For a regenerator that concentrates a solution of lithium chloride initially at 90 F and 39% to 43%, the thermal energy to heat the weak desiccant up to an operating temperature of 200 F equals 70% of the thermal energy required to remove the water from the desiccant. Thus, it is essential that some of the thermal energy in the hot, strong desiccant that flows from the regenerator to the conditioner be recovered to preheat the weak desiccant flowing to the regenerator. In a well-designed liquid-desiccant air conditioner (LDAC), an interchange heat exchanger (IHX) provides this heat recover. (The concept drawing in the liquid desiccant tutorial shows the operation of the IHX within the LDAC.)
For an LDAC that uses lithium chloride, a metallic IHX can be expensive. In high chloride environments, stainless steels can be subject to stress-corrosion cracking and so cannot be used for the IHX. A titanium plate-and-frame heat exchanger is a viable option, but an expensive one.
A plastic heat exchanger can be a cost-effective alternative for the IHX. AILR has developed liquid-to-liquid heat exchangers with plates made from thin, thermoformed films of polymers that can operate at over 200 F. The thin films (typically 5 mil thick) do not significantly impede heat transfer allowing the IHX to have a high effectiveness (i.e., plastic-film IHX with about 50 square feet of surface area that exchanges heat between two 4 gpm flows of lithium chloride will have an effectiveness of 80%). The neighboring figure shows a plastic-film IHX. (The IHX shell in this figure is polycarbonate so that the internal core is exposed. The IHXs now used in AILR's LDACs have G-10 fiberglass shells.)