Desiccant based rotary dehumidifiers are found most effective in humidity control of moist air, as air handler they are also advantageous in being free from CFCs, may be using low grade industrial waste heat or freely available renewable solar energy while controlling the ambient humidity. Compared with conventional vapor compression air conditioning system, it preserves the merits of environment-friendly, energy saving, healthy, comfortable, etc. Ongoing research and development works show that new desiccant materials and novel system configurations have significant potential for improving the performance and reliability and reducing the cost and size of rotary desiccant dehumidification and air handling system, thereby, increasing its market competitiveness and breaking out the current fairly small niche market. The purpose of providing this overview is to study the utility of desiccant based rotary air handlers that can ameliorates the comfort, cost and energy savings in the field of space conditioning.
Desiccant materials attract moisture based on differences in vapor pressure between desiccant substrate and moist air. Due to their enormous affinity to adsorb water and considerable ability to hold water, desiccants have been widely applied to marine cargo, pharmaceutical, electronics, plastics, food, storage, etc. Recently, the rapid development of desiccant dehumidification and space cooling technology, which can handle sensible and latent heat loads independently without using CFCs and without consuming a large amount of electric power, and thus, meet the current demands of occupant comfort, energy saving and environmental protection, has expanded desiccant industry to a broader niche applications, such as hospitals, supermarkets, indoor swimming pools, restaurants, theaters, schools and office buildings. Since desiccants remove moisture in the vapor phase without liquid condensate, desiccant dehumidification can continue even when the dew point of the air is blew freezing; in contrast, cooling-based dehumidification is limited by freezing phenomenon occurring at 0-8°C. As a result, desiccant assisted dehumidification is capable of handling the dew point of the air to 40°C. As desiccants can be either solid or liquid, desiccant air conditioning systems can be classified into two categories, namely, solid desiccant air conditioning systems, which consist of fixed bed type and rotary wheel type, and liquid desiccant air conditioning systems. Due to being advantageous in handling latent heat load, all these technologies have been used widely. Especially, rotary desiccant air conditioning systems, which are compact and less subject to corrosion and can work continuously, attract more attention. Desiccant materials have been playing a crucial role in the development of desiccant assisted air handling especially, during dehumidification of moist air. The characteristics of the desiccant material being utilized impact the performance of the desiccant assisted dehumidification systems significantly. Commonly used desiccant materials include activated carbon, activated alumina, molecular sieve, silica gel, lithium chloride, calcium chloride and etc. Two key principles for selecting appropriate desiccant materials are: the desiccant materials should possess large saturated adsorption amount and can be reactivated easily.
The desiccant wheel is a rotor, filled with a desiccant material (i.e. silica gel), in which humid air is dehumidified by the desiccant material, to balance latent loads of the ambient. To guarantee continuous operation, the wheel has to be regenerated by a hot air stream. The waste heat of a small cogeneration plant can be effectively used to regenerate the desiccant material, while the cogenerated electricity can drive a chiller to meet the room sensible load. The main advantages of this technology, with respect to conventional vapor compression based systems based on cooling dehumidification, are:
– The sensible and latent loads can be separately controlled;
– Very low dew point temperatures of process air, lower than -6°C, can be achieved;
– The cooling can be achieved with a higher chilled water temperature, which ultimately results in a higher COP of system;
– Due to the higher value of the COP, electric energy requirement for producing the cooling is lowered;
– As the sensible cooling coil has to handle only the sensible load of the process air, a reduction of its size is obtained; this consequently determines a lower environmental impact, both in terms of direct impact (ozone layer reduction and greenhouse effect due to refrigerant fluids) and indirect one (the reduced electric energy use determines lower equivalent CO2 emissions of the power plants);
– Consistent energy savings can be obtained to the increase in the overall energy efficiency;
– A better indoor air quality (IAQ) can be obtained, due to sanitizing effects of desiccants. Indeed, desiccant systems avoid the formation of condensed water; this strongly reduces the presence of microorganisms as bacteria, viruses and fungi.
