@classmethod
def _alfa_sat(cls, T):
    """Auxiliary equation for the alfa coefficient for calculate the
        enthalpy along the saturation line

        Parameters
        ----------
        T : float
            Temperature, [K]

        Returns
        -------
        alfa : float
            alfa coefficient, [kJ/kg]

        References
        ----------
        IAPWS, Revised Supplementary Release on Saturation Properties of
        Ordinary Water Substance September 1992,
        http://www.iapws.org/relguide/Supp-sat.html, Eq.4
        """
    di = [-1135.905627715, -5.65134998e-08, 2690.66631, 127.287297, -
        135.003439, 0.981825814]
    expi = [0, -19, 1, 4.5, 5, 54.5]
    Tita = T / cls.Tc
    alfa = 0
    for d, x in zip(di, expi):
        alfa += d * Tita ** x
    return alfa
