Pipelines

Objects of the type "pipeline" are used to indicate the relations between the components in the model.

These are actually ideal connection lines, which transfer the thermodynamic properties from one point of the cycle to another point, without any losses. If you want to model a real pipe with pressure and heat loss, you have to insert a Component 13.

The main task of  EBSILON^®Professional is the calculation of the following three basic quantities     

• Pressure,
• Enthalpy, and
• Mass flow

for each pipeline. The calculation is considered as successful, when this goal is reached.

A fluid-type is assigned to each pipeline..

The fluid-type determines, how the following derived thermodynamic quantities

• Temperature
• Heat flow (or power)
• Volume flow
• Entropy
• Steam content (only for water/steam pipelines)
• Exergy

are calculated from the three basic quantities.  

Each pipeline has two connection points that can be connected to components. When one connection point is connected to a component outlet, the other must be connected to a component inlet, and vice versa. The fluid type on both connection points must match the fluid type on the component connection point.

It is possible to leave one or both ends unconnected. In the latter case, the calculation kernel will not take notice of that line unless values are specified on the line by component 33 or 46.

Logical lines, i.e. pipelines with the fluid type logic, scheduled value or actual value, can be connected either to a component or to another line. To establish such a connection, place the ending point of the logical line somewhere in the middle on the other line. If there is such connection, the logical line transfers the values from or to the other line. Sometimes, the values of such a logical line are not displayed in the model (owing to an internal optimization that discards that line).

If you connect two pipelines of the same fluid-type at their ending points (with the correct flow direction), EBSILON^®Professional unifies both lines and you have one long line afterwards.

Pressure, enthalpy and mass flow are calculated for each pipe. For non-material pipes like shaft or electric line, internal dummy values for the pressure (0.01 bar) and the mass flow (1 kg/s) are used internally, while the enthalpy corresponds to the transferred power. To avoid confusion, these dummy values are not displayed generally.

The calculation of other quantities depends on the line type. The visible result values are:

Result value

Symbol

Air

Steam

Water(liquid)

Gas

Oil

Fluegas

Crudegas

Coal / Ash

User defined

Electric line

Mech. shaft

Scheduled value

Actual value

Logic

saltwater

2-Phase (liquid)

2-Phase (gas)

Pressure

P

x

x

x

x

x

x

x

x

x

x

x

x

Temperature

T

x

x

x

x

x

x

x

x

x

x

x

x

Enthalpy

H

x

x

x

x

x

x

x

x

x

x

x

x

Mass flow

M

x

x

x

x

x

x

x

x

x

x

x

x

Heat flow

Q

x

x

x

x

x

x

x

x

x

x

x

x

x

x

Steam content

X

x

x

Density

RHO

x

x

x

x

x

x

x

x

x

x

x

x

Specific volume

V

x

x

x

x

x

x

x

x

x

x

x

x

Volume flow

VM

x

x

x

x

x

x

x

x

x

x

x

x

Entropy

S

x

x

x

x

x

x

x

x

x

x

x

x

Exergy

E

x

x

x

x

x

x

x

x

x

x

x

x

The results can be displayed with value crosses or in value fields.

For calculating the exergy as per the following formula

E = H - Href - Tref (S - Sref)

a reference state is necessary, which is defined with the help of two measurement points having the type reference pressure and reference temperature, which can be present at any point in the cycle. Reference enthalpy and entropy are then calculated from the pressure and the temperature. Chemical and electrical exergies are not taken into account.

Other pipe attributes are the confidence intervals of the basic quantities pressure (DP), enthalpy (DH) and mass flow (DM). During validation, these are calculated according to VDI 2048.

The accuracy of the iteration

After the calculation, the relative deviations between the second-last and the last iteration step for mass flow, pressure and enthalpy is present on each line as result values DITM, DITP and DITH.

