def Ergun(dp, voidage, vs, rho, mu, L=1):
    r'''Calculates pressure drop across a packed bed of spheres using a
    correlation developed in [1]_, as shown in [2]_ and [3]_. Eighteenth most
    accurate correlation overall in the review of [2]_.

    Most often presented in the following form:

    .. math::
        \Delta P = \frac{150\mu (1-\epsilon)^2 v_s L}{\epsilon^3 d_p^2}
        + \frac{1.75 (1-\epsilon) \rho v_s^2 L}{\epsilon^3 d_p}

    It is also often presented with a term for sphericity, which is multiplied
    by particle diameter everywhere in the equation. However, this is highly
    empirical and better correlations for beds of differently-shaped particles
    exist. To use sphericity in this model, multiple the input particle
    diameter by the spericity separately.

    In the review of [2]_, it is expressed in terms of a parameter `fp`, shown
    below. This is a convenient means of expressing all forms of pressure drop
    in packed beds correlations in a way that allows for easy comparison.

    .. math::
        f_p = \left(150 + 1.75\left(\frac{Re}{1-\epsilon}\right)\right)
        \frac{(1-\epsilon)^2}{\epsilon^3 Re}

    .. math::
        f_p = \frac{\Delta P d_p}{\rho v_s^2 L}

    .. math::
        Re = \frac{\rho v_s  d_p}{\mu}

    Parameters
    ----------
    dp : float
        Particle diameter of spheres [m]
    voidage : float
        Void fraction of bed packing [-]
    vs : float
        Superficial velocity of the fluid (volumetric flow rate/cross-sectional 
        area)[m/s]
    rho : float
        Density of the fluid [kg/m^3]
    mu : float
        Viscosity of the fluid, [Pa*s]
    L : float, optional
        Length the fluid flows in the packed bed [m]

    Returns
    -------
    dP : float
        Pressure drop across the bed [Pa]

    Notes
    -----
    The first term in this equation represents laminar loses, and the second,
    turbulent loses. Developed with data from spheres, sand, and pulverized
    coke. Fluids tested were carbon dioxide, nitrogen, methane, and hydrogen.

    Validity range shown in [3]_ is :math:`1 < Re_{Erg} < 2300`.
    Over predicts pressure drop for :math:`Re_{Erg} > 700`.

    Examples
    --------
    >>> Ergun(dp=8E-4, voidage=0.4, vs=1E-3, rho=1E3, mu=1E-3)
    1338.8671874999995

    References
    ----------
    .. [1] Ergun, S. (1952) "Fluid flow through packed columns",
       Chem. Eng. Prog., 48, 89-94.
    .. [2] Erdim, Esra, Ömer Akgiray, and İbrahim Demir. "A Revisit of Pressure
       Drop-Flow Rate Correlations for Packed Beds of Spheres." Powder
       Technology 283 (October 2015): 488-504. doi:10.1016/j.powtec.2015.06.017.
    .. [3] Jones, D. P., and H. Krier. "Gas Flow Resistance Measurements
       Through Packed Beds at High Reynolds Numbers." Journal of Fluids
       Engineering 105, no. 2 (June 1, 1983): 168-172. doi:10.1115/1.3240959.
    '''
    Re = dp*rho*vs/mu
    fp = (150 + 1.75*(Re/(1-voidage)))*(1-voidage)**2/(voidage**3*Re)
    return fp*rho*vs**2*L/dp