APPENDIX C (Continued)
accurate only down to about 400 fpm (which unfortunately might be
as high as 33 to 50 percent of maximum flow in many cases).
Since desired minimum flow will usually be less than this, the
differential pressure PI is not accurate for controlling minimum
flow. The other common method uses a hot wire anemometer or
similar device for sensing. This is not very good because the
sensor is a single point in the inlet duct rather than a
multi-point device (as used by the flow ring described above).
It would only be accurate for a box that had many diameters of
straight duct upstream (not the usual case). In addition, the
response time is too slow for good PI control.
d) Minimum flow can be set on PD units by installing
mechanical stops, by setting the actuator stroke through linkage
adjustments, or by other similar means. These are
straightforward testing and balancing procedures.
e) PI costs somewhat more than PD on the average, is
much more complicated for maintenance personnel, and the
additional control components are more prone to failure.
f) If PI is used, however, the designer must be
careful to specify that supply maximums and minimums are checked
after the equipment is installed. Factory adjustments have
proven to not be an adequate guarantee that the installed
equipment will function as needed.
Carefully consider the throttling characteristics when
selecting the type and size of VAV terminal units. The installed
characteristic curve of a throttling damper expected to modulate
supply air to a space, is a function of the inherent
characteristic curve of the device and the ratio of the system
pressure drop to the drop across the damper at maximum flow. As
the pressure drop of the damper at maximum flow is reduced (by
selecting larger and larger dampers for a given flow rate--a
practice promoted by the desire to save fan energy), the
installed characteristic tends to move across the spectrum toward
quick opening characteristics. The geometry of butterfly dampers
tends to exaggerate this shift. With dampers specially designed
to retain linearity, this shift can be greatly reduced.
a) A shift toward quick opening increases the gain of
the control component, i.e., a smaller part of its stroke is
actually used for control. This is detrimental to the stability
of the control system. The common tendency of selecting dampers
with very low pressure drops can result in control that is nearly