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Types of Switches
The following sections outlines common switching devices currently used by the
industry.
Hand Switches
Hand switches are used as digital input devices and in hardwired electrical
control circuits associated with digital outputs. Hand switches come in
numerous sizes, shapes, and configurations. Common switch types include rotary,
selector type switches, toggle switches, and pushbuttons. Selector and toggle
switches are almost always maintained contact type. Pushbuttons may be
momentary or maintained contact type. Selector switches can have key operators
to prevent tampering.
Figure 2.2- Pushbuttons and Selector Switches (courtesy IDEC)
Limit Switches
Limit switches convert mechanical motion or proximity into a switching action.
Limit switches are most commonly used in DDC control systems for HVAC to
provide position status feedback to the controller for valve and damper
positions. A wide variety of configurations are available. Common types include
industrial limit switches, mercury, and proximity switches.
Figure 2.3-Industrial Limit Switches
Figure 2.4-Mercury Limit Switches
Figure 2.5-Proximity Switches
Temperature Switches
Temperature switches (also called thermostats, aquastats or freezestats
depending on application) are commonly used in DDC control systems to provide a
digital input when a process medium temperature rises or falls to a set
temperature. Switches with a number of different operating principles are
manufactured. Some of the common types include bimetallic, fluid thermal
expansion, freezestat and electronic.
Bimetallic temperature switches use a bonded "bimetal" strip
consisting of two dissimilar metals with different thermal coefficients of
expansion. When the temperature changes, the metals expand or contract at
different rates causing the strip to bend. Various configurations such as
coiled elements are used to increase the thermal movement to cause two contacts
to come together or separate. Some configurations use the bimetallic principle
to change the orientation of a bulb containing liquid mercury so that the
mercury flows into contact with two electrodes, completing the circuit.
Fluid thermal expansion temperature switches use the principle of
thermal expansion of a fluid to cause displacement of a bellows, diaphragm,
bourdon tube, or piston to open or close a set of contacts. Fluid system based
temperature switches can be connected to a remote fluid containing bulb by a
capillary tube, allowing the switch element to be remote from the sensing bulb.
Figure 2.6- Remote Bulb Thermostat
The freezestat is commonly used to prevent water or steam coils in
air handling units from freezing. Freezestats use a fluid that is a saturated
vapor at the switch set point temperature. This fluid is confined within a long
capillary tube. The tube is installed in a serpentine fashion over the area of
the air stream to being monitored. If any point along the tube falls below the
saturation temperature, the vapor begins to condense causing a rapid change in
pressure in the system and actuating the switch mechanism.
Electronic temperature switches use the same sensing technologies
used for analog temperature sensing to electronically operate a set of output
contacts. Refer to the Temperature Measurement portion of the Analog Input
Device Section for more details of sensing technology.
Figure 2.7-Freezestat
Humidity Switches
Humidity switches, or humidistats, are used in DDC control systems to provide a
digital input when a process or space humidity level rises or falls to a set
level. Common applications are high limit safety interlocks for humidifiers,
space or process humidity alarms, and simple on-off humidity control.
Mechanical humidistats use a hygroscopic material such as animal
hair, nylon or other plastic material that changes dimension with changes in
relative humidity. The dimensional change is amplified via a mechanical link to
causing a switch to operate.
Mechanical humidistats are rapidly being replaced by electronic
humidistats that use thin film capacitance or bulk polymer resistance analog
humidity sensing technologies combined with electronic switching circuitry to
produce a switching action at an adjustable set point. These sensing
technologies are described in the Humidity Measurement portion of the Analog
Input Device Section.
Flow Switches
Flow switches are used to provide a digital input to DDC controls systems when
a fluid flow rate has risen above or fallen below the set value. Common
applications include safety air and water flow interlocks for electric heaters
and humidifiers, chiller safety interlocks, and burner safety interlocks.
Numerous technologies are available, but the most common types used in DDC
systems for HVAC control are mechanical and differential pressure types.
