def shell_clearance(DBundle=None, DShell=None):
    """Looks up the recommended clearance between a shell and tube bundle in 
    a TEMA HX [1]. Either the bundle diameter or the shell diameter are needed 
    provided.

    Parameters
    ----------
    DBundle : float, optional
        Outer diameter of tube bundle, [m]
    DShell : float, optional
        Shell inner diameter, [m]

    Returns
    -------
    c : float
        Shell-tube bundle clearance, [m]

    Notes
    -----
    Lower limits are extended up to the next limit where intermediate limits
    are not provided. 
    
    Examples
    --------
    >>> shell_clearance(DBundle=1.245)
    0.0064

    References
    ----------
    .. [1] Standards of the Tubular Exchanger Manufacturers Association,
       Ninth edition, 2007, TEMA, New York.
    """
    DShell_data = [(0.457, 0.0032), (1.016, 0.0048), (1.397, 0.0064), (
        1.778, 0.0079), (2.159, 0.0095)]
    DBundle_data = [(0.457 - 0.0048, 0.0032), (1.016 - 0.0064, 0.0048), (
        1.397 - 0.0079, 0.0064), (1.778 - 0.0095, 0.0079), (2.159 - 0.011, 
        0.0095)]
    if DShell:
        for DShell_tabulated, c in DShell_data:
            if DShell < DShell_tabulated:
                return c
        return 0.011
    elif DBundle:
        for DBundle_tabulated, c in DBundle_data:
            if DBundle < DBundle_tabulated:
                return c
        return 0.011
    else:
        raise Exception('Either DShell or DBundle must be specified')
