12 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN
Designing a Dedicated OA System
Classroom 102 because it requires the driest air (lowest humidity ratio, W
ca
).
Supplying the conditioned outdoor air at a humidity ratio of 47.3 gr/lb
(6.76 g/kg) will offset the latent load in each classroom and assure that the
humidity in Classroom 102 does not exceed the maximum limit; lower
humidities will result in the other classrooms.
Step 6: Determine the required dew point for the conditioned
outdoor air.
With the help of a psychrometric chart (Figure 7), we find that a
humidity ratio of 47.3 grains/lb (6.76 g/kg) is equivalent to a dew-point
temperature of 46.7°F (8.2°C).
Step 7: Determine the supply-air dry-bulb temperature for the
dedicated outdoor air handler.
If the system design requires neutral-
temperature conditioned air, then the air leaving the dedicated outdoor air
unit must be reheated to the desired dry-bulb temperature. This is typically
between 70°F and 75°F (21°C and 24°C).
If the system design is based on cold conditioned air rather than neutral-
temperature air, then the dry-bulb temperature from the dedicated outdoor
air unit depends on the supply-air dew point. In our example, assuming that
saturated air leaves the cooling coil, then the leaving-air dry-bulb
temperature may be delivered as cold as 47°F (8.3°C), or might be reheated to
a more conventional air-delivery temperature, such as 55°F (12.8°C).
Note: For simplicity, our example does not include the effect of fan heat. A
draw-through fan arrangement will increase the dry-bulb temperature
of the conditioned outdoor air. The slightly warmer air offsets less of the
sensible load in the space, which will affect the selection criteria for the
local HVAC terminals.
To Next Page
Loading...
