Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSFLAPPERSPRINGSIGNAL PORTBRASS TUBEFig. 50. Rod-and-Tube Element.In a remote-bulb controller (Fig. 51), a remote capsule, orbulb, is attached to a bellows housing by a capillary. The remotebulb is placed in the controlled medium where changes intemperature cause changes in pressure of the fill. The capillarytransmits changes in fill pressure to the bellows housing andthe bellows expands or contracts to operate the mechanicaloutput to the controller. The bellows and capillary also sensetemperature, but because of their small volume compared tothe bulb, the bulb provides the control.MECHANICAL OUTPUTTO CONTROLLEREXTENSION SPRINGSENSOR BODYLIQUIDFILLBELLOWSCAPILLARYINVAR RODFig. 51. Typical Remote-Bulb Element.C2081CONTROLLEDMEDIUM(E.G., WATER)BULBC2083Two specialized versions of the remote bulb controller areavailable. They both have no bulb and use a long capillary(4.5 to 8.5 meters) as the sensor. One uses an averaging sensorthat is liquid filled and averages the temperature over the fulllength of the capillary. The other uses a cold spot or lowtemperature sensor and is vapor filled and senses the coldestspot (300 mm or more) along its length.Electronic temperature controllers use low-mass sensingelements that respond quickly to changes in the controlledcondition. A signal sent by the sensor is relatively weak, but isamplified to a usable strength by an electronic circuit.The temperature sensor for an electronic controller may be alength of wire or a thin metallic film (called a resistancetemperature device or RTD) or a thermistor. Both types ofresistance elements change electrical resistance as temperaturechanges. The wire increases resistance as its temperatureincreases. The thermistor is a semiconductor that decreases inresistance as the temperature increases.Because electronic sensors use extremely low mass, theyrespond to temperature changes more rapidly than bimetal orsealed-fluid sensors. The resistance change is detected by abridge circuit. Nickel “A”, BALCO, and platinum are typicalmaterials used for this type of sensor.In thermocouple temperature-sensing elements, twodissimilar metals (e.g., iron and nickel, copper and constantan,iron and constantan) are welded together. The junction of thetwo metals produces a small voltage when exposed to heat.Connecting two such junctions in series doubles the generatedvoltage. Thermocouples are used primarily for high-temperatureapplications.Many special application sensors are available, includingcarbon dioxide sensors and photoelectric sensors used insecurity, lighting control, and boiler flame safeguard controllers.PRESSURE SENSING ELEMENTSPressure sensing elements respond to pressure relative to aperfect vacuum (absolute pressure sensors), atmosphericpressure (gage pressure sensors), or a second system pressure(differential pressure sensors), such as across a coil or filter.Pressure sensors measure pressure in a gas or liquid inkilopascals (kPa). Low pressures are typically measured inpascals (Pa). Pressure can be generated by a fan, a pump orcompressor, a boiler, or other means.Pressure controllers use bellows, diaphragms, and a numberof other electronic pressure sensitive devices. The medium underpressure is transmitted directly to the device, and the movementof the pressure sensitive device operates the mechanism of apneumatic or electric switching controller. Variations of thepressure control sensors measure rate of flow, quantity of flow,liquid level, and static pressure. Solid state sensors may usethe piezoresistive effect in which increased pressure on siliconcrystals causes resistive changes in the crystals.ENGINEERING MANUAL OF AUTOMATIC CONTROL31