Handbook of air conditioning and refrigeration / Shan K

FIGURE 17.3 Pressure characteristics of a fan duct system.
AIR SYSTEMS: AIR DUCT DESIGN 17.9
combined airstream then becomes the main stream. Total pressure pt usually decreases when
the straight-through stream flows through the converging tee. It may increase if the velocity of the
branch stream is much higher than that of the straight-through stream. However, the sum of the energies
of the straight-through and the branch streams at the upstream side of the converging tee is
always higher than the energy at the downstream side because of the pressure loss of the converging
tee.
In the main duct section 1-21, pt and ps drop further while pv increases because of the higher air
velocity. This is mainly because of the greater volume flow rate in main duct section 1-21. When the
recirculating air enters the air-handling unit, its velocity drops sharply to a value between 400 and
600 fpm (2 and 3 m/s). The air is then mixed with the outdoor airstream from the fresh air duct at
node 2. After the mixing section, pt and ps drop sharply when the mixture of recirculating and outdoor
air flows through the filter and the coil. Total pressure pt and static pressure ps drop to their
minimum values before the inlet of the supply fan Fi. At the supply fan, pt is raised to its highest value at the fan outlet Fo. Both pt and ps decrease in
duct section Fo-3. At junction 3, the airstream diverges into the main stream or straight-through
stream and the branch stream. Although there is a drop in pt after the main stream passing through
the diverging tee 3-31, ps increases because of the smaller pv after 31. The increase in ps due to the
decrease of pv is known as the static regain �ps,r in in. WC (Pa). It can be expressed as
�ps,r �� v3 4005� 2
�� v31 4005� 2
� (17.22)
where v3, v31 � air velocity at cross section 3 and 31, respectively, fpm (m/s). In duct sections 3-4
and 4-S3, pt decreases gradually along the direction of airflow. Finally, pt, ps,and pv all drop to 0
after the supply air is discharged to the conditioned space.
�(v3 2 2 � v31) 2