SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 9
Designing a Dedicated OA System
In most climates, the peak latent ventilation load occurs at a lower dry-bulb
temperature and higher dew point than the outdoor air condition that
produces the peak sensible ventilation load. The ASHRAE Handbook—
Fundamentals is a popular source for climatic data representing the outdoor
design conditions for many locations. To aid the design of cooling and
dehumidifying systems, the handbook includes:
• Peak dry-bulb and mean-coincident wet-bulb temperatures
(sensible-design condition)
• Peak dew-point and mean-coincident dry-bulb temperatures
(latent-design condition)
• Peak wet-bulb and mean-coincident dry-bulb temperatures
(enthalpy-design condition)
Table 2 (p. 10) lists the 0.4 percent, cooling-design data for Jacksonville, Fla.
Plotting these values on the psychrometric chart (Figure 8, p. 10) illustrates
that the highest outdoor enthalpy exists at the peak wet-bulb condition. In
this case, the enthalpy of the outdoor air is 9 percent higher than it is at the
peak dry-bulb (sensible-cooling design) condition.
Note: Using the peak dry-bulb condition as the basis of design will undersize
the dedicated outdoor air unit, making it unable to properly dehumidify
the outdoor air at certain part-load conditions. Remember that the
primary purpose of the dedicated outdoor air system is to properly
control space humidity at all load conditions.
Step 2: Choose the maximum limit for space humidity. The leaving-
air dew point is determined so that the space humidity level does not exceed
some defined upper limit at worst-case conditions. Some design engineers
might choose 50 percent relative humidity (RH) for the upper limit; others
might choose to design the system to allow the humidity to rise a little higher
(e.g., 60 percent RH) at worst-case conditions.
Note: Some types of local cooling equipment, such as chilled beams or radiant
cooling panels, cannot handle any condensation. If this type of
equipment is used, the outdoor air must be dehumidified to a dew point
low enough to remove all of the space latent load plus some margin of
safety to prevent condensation from forming on the local equipment.
As an example, the upper humidity limit might be 55°F (13°C) dew point.
This allows water at about 57°F (14°C) to be sent to the chilled beams or
radiant panels without condensation.
In this example, combining the 75°F (23.9°C) setpoint for the space with a
maximum relative humidity of 50 percent corresponds to a humidity ratio of
64.9 grains/lb (9.3 g/kg) or a dew point of approximately 55°F (13°C).
The examples throughout this guide
are based on “0.4 percent” data from
the ASHRAE climatic data tables. This
percentage indicates that the temperature
is likely to equal or exceed the design value
for 35 hours each year. Some design
engineers choose to use more extreme
conditions; others base their designs on
the “1 percent” or “2 percent” values,
which represent more hours.
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