 

Performance results - Temperature difference  DELTA_T:

A line result value DELTA_T has been introduced to be able to treat temperature differences in logical constructions correctly. When using EBSILON^®Professional standard units (°C for temperature, K for temperature difference), the numerical value equals that of T. For the unit conversion, however, it will behave differently. Example: T=0°C becomes 32°F, but DELTA_T=0 K becomes 0 Rk.

Performance results - NCV0:

There is a result value NCV0 (NCV at a reference temperature of 0°C) on each line with composition. As Ebsilon refers the enthalpy zero point to 0°C, the NCV at 0°C also has to be used for the consideration of the energy balance.

 

Apart from the basic quantities mentioned, the calculation kernel determines the chemical composition for certain types of fluids. Pipelines of such a type have additional attributes:

• Mass fraction of each material contained in the fluid, denoted by X and the name of the material
• Net calorific value NCV ,  Net calorific value NCV0 (at  reference temperature of 0 °C).
  As Ebsilon refers the enthalpy zero point to 0°C, the NCV at 0°C also has to be used for the consideration of the energy balance.
• Volatile fraction (for coal only)
• Coal type (for coal only)
• Density (for oil only)
• Z-factor (for oil only)
• Normalized fraction of solid particles [mg/nm³] (for air, flue gas and crude gas)

For solids (coal, ash) and liquids (oil, user-defined fluid), the composition is determined by an elementary analysis (fractions of C, H, O, N, S, and Cl).

For gases, the molecular composition is specified.

In certain fluid types,  EBSILON^®Professional is able to handle mixtures of gases and solids. For instance, there may be solid particles in the flue gas. In this case, the values XC, XH, XO, XS, XCl, XASH and XLIME describe the solid (or liquid) part, while XN2, XO2 etc. describe the gaseous part.

A special case is water. In a water line (either liquid water or steam), no chemical composition is defined. 

For the water content in other fluids, there were three sizes until Release 10:

• XH2OL indicates the fraction of liquid water in a gas flow (as the default value , now only as a result of line)
• XH2OG indicates the fraction of gaseous water (steam) in a gas flow (as the default value only, only as a result of line)
• XH2OB indicates the fraction of bonded water in coal or oil i.e. the fraction of water that is considered as part of the coal or the oil fluid and will have an effect on the calculation of the net calorific value (it actually decreases it).

Due to the combination of XH2OG and XH2OL to XH2O (accordingly also XNH3L and XNH3G to XNH3 as well as XCO2 and XCO2L to XCO2), allocations to the old variables are no longer possible in the specification values (Components 1 and 33).

In the line results, it is possible to access the phase information via the result values XL_H2O, XL_CO2, XL_NH3. These values show the liquid fraction, e.g. XL_H2O = XH2OL/XH2O. For reasons of compatibility, XH2OG and XH2OL still exist.

Since EBSILON^®Professional allows gaseous fractions in coal , oil or user defined, there may be H2O in coal, oil or user defined . Note that this water fraction is handled differently from the XH2OB fraction.

Depending on the type of fluid, the following attributes are available:

Quantity

Symbol

Air

Steam

Water(liquid)

Gas

Oil

Flue gas

Crude gas

Coal/ Ash

User defined

Electric line

Mech. shaft

Scheduled value

Actual value

Logic

saltwater

2-Phase

Binary mixture

Net calorific value

NCV

x

x

x

x

x

x

x

x

Mass fraction of each material contained in the fluid, denoted by X and the name of the material (see "Pipe properties" - "Composition")

