FAQ - Flash Train related questions

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Navigation: User Guide -> Frequently Asked Questions


See also Configuring and Using Flash Trains for more help on a Flash Train.

Why is the display colour of some models dark-blue?

SysCAD uses Colours to display the unit operation's status.

Generally, while using the default white SysCAD background Scheme colour, when the display colour of a model is dark blue, they are part of a macro model, or a Flash Train model.

With a black SysCAD background Scheme colour (default for earlier versions of SysCAD), the flash train will be displayed in white.


What are macro models?

Macro model is the name given to a Flash Train. When models such as Flash Tanks, heat exchanger are correctly connected together, a flash train will be formed.

For example, a flash Tank and a shell and tube heat exchanger has formed a Flash Train, these two unit operations will be displayed in dark blue (with white background scheme colour). When models are under the flash train mode, SysCAD calculates the amount of steam being flashed off the flash tank based on the shell and tube exchanger configuration.

For more information see Flash Train.


The LMTD displayed is wrong

Its not wrong. It is calculated from the condensing temperature of the vapor, rather than the actual terminal temperatures. Here is what is going on - this shows the temperature profiles along the length of the exchanger for the cold and hot sides:

St-pr1-7.png

The calculation of heat transfer based on LMTD makes a number of assumptions, for example that that specific heat is constant and that the heat transfer coefficient doesn't vary along the exchanger. In the picture, the hot side profile would roughly follow the dotted line.

Based on these assumptions we can determine the temperature profiles and then calculate the net driving force, the LMTD equation:

[math] \Delta T_1 = T_{H_i} - T_{C_o}, \qquad \Delta T_2 = T_{H_o} - T_{C_i} [/math]

[math] \Delta T_{LM} = \frac{\Delta T_1 - \Delta T_2}{\ln \Delta T_1/\Delta T_2} [/math]

In flash train applications, the steam coming off the liquor is superheated because of boiling point elevation, so enters the condenser at a higher temperature than the final condensing temperature. So it must initially be desuperheated over some area of the exchanger, we refer to this as the desuperheating zone. The temperature profile follows the solid line.

In the desuperheating zone, the heat transfer coefficient is much lower than in the condensing zone, though the temperature difference between the steam and the liquor may be higher. These two effects tend to cancel each other out in calculating the overall temperature driving force for heat transfer.

So determining the true LMTD for a desuperheater-condenser is a complicated calculation. In practice it is adequate to calculate the LMTD based on the condensing temperature since the desuperheating zone is small and the two effects (higher temperature split and lower HTC) tend to cancel each other out. If you calculate LMTD based on the actual terminal temperatures, you are effectively adding in additional driving force (the gray area in the figure) and the number you calculate will be too large. The models use the condensing temperature at both terminals:

[math] \Delta T_1 = T_{H_c} - T_{C_o}, \qquad \Delta T_2 = T_{H_c} - T_{C_i} [/math]

In summary, the LMTD displayed in the condenser model is calculated from the liquor terminal temperatures and the vapor condensing temperature and that is a perfectly adequate assumption for basic design. In the real world, you are lucky if you know your heat transfer coefficients to within 20%.


Why is the steam vapour pressure not equal to atmospheric pressure at 100°C?

If you have a saturated steam stream at 100°C you find the pressure is actually 101.418kPa, or 1.0009 atm, which differs from 1atm by one tenth of one percent. Or if you set the pressure to 0barg (1atm), the temperature is not exactly 100°C.

The Celsius scale has been refined over the years from its original incantation, where the freezing point of water was set at 0°C, and the boiling point of water at atmospheric pressure to 100°C. The latest temperature scale defines temperatures at a number of points such as the melting point of gallium and the triple point of hydrogen. The only water value it uses is the triple point of water or 273.16°C. On this new scale, water actually boils at 99.974°C. SysCAD implements the IFS97 steam model, which uses this revised temperature scale.

In practical terms this difference is minute: it would be the difference in boiling point due to an elevation change of about 8 m.


What is the difference between BPE@T and BPE@P

There are two different ways to display Boiling Point Elevation (BPE) - as a function of pressure and as a function of temperature. For saturated liquids the two give identical results, they differ when you have a nonsaturated (subcooled) liquid and only if the boiling point elevation has a temperature dependency as well. The Alumina3 properties model displays both BPE@T and BPE@P.

For a subcooled liquid, we can get to saturation (boiling/flashing) by either

  • Raising the temperature at a fixed pressure
  • Lowering the pressure at a fixed temperature.

The two different BPE displays relate to these two scenarios -- once we reach saturation, either the liquor temperature has changed or the pressure has changed, and because of the temperature dependence of BPE, the liquors are in different thermophysical states and we should expect the properties (including BPE) to be different as well.

For example, suppose you are specifying operating conditions for a charge pump in a digestion system where live steam is used to heat an incoming liquor in a heat exchanger. To avoid boiling in the tubes (with resulting loss of efficiency and possible pitting and damage to heat transfer surfaces), the charge pump is operated at some margin over the liquor vapor pressure. In this case it is the BPE@P which is relevant to the calculation - if BPE@P is ten degrees and we are operating the charge pump at 6MPa, then the liquor boiling point of 285C is immediately determined from the water boiling point of 275C, and we can control live steam flow to provide a suitable margin.