Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
32
FLOW SENSORS
Flow sensors sense the rate of liquid and gas flow in volume
per unit of time. Flow is difficult to sense accurately under all
conditions. Selecting the best flow-sensing technique for an
application requires considering many aspects, especially the
level of accuracy required, the medium being measured, and
the degree of variation in the measured flow.
A simple flow sensor is a vane or paddle inserted into the
medium (Fig. 53) and generally called a flow switch. The paddle
is deflected as the medium flows and indicates that the medium
is in motion and is flowing in a certain direction. Vane or paddle
flow sensors are used for flow indication and interlock purposes
(e.g., a system requires an indication that water is flowing before
the system starts the chiller).
MOISTURE SENSING ELEMENTS
Elements that sense relative humidity fall generally into two
classes: mechanical and electronic. Mechanical elements
expand and contract as the moisture level changes and are called
“hygroscopic” elements. Several hygroscopic elements can be
used to produce mechanical output, but nylon is the most
commonly used element (Fig. 52). As the moisture content of
the surrounding air changes, the nylon element absorbs or
releases moisture, expanding or contracting, respectively. The
movement of the element operates the controller mechanism.
NYLON ELEMENT
RELATIVE HUMIDITY SCALE
C2084
LOW HIGH
ON/OFF SIGNAL
TO CONTROLLER
SENSOR
PIVOT
FLOW
PADDLE (PERPENDICULAR TO FLOW)
C2085
Fig. 52. Typical Nylon Humidity Sensing Element.
Electronic sensing of relative humidity is fast and accurate.
An electronic relative humidity sensor responds to a change in
humidity by a change in either the resistance or capacitance of
the element.
If the moisture content of the air remains constant, the relative
humidity of the air increases as temperature decreases and
decreases as temperature increases. Humidity sensors also respond
to changes in temperature. If the relative humidity is held constant,
the sensor reading can be affected by temperature changes.
Because of this characteristic, humidity sensors should not be
used in atmospheres that experience wide temperature variations
unless temperature compensation is provided. Temperature
compensation is usually provided with nylon elements and can
be factored into electronic sensor values, if required.
Dew point is the temperature at which vapor condenses. A
dew point sensor senses dew point directly. A typical sensor
uses a heated, permeable membrane to establish an equilibrium
condition in which the dry-bulb temperature of a cavity in the
sensor is proportional to the dew point temperature of the
ambient air. Another type of sensor senses condensation on a
cooled surface. If the ambient dry-bulb and dew point
temperature are known, the relative humidity, total heat, and
specific humidity can be calculated. Refer to the Psychrometric
Chart Fundamentals section of this manual.
Fig. 53. Paddle Flow Sensor.
Flow meters measure the rate of fluid flow. Principle types
of flow meters use orifice plates or vortex nozzles which
generate pressure drops proportional to the square of fluid
velocity. Other types of flow meters sense both total and static
pressure, the difference of which is velocity pressure, thus
providing a differential pressure measurement. Paddle wheels
and turbines respond directly to fluid velocity and are useful
over wide ranges of velocity.
In a commercial building or industrial process, flow meters
can measure the flow of steam, water, air, or fuel to enable
calculation of energy usage needs.
Airflow pickups, such as a pitot tube or flow measuring station
(an array of pitot tubes), measure static and total pressures in a
duct. Subtracting static pressure from total pressure yields
velocity pressure, from which velocity can be calculated.
Multiplying the velocity by the duct area yields flow. For
additional information, refer to the Building Airflow System
Control Applications section of this manual.