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amplified
during an economizer cycle on supply/return fan systems since the
damper’s non linear control characteristics complicates the
pressure effects on both the supply and return fans 27, 28 .
Ideally,
both the supply fan and return fan follow each other when the drive
signals are changed. For example, if during system setup the supply
fan speed is reduced from 100% to 50%, it is generally assumed that
the same (or proportional) reduction to the return fan VFD (or increase
to the relief fan VFD) will result. Unfortunately, the supply fan
is reacting to both changes to RPM and changes to system resistance
due to the variable positions of VAV boxes. Remember that the return
fan is essentially reacting to a fixed resistance system. As a result,
the supply fan operates within an "area" of the fan curve,
rather than along a single curve as the return fan does. The resulting
errors during operation are significant, although conditions appear
to track properly during setup 27 (Figure #3).
Figure #3 - VFD Proportional Drive Slaving (operational example)
Typical fan
curve illustrates the changes in airflow rates at a given RPM
on VAV systems. The supply fan responds within an area of the
fan curve (shaded oval) rather than simply a line or curve. As
a result, systems that use VFD proportional drive slaving techniques
can have dramatic building pressure variations, as indicated in
this example.27
Control
Scheme #3:
Space Static Pressure Control of the Return Fan (Systems with a
Return or Relief Fan)
Some
design engineers argue that controlling the return fan to space
static pressure assures positive building pressure. Static pressure
control is susceptible to the following factors, which should be
seriously considered prior to the selection of this technique for
control.
- Differential
pressures less than 0.05 in. WG are very difficult to control
in the field.
- The outside
pressure tap can not be placed where it is unaffected by wind.
- A single
inside pressure tap can not be located to assure positive pressure
throughout the building.
- Affordable
static pressure sensors can not provide the accuracy and stability
required for proper control.
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