Balancing of a Water and Air System (PDF

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Enthalpy
(H1-H2)
Enthalpy of air is defined as the sum of the internal energy of the air. Air heat is called
Enthalpy because it is a combination of sensible and latent heat. Sensible
heat raises or lowers the temperature of the air. Latent heat adds or removes the water
from the air.
The air that enters an evaporator has heat and that heat can be measured by the wet
bulb temperature. Wet bulb is a good indication of the total heat of the air the enthalpy.
• Air at 67ºF dry bulb and 67ºF wet bulb has an enthalpy of 31.62.
• Air at 70ºF dry bulb and 67ºF wet bulb has an enthalpy of 31.59.
• Air at 80ºF dry bulb and 67ºF wet bulb has an enthalpy of 31.51.
• Air at 90ºF dry bulb and 67ºF wet bulb has an enthalpy of 31.43.
When calculating the total heat that an evaporator has removed from the air the wet bulb
is taken from the air going in and out of the evaporator (∆ enthalpy (h1-h2)).
Example 5: using table 11
• Air going in to an evaporator coil at 80ºF dry bulb and 67ºF wet bulb has an enthalpy of
31.62.
• Air coming out of an evaporator coil at 62ºF dry bulb and 57ºF wet bulb has an enthalpy
of 24.48.The ∆ enthalpy (h1-h2) is 31.62-24.48= 7.14 ∆ enthalpy.
Table 11
Enthalpy at saturation, Btu per pound of dry air at 29.92”
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