Pressure Changer#
The IDAES Pressure Changer model represents a unit operation with a single stream of material which undergoes a change in pressure due to the application of a work. The Pressure Changer model contains support for a number of different thermodynamic assumptions regarding the working fluid.
Degrees of Freedom#
Pressure Changer units generally have one or more degrees of freedom, depending on the thermodynamic assumption used.
Typical fixed variables are:
outlet pressure, \(P_{ratio}\) or \(\Delta P\),
unit efficiency (isentropic or pump assumption).
Model Structure#
The core Pressure Changer unit model consists of a single ControlVolume0D
(named control_volume
) with one Inlet Port (named inlet
) and one Outlet Port (named outlet
). Additionally, if an isentropic pressure changer is used, the unit model contains an additional StateBlock
named properties_isentropic
at the unit model level.
Variables#
Pressure Changers contain the following Variables (not including those contained within the control volume Block):
Variable |
Name |
Notes |
---|---|---|
\(P_{ratio}\) |
ratioP |
|
\(V_t\) |
volume |
Only if has_rate_reactions = True, reference to control_volume.rate_reaction_extent |
\(W_{mechanical,t}\) |
work_mechanical |
Reference to control_volume.work * |
\(W_{fluid,t}\) |
work_fluid |
Pump assumption only |
\(\eta_{pump,t}\) |
efficiency_pump |
Pump assumption only |
\(W_{isentropic,t}\) |
work_isentropic |
Isentropic assumption only |
\(\eta_{isentropic,t}\) |
efficiency_isentropic |
Isentropic assumption only |
* By default, control_volume.work
is built only when the EnergyBalanceType
of control_volume
is not None
and has_work_transfer
. However, when modeling a Pump
there are cases where the mechanical work is calculated without demanding a full energy balance constraint. In these cases (when thermodynamic_assumption=ThermodynamicAssumption.pump
and energy_balance_type=none
), a variable control_volume.work
is constructed without being coupled to the usual energy balance constraint. work_mechanical
is then referenced to control_volume.work
and used to build constraints under the add_pump()
method to simulate essential pump properties.
Isentropic Pressure Changers also have an additional Property Block named properties_isentropic (attached to the Unit Model).
Constraints#
In addition to the Constraints written by the Control Volume block, Pressure Changer writes additional Constraints which depend on the thermodynamic assumption chosen. All Pressure Changers add the following Constraint to calculate the pressure ratio:
Isothermal Assumption#
The isothermal assumption writes one additional Constraint:
Adiabatic Assumption#
The isothermal assumption writes one additional Constraint:
Isentropic Assumption#
The isentropic assumption creates an additional set of Property Blocks (indexed by time) for the isentropic fluid calculations (named properties_isentropic). This requires a set of balance equations relating the inlet state to the isentropic conditions, which are shown below:
where \(F_{t,p,j}\) is the flow of component \(j\) in phase \(p\) at time \(t\) and \(s\) is the specific entropy of the fluid at time \(t\).
Next, the isentropic work is calculated as follows:
where \(H_{t,p}\) is the total energy flow of phase \(p\) at time \(t\). Finally, a constraint which relates the fluid work to the actual mechanical work via an efficiency term \(\eta\).
If compressor is True, \(W_{isentropic,t} = W_{mechanical,t} \times \eta_t\)
If compressor is False, \(W_{isentropic,t} \times \eta_t = W_{mechanical,t}\)
Pump (Incompressible Fluid) Assumption#
The incompressible fluid assumption writes two additional constraints. Firstly, a Constraint is written which relates fluid work to the pressure change of the fluid.
where \(F_{vol,t}\) is the total volumetric flowrate of material at time \(t\) (from the outlet Property Block). Secondly, a constraint which relates the fluid work to the actual mechanical work via an efficiency term \(\eta\).
If compressor is True, \(W_{fluid,t} = W_{mechanical,t} \times \eta_t\)
If compressor is False, \(W_{fluid,t} \times \eta_t = W_{mechanical,t}\)
Initialization#
For simpler pressure changer models (isothermal, adiabatic and pump assumptions), the default SingleControlVolumeInitializer is applicable.
