze is a powerful desiccant which can absorb moisture. Liquid and solid desiccants both behave in the same way - their surface vapour pressure 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 usually solid materials. Silica gel is an example of a solid 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 which collects water vapour by absorption, sodium chloride - common salt - is another.
When moisture is removed from air the combined gas laws apply and heat is liberated. In a cooling-based system it is less apparent because the heat is removed immediately by the cooling coil.
In a desiccant system, the heat is transferred to the air and to the desiccant so the dehumidified air usually leaves the dehumidifier warmer than when it entered the desiccant unit.
The temperature rise is directly proportional to the amount of moisture removed from the air - the more moisture removed by the dehumidifier the warmer the air leaving will be.
Comparing the desiccant process on the psychrometric chart clearly illustrates how desiccant dehumidification differs from cooling based systems. Using the same values as the previous example air entering the dehumidifier at 25ºC and 50% relative humidity, the dry bulb temperature rises as the moisture falls so that the total energy (enthalpy) of the air stays the same. In practice the total energy increases slightly because of waste heat transferred to the air from the reactivation process.
In many applications - notably product drying and unheated storage - this temperature rise is desirable. In other cases the additional sensible heat is not an advantage and the dry air has to be sensibly cooled before being delivered to the point of use.
We have mentioned four types of desiccant systems used in Industry. All have advantages and disadvantages but all types have been widely applied. By far the most common is the rotating honeycomb.
Fig1: Illustration of desiccant rotor with reactivation
The desiccant rotor
The renowned Swedish engineer and scientist Carl Munter first patented the desiccant rotor over 50 years ago. Since then rotor technology has advanced significantly with dewpoint temperatures as low as -70ºC (0.002 g/kg) having been achieved.
The restrictions on rotor design are not due to the materials utilised but the mechanical process involved in preventing leakage across seals in the equipment.
Whatever desiccant or combination of desiccant rotor is used, the principle of operation is the same.
The desiccant is impregnated into the semi - ceramic structure, which in appearance resembles corrugated cardboard that has been rolled up into the shape of a wheel. The wheel rotates slowly between the process and reactivation airstream.
The process air flows through the flutes formed by the corrugations and the desiccant impregnated into the structure absorbs/adsorbs the moisture from the air. As the desiccant picks up moisture it becomes saturated and its surface vapour pressure rises.
Then, as the wheel rotates into the reactivation airstream the desiccant is heated by the hot reactivation air, and the surface vapour pressure further rises. This allows the desiccant to release its moisture into the reactivation air.
Following reactivation, the hot desiccant rotates back into the process air where a small proportion of the process air cools the desiccant so it can collect more moisture from the balance of the process airstream. This is known as the cooling section and most desiccant rotors have this fitted.
This type of system has several advantages over other desiccant systems. The structure is lightweight and porous. Different kinds of desiccants - both solid and liquid - can be impregnated into the structure which allows a rotor to be tailored for specific applications. Because the flutes of the structure are like individual desiccant lined air ducts, the surface area available for drying is maximised. This reduces the static resistance significantly.
Low dewpoint and high capacity - the two mutually exclusive goals - can be achieved by combining different desiccants in the same rotor. And, since the rotating mass is low compared to its moisture removal capacity the design is highly energy efficient.
The one concern with this type of desiccant is the cost of the rotor. It is expensive to manufacture and care must be taken to ensure the rotor is not damaged during maintenance. However, the first cost is generally balanced by operational advantages.
Efficiency, simplicity, reliability and easy maintenance make this the system the most widely installed type of all desiccant dehumidifiers.
Choosing between desiccant and cooling dehumidifiers
In many situations, both desiccants and cooling- based dehumidifiers can remove moisture from air, so the question arises - which type to use? Like choosing between different types of desiccants there are no simple answers but there are some general guidelines that have emerged in the industry.
· Cooling and desiccant based dehumidification systems are most economical when used together. The technologies complement each other; each strength of the desiccant covers a weakness of cooling systems and vice versa.
· The difference in cost of electrical power and thermal energy will determine the optimum mix of desiccant to cooling based dehumidification. Desiccant dehumidifiers can use steam, gas or electricity for reactivation energy. If thermal energy is cheap and power costs high then economics favour the desiccant to remove the bulk of the moisture.
· Cooling based dehumidification systems are more economical than desiccants at high air temperatures and moisture levels. They are very seldom used to dry air below 5°C dewpoint because condensate freezes on the coil.
· Desiccant systems should always be used when dewpoint of 5°C or lower are required with close control. It is relatively easy to control desiccant systems at low dewpoint when the process or application requires close control.
With thanks to Mike Creamer of Business Edge who revisits his Masterclass series of articles, updating and adding to the information which proved so useful to readers when the series was first published over ten years ago. In this reincarnation, the series will cover both air conditioning and refrigeration and serve as an on-going source of technical reference for experienced personnel as well as providing a solid educational grounding for newcomers to our industry.