ASHRAE Hong Kong Chapter Technical Workshop Fundamentals of HVAC Control Systems 18, 19, 25, 26 April 2007 2007 ASHRAE Hong Kong Chapter Slide 1
Chapter 4 Sensors and Auxiliary Devices 2007 ASHRAE Hong Kong Chapter Slide 2
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 controlled device 2007 ASHRAE Hong Kong Chapter Slide 3
Control System Components 4. Actuators A device that converts electric or pneumatic energy into a 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 control system Indicating devices to facilitate monitoring of control system activity 2007 ASHRAE Hong Kong Chapter Slide 4
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] 2007 ASHRAE Hong Kong Chapter Slide 5
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] 2007 ASHRAE Hong Kong Chapter Slide 6
Types of Sensors Temperature sensors Humidity sensors Pressure sensors Flow sensors 2007 ASHRAE Hong Kong Chapter Slide 7
Types of Temperature Sensors Bimetallic Fluid expansion Seebeck effect Electrical resistance 2007 ASHRAE Hong Kong Chapter Slide 8
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] 2007 ASHRAE Hong Kong Chapter Slide 9
Bimetallic Temperature Sensor 2007 ASHRAE Hong Kong Chapter Slide 10
Mercury Switch 2007 ASHRAE Hong Kong Chapter Slide 11
Rod-and-Tube Sensors 2007 ASHRAE Hong Kong Chapter Slide 12
Capillary Sensor 2007 ASHRAE Hong Kong Chapter Slide 13
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 2007 ASHRAE Hong Kong Chapter Slide 14
Chilled Mirror Dewpoint Sensor 2007 ASHRAE Hong Kong Chapter Slide 15
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 point directly or detects condensation on a cooled surface 2007 ASHRAE Hong Kong Chapter Slide 16
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] 2007 ASHRAE Hong Kong Chapter Slide 17
Types of Pressure Sensors Bourdon tube Spiral tube Diaphragm sensor Strain gauge 2007 ASHRAE Hong Kong Chapter Slide 18
Bourdon Tube Pressure Sensor 2007 ASHRAE Hong Kong Chapter Slide 19
Diaphragm Pressure Sensor 2007 ASHRAE Hong Kong Chapter Slide 20
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 2007 ASHRAE Hong Kong Chapter Slide 21
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] 2007 ASHRAE Hong Kong Chapter Slide 22
Types of Flow Sensors Differential pressure flow sensors Displacement flow sensors Passive flow sensors Mass flow sensors 2007 ASHRAE Hong Kong Chapter Slide 23
Orifice Plate Flow Sensor 2007 ASHRAE Hong Kong Chapter Slide 24
Venturi Flow Meter 2007 ASHRAE Hong Kong Chapter Slide 25
Pitot Tube Sensor 2007 ASHRAE Hong Kong Chapter Slide 26
Air Flow Measuring Station 2007 ASHRAE Hong Kong Chapter Slide 27
Averaging, Pipe-Mounted Pitot Meter 2007 ASHRAE Hong Kong Chapter Slide 28
Annubar Flow Sensor 2007 ASHRAE Hong Kong Chapter Slide 29
Paddle Flow Switch 2007 ASHRAE Hong Kong Chapter Slide 30
Inline Turbine Flow Meter 2007 ASHRAE Hong Kong Chapter Slide 31
Target Meter 2007 ASHRAE Hong Kong Chapter Slide 32
Ultrasonic Meter 2007 ASHRAE Hong Kong Chapter Slide 33
Thermal Anemometer 2007 ASHRAE Hong Kong Chapter Slide 34
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 2007 ASHRAE Hong Kong Chapter Slide 35
Types of Auxiliary Devices Relays Fan and pump status switches Timeclocks Wind-up timers Limit switches Manual switches High and low limit switches Fire and smoke detectors Indicating lights Audible alarms Monitoring instrumentation 2007 ASHRAE Hong Kong Chapter Slide 36
Time-Delay Relay Wiring 2007 ASHRAE Hong Kong Chapter Slide 37
Limit Switch 2007 ASHRAE Hong Kong Chapter Slide 38
Manual Rotary Switch Configurations 2007 ASHRAE Hong Kong Chapter Slide 39
Duct Smoke Detector 2007 ASHRAE Hong Kong Chapter Slide 40
Alarm Horn and Light 2007 ASHRAE Hong Kong Chapter Slide 41