Desiccant systems are especially, useful when the latent load is high (i.e., when the latent-to-sensible heat ratio is high), because they remove moisture more economically than they remove sensible heat. Another desirable situation is when the cost of dehumidification with a desiccant is lower than the cost of dehumidification with a traditional vapor compression-based refrigeration system. This is where thermal energy comes into the picture: there are instances where desiccant regeneration done by waste heat, natural gas, or off-peak electricity is more economical compared to regular electric refrigeration. Because there is no need for reheating with desiccant dehumidification systems, another appropriate use is when conditioned air must be reheated after coming out of a coil to reach a comfortable dry-bulb temperature. Finally, the use of a desiccant is well-suited to the case where dehumidification is required at levels below freezing dew-point temperatures.
Working of Desiccant Assisted Air Handling System for Effective Moisture Control
For industrial applications, solid desiccant dehumidification cycles use dual-column packed-bed dehumidifiers; however, the most appropriate dehumidifier configuration for air dehumidification applications is the rotary wheel (Fig. 1). The most desiccant dehumidifiers use the desiccant material in a rotor form. Rotors are manufactured from alternate layers of flat and corrugated sheets impregnated with the active component (desiccant). This forms a vast number of axial air channels running parallel through the rotor structure. As air passes through these channels, moisture is transferred between the air and the desiccant. The air to be dehumidified enters the system, comes into contact with the desiccant wheel, and exits the dehumidifier hot and dry. The wheel is then rotated so that the desiccant portion that has picked up moisture is exposed to hot reactivation air and its moisture removed. Since the air leaving the desiccant is heated because of the release of heat adsorption, there is a need for cooling the dried air in cooling applications. This can be accomplished with a sensible heat exchanger such as a heat pipe or with a standard vaporcompression cooling coil. To re-use the desiccant, it must be regenerated or reactivated through a process in which moisture is driven off by heat from an energy source such as electricity, waste heat, natural gas, or solar energy.
Generally, our desiccant dehumidifiers comprise of five major components:
– The component that contains the desiccant material (rotor), of which there are several types.
– A fan to move the air to be dehumidified (process air) through the desiccant rotor or material.
– A fan to move hot air (reactivation air) through the desiccant rotor or material.
– A heater to heat the air that is used to regenerate the desiccant material.
– A mechanical device to slowly rotate the desiccant rotor or material bed.
Desiccant units are typically used in areas where dry conditions are essential –
such as pharmaceutical and petrochemical applications – due to their ability to produce extremely low levels of humidity. The desiccant types are also purposefully selected for drying out areas where access is restricted, such as storage tanks and marine environments. Figure 2 shows the water vapor adsorption isotherms of several desiccant materials mentioned above. It is obvious that silica gel as desiccants improve adsorption capacity significantly.
Due to the effect of adsorption heat released during the dehumidification process, the temperature of the process air is increased and its relative humidity is decreased. As a result, the vapor difference, which is actually the driving force for dehumidification, is reduced, and corresponding dehumidification ability is limited then. Because of this, much higher regeneration temperature is required to obtain desired dehumidification capacity, especially for high humid climates.
