def Re(F_mass, z_way, d_inner, n_pipe, mu_mix):
    """
    Calculates the Reynold criterion.
    Parameters
    ----------
    F_mass : float
        The mass flow rate of feed [kg/s]
    z_way : float
        The number of ways in heat exchanger [dimensionless]
    d_inner : float
        The diametr of inner pipe, [m]
    n_pipe : float
        The number of pipes in heat exchanger, [dimensionless]
    mu_mix : float
        The mix viscocity of liquid, [Pa/s]
    Returns
    -------
    Re : float
        The Reynold criterion, [dimensionless]
    References
    ----------
    &&&&&&
    """         
    return 0.785 * F_mass * z_way / (d_inner * n_pipe * mu_mix)