Engineering task and calculation objective
The KOKU module generates the condensation curve of a fluid mixture: starting from a stored mixture definition (file name of the material mixture), the relationship between temperature, pressure and condensed fraction is calculated between the inlet and outlet state and saved as a curve file. The inputs are the inlet and outlet temperature, the inlet and outlet pressure, and the step widths used to subdivide the temperature and pressure range into intervals.
Such a condensation curve is needed whenever a multi-component mixture is to be condensed, for example vapors containing inert gas, hydrocarbon mixtures or solvent vapors: unlike a pure substance, a mixture does not condense at a fixed temperature but glides between its dew point and bubble point. For the design of a condenser it must therefore be known how much heat is released in which temperature interval.
The generated curve serves as the property-data basis for downstream heat exchanger calculations, in particular for the incremental condensation modules for horizontal and vertical tubes (KON1, KON2), which work through the condenser section by section along this curve.
Calculation workflow
- Load the fluid mixture: Via the file name of the material mixture, the composition with the property data of the components is read in; it determines the phase equilibrium and thus the course of condensation.
- Define the state range: The inlet temperature and outlet temperature together with the inlet pressure and outlet pressure span the range over which condensation is tracked, from the superheated or near-dew-point inlet to the desired outlet state.
- Choose the interval subdivision: Using the temperature interval width and the pressure interval width, the range is subdivided into grid points. A finer subdivision resolves strongly curved sections of the curve, for example near the dew point, more accurately, but increases the number of calculation points.
- Evaluate the phase equilibrium at each grid point: For each grid point, the vapor-liquid equilibrium of the mixture is evaluated and from it the condensed mass fraction and the associated heat release are determined; this yields the condensation profile as a function of temperature.
- Save the curve: On request (saving of the data), the result is stored under the specified file name of the condensation curve and is then available as an input to the incremental condenser calculations.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| File name of condensation curve: | ('.tab') | - |
| Inlet temperature Outlet temperature | ϑE ϑA | °C |
| Inlet temperature Outlet temperature | ϑE ϑA | °C |
| Inlet pressure Outlet pressure | pE pA | Pa |
| Inlet pressure Outlet pressure | pE pA | Pa |
| ⇒ Temperature interval width | Δϑ | K (diff) |
| ⇒ Pressure interval width | Δp | Pa |
| - | - | - |
| Saving of the data: (Yes = J / No = -) | <J> | - |
| File name of material mixture | ('.str') | - |
| T1 | T1 | °C |
| T2 | T2 | °C |
| T3 | T3 | °C |
| T4 | T4 | °C |
| T5 | T5 | °C |
| T6 | T6 | °C |
| T7 | T7 | °C |
| T8 | T8 | °C |
| T9 | T9 | °C |
| T10 | T10 | °C |
| T11 | T11 | °C |
| T12 | T12 | °C |
| T13 | T13 | °C |
| T14 | T14 | °C |
Frequently asked questions
Why is a single condensation temperature not sufficient for a mixture?
At a given pressure, a pure substance condenses isothermally at its saturation temperature. A mixture, by contrast, condenses over a gliding temperature range: at the dew point the heavier-boiling components condense first, the vapor composition shifts, and the condensation temperature falls continuously down to the bubble point of the mixture. For condenser design, the entire curve of heat release versus temperature is therefore required, not a single value.
How fine should the temperature and pressure intervals be chosen?
The subdivision must be fine enough that the condensation curve runs approximately linearly between the grid points. The critical region is above all the range immediately below the dew point, where for close-boiling or inert-gas-containing mixtures a large amount of condensate forms over a small temperature interval. A sensible check is to repeat the calculation with the interval width halved and to verify whether the result still changes noticeably.
What is the purpose of the pressure profile between inlet and outlet?
In real condensers, the pressure drops between inlet and outlet due to friction and flow redirection. Since the equilibrium temperatures depend on pressure, the pressure loss shifts the condensation curve. By specifying the inlet and outlet pressure with the corresponding interval width, this effect is approximately captured, instead of generating the curve for a constant pressure only.