HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Advanced HVAC Control, Information Technology and Open Systemsdetermines the saturation partial pressure of the water vapor. This relationshipis used by the National Bureau of Standards for humidity sensorcalibration. The saturation vapor pressures over plane surfaces ofpure water and pure ice correspond to equivalent dew points or frostpoints.The equilibrium partial pressure is illustrated in Figure 4-2. At thesaturation temperature, or dew point, the gas mixture is saturated withrespect to water or ice and the rate of water molecules leaving the atmosphereand condensing on the chilled surface is the same as the rate ofwater molecules leaving the chilled surface and reentering the atmosphere.At equilibrium saturation, the water vapor partial pressure ofthe condensate is equal to the water vapor partial pressure of the gasatmosphere.This equilibrium at the mirror surface is obtained by cooling themirror to the saturation temperature. A temperature sensor is in directthermal contact with the mirror, and the mirror temperature is usedindirectly as the dew point or saturation temperature.The cooling of the mirror surface may be accomplished with mechanicalrefrigeration. More recent units use thermoelectric heat pumps.Detection of the condensation is done with optical phototransistor detection.The newer units automatically control the surface at the dew pointor frost point.Manually cooled, visually observed hygrometers are known asdew cups. These devices have been replaced by the thermoelectricallycooled, optically observed dew point hygrometer for on-line measurement.Here, the mirror surface is chilled to the dew point by a thermo-Figure 4-2. Equilibrium partial pressure.©2001 by The Fairmont Press, Inc. All rights reserved.