Degrees of Freedom, Gain, Time Constants

The time constant of any process is the result of its capacitance and resistance. Usually, the heat exchanger outlet temperature is the controlled variable, and the flow rate of the heat transfer fluid is the manipulated variable. The time constant of an exchanger is a function of the mass and the specific heat of the tube material, the mass flow, and the specific heat of the process and utility streams and their heat transfer coefficients. 

Dead time is also called transportation lag, because it is the time required for fresh heat transfer fluid to displace the contents of the exchanger and its associated piping. The dead time is the worst enemy of control, because until it has expired, a change in the heat transfer fluid flow (or temperature) will not even begin to have an observable effect. For a heat exchanger, the dead time is usually between 1 and 30 seconds. When the equipment is correctly designed, the dead time is much less than the time constant. 

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