3ashrae workshop control samhui part 3

8/11/2019 3ASHRAE Workshop Control SamHui Part 3

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2007 ASHRAE Hong Kong Chapter Slide 1

Fundamentals of HVAC

Control Systems

18, 19, 25, 26 April 2007

ASHRAE Hong Kong Chapter Technical Workshop


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Chapter 4

Sensors and Auxiliary Devices


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Control System Components

1. Sensing elements Temperature, humidity/moisture, pressure, flow, proof of

operation (e.g. for safety interlock)

2. Transducers Convert (change) sensor inputs and controller outputs

from one analogue form to another, more usable,analogue form, e.g. pressure-to-voltage

3. Controllers Receive inputs from sensors

Compares the input signal with the setpoint

Generates an output signal to operate a controlleddevice


4/41 2007 ASHRAE Hong Kong Chapter Slide 4

Control System Components

4. Actuators

A device that converts electric or pneumatic energy intoa rotary or linear action, e.g. for valves and dampers

(can be pneumatic or electrical controlled)

5. Auxiliary elements Transducers to convert signals from one type to another

(e.g. from pneumatic to electric)

Relays and switches to manipulate signals, electric

power and compressed air supplies to power the controlsystem

Indicating devices to facilitate monitoring of control

system activity


5/41 2007 ASHRAE Hong Kong Chapter Slide 5

[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]


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Types of Sensors

Temperature sensors

Humidity sensors Pressure sensors

Flow sensors


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Types of Temperature Sensors

Bimetallic

Fluid expansion Seebeck effect

Electrical resistance


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Bimetallic Temperature Sensor


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Mercury Switch


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Rod-and-Tube Sensors


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Capillary Sensor


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Types of Humidity Sensors

Relative humidity sensors

Mechanical sensors Resistance-type sensors

Capacitance-type sensors

Dewpoint temperature sensors Saturated salt solution type Chilled mirror type

Wetbulb temperature sensors

Psychrometers


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Chilled Mirror Dewpoint Sensor


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Moisture Sensing Elements

Mechanical expand and contract as the

moisture level change ( hygroscopic ),e.g. nylon

Electronic change in either the

resistance or capacitance of the element Can be affected by temperature changes

Temperature compensation may be needed

A dew point sensor senses dew pointdirectly or detects condensation on a

cooled surface


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Types of Pressure Sensors

Bourdon tube

Spiral tube Diaphragm sensor

Strain gauge


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Bourdon Tube Pressure Sensor


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Diaphragm Pressure Sensor


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Flow Sensors

Sense the rate of liquid and gas flow Flow is difficult to sense accurately under all conditions

Selecting the best flow-sensing technique

for an application requires considering

many aspects Level of accuracy required

The medium being measured

The degree of variation in the measured flow


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Types of Flow Sensors

Differential pressure flow sensors

Displacement flow sensors Passive flow sensors

Mass flow sensors


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Orifice Plate Flow Sensor


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Venturi Flow Meter


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Pitot Tube Sensor


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Air Flow Measuring Station


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Averaging Pipe-Mounted

Pitot Meter


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Paddle Flow Switch


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Inline Turbine Flow Meter


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Target Meter


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Ultrasonic Meter

Thermal Anemometer


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Flow Measurement Devices

Selecting flow measurement devices Ref.: see the technical article

Spitzer, D. W., 2002. Selecting flow measurement

devices, HPAC Engineering, 74 (12): 52-59.

What flow meters measure (volume/mass)

Chilled water flow meters Differential pressure

Magnetic

Turbine Ultrasonic

Vortex shedding & fluidic

Selection criteria & considerations

Types of Auxiliary Devices


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Relays

Fan and pump status switches

Timeclocks Wind-up t imers

Limit switches

Manual switches

High and low limit switches

Fire and smoke detectors

Indicating lights

Audible alarms Monitoring instrumentation

Time-Delay Relay Wiring


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Limit Switch


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Manual Rotary


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Switch Configurations

Duct Smoke Detector


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Alarm Horn and Light


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3ashrae workshop control samhui part 3

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