Assignment_3_2007_Solutions

For the disturbances changes to be able to represent

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Unformatted text preview: e Linear model for disturbance Linear model simulation Fitting Alternat ive Linear model for disturbance Linear model simulation Alternat ive Linear model for disturbance Process model for simulat ion Disturbance model for simulation Process response for a disturbance change Process response for an input change Final verdict on model selection: For the manipulated variable changes a simple first order plus t ime delay model is enough. For the disturbances changes, to be able to represent the more co mplex dynamics invo lved, a first order model wit h a lead and a delay terms is necessary. This is the only model to be able to represent the inverse response behavior. Why these types of response for a disturbance change? We have two effects act ing together in opposite ways and at different time scales. One is a convect ive effect because the change of the amount of total flow coming in into the heat exchanger and the other is a thermal effect since the temperature if the inco ming feed has changed. The convect ive effect is immediate (incompressible fluid) and acts in the first instance and the thermal effect take some t ime to have an impact on the system (the t ime for the change to travel through the heat exchanger). For example, if we introduce a posit ive step change in the disturbance, that is we increase the warm liquid flow, we have and increase on the amount to total flow co ming in, but also a decrease on the temperature of the inco ming stream. Because the increase on the feed stream flow (convective effect) and since the amount of heat taken by the cooling water is constant, then init ially the outlet temperature will go up. After so me t ime, because the feed stream temperature decreased (thermal effect) the outlet temperature will go down....
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This note was uploaded on 01/12/2014 for the course CHE 4198 taught by Professor Hjortso,m during the Fall '08 term at LSU.

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