Catalogue

8 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001-EN
Designing a Dedicated OA System
In most applications, in most climates, the dedicated OA unit is sized to
dehumidify the outdoor air to remove the moisture, or latent load, from the
entering outdoor air, and is often then dehumidified a little further. In this case,
the resulting dew point of the conditioned air is drier than the space, dry
enough that this quantity of outdoor air also removes most, or all, of the space
latent loads (Figure 7).
Figure 7. Sizing the dedicated OA unit to offset space latent loads
In some cases, the local HVAC terminals may also help to dehumidify the
space when the sensible-cooling load is high, yielding an indoor humidity that
is drier than the maximum upper limit. As a rule of thumb, size the dedicated
outdoor air unit so that it offsets both the ventilation load and the space latent
loads at the peak outdoor-enthalpy condition.
Selecting the Dedicated OA Unit
The following steps establish the required airflow, dew point, and dry-bulb
temperature for the conditioned air.
Step 1: Determine the entering-air condition. Three factors dictate the
capacity required from the dedicated outdoor air unit: airflow, the enthalpy of
the entering outdoor air, and the enthalpy of the conditioned air leaving the
cooling coil. If the outdoor airflow is constant, then the basis of design is the
condition resulting in the greatest difference in enthalpy across the cooling
coil.
Indoor latent loads fluctuate with occupancy and processes, as well as with
ambient conditions and wind through infiltration. These variables can make it
difficult to discover when the greatest enthalpy difference occurs. However, if
the latent loads within the space are relatively constant and infiltration is
minimal, assume that the greatest enthalpy difference occurs at the highest
outdoor air enthalpy.
CA
space
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
dry enough to remove
space latent load