The main function is HeatExchanger.m that calculates the outlet temperatures of a heat exchanger using Epsilon-NTU (E-NTU) method. This function uses effectiveness.m as a function and should have access to it.
The inputs are as follows:
Hot Flow: c_p_hot, m_dot_hot, T_hot_in.
Cold Flow: c_p_cold, m_dot_cold, T_cold_in.
Heat exchanger design parameters: U,A, HE_Type.
HE_Type defines the type of heat exchanger: (see reference)
  'Parallel Flow'
  'Counter Flow'
  'One Shell Pass'
  'N Shell Pass'
  'Cross Both Unmixed'
  'Cross Cmax Mixed'
  'Cross Cmin Mixed'
The second function is effectiveness.m which calculates the effectiveness of a heat exchanger : epsilon = f(NTU,C_r)
See Reference for definitions.
Third file is a test script that shows an example of using this function.
Reference:
Frank P. Incropera, Introduction to heat transfer. New York:Wiley, 1985, Section 11.4.