Where:
Q
aproximately account for the losses caused for typical
fittings in a duct run and multiply by 0.1 in. wg/100
ft (0.8 Pa/m) and compare with the available external
static pressure of the HVAC unit.
= Heat flow (watts)
Lńs = Airflow (liters per second)
Dt
Example 5−10 (I−P)
= Temperature difference (°C)
A heating unit has an airflow rating of 4000 cfm and
an output of 150,000 Btuh. Find the final discharge
temperature with 75 degree F inlet air.
Using Equation 5−10:
AF
Example 5−10 (SI)
A heating unit has airflow of 1000 L/s and an output
of 50 kW. Find the final discharge temperature with 24
degree C inlet air.
1.23
Dt = 20.3°C
Q
lńs
An air handling unit has an external static pressure of
125 to 375 Pa depending on the fan rpm and motor
power (kW). Estimate if an 80 m duct run with many
fittings would be feasible.
80 m × 4 (1 + 3) = 320 m equivalent length
Q + 1.23
DT +
0.1 in.wg
= 0.9 in. wg
100 ft
The estimated system loss falls about half way within
the HVAC unit pressure capacity.
Final temperature = 75° + 34.7° = 109.7°F
Using Equation 5−10:
225 ft × 4 (1 + 3) = 900 ft equivalent length
900 ft
Q + 1.08 cfm Dt
Q
150, 000
Dt +
+
1.08 cfm
1.08 4000
Dt = 34.7°F
Example 5−9 (SI)
An air handling unit has an external static pressure
range of 0.50 to 1.50 in. wg depending on the fan rpm
and motor horsepower. Estimate if a 225 ft duct run
with many fittings would be feasible.
T
Example 5−9 (I−P)
+
lńs
Dt
320 m × 0.8 Pa/m = 256 Pa
50kW 1000WńkW
1.23 2000lńs
The estimated system loss falls about halfway within
the HVAC unit pressure capacity.
5.9.3
Friction Loss Chart
Final temperature = 24° + 20.3°= 44.3°C
HVAC Unit/Duct Relationship
DR
5.9.2
The selection of HVAC units is important to the design
of the connected duct systems. A lower airflow will result in higher temperature rises. The air velocity and
length of the duct system also affects the temperature
losses through the duct walls as well as the straight
duct and fitting pressure losses.
Once the HVAC air handling unit has been selected,
the individual space loads are proportioned from the
total airflow and the airflow for each section is marked
on the duct layout. The range of the HVAC unit external static pressure may limit the velocities and pressure drops that may be used in the individual duct sections. For lower pressure systems, a pressure drop of
0.1 in. wg per 100 in. (0.8 Pa/m) is a good place to start.
Measure the longest duct run (an assumption is made
that it has the highest pressure drop). For a rough estimate, double or triple the duct run length this will
5.14
Figures 5−8 and 5−9 are part charts from Figures A−1
and A−1M. If data for any two conditions of the duct
are known, the other two unknowns may be obtained
from the charts. The shaded area indicates the recommended design range.
Example 5−11 (I−P)
Using Figure A−1 find the duct diameter and airflow
velocity for 700 cfm at 0.1 in. wg/100 ft duct friction
loss.
The plotted solution can be found in Figure A−1. The
duct velocity = 900 fpm and the duct size = 12 in. diameter.
Example 5−11 (SI)
Using Figure A−1M, find the duct diameter and airflow
velocity for 200 L/s at 0.9 Pa/m duct friction loss.
The plotted solution can be found in Figure A−1M. The
duct velocity = 4.2 m/s and the diameter = 250 mm.
HVAC Systems Duct Design • Fourth Edition