The rotating wheel heat exchanger, or energy recovery wheel, is
comprised of a rotating disk filled with an air permeable material resulting in
an extremely large surface area. The surface area is the material is utilized
for the sensible energy transfer. As the wheel rotates between the ventilation
and exhaust air streams it picks up heat energy and releases it into the colder
air stream. The energy transfer is simple as the difference in temperatures
between the opposing air streams which is also called the thermal gradient.
Typical media used consists of polymer(plastic) and aluminum.
The Enthalpy Exchange is accomplished through the use of desiccants
embedded in the surface of the wheel media. Desiccants transfer moisture through
the process of adsorption which is primarily caused by the difference in
the partial pressure of humidity between the supply and exhaust
air-streams. The most common desiccants in use consist of a silica gel
compound and molecular sieves compounds that bind at the molecular
level with the water molecule.
A typical wheel has two air streams. The first is the fresh air
stream. The second is the exhaust air stream. The fresh air stream
can be divided into two smaller components; the first is the outdoor air
component. This is the fresh unconditioned air entering the energy
recovery wheel. It will eventually be delivered to the space. The
second component of the fresh air stream is the supply air component The
supply air component of the fresh air stream is the air leaving the wheel.
It has been conditioned i.e. either warmed up or cooled down and is
closer in energy to the temperature of the space than unconditioned outdoor
air.
The second airstream can also be divided into two subsections. The
first of these two subsections is the return air. This is the air coming
back from the space. It is eventually destined for the outdoors once the
energy present in it has been recovered. The second of these subsections
is the exhaust air. This is the air that was return air before it passed
through the energy recovery wheel. All available energy has been
extracted from it and it is now a waste product.
Another concept important to understand about energy recovery wheels is
concurrent flow vs. counter flow. Concurrent flow occurs when the two
airflows are physically moving in the same direction. The entering side
of the wheel is the same for both air paths. Counterflow is the opposite
of this. It is more efficient and leads to better energy recovery
efficiencies.
Some energy recovery wheels will have a device known as a purge plate.
I will cover its exact operation in a dedicated blog entry, but for now
understand that the purge plate is used to help prevent cross-contamination
between the two air streams.
As stated earlier, energy wheels rotate to exchange energy between the
two streams. This rotation can either be at a constant speed or at a
variable speed. If the wheel is rotating at a constant speed, the wheel
leaving temperature is determined by the difference in temperature and humidity
of the two air streams. If the wheel will rotate at a variable speed the
temperature of the leaving supply air can actually be modulated up and down, by
spinning the wheel faster and slower through the use of a variable frequency
drive.
