MIL-HDBK-1003/3
APPENDIX C (Continued)
designer must be careful to fit his design to the area where he
is working. Typical conditions in dry and cold climates are
that: the relative humidity is low; the daily dry bulb range is
high; the outdoor air economizer is effective (at least for night
purging); the sensible heat ratio is high and systems are often
designed primarily to handle the sensible load and to let the
room relative humidity float as it will. Naval facilities
located in dry and cold climates may require modifications to the
criteria listed below.
C-1.07
Selection of Type of Control System. Design control
systems as simple as possible to provide adequate control and
give careful consideration to the following when selecting the
type of control system:
a) Pneumatic Controls. Pneumatic controls seldom work
consistently well in Naval facilities. Pneumatic controls
require frequent maintenance and calibration at a level that is
not usually performed by Naval personnel. Pneumatic systems are
prone to fail from water or oil in the compressed air. Many
pneumatic control systems are never set up properly by the
installing contractor. Even hybrid control systems (e.g., DDC
with pneumatic actuators) can have similar problems because the
electric-to-pneumatic transducers have small passages that are
vulnerable to moisture and oil in the compressed air. All other
things being equal, pneumatic control is not a good choice for
the control system; but if operation and maintenance personnel
are expert in pneumatic controls, have adequate funds for
maintenance, and refuse to use state-of-art systems, pneumatic
systems may be justified.
b) Electric Control. For the sake of simplicity,
electric VAV terminal unit controls may be used in conjunction
with pressure dependent (PD) terminal units (refer to
par. C-2.10). This type of zone temperature control requires
only the simplest of control sequences and therefore, will be
easier for Naval operation and maintenance personnel to
understand and maintain. Electric controls for the central
equipment (e.g., CHW valve and control, etc.) are in common use
and are suitably rugged, however, may not be accurate enough for
the application.
c) Analog Electronic Control. If pressure independent
(PI) terminal units are used, it will be necessary to have
electronic controls. Analog electronics usually introduce
additional levels of complexity whose advantages can be
outweighed by the likelihood that they will not be understood by
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