Enthalpy is one of the core thermodynamic parameters in the field of air separation, which represents the total energy carried by the unit mass of flowing working medium (air, oxygen, nitrogen, etc.). Engineering definition is the sum of internal energy and flow pushing work, i.e. H=u+PV, where u is internal energy and PV is flow pushing work.
Popular understanding: under the same pressure, the higher the temperature, the greater the enthalpy; Liquid vaporization endothermic enthalpy increases significantly, while gas liquefaction exothermic enthalpy decreases. The core calculations such as the cooling capacity of the expander, the heat exchange capacity of the heat exchanger and the heat balance of the distillation column in the air separation unit are all completed by the enthalpy difference. The formula is q=m ・ Δ h, that is, the heat exchange capacity is equal to the mass flow multiplied by the enthalpy difference between the inlet and outlet. For example, the normal temperature and high pressure air is cooled and depressurized by the expander, and the unit refrigeration capacity is the inlet enthalpy minus the outlet enthalpy.
There are mainly two sets of enthalpy units in the air separation industry: one is the international standard unit kj/kg (kilojoules per kilogram), which is the common unit for current drawings and thermal calculation, representing the total enthalpy value contained in each kilogram of working fluid; The second is the old engineering unit kcal/kg (kcal/kg), which is occasionally seen in the old equipment data, and the conversion relationship is 1 kcal=4.1868 kJ.
In practice, check the air T-H diagram and the oxygen nitrogen enthalpy entropy diagram with the specific enthalpy kj/kg as the coordinate, which should be distinguished from the entropy S (unit kj/(kg ・ K)). Enthalpy represents the amount of energy, while entropy is used to judge the reversibility of the process and the size of cold loss.
