def compute_n_trays_factor(N_T):
    """
    Return the cost factor for number of trays, F_NT.
    
    Parameters
    ----------
    N_T: Number of trays
    
    Notes
    -----
    The cost factor is given by [1]_. See source code for details.
    
    """
    if N_T < 20:
        F_NT = 2.25/1.0414**N_T
    else:
        F_NT = 1
    return F_NT