Generally, substance can be dried with the help of supplying hot air stream (air at high temperature) & dry air. Humidity, temperature, air flow rate are the vital parameters of the drying process. With this method, products’ quality may be affected, because of high temperature. Now, if we want the drying process to take place at lower temperature then the solution is desiccant dehumidification – a novel technique.
Desiccants are the materials which attract the moisture from both liquids and gases (air) naturally. This moisture (which is attracted and retained by the desiccant) is then released to the atmosphere. Desiccants may be solid or liquid ones. Some items that may be damaged at higher temperatures can be kept dry by using these techniques. These techniques are cost effective too.
Other drying techniques reduce air’s humidity at higher temperatures. There are three methods of lowering the humidity ratio of the air:- (1) Cool it to condense the water vapour (a conventional method, used by vapour compression air conditioners). (2) Increase its total pressure, which also leads to the condensation. (3) Flowing the air over the desiccant, which sorbs its moisture, this happens due to the difference in their vapor pressures.
Dry (conditioned) air can be used for human comfort or for preservation of vital goods. The most common desiccants are of two types – solid as well as liquid, both collect moisture. Solid desiccants are silica gel, polymer sorbent, alumina silicate, zeolite etc., and liquid desiccants include lithium chloride (LiCl), calcium chloride (CaCl2), tri-ethylene glycol (TEG). Their surface vapour pressure is a function of their moisture content and temperature.
One important difference between these two types of desiccants is their reaction with moisture. Some collect the moisture upon their surfaces. The water is held in between small pores, these types are called adsorbents and these are usually solid in nature. On the other hand, those desiccants undergo chemical or physical change when they collect moisture i.e., LiCl, CaCl2, sodium chloride- table salt etc. These are called absorbents. They may be solid or liquid in nature, but after absorbing the moisture they become liquids.
One more important difference between conventional dehumidification or desiccant dehumidification is that the operating temperature of dehumidifier is not necessary to be lower than DPT (Dew Point Temperature) of the process air (which has to be dehumidified), as in case of conventional dehumidification system and also low grade heat energy (waste heat) can be used for desiccant dehumidification system. However, there are some problems with the desiccant dehumidification system which are: – pressure drop in solid desiccant, liquid desiccant carryover by air stream, low moisture adsorption capacity etc. These problems may be eliminated in future by using improved designs of desiccant systems.
Desiccant assisted air conditioning system
A dedicated system comprises a solid silica gel-based dehumidification unit and a sensible heat exchanger and a heat source. Ambient air travels through a desiccant dehumidifier, which dehumidifies it and raises its temperature to near the desiccant dehumidifier regeneration temperature. Similarly, hot and dehumidified air travels through a sensible heat exchanger (heat recovery), which reduces the temperature of the air somewhat while maintaining the humidity ratio. The process air then goes through the sensible cooling device, which reduces the temperature to that of the designed room supply condition, which is optimal. Ambient air conditions are intended for the regeneration of desiccant dehumidifier (regeneration side). The return room air is gone through a heat exchanger as a return air that transfers the heat of process air into the regeneration air, slightly increasing its temperature without influencing the humidity ratio.
Fig. 1 shows a schematic design of the system. The regeneration air is heated by a low-grade heating source (ideally a solar collector, although biomass or waste heat from operations can also be employed), which raises the temperature of the regeneration air even more. The desiccant dehumidifier desorbs the solid desiccant (i.e., silica gel) and absorbs its moisture, transferring the sensible heat into the desiccant unit, which is subsequently delivered to the process air, completing the cycle.
How is desiccant cooling better than the conventional cooling?
Desiccant dehumidifiers’ working principle is different from conventional cooling based dehumidifiers. Desiccant dehumidifiers perform two functions (1) Dehumidification (or conditioning) of air and (2) regeneration of desiccant, thus cycle is completed. They do not directly cool the air to remove its moisture, when the vapour pressure is low at the surface of desiccant, they attract moisture. At that time the vapour pressure exerted by molecules of water is higher, so the water molecules enter into the desiccant – and thus air becomes dry (this is known as dehumidification process). The required characteristic of desiccant is their low surface vapour pressure, when it is cool and dry. Its vapour pressure increases when it is wet and hot. This characteristic of desiccant is exploited to absorb the moisture of air for the dehumidification purpose and again releases the moisture to the air for the regeneration (when vapour pressure of desiccant is higher than the air) of desiccant. This system also improves the quality of conditioned air because we can use fresh air instead of used air as in conventional air conditioning.
Conventional dehumidifiers are best when used for higher temperatures and moisture levels. But, they are not suitable to dry air below 8°C dew point because condensation freezes on the coil, thus slowing the moisture removal process.
If dehumidified air with relatively high humidity is required then solution is conventional dehumidification, on the other hand if desired result is conditioned air whose RH is less than 45% then desiccant humidifier is the only solution. Generally, desiccant dehumidification system is used for applications below 45% RH down to less than 1% RH.
Desiccant dehumidification is the best suitable when air is cold around 8°C or below and humid or when low dew point is required.
Conclusion
Desiccant-based humidifiers and cooling systems are environment friendly and provide an added advantage of simple technology, robustness with low power consumption. The main objective is to get desired humidity and comfortable temperature. This aim can be accomplished by selecting appropriate desiccant material and regeneration heat source at suitable geographic conditions or by opting hybrid of techniques to save energy further. After understanding the basic operating principle of this technique, it’s clear that today the user has number of options in desiccant dehumidification systems to choose from. By using right technology in right condition one can conserve a lot of energy and obtain better economy.
Dr. (Prof.) D.B. Jani received Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently he is a recognized Ph.D. Supervisor at Gujarat Technological University (GTU). He has published more than 180 Research Articles in reputed International Conferences and Journals. He has also published 5 reputed books in the area of thermal engineering. Presently, he is an Associate Professor at GEC, Dahod, Gujarat Technological University, GTU, Ahmedabad (Education Department, State of
Gujarat, India). His area of research is Desiccant cooling, ANN, TRNSYS, and Exergy.
