Below is a presentation of concepts and terminology used in the FrigoSim program.
The quantities that describe the size and efficiencies of the components. They are either constants (integers or real numbers) or functions. See basis components for input description.
A collective term of the set of ways a user may specify functional dependencies. (Ref. description of functions input)
Redefinition of quantities used as input variables to functions. Read more in Modifier specification.
The quantities that are chosen to represent the fluid state or power shared by adjacent components. These are the unknowns of the systems of algebraic equations to be solved. They are quantities of type temperature, specific enthalpy, power, pressure, mass flow rate or mass fraction.
Quantities derived from variables or data, giving supplementary information on the component state or properties.
These are the building elements. Each model approximates a physical component type that is found in process plants. Every model is represented by a procedure (Pascal terminology). The procedures are contained in a module called UnitMdl. In addition a set of properties for each of the components is found in a system file called Models.ini . These are properties that can be handled by generic system code independently of the specific model, like number of and names of data, variables, parameters, connectors etc.
Most components have a set of quantities like size and efficiency that need to be specified. Examples are area, heat transfer coefficients, piston displacement, volumetric and isentropic efficiencies. A specific model with all of these data given, is called a basis component. A price may be attached to each one. (See input description).
There are two kinds of fluids, refrigerants and secondary fluids. For both kinds of fluids the properties are supplied by a function library. If you need to specify a secondary fluid not present in the library, you may enter your own fluid data. The required data are specific heat capacity, density, dynamic viscosity, thermal conductivity and relative thermal expansion. The refrigerant library is called RnLib, while the secondary fluid library is called FluidLib.
A basis component connected to other components and given the required initial values (as specified by the corresponding model), is called a plant component, or component for short. These are referred to in boundary conditions and control specifications and when selecting quantities for output. (See input description).
When connecting components to constitute a plant, we refer to connectors. These are the points or intersections where components are attached to each other. The connector numbers are generated automatically and are by default hidden from the user. The visibility of connectors is affected by the menu command .
Each component requires initialisation of a set of quantities. These supplies FrigoSim with a reasonable start in the search for a solution. Most initial values are only used in the very first iteration. All subsequent iterations use the previous values, not the initial values. Normally the suggested values can be rather coarsely specified, if only they are reasonable.
Some initial values are called controllable parameters. These are not recalculated by model code.
They stay constant, unless modified by some control, either
a parameter control,
a regulator control,
a valve control,
a bypass control,
or a capacity control.
All circuits formed by connecting the components into a plant have to be "filled" with a fluid. A circuit is specified by reference to an arbitrary connector of the circuit, and tying this connector to a fluid. A circuit is "the closed volume a fluid may flow into". In addition, power-transmission lines are called circuits. The fluid reference is in this case changed to the name "Power ". (See the input description.)
There has to be drawn a boundary somewhere to limit the considered system (plant) from its surroundings. The variables on this boundary may have system independent values. Therefore, they have to be specified explicitly. A typical example is the entry fluid temperatures of open circuits. These are specified through boundary conditions. (See the input description.)
In some cases, there is a need to control the plant or a part of it by a set of criteria. Examples are superheat/subcooling in certain connectors, control of mass flow distribution by three-way valves etc. It is also possible to control a set of components to achieve a given capacity by one or several heat exchangers. All these types of controls are covered by a part of the data set just called control, under which there is a range of possibilities. (See the input description.)
The user may influence the way the simulation is performed. This includes start and stop time, time step and how this is may vary, convergence criteria, variable limits, relaxation, time unit and additional output for testing. All this is covered by the collective term simulation conditions.
The user has several ways of specifying output quantities. These are component quantities, derived quantities and result conditions.
In this part, you specify component variables, parameters or data. A set of these may be selected for further use in derived quantities. (See the input description.)
Here you may operate on selected component quantities by standard operations like addition, subtraction, multiplication, division, minimum, maximum, time integration, derivation and mean value. In this way, you may compute e.g. energy consumption, coefficients of performance, maximum capacities etc. You may also introduce external quantities. By entering prices, various costs may be calculated. A set of predefined special output quantities is available for output. One of these is plant equipment cost derived from the basis component prices.
In this part, you may specify output time points and whether to have a complete output to the result file.
If you want to perform a number of related simulations in one run, use variant conditions. This enables variation of of plant properties that are fixed during a single simulation. This mechanism has been disabled.
Enthalpy flow rate
This is a quantity of a connector being the product of the mass flow rate and the specific enthalpy. In an enthalpy flow rate difference specification (used in capacity control) one must use two connectors of the same circuit, since an enthalpy flow rate value depends on a chosen reference enthalpy value ("zero enthalpy").
Some parameters relates to the fluid flowing through the component, as well as being influenced by the state of the component. These parameters are of type specific heat capacity, density, dynamic viscosity, thermal conductivity and volumetric thermal expansion coefficient of the fluid. They are functions of some local temperature. Such parameters are called fluid parameters.
The majority of components is represented by a set of equations relating variables of adjacent connectors. These equations, called component equations, describe the properties of the component model. The equations often contain functions (typically fluid properties functions) and need to be transformed into (non-linear) algebraic equations. Components that are not contributing by equations, may in stead influence through affecting the system variable structure.
Through the component connections, the component equations form plant equations. These are solved at every iteration step. FrigoSim uses Gauss elimination with partial pivoting. Heat balances are represented by one equation system, while mass flow balances form one equation system per circuit. The plant equation structure is viewed by the menu command .
All elements of the plant, like components, fluids, boundary conditions, controls, etc. are given an identification tag simply called a name. This name may contain any printing character, except a blank and consist of a maximum of 8 characters.
Note though: Names of basis components, fluids and functions should have names being valid file names in case you would like to store them on separate files, as the name is used as file name.
This name is subsequently used in all input references to the element.
In addition, FrigoSim uses such names for data, variables, parameters and models.
In the specification file each data group and some other data are ended by the name '$'.
In addition to names, characters are used for selection of options.
Also in this case '$' is used as terminator.
Note that the program distinguishes between uppercase and lowercase characters, i.e. use of case-sensitive names.
Clicking outside any component or other plant element and dragging over a number of components will "frame" these components. You may also use for this purpose. Framed components will be marked as individually marked components, but will be considered in context: If you copy a number of framed components, then also connections and related boundary conditions and controls will be copied. This also applies to deletion. You may drag framed components to a new position.
Filling a circuit with a fluid from the fluid windows can be done in a number of ways:
Double-click a connector of the circuit and select a fluid. Create a fill object, drag it close to a connector and drop the object when it turns white. A fill object is created by marking a fluid and selecting the menu command . Alternatively click the right mouse-button on the fluid and select Fill .
See also heat transfer boundary.
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