- #1
Soumalya
- 183
- 2
I am facing some overwhelming doubts while trying to study 'Psychrometrics'.Currently I am bamboozled trying to understand the process of adiabatic saturation of moist air.
Some of the textbooks claim that a true adiabatic saturation process proceeds along the line of constant enthalpy of moist air in a psychrometric chart.But again from an energy analysis applied to an adiabatic saturator as a control volume the exit enthalpy of moist air is greater than the inlet enthalpy of moist air by an amount equal to the product of the difference in specific humidities at inlet and exit conditions and the enthalpy of liquid water supplied at the adiabatic saturation temperature.
Also I am confused when a textbook says " The adiabatic evaporative cooling process involves a decrease in sensible enthalpy, which is exactly offset by an increase in latent enthalpy" which again implies the total enthalpy remains same.
The same textbook says," In a perfectly insulated adiabatic saturator the moist air passing through the saturator will take on additional water vapor (the dew-point temperature and humidity ratio will increase) and the dry-bulb temperature of the air will decrease until the air is fully saturated with water vapor. The latent energy necessary to evaporate the water comes from the sensible energy of the moist air passing through the saturator."
The two quoted statements keeps me wondering if the 'sensible and latent enthalpies' and 'sensible and latent energies' of moist air are the same thing.But this seems contradictory as sensible energy and latent energy in thermodynamics are part of internal energy of a fluid and they cannot be the same as sensible enthalpy and latent enthalpy.
Considering that under steady operating conditions of an adiabatic saturator the 'sensible energy' released from moist air is exactly equal to the latent heat of vaporization of water at the water temperature(adiabatic saturation temperature)this 'sensible energy' would be equal to a change in 'sensible enthalpy' of air only if the partial pressures of dry air and water vapor in moist air were constant during the flow as energy released during a constant pressure process is equal to a change in enthalpy.But the partial pressures of both dry air and water vapor in an adiabatic saturation process varies from inlet to exit of the saturator and 'sensible energy' released from air cannot be equal to its 'sensible enthalpy' change!
Can somebody help me resolve my confusions?
Some of the textbooks claim that a true adiabatic saturation process proceeds along the line of constant enthalpy of moist air in a psychrometric chart.But again from an energy analysis applied to an adiabatic saturator as a control volume the exit enthalpy of moist air is greater than the inlet enthalpy of moist air by an amount equal to the product of the difference in specific humidities at inlet and exit conditions and the enthalpy of liquid water supplied at the adiabatic saturation temperature.
Also I am confused when a textbook says " The adiabatic evaporative cooling process involves a decrease in sensible enthalpy, which is exactly offset by an increase in latent enthalpy" which again implies the total enthalpy remains same.
The same textbook says," In a perfectly insulated adiabatic saturator the moist air passing through the saturator will take on additional water vapor (the dew-point temperature and humidity ratio will increase) and the dry-bulb temperature of the air will decrease until the air is fully saturated with water vapor. The latent energy necessary to evaporate the water comes from the sensible energy of the moist air passing through the saturator."
The two quoted statements keeps me wondering if the 'sensible and latent enthalpies' and 'sensible and latent energies' of moist air are the same thing.But this seems contradictory as sensible energy and latent energy in thermodynamics are part of internal energy of a fluid and they cannot be the same as sensible enthalpy and latent enthalpy.
Considering that under steady operating conditions of an adiabatic saturator the 'sensible energy' released from moist air is exactly equal to the latent heat of vaporization of water at the water temperature(adiabatic saturation temperature)this 'sensible energy' would be equal to a change in 'sensible enthalpy' of air only if the partial pressures of dry air and water vapor in moist air were constant during the flow as energy released during a constant pressure process is equal to a change in enthalpy.But the partial pressures of both dry air and water vapor in an adiabatic saturation process varies from inlet to exit of the saturator and 'sensible energy' released from air cannot be equal to its 'sensible enthalpy' change!
Can somebody help me resolve my confusions?