MIL-HDBK-1003/3
APPENDIX C (Continued)
To maintain a constant outside air quantity being drawn
in and supplied by the AHU, a separate outside air injection fan
or PI VAV box should be used. Use of an injection fan is
encouraged rather than a PI VAV box because it is a more reliable
method of providing constant ventilation from a simplicity and
maintainability viewpoint. The designer should be aware that PI
units rarely work properly in Navy installations because of
maintenance problems with PI velocity sensing hardware/controls.
Properly working PI units, however, would be a more accurate
means of providing a constant flow throughout a given range of
mixing box pressures. A PI unit should be considered if the
range of mixing box pressures expected is such that it would be
difficult to specify a fan which would deliver acceptable flows
throughout the range of pressure.
C-2.10
Pressure Dependent (PD) Boxes. The use of PD VAV boxes
rather than PI boxes is recommended. Feedback: PI velocity
resetting controls are often a significant contributor to failure
of Navy VAV systems. When determining the type of VAV terminal
units to use in a system, give careful consideration of the
following feedbacks:
a) The most important feature touted for PI units is
that they respond to fluctuations in system pressure and thus
enhance control stability by reducing "hunting." However, when
the supply duct system has been properly designed for static
regain, there is no clear evidence that the use of PI units
results in greater zone temperature control stability than when
using PD units. The transfer functions and time constants in a
typical VAV unit control are such that the potential benefits of
velocity resetting are nullified by the disadvantages of the
additional and complicated control loop.
b) PI controls claim to render the whole system
virtually self-balancing. However, in a system with PD controls
and a well designed static regain duct system, the VAV dampers
respond to changes in load sensed by the thermostat and respond
to balance the air in a similar way.
c) Maximum and minimum airflow limiting is a feature
that is inherent with PI control units. However, there is some
question as to how well the velocity sensors used for this
purpose actually measure flow (setpoints are at the extreme low
end of their range). Some manufacturers say the velocity loop
readily drifts out of calibration and must be auto-calibrated
regularly by using a DDC system. The problem is worse at lower
airflow. A differential pressure flow ring or flow cross is
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