The water vapor transfer across a membrane exhibitsnon-minimum phase behavior. This paper shows that thecompeting dynamics of heat and mass transfer cause themembrane humidifier to have a non-minimum phase zero. Eventhough the non-minimum phase zero exists in the disturbance-output loop, it will limit the feedback controller gain becausethe disturbance-output loop is coupled with the input-outputloop. The membrane properties and heat transfer parametersaffect the non-minimum phase zero location. The impact onavailable feedback control gain and system bandwidth isanalyzed in relation to changes of the non-minimum phase zeroduring hardware design.
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