On observability of a parallel-flow three-fluid heat exchange process

This paper is concerned with the observability problem of a parallel-flow three-fluid heat exchange process. First, the case where three fluid temperatures are measured at the outlet is considered. It is shown that the dual system, which is formulated as an well-defined boundary control system, is reachable through a concrete expression of the solution. In other words, the parallel-flow heat exchange process with the measurements becomes observable. Next, the unobservable subspace is characterized for the case where only one temperature is measured at the outlet.