Reading the Psychrometric Chart: Part Two

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Below is the brief and helpful summary that Dr.  John Watt provided  for using the psychrometric chart.

“Typical thermodynamic processes trace paths on the chart from their initial states points as follows:

  1. A simple heating process moves horizontally to the right; a simple cooling process horizontally to the left.  The dew point temperature and specific humidity remain constant for both.
  2. A heating-and-humidifying process moves horizontally to the right to its final dry-bulb temperature and then upward at constant dbt to a final state point indicated by any other psychrometric property.
  3. A cooling-and-dehumidifying process moves leftward horizontally to the saturation line and follows it downward to a final state point located by other terminal data.  The relative humidity rises during the sensible cooling until it reaches 100 percent at the saturation line; it remains at 100 percent during the latent cooling along the saturation line, though during that phase vapor is being steadily condensed from the air.
  4. Complex processes, like the above, are often illustrated by straight lines directly connecting their initial and final state points.”*

*  Watt, John R., Richard L. Koral, and John R. Watt. Evaporative Air Conditioning Handbook. (New York: Chapman and Hall, 1986) 17.