Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSMOISTURE SENSING ELEMENTSElements that sense relative humidity fall generally into twoclasses: mechanical and electronic. Mechanical elementsexpand and contract as the moisture level changes and are called“hygroscopic” elements. Several hygroscopic elements can beused to produce mechanical output, but nylon is the mostcommonly used element (Fig. 52). As the moisture content ofthe surrounding air changes, the nylon element absorbs orreleases moisture, expanding or contracting, respectively. Themovement of the element operates the controller mechanism.NYLON ELEMENTFLOW SENSORSFlow sensors sense the rate of liquid and gas flow in volumeper unit of time. Flow is difficult to sense accurately under allconditions. Selecting the best flow-sensing technique for anapplication requires considering many aspects, especially thelevel of accuracy required, the medium being measured, andthe degree of variation in the measured flow.A simple flow sensor is a vane or paddle inserted into themedium (Fig. 53) and generally called a flow switch. The paddleis deflected as the medium flows and indicates that the mediumis in motion and is flowing in a certain direction. Vane or paddleflow sensors are used for flow indication and interlock purposes(e.g., a system requires an indication that water is flowing beforethe system starts the chiller).LOWHIGHON/OFF SIGNALTO CONTROLLERSENSORRELATIVE HUMIDITY SCALEC2084PIVOTFig. 52. Typical Nylon Humidity Sensing Element.Electronic sensing of relative humidity is fast and accurate.An electronic relative humidity sensor responds to a change inhumidity by a change in either the resistance or capacitance ofthe element.If the moisture content of the air remains constant, the relativehumidity of the air increases as temperature decreases anddecreases as temperature increases. Humidity sensors also respondto 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 beused in atmospheres that experience wide temperature variationsunless temperature compensation is provided. Temperaturecompensation is usually provided with nylon elements and canbe factored into electronic sensor values, if required.Dew point is the temperature at which vapor condenses. Adew point sensor senses dew point directly. A typical sensoruses a heated, permeable membrane to establish an equilibriumcondition in which the dry-bulb temperature of a cavity in thesensor is proportional to the dew point temperature of theambient air. Another type of sensor senses condensation on acooled surface. If the ambient dry-bulb and dew pointtemperature are known, the relative humidity, total heat, andspecific humidity can be calculated. Refer to the PsychrometricChart Fundamentals section of this manual.FLOWPADDLE (PERPENDICULAR TO FLOW)Fig. 53. Paddle Flow Sensor.C2085Flow meters measure the rate of fluid flow. Principle typesof flow meters use orifice plates or vortex nozzles whichgenerate pressure drops proportional to the square of fluidvelocity. Other types of flow meters sense both total and staticpressure, the difference of which is velocity pressure, thusproviding a differential pressure measurement. Paddle wheelsand turbines respond directly to fluid velocity and are usefulover wide ranges of velocity.In a commercial building or industrial process, flow meterscan measure the flow of steam, water, air, or fuel to enablecalculation 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 aduct. Subtracting static pressure from total pressure yieldsvelocity pressure, from which velocity can be calculated.Multiplying the velocity by the duct area yields flow. Foradditional information, refer to the Building Airflow SystemControl Applications section of this manual.32ENGINEERING MANUAL OF AUTOMATIC CONTROL