Trane DX - Control of the Dedicated OA Unit

SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 13
Control of the Dedicated OA Unit
The most common approach to controlling the dedicated OA system is to turn
it on when the building is expected to be occupied. The same time-of-day
schedule that is used to start and stop the local HVAC terminal equipment is
used to start and stop the dedicated OA system.
The fan in the dedicated OA unit is activated to bring in the required amount of
outdoor air for ventilation, and cooling, dehumidification, or heating is
modulated to maintain the discharge air at the desired conditions.
The operating mode for the dedicated OA unit is based on the current outdoor
air conditions. Outdoor temperature and humidity sensors are used to
calculate the outdoor air dew point, and compare it to the desired leaving-air
conditions. This determines whether the unit operates in Dehumidification
Mode, Sensible Cooling Mode, Sensible Heating Mode, or if conditions are
sufficient for the unit to operate in Ventilation Only mode (Figure 9 and
Table 4).
Figure 9. Dedicated OA unit control modes
Table 4. Dedicated OA unit control modes
Control mode Outdoor conditions
Dehumidification Outdoor Air Dew Point > Dehumidification Enable Setpoint
Sensible cooling
Outdoor Air Dew Point ≤ Dehumidification Enable Setpoint
Outdoor Air Dry-Bulb Temperature > Cooling Enable Setpoint
Ventilation only
Outdoor Air Dew Point ≤ Dehumidification Enable Setpoint
Heating Enable Setpoint ≤ Outdoor Air Dry-Bulb Temperature ≤
Cooling Enable Setpoint
Sensible heating
Outdoor Air Dew Point ≤ Dehumidification Enable Setpoint
Outdoor Air Dry-Bulb Temperature < Heating Enable Setpoint
sensible cooling
30 35 40 45
50
55 65 70 80 90 100 11075 85 95 105
30
35
40
45
50
55
65
70
80
75
85
60
60
wet-bulb temperature, °F
dry-bulb temperature, °F
humidity ratio, grains/lb of dry air
0
20
40
60
80
100
120
140
160
180
dew-point temperature, °F
40
45
50
55
65
70
80
75
60
30
20
0
sensible cooling mode
sensible heating mode
dehumidification mode
ventilation
only
dehumidification enable setpoint
cooling
enable setpoint
heating
enable setpoint