Trane DX - Page 14

SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 11
Designing a Dedicated OA System
Step 4: Determine the total airflow that the dedicated outdoor air
unit must deliver.
If a centralized piece of equipment brings in outdoor air,
and then delivers only outdoor air (not mixed with any recirculated air) to
one or more ventilation zones, ASHRAE Standard 62.1 classifies this as a
“100-percent outdoor air ventilation system.” Accordingly, the system-level
intake airflow (V
ot
) delivered by the dedicated OA unit should be the sum of
the calculated zone outdoor airflows (V
oz
):
V
ot
= Σ V
oz
Given the zone outdoor airflow requirements (V
oz
) listed in Table 3, the
dedicated OA unit in this example must deliver a total outdoor airflow of 1815
cfm (0.86 m
3
/s).
Step 5: Determine which zone requires the driest conditioned
outdoor air.
Because the dedicated outdoor air unit will offset the latent
loads in each space (as well as the total ventilation load), the conditioned
outdoor air must be dry enough to enforce the maximum humidity limit in
the worst-case space. Use the following equation to calculate the required
conditioned-air humidity ratio, W
ca
, for each space:
where,
Q
L
= latent load in the space, Btu/hr (kW)
V
oa
= conditioned outdoor airflow, cfm (m³/s), which is supplied to the
space by the dedicated outdoor air handler
W
ca
= humidity ratio of the conditioned outdoor air, grains/lb (grams/kg)
W
sp
= maximum limit for the humidity ratio in the space,
grains/lb (grams/kg)
For example, to ensure that the humidity (W
sp
) in Classroom 101 does not
exceed the maximum limit of 64.9 grains/lb (9.3 g/kg), the humidity ratio of
the conditioned outdoor air, W
ca
, must be 48.0 grains/lb (6.92 g/kg).
Table 3 shows the results of this calculation for all four classrooms. Although
the highest latent load exists in Classroom 103, the “critical space” is
Table 3. Design criteria for a DOAS serving four classrooms in Jacksonville,
Fla. (example)
Space characteristics Classroom 101 Classroom 102 Classroom 103 Classroom 104
Sensible load
29,750 Btu/hr
(8.7 kW)
26,775 Btu/hr
(7.8 kW)
26,927 Btu/hr
(7.9 kW)
28,262 Btu/hr
(8.3 kW)
Latent load
5,250 Btu/hr
(1.5 kW)
5,465 Btu/hr
(1.6 kW)
5,697 Btu/hr
(1.7 kW)
5,250 Btu/hr
(1.5 kW)
Sensible-heat ratio
(SHR)
0.85 0.83 0.83 0.84
Required outdoor airflow
450 cfm
(0.21 m³/s)
450 cfm
(0.21 m³/s)
480 cfm
(0.23 m³/s)
435 cfm
(0.20 m³/s)
Humidity ratio of
conditioned air, W
ca
48.0 grains/lb
(6.92 g/kg)
47.3 grains/lb
(6.76 g/kg)
47.7 grains/lb
(6.84 g/kg)
47.4 grains/lb
(6.80 g/kg)
Q
L
0.69 V
oa
× W
sp
W
ca
–()×=
Q
L
3.0 V
oa
× W
sp
W
ca
–[]×=()
In the Q
L
equations at right, 0.69 and 3.0
are derived from the properties of air; they
are not constants. At the “standard” air
condition, which is 69°F (21°C) dry air at
sea level, the product of density, the latent
heat of water vapor, and a conversion
factor for units—7,000 grains/lb (1,000
grams/kg) and 60 min/hr—equals 0.69
(3.0). A different air condition or elevation
will result in a different value.
Step 5 (Classroom 101):
Q
L
= 5,250 Btu/hr
= 0.69 x 450 cfm x (64.9 gr/lb - W
ca
)
∴
W
ca
= 48.0 gr/lb
(Q
L
= 1.5 kW)
(= 3.0 x 0.21 m
3
/s x [9.3 g/kg - W
ca
])
(∴ W
ca
= 6.92g/kg)