XN2 bis XL_NH3

x

x

x

x

x

x

x

Fraction volatiles

VOLA

x

Z-Factor

ZFAC

x

Coal type

FCOAL

x

Correction factor for cp ash

CPCORR

x

Net calorific value at 0°C

NCV0

x

x

x

x

x

x

x

x

Fraction (dry) of solid particles in mg/Nm3

MGNM3

x

x

Density for fraction defined by elementary analysis

RHOELEM

x

x

x

x

x

x

x

Volume fraction (dry) of NOx (normalized to reference O2 - concentration

NOXP

x

x

Fraction of NOx (dry) in mg/Nm3

NOXM

x

x

Volume fraction (dry) of NH3

NH3P

x

x

Fraction of NH3 (dry) in mg/Nm3

NH3M

x

x

Mass fraction of salt from total mass

SALT

x

Medium type

FMED

x

Medium type

FBIN

x

Fraction of refrigerant medium / water in air

XI

x

x

x

Gaseous water fraction

XH2OG

x

x

x

x

x

x

x

Liquid water fraction

XH2OL

x

x

x

x

x

x

x

The chemical composition in the pipelines is calculated by the calculation kernel. To specify a composition, you have to use component 1 or 33. See Definition of Material Composition for details.

Line results MGNM3, NOXP, NOXM, NH3P, NH3M

These values were converted to the release level 10 for fixed reference conditions (1 bar, 0°C) considering the water fraction, with water occurring mostly in the liquid phase under these conditions.
As its density is  considered  (see paragraph "Consideration of non-gaseous components for the specific volume of gases"), the results are probably difficult to interpret.
As these quantities are usually related to the dry portion, this is done in Ebsilon as well. Moreover, the reference conditions defined by measured values of the type “Reference pressure” (FTYP=13) and “Reference temperature (for norm conditions and exergy)” (FTYP=26) are used here. 

 

Result values standardized concentrations

In the sheet "composition" in the flue gas lines some values  at the output, the output concentrations of pollutants:

• MGNM3
  specifies how much solid (in milligrams) substances (ash and unburned fuel) and gaseous ash in a flue gas m³ (dry) is under reference conditions (pressure and temperature)
  
• NH3P
  indicates the mole fraction of NH3 when the flue gas (dry) to the reference oxygen concentration converts.  
  
• NH3M
  indicates how much NH3 (in milligrams) in a m³ flue gas (dry) when the flue gas is converts to the reference oxygen concentration and the reference conditions (pressure and temperature). Between NH3P and NH3M is a fixed conversion factor of 0.759629 (NH3M = 0.759629 * NH3P). 
  
• NOXP
  indicates the mole fraction of NOX when the flue gas (dry) converts the reference oxygen concentration. Here NO as NO2 is treated, since it is assumed that all NO is converted into NO2 in the air.
  
• NOX   
  indicates how much NOX is (in milligrams) in a m³ flue gas (dry) when the flue gas to the reference oxygen concentration and the reference conditions (pressure and temperature) converts. Between NOXP and NOxM is a fixed conversion factor of 2.05204 (NOxM = 2.05204 * NOXP). 

 

Output Molar Mass

In the case of streams with composition, the molar mass is output as result value MOLM.

If an elementary analysis is specified, the molar mass of the atoms is used here.

As no molar mass is defined for the component “ash“, no molar mass is calculated for streams with a significant (≥10-5) ash fraction.

 

Consideration of non-gaseous components for the specific volume of gases

Only the gaseous components were considered for the calculation of the specific volume (and thus also the density) of gases (air, flue gas, gas, crude gas), because the fraction of the liquid
and solid components to the specific volume is generally negligible due to the higher density.

Generally the specific volume of these components cannot be calculated because usually only the elementary composition or the general specification “ash“ is specified for this.

Now the option to specify a density for this fraction (liquid and solid parts) in the specification value “Density for fraction defined by elementary analysis“ (RHOELEM) has been created.
Up to Release 10, the density specification (specification value RHO) was only used for oil. For standardization purposes, RHOELEM has to be used for oil, too. RHO is no longer available
as a specification value.

As result values on the lines there are both RHO (mean density of the total flow) and RHOELEM (Density for fraction defined by elementary analysis).

If a value of 0 is entered for the density (RHOELEM), the fraction of the substances given as elementary analysis will be neglected when determining the specific volume.

