docs: def calc_Cr(mh, mc, Cph, Cpc):
      r'''Returns the heat capacity rate ratio for a heat exchanger
      having flows `mh` and `mc`, with averaged heat capacities `Cph` and `Cpc`.
  
      .. math::
          C_r=C^*=\frac{C_{min}}{C_{max}}
  
      Parameters
      ----------
      mh : float
          Mass flow rate of hot stream, [kg/s]
      mc : float
          Mass flow rate of cold stream, [kg/s]
      Cph : float
          Averaged heat capacity of hot stream, [J/kg/K]
      Cpc : float
          Averaged heat capacity of cold stream, [J/kg/K]
  
      Returns
      -------
      Cr : float
          The heat capacity rate ratio, of the smaller fluid to the larger
          fluid, [W/K]
  
      Notes
      -----
      Used with the effectiveness method for heat exchanger design.
      Technically, it doesn't matter if the hot and cold streams are in the right
      order for the input, but it is easiest to use this function when the order
      is specified.
  
      Examples
      --------
      >>> calc_Cr(mh=22., mc=5.5, Cph=2200, Cpc=4400.)
      0.5
  
      References
      ----------
      .. [1] Bergman, Theodore L., Adrienne S. Lavine, Frank P. Incropera, and
         David P. DeWitt. Introduction to Heat Transfer. 6E. Hoboken, NJ:
         Wiley, 2011.
      '''
      Ch = mh*Cph
      Cc = mc*Cpc
      Cmin = min(Ch, Cc)
      Cmax = max(Ch, Cc)
      return Cmin/Cmax