This is part three of three in a series on making sense of the psychrometric chart. Dr. John Watt gave a very helpful explanation for how to read the chart below is step 5-8. (To read the previous posts click here and here)
5. “A process in which air of two different states is mixed is shown by a straight line connecting them. The final state point location between them is usually calculated but may be indicated by intersecting data.
6. An air-drying process in, using moisture absorbing chemicals such as silica gel or calcium chloride, moves diagonally downward to the right along constant wet-bulb or enthalpy lines to the final state point, indicated by other intersecting data. Those processes are normally adiabatic, without loss or gain of enthalpy.
7. AN ideal adiabatic saturation process, as in the best direct evaporative cooling, moves diagonally upward to the left along constant wet-bulb or enthalpy lines to the state point shown by terminal conditions such as dry-bulb temperature, relative or specific humidity, or dew point. That path indicates the fall in the air’s dry-bulb temperature and the corresponding gain in dew point temperature and relative and specific humidity.
8. An ordinary adiabatic saturation process, as in cooling towers and in direct evaporative coolers with nonrecirculating water, has two motions. It moves diagonally upward to the left along constant wet-bulb or enthalpy lines to a point indicated by the cooled air’s dry-bulb temperature. Then, with the latter constant, it moves upward to the state point shown by such terminal conditions as relative or specific humidity or wet-bulb temperature. The second motion represents the gains in the air’s enthalpy, relative and specific humidity, and wet-bulb temperature resulting from cooling the water from its entering to final temperature. “*
* Watt, John R., Richard L. Koral, and John R. Watt. Evaporative Air Conditioning Handbook. (New York: Chapman and Hall, 1986) 17.