CAPE-OPEN

On the attached flowsheet I am heating an air stream and then humidifying with steam. When I change the Air heater exchanger outlet temperature, the controller C1 has trouble converging. I am using this controller C1 to make sure I have mostly condensate leaving the exchanger. Any recommendations?

The problem is that you your measured variable is vapor fraction, and if you change the temperature, I suppose you are at a stream where vapor fraction is either zero or unity. In both cases, the sensitivity of vapor fraction w.r.t. the controlled variable is zero.

You can get around this by changing the measured variable to something continuous, e.g. enthalpy (0th order continuous, 1st order discontinuous at the phase boundaries). Now of course your problem is that you need to know what enthalpy matches a vapor fraction of 0.1. If pressure is constant, this is a constant value for steam. Otherwise you could get this enthalpy from a stream that is flashed at VF = 0.01 (which you would need to add to the flowsheet, measure the enthalpy and use to connect to the set point).