In the case of the non-gaseous components the chemical composition of which is known, the specific volume is determined from the corresponding material data. This applies to liquid
H2O, NH3, and CO2, for which libraries are integrated in Ebsilon, as well as the new substances for the direct desulfurization, for which the following constants are used:

• CaSO4 2960 kg/m³
• CaCO3 2730 kg/m³
• CaO 3370 kg/m³
• Ca(OH)2 2240 kg/m³
• MgCO3 2960 kg/m³
• MgO 3580 kg/m³

If water bound in the coal (H2OB) is present, this will be considered as a component of the coal, i.e. it is assumed that this fraction is already contained in RHOELEM. H2O on a coal line (e.g. rain water between the coal chunks), however, will be calculated separately with the material data for H2O.

 

Specific volume (V)

As described in Consideration of non-gaseous components for the specific volume of gases, the non-gaseous components are considered when calculating the specific volume as well.
In the case of gaseous flows this leads to an increase in the specific volume and thus to a reduction of the density. As the liquid fraction in the gas flow is usually small and the specific volume
of the liquid significantly lower than that of the components, only minor changes result from this. In order to prevent inaccuracies, however, it will be a good idea to repeat the design
calculation and to save the reference values anew if components with specific volume or volume flow as nominal values (e.g. Throttle) are flown through by gas streams with liquid portions.

In the case of lines of the types “Coal“, “Oil“, and “User-defined“, now contrariwise the entered density is only related to the portion that is defined by the elementary analysis, whereas the
respective material data are consulted for the other fractions. As far as gaseous components exist in such lines, a correspondingly higher specific volume and thus a lower density will result.

 

Note :Warning in the case of double specification of material data

There is a warning when a composition is specified along with a start value on a line where the composition has already been specified. Often this happens when a start value is placed on a line with composition in order to specify pressure, temperature, or mass flow there without setting the flag to “Do not specify composition“ on the sheet “Material fractions“: by default, the composition is specified.
This has no impact on the calculation because the double specification of the composition is overwritten in the course of the iteration anyway.
Up to Release 7, deactivating the specification of the composition was impossible anyway.

 

Mechanical Shafts and Electric Streams

---

 

For the type of lines

- Mechanical Shaft
- Electric Streams

Only one attribute, namely the performance Q, has been considered.Therefore components like gears and transformers could only be represented by means of more complex
logical constructions.
In order to simplify this, the following additional attributes have been introduced for these type of lines:

• F: rotational speed and frequency respectively (for mechanical shafts and electric streams)
• I: current (only for electric streams)
• U: voltage (only for electric streams)
• PHEL: phase shift between voltage and current (only for electric streams)

additionally

• COSP: Power factor (for electric cables only)
• NPHAS: Type of current type (for electric cables only)

As output and current are linked via the correlation

            Q = U * I * cos(PHEL),

only either the current or the output can be specified on an electric line. If both variables are specified, a double entry will be signalized: a double entry in the enthalpy as both the output
and also the current are mapped onto the internal variable “enthalpy” in the calculation kernel. Accordingly, the frequency is mapped onto the mass flow, the voltage onto the pressure, and the phase onto the NCV. This needs to be considered when programming own components (KernelScripting) if equations are to be created for these variables.

The variable COSP=cos(PHEL) is also called power factor.

It is possible to set for electric lines whether direct current, one-phase alternating current or three-phase alternating current (three-phase current) applies.
The setting is made via the specification value NPHAS in the boundary value (Component 1) and start value (Component 33) respectively or in components with electric outlets.

In Ebsilon, this setting only affects the correlation of power output and current.

For direct current applies: active power Q = U * I

For one-phase alternating current applies: active power Q = U * I * cos (phi)

For three-phase alternating current applies: active power Q = U * I * cos (phi) * SQRT (3)

Thus when specifying the current, a power output increased by a factor of SQRT(3) is achieved for three-phase current.

