def Pr_deph(C_coolwater, mu_coolwater, lyambda_coolwater):
    """
    Calculates the Prandtl criterion.
    Parameters
    ----------
    C_capacity_water : float
        The heat capacity of cool water [J/(kg * degrees celcium)]
    mu_water : float
        The viscocity of cool water, [Pa/s]
    lyambda_coolwater : float
        The thermal conductivity of cool water, [W / (m * degreec celcium)]
    Returns
    -------
    Pr_deph : float
        The Prandtl criterion, [dimensionless]
    References
    ----------
    Романков, формула 4.12, стр.151
    """        
    return C_coolwater * mu_coolwater / lyambda_coolwater