Flash Train
From SysCAD Documentation
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General Description
The Flash Train is not a unit as such, but rather it consists of a number of units, which together comprise a 'Flash Train'. The user cannot specify which units will comprise a Flash Train, the SysCAD software assesses the units that are inserted and the manner in which they are joined together and determines whether a flash train exists.
The main units that can comprise a Flash Train are: Flash Tank, Barometric Condenser, Shell and Tube Heat Exchanger, Shell and Tube Heat Exchanger 2 and Direct Contact Heater. A Desuperheater can also be included in a steam line for a flash train model. The Tie model (see note below) and Cross Page Connector are allowable units in the Flash Train.
For example, if a Flash Tank is connected to other Flash Tanks, Barometric Condensers and/or either Shell and Tube or Direct Contact Heat exchangers, the model will become part of the entire Flash Train. The user does not have to specify that the unit is part of the Flash Train; SysCAD will do this automatically.
The user must still configure the individual units within the Flash Train. However, the variables that can be configured will be different from the 'stand alone' unit. For example, the user cannot configure the flash pressure required in the Flash Tank, as the Flash Train will calculate this.
The Flash Train uses the Demand functionality to transfer the steam demand between units.
NOTES:
- If a Tie is inserted between two Flash Train units, the Flash train will be broken. However, if the user ticks the SplitFlows (GM) option in the Tie and "Operation" on the "GM" tab page is set to "General", with QmMode for all links set at (Demand), a flash train will be formed again.
- The Evaporation Project and Digestion Projects, which are distributed with SysCAD in the Examples Folder, demonstrate the use of a Flash Train in SysCAD projects.
The diagram below shows an example of the main units connected in such a way as to form three individual Flash Trains. These are NOT the only configurations for a Flash Train.
Diagram
The diagram shows a configuration that will result in three individual Flash Trains. The first Flash Train consists of Flash Tank 1 and the Direct Contact Heater. Flash Tank 2 and Shell and Tube Heat Exchanger 1 make up the next Flash Train, and Flash Tank 3 and the Barometric Condenser are joined in the third flash train. Heat Exchanger 2 is not part of a flash train.
Not all of the above units need to be inserted for SysCAD to configure a Flash Train. If a Flash Tank is connected to another Flash Tank, a Shell and Tube Heat Exchanger, a Direct Contact Heat Exchanger or a Barometric Condenser, SysCAD will automatically form a Flash Train.
Inputs and Outputs
The Flash Train does not have configurable Inputs or Outputs. The user must connect the required Inputs and Outputs on each individual unit.
Model Theory
The theory for each individual unit is described in the documentation (see the relevant sections for Flash Tank, Barometric Condenser, Shell and Tube Heat Exchanger 2, Shell and Tube Heat Exchanger, Direct Contact Heater and Desuperheater.)
The object of the Flash Train is to emulate a plant system without having to hard-wire the temperatures and pressures of each individual unit. The Flash Tanks within the Flash Train will attempt to produce sufficient steam to satisfy the demands of the connected units. For example, if the vapour line from the Flash Tank is connected to a Shell and Tube Heat Exchanger, then the heat exchanger will 'demand' a certain amount of steam. SysCAD will adjust the pressure in the flash tank in an attempt to produce sufficient steam to satisfy the Heat Exchanger.
Data Sections
The data sections for each unit within the Flash Train are described in detail elsewhere (see the relevant sections for Flash Tank, Barometric Condenser, Shell and Tube Heat Exchanger 2, Shell and Tube Heat Exchanger and Direct Contact Heater.)
Hints and Comments
- The information in the Vapour Liquid Equilibrium (VLE) must be correct for the units to operate as required. If the VLE configuration requires information that is not available, then the flashing will not occur.
- If the Flash Train includes a Shell and Tube Heat Exchanger, the user must ensure that the area and heat transfer coefficients of the unit are correctly specified.
- The Flash Train requires a feed pressure higher than the expected pressure in the flash tank. If required, this could be achieved either in the feeder to the Flash train, or with a boost in the pipe prior to the train. In the diagram above, the user need only ensure that there is a pressure boost to the first flash tank. The software will calculate the pressure profile through the remaining units.
- The Flash Train attempts to simulate a real plant environment. Therefore, the connections are very important. If the user configures the connections such that the fluids would not flow in a real plant, then the Flash Train will not function correctly.
- The user can make use of the SysCAD display colours to easily identify the flash train. From Tools | General Options | Colours, switch on the Macro Model option.
