pair: idaes.models.unit_models.heat_exchanger;HeatExchanger
idaes.models.unit_models.heat_exchanger
The HeatExchanger model can be imported from idaes.models.unit_models, while additional rules and utility functions can be imported from idaes.models.unit_models.heat_exchanger.
The example below demonstrates how to initialize the HeatExchanger model, and override the default temperature difference calculation. This example also demonstrates how to assign custom names to the hot and cold sides of the heat exchanger.
import pyomo.environ as pe # Pyomo environment from idaes.core import FlowsheetBlock, StateBlock from idaes.models.unit_models import HeatExchanger from idaes.models.unit_models.heat_exchanger import delta_temperature_amtd_callback from idaes.models.properties import iapws95
# Create an empty flowsheet and steam property parameter block. model = pe.ConcreteModel() model.fs = FlowsheetBlock(dynamic=False) model.fs.properties = iapws95.Iapws95ParameterBlock()
# Add a Heater model to the flowsheet. model.fs.heat_exchanger = HeatExchanger( delta_temperature_callback=delta_temperature_amtd_callback, hot_side_name="shell", cold_side_name="tube", shell={"property_package": model.fs.properties}, tube={"property_package": model.fs.properties} )
model.fs.heat_exchanger.area.fix(1000) model.fs.heat_exchanger.overall_heat_transfer_coefficient[0].fix(100) model.fs.heat_exchanger.shell_inlet.flow_mol.fix(100) model.fs.heat_exchanger.shell_inlet.pressure.fix(101325) model.fs.heat_exchanger.shell_inlet.enth_mol.fix(4000) model.fs.heat_exchanger.tube_inlet.flow_mol.fix(100) model.fs.heat_exchanger.tube_inlet.pressure.fix(101325) model.fs.heat_exchanger.tube_inlet.enth_mol.fix(3000)
# Initialize the model model.fs.heat_exchanger.initialize()
Aside from the inlet conditions, a heat exchanger model usually has two degrees of freedom, which can be fixed for it to be fully specified. Things that are frequently fixed are two of:
- heat transfer area,
- heat transfer coefficient, or
- temperature approach.
The user may also provide constraints to calculate the heat transfer coefficient.
Note
For model initialization, the in-built routine currently only supports defining two of area, heat transfer coefficient and heat duty as fixed variables. Fixing of temperature differences or driving forces is not currently supported. Also note that it is assumed that if a time-indexed variable (i.e. heat duty) is fixed that it will be fixed at all points in time during initialization. Users wishing to solve models with alternative specifications should first initialize the model using one of the supported options, or write a custom initialization routine for their specification.
The HeatExchanger model contains two ControlVolume0DBlock blocks which are named hot_side and cold_side. These names are configurable using the hot_side_name and cold_side_name configuration arguments, in which case aliases are assigned to the control volumes and associated Ports using the names provided (note that hot_side and cold_side will always work). The sign convention is that duty is positive for heat flowing from the hot side to the cold side. Aside from the sign convention there is no requirement that the hot side be hotter than the cold side, however some formulations for the average temperature driving force may require that hte hot side be hotter than the cold side.
The control volumes are configured the same as the ControlVolume0DBlock in the Heater model <reference_guides/model_libraries/generic/unit_models/heater:Heater>. The HeatExchanger model contains additional constraints that calculate the amount of heat transferred from the hot side to the cold side.
The HeatExchanger has two inlet ports and two outlet ports. By default these are hot_side_inlet, cold_side_inlet, hot_side_outlet, and cold_side_outlet. If the user supplies different hot and cold side names the inlet and outlets are named accordingly.
| Variable | Symbol | Index Sets | Doc |
|---|---|---|---|
| heat_duty | Q | t | Heat transferred from hot side to the cold side |
| area | A | None | Heat transfer area |
| heat_transfer_coefficient | U | t | Heat transfer coefficient |
| delta_temperature | ΔT | t | Temperature difference, defaults to LMTD |
Note: delta_temperature may be either a variable or expression depending on the callback used. If the specified cold side is hotter than the specified hot side this value will be negative.
The default constants can be overridden by providing alternative rules <reference_guides/model_libraries/generic/unit_models/heat_exchanger:Callbacks> for the heat transfer equation, temperature difference, and heat transfer coefficient. The section describes the default constraints.
Heat transfer from shell to tube:
Q = UAΔT
Temperature difference is an expression:
The heat transfer coefficient is a variable with no associated constraints by default.
Note
The hot_side and cold_side can also be supplied using the name of the hot and cold sides as in the example <reference_guides/model_libraries/generic/unit_models/heat_exchanger:Example>.
HeatExchanger
HeatExchangerData
A selection of functions for constructing the delta_temperature variable or expression are provided in the idaes.models.unit_models.heat_exchanger module. The user may also provide their own function. These callbacks should all take one argument (the HeatExchanger block). With the block argument, the function can add any additional variables, constraints, and expressions needed. The only requirement is that either a variable or expression called delta_temperature must be added to the block.
These callbacks provide expressions for the temperature difference used in the heat transfer equations. There is a choice of three forms for the LMTD calculation. Depending on the expected approach temperatures, one form may be more favorable. The first two forms do not require one side or the other to be the hot side, while the third form requires the hot side to be the hot side to avoid an evaluation error in the log function.
delta_temperature_lmtd_callback
delta_temperature_lmtd2_callback
delta_temperature_lmtd3_callback
delta_temperature_amtd_callback
delta_temperature_underwood_callback
delta_temperature_lmtd_smooth_callback