  We present a model-based real-time method to monitor a counterflow heat
exchanger's thermal performance for all operating conditions. A first principle
reference model that describes the reference counterflow process in an accurate
manner is derived first. Real gas behavior is taken into account. Without
simplifications, the respective equations must be solved in an iterative,
computationally expensive manner, which prohibits their use for real-time
monitoring purposes. Therefore, we propose one-step-solvable model equations,
resulting in an approximate but quick model, which is able to track an
important thermal property reliably. The monitoring, i.e., the online
estimation of the thermal properties, is achieved via a nonlinear
Kalman-Filter. Due to the low-order dynamic model formulation, the overall
monitoring scheme is accompanied by an acceptable computational burden.
Moreover, it is easy to deploy and to adapt in industrial practice. Monitoring
results, where the reference model replaces a real process with supercritical
carbon dioxide, are given and discussed herein.
