Using a two stage heat exchanger with two independent heat exchanger designs to heat up HTL feedstock will drastically reduce the capital investment for a HTL plant. The first stage HTL heat exchanger will be rated for lower pressure, and maximize the heat exchanger area by reducing the tube size within the pressure drop limits of the system. The second heat exchanger will involve large tubes to produce a turbulent stream, thereby changing the heat transfer regime and drastically increasing the heat transfer. The second heat exchanger will be rated for high pressure.