MIL-HDBK-1003/3
Section 8:
CONTROLS AND INSTRUMENTATION
8.1
General Requirements. See pars. 2.2.6 and 2.2.8 for an
overview of temperature controls and instrumentation. Another
source of assistance that the designer may use is NFESC, Code
ESC23 at Port Hueneme, California. NFESC is a Navy organization
that has expertise in direct digital control (DDC) applications.
8.1.1
Choice of Controls. DDC is the control system of
choice for HVAC systems. Consider the advantages of DDC for new
and major renovation projects. Older conventional control
systems may be specified for small buildings where DDC is not
cost effective or where the customer refuses to accept DDC. Use
of pneumatic controllers is discouraged because of the high
maintenance required to keep them functional. Existing pneumatic
operators may be reused, if in good operating condition, as part
of a hybrid system with DDC sensors and controllers and the
pneumatic operators.
8.1.1.1
A Guide to Choose Control Systems. Consider the
following when selecting the type of control system (digital,
electric, analog electronic, or pneumatic):
a) Life cycle cost. In most cases, DDC will be the
lowest cost on a life cycle cost basis. The first cost of DDC and
conventional control systems may be comparable but DDC operating
and maintenance costs are significantly lower.
b) Customer preference of control type. The customer
may feel that his personnel are better qualified to operate and
maintain a particular type control system. Repair parts may be
an important consideration.
8.1.1.2
Factors to Select Control Systems. Control selection
may be determined by several factors as follows:
a) DDC systems use stand-alone digital controllers,
distributed throughout the building to provide control of HVAC
functions. The digital controller replaces conventional receiver
controllers, thermostats, switches, relays, and auxiliary
devices. Sensors are wired directly to the digital controller,
which performs the control logic in software, and outputs a
control signal directly to the actuator or relay. Terminal
control units control individual terminal HVAC equipment, such as
VAV boxes, fan coil units, and heat pumps. Distributed control
refers to locating the digital controllers near the equipment
being controlled.
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