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
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
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
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.