Enthalphy Wheel Desiccant media

D ESICCANT SELECTION

By their nature, all desiccants remove water from an air stream. Each desiccant has an equilibrium capacity which determines the amount of water that it can adsorb. Kinetics defines the rate at which this equilibrium capacity is achieved. Other factors that affect water adsorption are desiccant selectivity and desiccant life.

There are three classifications of desiccants used in dehumidification equipment within the HVAC industry:

1.) Lithium chloride
2.) Silica gel (including titanium silicate).
3.) Molecular sieves (e.g., 3Å, 4Å)

Lithium chloride was widely used 20 years ago in a majority of desiccant-based equipment. It is rarely used today
because it dissolves (deliquesces) as it removes moisture
from the wheel. This greatly limits the life of the product.

An enthalpy wheel manufactured with silica gel is an excellent desiccant for treating saturated or near-saturated air streams. Compared to most molecular sieves on an equal weight basis, silica gel can hold more water in air streams above 50% RH. It must be noted that in the case of an enthalpy exchanger, the wheel moves between two equilibrium relationships (isotherms), that of return air and outside air. The wheel never completely dries out, but in fact remains near saturated all the time. As a result, only a very small part (less than 1%) of the isotherm curve is used. In order to accurately measure the amount of moisture that’s adsorbed by the wheel in a given revolution and on an equal desiccant weight basis, you must consider the differential between the adsorption capacity at the outdoor air and return air conditions. While silica gel holds more water at RH’s above 50% than do
molecular sieves, it also holds more water under the return air condition. Molecular sieves and silica gel do approximately the same amount of work in an enthalpy exchange application on an equal desiccant weight basis.

Desert Aire’s enthalpy wheel uses molecular sieves made of porous crystalline aluminosilicates. This type of desiccant falls under a family classification called zeolites. On an atomic level,

the zeolite framework is an assemblage of silica and alumina tetrahedra joined together in various regular arrangements through shared oxygen atoms. This configuration forms an open crystal lattice of molecular-sized pores into which guest molecules can penetrate. The crystal lattice creates a micro- pore structure that is precisely uniform with no distribution of pore size. This single feature distinguishes zeolites from other microporous adsorbents. A 4 Angstrom sieve is a sodium aluminosilicate with an effective aperture size of 3.8 Angstroms. Since methane is the smallest of the organic molecules at a critical diameter of 4.2 Angstroms, molecular
sieves effectively screen out adsorption of all organic compounds. On the other hand, water is smaller than 3.8 Angstroms so it is readily adsorbed.

In addition, an enthalpy exchanger’s effectiveness is determined not only by the capacity of the desiccant it uses, but
also by the quantity of desiccant it exposes to the air stream.