Mechanical flow switches operate on the principle that the kinetic
energy of a flowing fluid creates a force on an object suspended in the flow
stream. The magnitude of the force varies with (the square of) the velocity of
the fluid. Various configurations are used to transfer this force into
operation of a switch. Common configurations include paddles or sails, pistons
or discs.
Differential pressure type flow switches (Figure 2.8) operate on
the principle that a difference in pressure is always associated with fluid
flow, or the principle that the total pressure of a flowing fluid is always
greater than the static pressure. These differences in pressure can be
accurately predicted for a given situation and related to the fluid flow rate.
For more information see the Flow Measurement portion of the Analog Input
Section.
Level Switches
Level switches are used in DDC control systems (for HVAC) to provide a digital
input when the fluid level in a tank, vessel or sump has reached a
predetermined height. Common applications include cooling tower sump level
control and monitoring, steam condensate tank level, storm water and sewage
sump level monitoring and control and thermal storage tank level monitoring.
Numerous mechanical and analog technologies are currently available. Some
analog technologies include capacitance, ultrasonic, and magnetostrictive-based
devices in combination with solid-state electronics to provide a switching
action based on level. More commonly used technologies include devices that
employ the use of a float (integral, rod and float, submersible), conductivity
probe, or differential pressure mechanism.
Integral float type level switches typically combine an metal or
plastic float attached to the arm of a submersible rotary switch mechanism, or
a float that encloses a magnet which slides on a hollow rod enclosing one or
more reed switches.
Submersible float switches utilize an encapsulated integral float
type switch or mercury switch suspended on a fluid tight cord in the vessel
being monitored. When the level is below the cord attachment, the float hangs
down and the switch is in its normally open or closed position. When the fluid
level rises, the float rises above the cord attachment point, changing the
float orientation. When the float has position has inverted sufficiently, the
internal switch changes position.
Conductivity probe-type level switches work for conductive liquids
only and use the liquid itself to conduct low level electrical signals between
two or more electrodes to operate higher level electronic switching devices
such as transistors or triacs.
Pressure Switches
Pressure switches are used in DDC systems to provide status indication for
fans, filters and pumps, and to provide flow and level status indication by
virtue of the predicable relationships between pressure and these values.
Pressure switches may be mechanical or electronic.
Mechanical pressure switches use a piston, bellows, bourdon tube or
diaphragm and a magnetic or mechanical linkage to convert the forces resulting
from the measured pressure into repeatable motions used to operate one or more
switches (Figure 2.3). Low pressure switches commonly used to measure air
pressures in the range of 0.05 inches water column to 1 psig typically use a
flexible diaphragm. Piston, bourdon tube and bellow type switches are available
for higher pressures ranging from 1 to over 100 psig.
Vibration Switches
Vibration switches are used to provide a signal when vibration levels in
rotating machinery such as fans, reach unsafe levels. Vibration switches are
commonly applied on large cooling tower and air handling unit fans.
Moisture Switches
Moisture detecting switches are commonly used to detect moisture under raised
floors, in piping and tank containment areas and in the drain pans of air
handling units to alert system operators before damage or flooding occurs. Most
moisture detecting switches are instruments of the float type or conductivity
type. Float types are adapted to actuate at very low levels. Conductivity types
may consist of point sensitive probes located very close to the bottom of a low
point or sump where water will collect, or they may be ribbons or strips with
wires separated by a non-conductive material, such that when any portion of the
ribbon is exposed to liquid moisture, the electrical circuit is completed and
the switch mechanism activates.
Current Switches
Current sensing relays are used in DDC systems to monitor the status of
electrical devices. The devices typically have one or more adjustable current
set points. Common applications include fan and pump on/off status feedback.
Current switches can detect broken fan belts if properly adjusted. Current
relays can also be used for phase monitoring.
Continue on to Chapter 2: Part 2: Analog
Devices
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