For isentropic pressure changers, an alternate IsentropicPressureChangerInitializer is available.
- class idaes.models.unit_models.pressure_changer.IsentropicPressureChangerInitializer(**kwargs)[source]#
Initializer for isentropic pressure changer models (and derived types).
- constraint_tolerance
Tolerance for checking constraint convergence
- output_level
Set output level for logging messages
- solver
Solver to use for initialization
- solver_options
Dict of options to pass to solver
- writer_config
Dict of writer_config arguments to pass to solver
- default_submodel_initializer
Default Initializer object to use for sub-models. Only used if no Initializer defined in submodel_initializers.
- always_estimate_states
Whether initialization routine should estimate values for state variables that already have values. Note that if set to True, this will overwrite any initial guesses provided.
- initialization_routine(model)[source]#
Initialization routine for isentropic pressure changers.
This routine starts by initializing the inlet and outlet states as usual, using the user provided operating conditions to estimate the outlet state. The isentropic state is then initialized at the same conditions as the outlet. Next, the pressure changer is solved with an isothermal assumption and fixed efficiency, followed by a second solve with the isentropic constraints. Finally, if user-provided performance constraints are present, these are activated and the model solved again.
- Parameters:
model (Block) – model to be initialized
- Returns:
Pyomo solver status object
Performance Curves#
Isentropic pressure changers support optional performance curve constraints. The exact form of these constraints is left to the user, but generally the constraints take the form of one or two equations which provide a correlation between head, efficiency, or pressure ratio and mass or volumetric flow. Additional variables such as compressor or turbine speed can be added if needed.
Performance curves should be added to the performance_curve
sub-block rather
than adding them elsewhere because it allows them to be integrated into the
unit model initialization. It also provides standardization for users and
provides a convenient way to turn the performance equations on and off by
activating and deactivating the block.
Usually there are one or two performance curve constraints. Either directly or indirectly, these curves specify an efficiency and pressure drop, so in adding performance curves the efficiency and/or pressure drop should be freed as appropriate.
Performance equations generally are in a simple form (e.g. efficiency = f(flow)), where no special initialization is needed. Performance curves also are specific to a particular property package selection and pressure changer, which allows the performance curve equations to be written in a well-scaled way since units of measure and magnitudes are known.
To add performance curves to an isentropic pressure changer, simply supply the
"support_isentropic_performance_curves": True
options in the pressure
changer config dict. This will create a performance_curve
sub-block of
the pressure changer model. By default this block will have the expressions
head
and heat_isentropic
for convenience, as these quantities often
appear in performance curves.
Two examples are provided below that demonstrate two ways to add performance curves. The first does not use a callback the second does.
from pyomo.environ import ConcreteModel, SolverFactory, units, value
from idaes.core import FlowsheetBlock
from idaes.models.unit_models.pressure_changer import Turbine
from idaes.models.properties import iapws95
import pytest
solver = SolverFactory('ipopt')
m = ConcreteModel()
m.fs = FlowsheetBlock(dynamic=False)
m.fs.properties = iapws95.Iapws95ParameterBlock()
m.fs.unit = Turbine(
property_package=m.fs.properties,
support_isentropic_performance_curves=True
)
# Add performance curves
@m.fs.unit.performance_curve.Constraint(m.fs.config.time)
def pc_isen_eff_eqn(b, t):
# main pressure changer block parent of performance_curve
prnt = b.parent_block()
return prnt.efficiency_isentropic[t] == 0.9
@m.fs.unit.performance_curve.Constraint(m.fs.config.time)
def pc_isen_head_eqn(b, t):
# divide both sides by 1000 for scaling
return b.head_isentropic[t]/1000 == -75530.8/1000*units.J/units.kg
# set inputs
m.fs.unit.inlet.flow_mol[0].fix(1000) # mol/s
Tin = 500 # K
Pin = 1000000 # Pa
Pout = 700000 # Pa
hin = iapws95.htpx(Tin*units.K, Pin*units.Pa)
m.fs.unit.inlet.enth_mol[0].fix(hin)
m.fs.unit.inlet.pressure[0].fix(Pin)
m.fs.unit.initialize()
solver.solve(m, tee=False)
assert value(m.fs.unit.efficiency_isentropic[0]) == pytest.approx(0.9, rel=1e-3)
assert value(m.fs.unit.deltaP[0]) == pytest.approx(-3e5, rel=1e-3)
The next example shows how to use a callback to add performance curves.