NPHAS is transferred in the direction of the flow. When connecting lines with different NPHAS, the value of the main inlet is used.

Specification of Frequency, Current, Voltage, and Phase 

These variables can be specified via

• the generator (Component 11)
• the pump with variable speed (Component 83) or via
• measured value inputs (Component 46)

The following measured value types are used for the specification with measured value inputs:

• FTYP=15 for the current (on electric lines),
• FTYP=16 for the frequency and rotational speed respectively (on shafts and electric lines),
• FTYP=20 for the voltage (on electric lines),
• FTYP=55 for the power factor cos(phi) phi>0 adopted,
• FTYP=57 for the phase.

FTYP=15, 16, and 20 were also available in older Ebsilon Releases, but were then mapped onto enthalpies. A conversion is therefore carried out when loading old models
(see changes in the results).

FTYP=55 and FTYP=57 enable different levels of detail when considering the phase:

• As long as the interest is limited to the electrical outputs, it is sufficient to specify the power factor (FTYP=55). Because of cos(phi)=cos(-phi), however, the algebraic sign of the phase cannot be determined unambiguously this way. Ebsilon assumes a positive algebraic sign in this case.
  
• If capacitive or inductive resistances are to be considered in the model, however, the algebraic sign of the phase is of importance as well. In this case FTYP=57 has to be used. Here “phase” is understood as the phase difference between voltage and current In the case of PHEL>0 the voltage runs ahead of the current (inductive resistance), in the case of PHEL<0 the voltage runs after the current (capacitive resistance).

In the generator, the specification values COSPHI (for the power factor) and GENF (for the frequency) as well as the new specification value VOLT are used for specifying these variables. For the specification values to be used, the flags FCOS, FGENF, and FVOLT are to be set accordingly. In this case, the current is calculated from the output. The positive algebraic sign is used for the phase.

The pump with variable speed can be used for specifying the rotational speed on the connected shaft. The specification value REVG is used for this, with corresponding setting
of the flag FSPEC.  

 

Component Physics:

In the case of components that have an inlet and an outlet for a mechanical shaft or an electric line, the frequencies, and in the case of electric streams also the voltages and phases of the
two pins are equalized.

However, there is a special implementation for Component 13 (“Piping“) on electric streams where it can model an electrical resistor. Please refer to Chapter Component 13 (“Piping“)
"Electrical Resistance"  for details.

In the case of splitters, the frequencies, and in the case of electric streams also the voltages and phases of the inlet and the two outlets are equalized.

In the case of mixers, the frequencies and the voltages of the main inlet (Pin 1) and outlet (Pin 2) are equalized. If the frequency and the voltage of the auxiliary inlet (Pin 3) coincide with the main inlet, the flows will be added as a vector, and the resulting phase and the output will be calculated. If the frequency or the voltage of the auxiliary inlet does not coincide with the main inlet, a simple summation of the outputs will be carried out . In Components 37 and 60 this will be indicated in a comment. In this case, the outlet line receives the phase of the main inlet line.

The generator forwards the frequency both to the electrical connection and to the mechanical shaft.

Additions for further components (like e.g. the motor (Component 29)) which allow to consider these new values are planned for future Ebsilon Releases.

A remark for users who want to apply the extension of the equation system by material equations:
As from Release 11 onwards no more material equations are triggered for mechanical shafts and electric streams in order to reduce the computing time, it is not possible to use the phase (which internally is mapped onto the NCV) in this mode.

 

Display:

The values are displayed as line attributes “F”, “I”, “U”, “COSP”, and “PHEL”. Moreover, a value indicator (Component 45) with corresponding FTYP can be used. 

 

Changes in the Results for Mechanical Shafts and Electric Streams 

Measured Value Input (Component 46):

In models (release 11 and older), measured values were displayed for current (FTYP=15), rotational speed and frequency respectively (FTYP=16) and voltage (FTYP=20) were mapped onto enthalpies and could be processed further in logic constructions.

