The fan is selected based on the minimum airflow
requirements for the space or the heating load required. In most cases the fan
can be downsized from the cooling flow requirement considerably, reducing both
first cost and operating cost. The fan is selected from the fan curves. The
downstream static pressure of the secondary air may not be the same as the
primary air, however. If the secondary airflow requirements are less than the
primary air requirements, the static pressure will be reduced. The following
equation can be used to determine the static pressure at reduced airflows. (Do
not forget to add water coil pressure drops to the fan requirement).
To select a Titus parallel fan powered terminal, refer to
the published fan curves and primary air pressure drop curves, together with
the application and sound power data.
In the parallel flow
type of unit, when the primary air is ON, the fan is typically OFF, and vice
versa. As shown in the Figure 1, the primary air and the fan discharge air
follow parallel paths into a common plenum. Therefore both airflows will
encounter the same downstream resistance at a given flow rate.
Since the primary and secondary airflows come from two
different sources-and often at two different specified flow rates-the volume
vs. pressure relationship in each of these airflows must be checked to ensure
adequate flow rates under actual job conditions.
Example: Select a Model DTQP for a maximum of 1400 cfm of
primary air with 1.00” wg inlet static pressure. The fan airflow required is
1150 cfm. The downstream resistance offered by the duct and diffusers has been
determined to be 0.30” static pressure at 1150 cfm.
Primary Air: From the air inlet pressure table, a size 4
with a 12” inlet will handle 1400 cfm of primary air with a minimum static
pressure drop of 0.23” through the primary air section. But since the
downstream resistance is 0.30” at 1150 cfm:
The overall primary air static pressure drop is:
0.23”+ 0.44”= 0.67” sp
Since a 1.0” static pressure is available at the inlet, the
selection will work. The damper in the primary air section will do some
throttling to hold the maximum air flow to 1400 cfm.
Secondary Air (Fan): From the fan curves, a size 4, without
coils, terminal will handle 1150 cfm at 0.30” static pressure, with the proper
setting of the standard SCR speed control.
Trenton Yarbrough - Director of Engineering