from pyomo.environ import ConcreteModel, SolverFactory, units, value
from idaes.core import FlowsheetBlock
from idaes.models.unit_models.pressure_changer import Turbine, IsentropicPressureChangerInitializer
from idaes.models.properties import iapws95
import pytest
solver = SolverFactory('ipopt')
m = ConcreteModel()
m.fs = FlowsheetBlock(dynamic=False)
m.fs.properties = iapws95.Iapws95ParameterBlock()
def perf_callback(blk):
# This callback adds constraints to the performance_cruve block. blk is the
# performance_curve block, but we also want to use quantities from the main
# pressure changer model, which is the parent block.
prnt = blk.parent_block()
# this is the pressure changer model block
@blk.Constraint(m.fs.config.time)
def pc_isen_eff_eqn(b, t):
return prnt.efficiency_isentropic[t] == 0.9
@blk.Constraint(m.fs.config.time)
def pc_isen_head_eqn(b, t):
return b.head_isentropic[t]/1000 == -75530.8/1000*units.J/units.kg
m.fs.unit = Turbine(
property_package=m.fs.properties,
support_isentropic_performance_curves=True,
isentropic_performance_curves={"build_callback": perf_callback}
)
# set inputs
m.fs.unit.inlet.flow_mol[0].fix(1000) # mol/s
Tin = 500 # K
Pin = 1000000 # Pa
Pout = 700000 # Pa
hin = iapws95.htpx(Tin*units.K, Pin*units.Pa)
m.fs.unit.inlet.enth_mol[0].fix(hin)
m.fs.unit.inlet.pressure[0].fix(Pin)
initializer = IsentropicPressureChangerInitializer()
initializer.initialize(m.fs.unit)
solver.solve(m, tee=False)
assert value(m.fs.unit.efficiency_isentropic[0]) == pytest.approx(0.9, rel=1e-3)
assert value(m.fs.unit.deltaP[0]) == pytest.approx(-3e5, rel=1e-3)
PressureChanger Class#
- class idaes.models.unit_models.pressure_changer.PressureChanger(*args, **kwds)#
- Parameters:
rule (function) – A rule function or None. Default rule calls build().
concrete (bool) – If True, make this a toplevel model. Default - False.
ctype (class) –
Pyomo ctype of the block. Default - pyomo.environ.Block
Config args
- dynamic
Indicates whether this model will be dynamic or not, default = useDefault. Valid values: { useDefault - get flag from parent (default = False), True - set as a dynamic model, False - set as a steady-state model.}
- has_holdup
Indicates whether holdup terms should be constructed or not. Must be True if dynamic = True, default - False. Valid values: { useDefault - get flag from parent (default = False), True - construct holdup terms, False - do not construct holdup terms}
- material_balance_type
Indicates what type of mass balance should be constructed, default - MaterialBalanceType.useDefault. Valid values: { MaterialBalanceType.useDefault - refer to property package for default balance type **MaterialBalanceType.none - exclude material balances, MaterialBalanceType.componentPhase - use phase component balances, MaterialBalanceType.componentTotal - use total component balances, MaterialBalanceType.elementTotal - use total element balances, MaterialBalanceType.total - use total material balance.}
- energy_balance_type
Indicates what type of energy balance should be constructed, default - EnergyBalanceType.useDefault. Valid values: { EnergyBalanceType.useDefault - refer to property package for default balance type **EnergyBalanceType.none - exclude energy balances, EnergyBalanceType.enthalpyTotal - single enthalpy balance for material, EnergyBalanceType.enthalpyPhase - enthalpy balances for each phase, EnergyBalanceType.energyTotal - single energy balance for material, EnergyBalanceType.energyPhase - energy balances for each phase.}
- momentum_balance_type
Indicates what type of momentum balance should be constructed, default - MomentumBalanceType.pressureTotal. Valid values: { MomentumBalanceType.none - exclude momentum balances, MomentumBalanceType.pressureTotal - single pressure balance for material, MomentumBalanceType.pressurePhase - pressure balances for each phase, MomentumBalanceType.momentumTotal - single momentum balance for material, MomentumBalanceType.momentumPhase - momentum balances for each phase.}
- has_phase_equilibrium
Indicates whether terms for phase equilibrium should be constructed, default = False. Valid values: { True - include phase equilibrium terms False - exclude phase equilibrium terms.}
- compressor
Indicates whether this unit should be considered a compressor (True (default), pressure increase) or an expander (False, pressure decrease).
