Counter-Current Heat Exchanger
Modeling a Counter-Current Flow Double-Pipe Heat Exchanger
This model describes heat transfer in a double-pipe heat exchanger (DPHX) operated in counter-current flow. By definition (cooling or heating) water flows through the core pipe whereas another (arbitrary) fluid is flowing through the annulus between core and shell pipe. Since physical properties of both fluid streams have to be entered as model parameters, the model can be used without modification if a fluid other than water is flowing through the core pipe.
The model is based on the usual assumptions (constant ambient temperature; no evaporation, condensation, or chemical reaction; constant physical properties and constant heat transfer coefficients over the whole DPHX). If variations in physical properties and/or heat transfer coefficients are expected to be large, the DPHX should be subdivided in smaller units so that the varations are not significant.
The model takes four mechanisms into account: (1) water flow through core pipe, (2) fluid flow through annulus as well as (3) heat transfer within the core pipe wall and (4) heat transfer to the shell pipe wall. Each subsystem is subdivided into 9 full-size finite volumes (FV) labelled "1" to "9" with half-size volumes (denoted by "0" and "10") at the inlet and outlet. Nodal points of FV "1" to "9" are equally spaced and assumed to be situated in the centre of the FV. Enthalpy flow rates at the boundaries of the FV are approximated by upward parabolic interpolation (UPI), and dispersion flow rates by central linear interpolation. Heat flow rates are calculated on the basis of a linear temperature change between the nodal points situated in the centre of the FVs.