Here we shall concentrat on the basics of the substances involved in Air Conditioning processes for comfort, their properties, and their fundamental characteristics required for operation in liquid desiccant-based systems, LDACS.
Atmospheric Air is a mixture of dry air with Water Vapour in a variable concentration. The composition of dryair may vary from place to place, and in time. We shall consider here the standard atmospheric dry air composition, however.
A common graphic representation of the state of atmospheric air takes the form of a h-x Mollier Diagram1,2, or of a psychrometric chart3. Both representations are for a single atmospheric pressure, which means that different representations are in principle necessary for different altitudes. A particularly interesting representation is obtained when the diagram also includes the equilibrium lines of a specific liquid desiccant, e.g. aqueous LiCl.
Diagrams, such as the h-x Mollier Diagram and its equivalents, are not anymore the preferred engineering tools in the design of air conditioning systems. However, they continue to represent an excellent tool for process visualization, far superior to the nakedness of a triplet of numbers! An image, even an abstract one such as a diagram with its criss-crossing lines, gives the educated eye an enormous richness of information, e.g. how far from crystallization is a regenerator operating if if you cannot ascertain the relative positions of the operating point and the crystallization boundary?
An air-desiccant equilibrium diagram would give an unequivocal hint for action, before catastrophe strikes.
Desiccants are substances characterized by a high affinity for water, and desiccant-based air conditioning systems are often designated as Sorption Systems.
Etymologically, to desiccate means drying out water from something, e.g. atmospheric air. Physically, this is limited by the capacity of desiccants to take-up water, which ends as the thermodynamic equilibrium with the medium or substance to be dehydrated is attained. In the case of atmospheric air in contact with a liquid ddesiccant, the water vapour condenses into the desiccant. In this process, the water enthalpy of condensation, and eventually some sensible heat, will increase significatively the temperature of the desiccant, thus reducing its potential to continue absorbing water vapour. The full drying potential of a desiccant can only be exhausted when the absorption process is cooled continuously. Furthermore, the absorption of water by the desiccant is accompanied by the liberation of the enthalpy of sorption, which further increases the need for cooling.
Eventually however, the water intake capacity of the desiccant will be exhausted. The desiccant needs to be regenerated, that is, reconcentrated to free it from water. The vaporization of water during regeneration cools-down the desiccant. An external supply of thermal energy is thus required to keep the regeneration process going. Besides the vaporization enthalpy, the energy required by the regeneration process has to include (de)sorption enthalpy as well (This is like a toll road: It costs in both directions!). As a rule of thumb, the sorption enthalpy is typically 10% of the enthalpy of vaporization, at the actual process temperature.
|Liquid Desiccants are Aqueous Solutions of:|
|1.||Alkali Halide Salts|
|2.||Metalic Bases, e.g.|
|NaOH||Sodium Hydroxide, Caustic Soda|
|4.||Organic Substances, e.g.|
Water is the key player in the operation of LDACS. Its large phase-change enthalpy means that only small amounts need to be vaporized, or condensed, to induce significant temperature changes in air — the temperature of 1 kg of air 2.5 K by the adiabatic evaporation of just one gram of water! — as in evaporative cooling, for example. And evaporative cooling is an integral part of any LDACS.
Water is used in conventional air conditioning systems in cooling towers, and in evaporative cooled condensers. These are direct contact evaporative coolers EC, with open residual pools of water. As such, they may bear significant risks to people and the environment, by offering excellent breeding conditions for insects (e.g. Denge transmitting mosquitoes), and bacteria, in particular for Legionella Pneumophila, which is at the origin of Legionaires Disease, an often fatal infection of the lungs.
In ALDACS indirect evaporative cooling IEC is the rule. The IEC process needs no open pool of residual water, and there needs not be any direct contact of the supply air with water at any point in an ALDACS Plant. The evaporative process takes place to the return air, which is discharged after passing through a heat recovery heat exchanger.
It is conceivable to isolate the water from the air in an indirect evaporative cooler (IEC), or even in an IEC Cooling Tower.
Notes & References
 Ramzin, L. K. 1927. Diagram for Humid Air, Izvestija Teplotechničeskogo Instituta, 24(1), 8-47.
(Рамзин, Д. К. 1927. Расчет сушилок и Ј — б Диаграмма, ИЗВЕСТИЯ ТЕПЛОТЕХНИЧЕСКОГО ИНСТИТУТА,
ВыПУСК № 1 (24), 8-47)
 Carrier, W. H. 1911. Rational psychrometric formulae, Trans. Am. Soc. Mech. Engineers, 33, 1005-1035.