Introduction; Definitions - Honeywell AUTOMATIC CONTROL Engineering Manual

INTRODUCTION

This section describes heating, ventilating, and air
conditioning (HVAC) systems and discusses characteristics and
components of automatic control systems. Cross-references
are made to sections that provide more detailed information.
A correctly designed HVAC control system can provide a
comfortable environment for occupants, optimize energy cost
and consumption, improve employee productivity, facilitate
efficient manufacturing, control smoke in the event of a fire,
and support the operation of computer and telecommunications
equipment. Controls are essential to the proper operation of
the system and should be considered as early in the design
process as possible.
Properly applied automatic controls ensure that a correctly
designed HVAC system will maintain a comfortable
environment and perform economically under a wide range of
operating conditions. Automatic controls regulate HVAC
system output in response to varying indoor and outdoor
conditions to maintain general comfort conditions in office
areas and provide narrow temperature and humidity limits
where required in production areas for product quality.

DEFINITIONS

The following terms are used in this manual. Figure 1 at the
end of this list illustrates a typical control loop with the
components identified using terms from this list.
Analog: Continuously variable (e.g., a faucet controlling water
from off to full flow).
Automatic control system: A system that reacts to a change
or imbalance in the variable it controls by adjusting
other variables to restore the system to the desired
balance.
Algorithm: A calculation method that produces a control
output by operating on an error signal or a time series
of error signals.
Compensation control: A process of automatically adjusting
the setpoint of a given controller to compensate for
changes in a second measured variable (e.g., outdoor
air temperature). For example, the hot deck setpoint
is normally reset upward as the outdoor air
temperature decreases. Also called "reset control".
Control agent: The medium in which the manipulated variable
exists. In a steam heating system, the control agent is
the steam and the manipulated variable is the flow of
the steam.
Control point: The actual value of the controlled variable
(setpoint plus or minus offset).
Automatic controls can optimize HVAC system operation.
They can adjust temperatures and pressures automatically to
reduce demand when spaces are unoccupied and regulate
heating and cooling to provide comfort conditions while
limiting energy usage. Limit controls ensure safe operation of
HVAC system equipment and prevent injury to personnel and
damage to the system. Examples of limit controls are low-
limit temperature controllers which help prevent water coils
or heat exchangers from freezing and flow sensors for safe
operation of some equipment (e.g., chillers). In the event of a
fire, controlled air distribution can provide smoke-free
evacuation passages, and smoke detection in ducts can close
dampers to prevent the spread of smoke and toxic gases.
HVAC control systems can also be integrated with security
access control systems, fire alarm systems, lighting control
systems, and building and facility management systems to
further optimize building comfort, safety, and efficiency.
Controlled medium: The medium in which the controlled
variable exists. In a space temperature control system,
the controlled variable is the space temperature and
the controlled medium is the air within the space.
Controlled Variable: The quantity or condition that is
measured and controlled.
Controller: A device that senses changes in the controlled
variable (or receives input from a remote sensor) and
derives the proper correction output.
Corrective action: Control action that results in a change of
the manipulated variable. Initiated when the
controlled variable deviates from setpoint.
Cycle: One complete execution of a repeatable process. In
basic heating operation, a cycle comprises one on
period and one off period in a two-position control
system.
Cycling: A periodic change in the controlled variable from
one value to another. Out-of-control analog cycling
is called "hunting". Too frequent on-off cycling is
called "short cycling". Short cycling can harm electric
motors, fans, and compressors.
Cycling rate: The number of cycles completed per time unit,
typically cycles per hour for a heating or cooling
system. The inverse of the length of the period of the
cycle.
5 ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS