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

HVAC Control in the New Millenniumelectric cooler while a continuous sample of the atmosphere passes overthe mirror. The mirror is illuminated by a light source and observed bya photodetector bridge network.As condensate forms on the mirror, the change in reflectance isdetected by a reduction in the reflected light level received by the photodetectorbecause of the light-scattering effect of the individual dewmolecules. This light reduction forces the optical bridge toward a balancepoint, reduces the input error signal to the amplifier, and proportionallycontrols the drive from the power supply to the thermoelectriccooler (Figure 4-3). This maintains the mirror at a temperature at whicha constant-thickness dew layer is retained. Embedded in the mirror, atemperature-measuring element measures the dew point temperature.Advances in the technology of thermoelectric coolers allow practicalinstruments with several stages of thermoelectric cooling for most dewpoint ranges.Relative humidity is controlled at the zone level. The control actionis based on the relative humidity reading in the combined return airstream from the workstations in that zone.Figure 4-3. Chilled mirror hygrometer.©2001 by The Fairmont Press, Inc. All rights reserved.