Comparison of Moist Air Handling by Desiccant Wheel and Conventional VCR System
Air dehumidification can be achieved usually by two methods: (1) cooling the air below its dew point and removing moisture by condensation or (2) sorption by a desiccant material. Conventionally, used VCR compressor dehumidifiers are the most popular systems. They create a cold surface, and once the warm and damp air inside the room hits the cold surface, condensation starts, then the water separates from the air. A desiccant dehumidifier has no compressor and does not use a cold surface to extract the excess moisture from the air. Instead, it has a desiccant wheel that absorbs the moisture from the air, in a similar way to a sponge. The desiccant is regenerated by an internal heater so that the process can be repeated time and time again. Any application whereby the room air temperature is likely to fall below 15°C then you should be looking at a desiccant dehumidifier. This is because the inside of the vapor compressor dehumidifier needs to be colder than the air within the room and the colder the room is the harder the dehumidifier has to work to create that cold surface. As the temperature starts to fall down towards 10°C then the chances are that the inside of the dehumidifier will get close to freezing, increasing the changes of ice forming on the dehumidifiers cooling coils. This is why below around 15°C the vapor compressor dehumidifiers are programmed to spend up to two thirds of their time defrosting themselves rather than dehumidifying. The desiccant dehumidifiers on the other hand have a consistent performance regardless of the temperature. Both types of dehumidifier will warm the air up slightly, that is not to say that they are heaters just that they warm the air up as it passes through the dehumidifier. The air coming out of the compressor dehumidifier will be about 2°C warmer while the air coming out of a desiccant dehumidifier will be about 10-12°C warmer, quite a difference between the two (Fig. 3 (a) and (b)). So, if you are putting the dehumidifier into a hallway that is on the chilly side then the desiccant makes sense as it will warm it up, but if the hallway is already nice and toasty then the compressor dehumidifier is the correct option.
a)
b)
Figure 3: Performance comparison between a VCR compressor and desiccant dehumidifier.
In low fan speed desiccant dehumidifiers are all just below 40db and you will not get a vapor compressor dehumidifier below 40dB. The important point is that at least with desiccant dehumidifier you have the option to switch to a quieter mode which you cannot do with a compressor dehumidifier. Compressor dehumidifiers work at temperatures between 5°C-35°C with the optimum operating temperature being 15°C. Compressor models do not work at 0°C-5°C because the cold coils cannot extract moisture from the air at these temperatures. Desiccant dehumidifiers work in temperatures from 0°C-35°C and they feature a stable humidity intake no matter what temperature so, despite at running 600w/h, a desiccant dehumidifier is cheaper to run than a comparable compressor dehumidifier. Another important note is that compressor dehumidifiers use major greenhouse gases whilst desiccant dehumidifiers do not. Therefore, desiccant dehumidifiers are eco-friendly whilst compressor dehumidifiers are not. The comparison between working of the two dehumidifiers is shown in Fig. 4. Moreover, desiccant units use no consumables, so the desiccant material will not expire, or have to be topped up or replaced.
Desiccant dehumidifiers range in size and configuration dramatically. Desiccants used in restorative drying can be as small as a roll-on suitcase or as large as two semi tractor-trailers. Large desiccants are often self-contained dehumidification systems, utilizing onboard generators and running on propane or diesel fuel. When large catastrophic events occur, such as hurricanes, large desiccants can provide a means of dehumidification when little or no power is available.
Conclusions
Desiccant assisted dehumidification is an established technology that has been used successfully for many years in institutional and industrial applications. Commercial applications are now gaining acceptance. Desiccant based air handling systems have been applied successfully in supermarkets and ice rinks. Hotels and motels, office buildings, and restaurants provide the next opportunity. Lowering the cost of desiccant dehumidification systems and improving their performance will clearly provide more opportunities for desiccant dehumidification technology. Currently, a number of cost-effective applications in the market will result in increased sales during the next several years; but as in other technologies, further R&D and demonstration programs will enhance broader applications of the technology. Low temperature desiccants can effectively use waste heat from electric air conditioners and improve their efficiency and effectiveness-an area that need to participate for further development. Desiccant dehumidification systems as add-on modules to electrical refrigeration systems could help solve the challenges facing the HVAC industry especially as increased ventilation rates, need for improved indoor air quality and better humidity controls, phase-out of CFCs, national standards requiring higher efficiency for cooling systems, and desire for lowered peak electric demands. These factors, and the ability for desiccant assisted air handling systems to solve specific problems, are driving these desiccant technologies to the mainstream of the air-conditioning market.
