AAON RK Series - Manual (Page 13)

13
bulk liquid water present which could support fungal
growth or dissolve other chemical species. The
transfer of water onto and off of the wheel’s desiccant
surfaces occurs in the vapor or gas phase. There are
no “wet” surfaces and liquid water does not enter the
air stream.
The sensible (non-desiccant coated) wheel can also
transfer water through the different mechanism of
condensation and re-evaporation, however; again,
there is no accumulation of water, unless the frosting
threshold is violated through misapplication of the
component. In this case, the water is in the form of
frost or ice which does not support fungal growth.
Sensible (uncoated) wheels from all manufacturers are
identical in this regard.
Both moisture and nutrients are required to support
fungal growth. Therefore dirt accumulation on heat
wheels is of potential concern. It is also true that any
heat wheel can accumulate semi-volatile compounds
like tars and grease which are deposited on surfaces.
These surfaces can then become odor and
contaminant sources, in the same way that a filter or
any other element of an air handling system can
become a source of compounds accumulated over
time.
The heatwheel was designed to respond to these
issues over the life of the system by providing for
cleaning and maintenance with washable desiccant
surfaces, removable segments and easy to access
cassettes. Many aspects of this technology are
patented and are unique in the industry.
Silica Gel Desiccant
Silica gel is an inert, highly porous solid adsorbent
material that structurally resembles a rigid sponge. It
has a very large internal surface composed of myriad
microscopic cavities and a vast system of capillary
channels that provide pathways connecting the
internal microscopic cavities to the outside surface of
the “sponge”.
The characteristic curve for adsorption of water on
silica gel is shown in Figure 1 (page 13), as % weight
adsorbed versus relative humidity of the air stream in
contact with the silica gel. The amount of water
adsorbed rises almost linearly with increasing relative
humidity until RH reaches about 60%. It then plateaus
out at about 40% adsorbed as relative humidity
approaches 100%. (The curve for molecular sieves, by
contrast, rises rapidly to plateau at about 20%
adsorbed at 20% relative humidity. This helps to
explain why the molecular sieve is an excellent choice
for regenerated applications such as desiccant cooling
and dehumidification systems which are designed to
reduce processed airstreams to very low relative
humidity. On the other hand, silica gel has superior
characteristics for the recovery of space conditioning
energy from exhaust air.)
The use of silica gel on rotary regenerators for energy
recovery ventilation applications involves a process
cycle where the silica gel is alternately exposed to
airstreams having nearly equal relative humidity
somewhere in the mid range of this curve (typically
between 40 and 60%). When the air stream with the
higher relative humidity passes over the silica gel
coated wheel, moisture is adsorbed from the air
stream into the silica gel. Then when the air stream
with the lower relative humidity contacts the silica gel,
moisture is desorbed (removed) from the silica gel and
put into the air stream.
In this ventilation energy recovery application, the
silica gel has all of its surface area covered with at
least a monomolecular layer of water because it has a
greater affinity for water than any other chemical
species. With all of the adsorption sites occupied by
water, the silica gel will not be able to transfer other
chemical species by adsorption and desorption in its
normal form. Species that are soluble in water could
become dissolved in the adsorbed water and then
released when the water is desorbed but this process
is limited by kinetics and does not present a very
efficient mechanism for contaminant transfer.
An example of this phenomenon is formaldehyde, a
gas which is very highly soluble in water.
In the early 1980’s when energy recovery ventilators
were being used to mitigate excessive formaldehyde
levels in mobile homes, concern was expressed by
some people that enthalpy type heat exchangers that
transferred moisture as well as heat might also
transfer excess amounts of formaldehyde gas due to
its high solubility in water. Accordingly, tests were
conducted by the Lawrence Berkeley Laboratories of
the U.S.D.O.E., on two enthalpy type exchangers to
determine whether this suspicion was justified. Results
were presented in ASHRAE paper No. CH85-03 No. 3
which reported that the rotary type enthalpy heat
exchanger transferred formaldehyde with only 3-6%
efficiency. They concluded that “formaldehyde transfer
between airstreams by processes other than air
leakage does not seriously compromise the
performance of these enthalpy exchangers”.