Control of Heat Exchangers
Heat exchangers are used to control fluid temperatures in a wide variety of applications. This is normally achieved by controlling the exit temperature of one of the fluids by varying the inlet temperature or flow rate of another fluid. Accurate prediction of the dynamic and steady-state thermal behaviour of heat exchangers is extremely difficult because of the complex nature of their geometries and the non-linear spatially varying nature of the associated thermo-fluid processes (Pacheco-Vega et al., 2009). Many assumptions must be made (e.g. lumped parameter simplification, one-dimensional heat transfer, constant fluid properties and heat transfer coefficients) when generating mathematical models that are suitable for use in model-based controllers (Maidi et al., 2008).
The example application considered in this chapter is the use of an air-to-water heat exchanger to control the supply air temperature in an air-conditioning system. This is a particularly challenging control problem because of the uncertainties that normally characterize the operation of its air-handling units, for example: uncertain process gain (Huang et al., 2009); uncertain dynamics (Huang and Dexter, 2008; Xu et al., 2010); unmeasured disturbances (Lu et al., 2007); uncertain process interactions (Lygouras et al., 2008); and uncertain non-linear behaviour (He et al., 2005; Zhao et al., 2011).
16.1 Main Sources of Uncertainty and Practical Considerations
Because there is usually ...