Air dew point temperature Tdewpoint is the temperature when air is 100% saturated and water begins to condense out of the air. When warm wet air is cooled as it passes over a cooling coil, the temperature begins to decrease. If the air leaves the coil at a temperature above its dew point temperature, then no condensation occurs. On a psychrometric chart, the exiting air is at a lower temperature than the incoming air while the humidity ratio remains constant since no moisture is condensed from the air. Reducing the temperature of air without changing the quantity of water in the air is called sensible cooling, and is shown schematically below.
Since the humidity ratio remains unchanged, and so we use a horizontal line on the psychrometric chart to represent this process. This provides no control over the relative humidity and may produce conditions that are uncomfortable. The relative humidity increases with sensible cooling.
Observable features of a sensible cooling process
• the dry bulb temperature decreases
• the relative humidity increases
• the enthalpy decreases
• the wet bulb temperature decreases
• the specific volume decreases
• the humidity ratio, vapor pressure and dew point remains constant
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