Pr
o
-
C
Series
9
Controls
Basic Package
The basic PRO-C unit ships with no controls. The standard
wiring package provides connections for the starting/
stopping of the complete unit, supply fan and the energy
wheel. The connections are shipped with factory jumpers
installed. Remote control of any of these options can
be achieved by removing the correct factory jumper and
installing a contact in its place. The contact should be capable
of handling 24V power at 2 amps (PRO-C-1000) or 3.5 amps
(PRO-C-2400). (See appropriate 1Ø or 3Ø circuit diagram
on pages 11 through 13). It is strongly recommended that
a remote unit start/stop contactor (supplied by others) be
used to turn the unit on and off. This allows the outdoor
air damper to fully close when the unit is off.
Optional Electric Preheat Frost Control
For applications where the outdoor conditions do not exceed
-10°F and where the indoor design conditions do not exceed
70°F and 25 percent RH, the energy wheel can operate at full
capacity and will not frost. For colder design conditions or
buildings with higher humidity levels, frosting of the wheel
can be prevented by providing a modest amount of preheat
to the outdoor air. The amount of preheat required is small
and is not intended to raise the outdoor air temperature
above the freezing point. It is only necessary to keep the
exhaust air temperature above the dew point. This prevents
condensation on the wheel so that all the moisture transfer
occurs in the vapor phase.
The preheat control option includes a fi nned tube
electric coil mounted on the outdoor air intake of the unit,
an SCR controller and a temperature sensor mounted in the
outdoor air plenum. The temperature for the controller is set
to the minimum temperature of the outdoor air required to
prevent condensation at the design indoor temperature and
humidity. This is done by plotting a line on the psychrometric
chart from the indoor design condition down to the coldest
temperature that does not cause the operating line to
intersect the saturation curve on the chart. As stated above,
for inside conditions of 70°F and 25 percent RH, this
temperature is about -10°F.
60
50
40
80
40
30
20
Dry Bulb Temperature (°F)
60
30
20
10
0
10
50
Exhaust Air EA
2
-5
20.2
-10
1.5
Humidity Ratio
(grains of moisture per pound of dry air)
38
27
70
Return Air RA
1
Return Air RA
2
Outside Air OA
2
Outside
Air OA
1
Example 1:
The return air condition is 70°F and
25% relative humidity (27gr/lb).
Line RA
1
-OA: No frosting occurs at full recovery
Example 2:
The return air condition is 70°F and
35% relative humidity (38 gr/lb).
Line RA -OA: Frosting occurs at full
recovery at exhaust air condition EA .
Therefore, frost preheat is required
to bring the outside air to condition OA .
2
2
2
Figure 8. SMX70 controller as installed on the electric pan-
el.
