def Pr(C_capacity, mu_mix, lyambda_feed):
    """
    Calculates the Prandtl criterion.
    Parameters
    ----------
    C_capacity : float
        The heat capacity of mix [J/(kg * degrees celcium)]
    mu_mix : float
        The mix viscocity of liquid, [Pa/s]
    lyambda_feed : float
        The thermal conductivity of feed, [W / (m * degreec celcium)]
    Returns
    -------
    Pr : float
        The Prandtl criterion, [dimensionless]
    References
    ----------
    Романков, формула 4.12, стр.151
    """        
    return C_capacity * mu_mix / lyambda_feed