AAON RK Series - Manual (Page 11)

11
DESIGN CONDITIONS & CONTROL STRATEGIES
Standard temperature control
The unit can be configured with normal air flows and
controls but still have the benefit of a large amount of
makeup air, better humidity control and lower
operating cost than a unit without a heat wheel. The
energy recovery unit operates in four (4) basic modes;
fan only; economizer; cooling and heating. Each of
these modes has specific functions as defined below.
Fan only mode: When the unit supply fan is started,
and there is no call for cooling or heating, the unit
economizer moves to its minimum position, the
heatwheel is activated and the heatwheel fan is
started. If the unit is equipped with heatwheel bypass
dampers, these are closed.
Economizer mode: With the unit supply fan in
operation and a call for cooling is made, if the outdoor
air temperature and humidity are below the enthalpy
setpoint, the heatwheel exhaust fan is activated, the
heatwheel is deactivated and the economizer
modulates to maintain the mixed air setpoint. If the unit
is equipped with heatwheel bypass dampers, these
are opened to accommodate the increase in outside
air volume.
Cooling mode: With the unit supply fan in operation
and a call for cooling is made, if the outdoor air
temperature and humidity are above the enthalpy
setpoint, the economizer moves to its minimum
position and mechanical cooling is activated. The
heatwheel is activated and the heatwheel exhaust fan
is started. If the unit is equipped with heatwheel
bypass dampers, these are closed.
Heating mode: Upon a call for heat, the heating
function is activated, the supply fan is activated and
the economizer moves to its minimum position. The
heatwheel is activated and the heatwheel exhaust fan
is started. If the unit is equipped with heatwheel
bypass dampers, these are closed.
Notice that in all four (4) basic above modes, the
operation of the heatwheel is determined by the
position of the economizer. With the exception of unit
shutdown or a night setback mode, the heatwheel
exhaust fan is in operation.
When control systems are "by others", all of the above
modes of operation must be considered.
Ventilation of Occupied Spaces
In Industrial Applications
General ventilation of occupied spaces in Industrial
facilities is an excellent application for energy
recovery. It can have many significant benefits
including: odor control, a better working environment
for employees, higher productivity, reduced risk from
exposure to volatile compounds and particulates in the
indoor air, improved humidity control (for process and
people) and reduced energy costs to condition the
ventilation air. General ventilation with energy recovery
is not a substitute for fume hood exhaust. The success
of the industrial application depends on proper design
and an understanding of the performance
characteristics of the enthalpy wheel.
Energy recovery wheels or enthalpy wheels have
some inherent exhaust air transfer due to the volume
of air carried by wheel rotation from one airstream to
the other. In addition, while wheels are highly resistant
to fouling due to the counter flowing airflow
arrangement, they can be plugged by large amounts of
semi-volatile compounds or aerosols, which are
allowed to impinge and/or condense on the wheel
surfaces. These characteristics affect the installation
and application as follows:
1. Use energy recovery for general dilution
ventilation of the occupied space, not for
recovering energy from dedicated, highly
concentrated or toxic exhaust.
Exhaust air transfer in the energy recovery system
results in a small amount of the exhaust air, typically
less than 5% for wheels operating in balanced flow,
returning to the space. This amount of exhaust air
transfer is appropriate to handling general exhaust in
an environment where continuous exhaust and supply
of outdoor air to the space achieves the required
dilution of contaminants. In space conditioning
applications, where the ventilation system is operating
to maintain acceptable indoor air quality, there should
not be contaminants in concentrations of concern. It is
not appropriate for recovering energy from highly
concentrated machine exhaust, such as hoods
installed on the print heads themselves. Even a small
amount of exhaust air transfer in this case can
increase contaminants and odors in the space. This air
is best exhausted directly outdoors and treated as may
be required by local code. If energy recovery is desired
in these environments, a “run around loop” approach is
suggested.