As in Release 12, however, these variables are available as attributes on shafts and electric streams, the behaviour of the measuring point for the FTYP values has been changed (see above).

Compatibility modes have been introduced to allow logic constructions from existing models to continue working; these are FTYP=-15, -16, and -20 that continue to be mapped onto dummy enthalpies. When loading a model created with Release 11 or older, measuring points with FTYP=15, 16, 20 are automatically shifted to -15, -16, -20, so that the measuring points provide the same equations as before.

If required, the model can be revised accordingly in order to use the new options.

 

Generator (Component 11):

If the flags FCOS and FGENF respectively are set accordingly, the specification values COSPHI and GENF will be transferred to the electric line. As in Release 11 the default setting for these
flags was “not used“, no changes occur for these flags.

Note: For models created with older versions of Ebsilon where the setting “not used” did not exist, an unintended transfer of the power factor and the frequency to the electric line may occur.
Usually this has no consequences. If, however, start values for mass flow or pressure were used on the shaft and electric line respectively (which was required in former versions of Ebsilon), double entries may occur, but these are easy to eliminate by removing the unnecessary specifications.

 

Transferring information about phases and resistances

In Ebsilon, only the power output, the frequency, the voltage, and the difference between the phase of the voltage and the phase of the current were transferred on electric lines in Ebsilon. However, this information is insufficient for modeling networks with splitters and mixers because

• for a mixer, also the phase shift between the currents of the two inlet lines is relevant
• for a splitter, the specification of the distribution of the power output is insufficient for unambiguously determining the currents and phases on both outlet lines.

In reality, the currents and phases downstream of a splitter will materialize due to the resistances and impedances respectively of the two sub-legs.

In the splitter (Component 18: Distribution of electric currents), the distribution can be calculated. In the following mixer, an addition of the currents according to phases is then effected.

To enable this, the following information required for the calculation by splitter and mixer has to be transferred internally on the electric lines:

• Real and imaginary part of the resistance in the leg:
  This information is written by the Components 13 used as electrical resistor (FDN=9) on their inlet lines. To do so, Component 13 adds its own resistance to the resistance it finds on its outlet line. In the course of the iteration, the information on the total resistance of the leg then arrives at the splitter (Component 18).
  
• Real and imaginary part of the current in the leg:
  This information is written on the two outlet lines by Component 18 and is transferred from the inlet to the outlet by Component 13. In the course of the iteration, the information on the current in the leg arrives at the mixer (Component 60).

The Components 80 (Separator) and 81 (Stream coupling) can be installed in the leg as well. They also transfer the information.

Please note: The transfer of real and imaginary part of the current represents a certain redundancy as the amount of the current can mostly be calculated from power output, voltage, and phase shift according to I = Q / (U*cos(phi)). However, also pure reactive current (cos(phi) = 0) and the current flow on a de-energized line (U=0) can be modelled by transferring the real and imaginary part of of the current. The summary then checks the consistency between power output and current. In the event of inconsistencies, the power output is prioritized (because compliance with the energy balance is considered more important by Ebsilon), and a comment is output.

In Component 45 (see also component 33: New start value inputs on electric lines) it is possible to query the data internally stored on the line. 

Multiply nested splitters and mixers cannot be treated by this mechanism. The transfer of the resistance information always ends at the innermost splitter. For modeling more complex networks, however, it is possible to transmit the resistance information to Component 18 using a logic line (see also component 18: Distribution of electric currents).

 

Initialization

As the usual start values for mass flow (1.0) and pressure (0.01) for frequencies and voltages are outside the usual range, frequencies are initialized with 3000/min and voltages with 1000 V. Especially too small values for voltage led to extremely high values for the current, which resulted in problems on electrical resistors in the initialization phase.