Demand
From SysCAD Documentation
The Demand Project is an example project distributed with SysCAD that shows the Demand functionality used in a project.
Contents |
Introduction
The Demand functionality is used in 2 distinct ways:
- The Flash Train macro model uses Demand to transfer steam flow demands from one unit, often a Flash Tank, to a another unit that consumes steam, e.g. a Shell and Tube Heat Exchanger, Barometric Condenser, etc. The user does not have to set any variables to make this functionality work. When the user connects the Flash Tank to an appropriate unit, SysCAD will recognise that these units form a Flash Train and will automatically pass the steam Demand between the units.
- Users can set up Demand requirements in Pipes (see Demand in Pipes) which connect back to Feeders which have the Demand functionality enabled, see Flow Requirements in a Feeder. The Demand required is transferred through Pipes and through Ties and Tanks by use of the General Model using the demand option.
- NOTE: The Makeup Block (MU) is often a simpler method to meet demand requirements.
This section will only discuss method 2 - the user defined Demand. The demand functionality in the Flash Train macro model is automatic and requires minimal user intervention.
Note: The Demand Project, which is distributed with SysCAD in the Examples folder, demonstrates the use of this functionality.
Setting up a demand stream (ProBal)
Example on setting up a tank input stream as demand stream (in ProBal):
The above example has a tank with three outlet streams, each with a known flowrate. There is also a side stream feed to the tank. The aim is to have the demand stream feed the correct amount to the tank to satisfy all the outlet requirements. When the project is solved, SysCAD will calculate the demand stream and input 13 t/h.
The above will also work for the tie model.
Setting a Feeder to Demand Mode
For the Demand functionality to work, the demand stream must be connected to a Feeder, with the Demand.On enabled, as shown in the figure below:
The user may also set the Maximum and the Minimum flows allowed from the Feeder, as shown above, where the maximum flow allowed is 1000kg/s.
Selecting Demand Options in Plant Model
- User can select to block the Demand in Tear streams (recommended)
- The amount available can be shown by selecting the ShowDemandAvail box. See the next heading for more information.
Setting up Demand in a Pipe
On the outlet streams, the user has to specify the Qm_Demand in the pipes, as shown below:
- The Qm_Avail.Src and Qm_Avail.Dst are only visible if the ShowDemandAvail tick box is selected in the PlantModel - Settings tab page. (See the previous heading.) The Available source will show the maximum amount it has available, normally set by the Demand.Max in the Feeder(s).
- In the pipe shown above, the user has set the Required Demand (Qm_Demand.Rqd) to 100kg/h
- The Source has provided this demand, Qm_Demand.Src = 100kg/h.
- There are no further Demands set downstream, and therefore Qm_Demand.Dst is not valid, i.e. a *.
The user may Block the Demand from perpetuating either up or downstream by enabling the Qm_Demand.Blk tick box. This is used as follows:
- To prevent the Demand from a Feeder from perpetuating downstream to a pipe where it is not required;
- To stop Demand in a recycle stream.
Hints and Comments
- When setting a Demand value in a pipe, the user must ensure that it is connected via the network to a Single source of material, i.e a Feeder with Demand enabled, see Flow Requirements in a Feeder. If there is no connection to a feeder with Demand enabled, then the Demand functionality will not operate correctly.
- Note: This is true in SysCAD 9.2, which has rigorous checks for the Demand functionality. SysCAD 9.1 and earlier may partially work without a Feeder in Demand mode, but the functionality may be compromised.
- If the user sees the warning More than one demand source available then this means that a unit is either connected to 2 Feeders with Demand enabled, or a recycle is repeating a Demand. The user must block the demand from one of the Feeders to the unit or in the recycle, as each unit may only have a single Demand source available if it is to work correctly.
- The demand logic often works better if the Qm_Rqd (Min) field is a small number rather than zero.
