Fundamentals of HVAC Control Systemsibse.hk/cpd/control/ASHRAE_Workshop_Control_SamHui_Part_3.pdf · A device that converts electric or pneumatic energy into a rotary or linear action,

© 2007 ASHRAE Hong Kong Chapter Slide 1

Fundamentals of HVAC Control Systems

18, 19, 25, 26 April 2007

ASHRAE Hong Kong Chapter Technical Workshop


Chapter 4Sensors and Auxiliary Devices


Control System Components

1. Sensing elementsTemperature, humidity/moisture, pressure, flow, proof of operation (e.g. for safety interlock)

2. TransducersConvert (change) sensor inputs and controller outputs from one analogue form to another, more usable, analogue form, e.g. pressure-to-voltage

3. ControllersReceive inputs from sensorsCompares the input signal with the setpointGenerates an output signal to operate a controlled device


Control System Components

4. ActuatorsA 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 elementsTransducers 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 systemIndicating devices to facilitate monitoring of control system activity


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




Types of Sensors

Temperature sensorsHumidity sensorsPressure sensorsFlow sensors


Types of Temperature Sensors

BimetallicFluid expansionSeebeck effectElectrical resistance

© 2007 ASHRAE Hong Kong Chapter Slide 9[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]


Bimetallic Temperature Sensor


Mercury Switch


Rod-and-Tube Sensors


Capillary Sensor


Types of Humidity Sensors

Relative humidity sensorsMechanical sensorsResistance-type sensorsCapacitance-type sensors

Dewpoint temperature sensorsSaturated salt solution typeChilled mirror type

Wetbulb temperature sensorsPsychrometers


Chilled Mirror Dewpoint Sensor


Moisture Sensing Elements

Mechanical – expand and contract as the moisture level change (“hygroscopic”), e.g. nylonElectronic – change in either the resistance or capacitance of the element

Can be affected by temperature changesTemperature compensation may be needed

A dew point sensor senses dew point directly or detects condensation on a cooled surface




Types of Pressure Sensors

Bourdon tubeSpiral tubeDiaphragm sensorStrain gauge


Bourdon Tube Pressure Sensor


Diaphragm Pressure Sensor


Flow Sensors

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

Selecting the best flow-sensing technique for an application requires considering many aspects

Level of accuracy requiredThe medium being measuredThe degree of variation in the measured flow




Types of Flow Sensors

Differential pressure flow sensorsDisplacement flow sensorsPassive flow sensorsMass flow sensors


Orifice Plate Flow Sensor


Venturi Flow Meter


Pitot Tube Sensor


Air Flow Measuring Station


Averaging, Pipe-MountedPitot Meter


Annubar Flow Sensor


Paddle Flow Switch


Inline Turbine Flow Meter


Target Meter


Ultrasonic Meter


Thermal Anemometer


Flow Measurement Devices

Selecting flow measurement devicesRef.: 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 pressureMagneticTurbineUltrasonicVortex shedding & fluidic

Selection criteria & considerations


Types of Auxiliary Devices

RelaysFan and pump status switchesTimeclocksWind-up timersLimit switchesManual switchesHigh and low limit switchesFire and smoke detectorsIndicating lightsAudible alarmsMonitoring instrumentation


Time-Delay Relay Wiring


Limit Switch


Manual RotarySwitch Configurations


Duct Smoke Detector


Alarm Horn and Light

Documents

Fundamentals of HVAC Control Systemsibse.hk/cpd/control/ASHRAE_Workshop_Control_SamHui_Part_3.pdf · A device that converts electric or pneumatic energy into a rotary or linear action,