Control System Components; Sensing Elements; Temperature Sensing Elements - Honeywell AUTOMATIC CONTROL Engineering Manual

CONTROL FUNDAMENTALS
Each control mode is applicable to processes having certain
combinations of the basic characteristics. The simplest mode
of control that meets application requirements is the best mode
to use, both for economy and for best results. Using a control
Control Application
Space Temperature
Mixed Air Temperature
Coil Discharge Temperature
Chiller Discharge Temperature
Hot Water Converter Discharge Temperature
Airflow
Fan Static Pressure
Humidity
Dewpoint Temperature
a PID, EPID control is used in digital systems.

CONTROL SYSTEM COMPONENTS

Control system components consist of sensing elements,
controllers, actuators, and auxiliary equipment.

SENSING ELEMENTS

A sensing element measures the value of the controlled
variable. Controlled variables most often sensed in HVAC
systems are temperature, pressure, relative humidity, and flow.

TEMPERATURE SENSING ELEMENTS

The sensing element in a temperature sensor can be a bimetal
strip, a rod-and-tube element, a sealed bellows, a sealed bellows
attached to a capillary or bulb, a resistive wire, or a thermistor.
Refer to the Electronic Control Fundamentals section of this
manual for Electronic Sensors for Microprocessor Based
Systems.
A bimetal element is a thin metallic strip composed of two
layers of different kinds of metal. Because the two metals have
different rates of heat expansion, the curvature of the bimetal
changes with changes in temperature. The resulting movement
of the bimetal can be used to open or close circuits in electric
control systems or regulate airflow through nozzles in
pneumatic control systems. Winding the bimetal in a coil
(Fig. 49) enables a greater length of the bimetal to be used in
a limited space.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Table 3. Control Applications and Recommended Control Modes.
P, PID
PI, EPID
PI, EPID
PI, EPID
PI, EPID
PI Use a wide proportional band and a fast reset rate. For some
PI , EPID
P, or if very tight control is required, PI
P, or if very tight control is required, PI
mode that is too complicated for the application may result in
poor rather than good control. Conversely, using a control
mode that is too basic for requirements can make adequate
control impossible. Table 3 lists typical control applications
and recommended control modes.
Recommended Control Mode
applications, PID may be required.
Fig. 49. Coiled Bimetal Element.
The rod-and-tube element (Fig. 50) also uses the principle
of expansion of metals. It is used primarily for insertion directly
into a controlled medium, such as water or air. In a typical
pneumatic device, a brass tube contains an Invar rod which is
fastened at one end to the tube and at the other end to a spring
and flapper. Brass has the higher expansion coefficient and is
placed outside to be in direct contact with the measured
medium. Invar does not expand noticeably with temperature
changes. As the brass tube expands lengthwise, it pulls the
Invar rod with it and changes the force on the flapper. The
flapper is used to generate a pneumatic signal. When the flapper
position changes, the signal changes correspondingly.
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