- thermodynamic_assumption
Flag to set the thermodynamic assumption to use for the unit. - ThermodynamicAssumption.isothermal (default) - ThermodynamicAssumption.isentropic - ThermodynamicAssumption.pump - ThermodynamicAssumption.adiabatic
- property_package
Property parameter object used to define property calculations, default - useDefault. Valid values: { useDefault - use default package from parent model or flowsheet, PropertyParameterObject - a PropertyParameterBlock object.}
- property_package_args
A ConfigBlock with arguments to be passed to a property block(s) and used when constructing these, default - None. Valid values: { see property package for documentation.}
- support_isentropic_performance_curves
Include a block for performance curves, configure via isentropic_performance_curves.
- isentropic_performance_curves
Configuration dictionary for the performance curve block.
- isentropic_performance_curves
- build_callback
Optional callback to add performance curve constraints
- build_head_expressions
If true add expressions for ‘head’ and ‘head_isentropic’. These expressions can be used in performance curve constraints.
initialize (dict) – ProcessBlockData config for individual elements. Keys are BlockData indexes and values are dictionaries with config arguments as keys.
idx_map (function) – Function to take the index of a BlockData element and return the index in the initialize dict from which to read arguments. This can be provided to override the default behavior of matching the BlockData index exactly to the index in initialize.
- Returns:
(PressureChanger) New instance
PressureChangerData Class#
- class idaes.models.unit_models.pressure_changer.PressureChangerData(component)[source]#
Standard Compressor/Expander Unit Model Class
- add_isentropic()[source]#
Add constraints for isentropic assumption.
- Parameters:
None –
- Returns:
None
- add_isothermal()[source]#
Add constraints for isothermal assumption.
- Parameters:
None –
- Returns:
None
- add_pump()[source]#
Add constraints for the incompressible fluid assumption
- Parameters:
None –
- Returns:
None
- init_adiabatic(state_args, outlvl, solver, optarg)[source]#
Initialization routine for adiabatic pressure changers.
- Keyword Arguments:
state_args – a dict of arguments to be passed to the property package(s) to provide an initial state for initialization (see documentation of the specific property package) (default = {}).
outlvl – sets output level of initialization routine
optarg – solver options dictionary object (default={})
solver – str indicating which solver to use during initialization (default = None)
- Returns:
None
- init_isentropic(state_args, outlvl, solver, optarg)[source]#
Initialization routine for isentropic pressure changers.
- Keyword Arguments:
state_args – a dict of arguments to be passed to the property package(s) to provide an initial state for initialization (see documentation of the specific property package) (default = {}).
outlvl – sets output level of initialization routine
optarg – solver options dictionary object (default={})
solver – str indicating which solver to use during initialization (default = None)
- Returns:
None
- initialize_build(state_args=None, routine=None, outlvl=0, solver=None, optarg=None)[source]#
General wrapper for pressure changer initialization routines
- Keyword Arguments:
routine – str stating which initialization routine to execute * None - use routine matching thermodynamic_assumption * ‘isentropic’ - use isentropic initialization routine * ‘isothermal’ - use isothermal initialization routine
state_args – a dict of arguments to be passed to the property package(s) to provide an initial state for initialization (see documentation of the specific property package) (default = {}).
outlvl – sets output level of initialization routine
optarg – solver options dictionary object (default=None, use default solver options)
solver – str indicating which solver to use during initialization (default = None, use default solver)
- Returns:
None