Heat Exchanger (G)

Parameters defining the mutual arrangement of the flows in the heat exchanger: direct flow, countercurrent, across each other (transverse), as well as the "pipe in shell" design, in which one flow passes inside the pipes and the other flows outside, in the shell.

Alternative flow patterns can be specified in an arbitrary efficiency table, which does not require a detailed specification of the heat exchanger.

The resistance of a wall to heat flow due to heat conduction and the inverse of thermal conductivity, or the product of thermal conductivity by the ratio of surface area to length. The wall resistance is added together with the convective and fouling resistance to determine the total heat transfer coefficient between the flows.

Number of flow passages through the shell in a shell and tube heat exchanger.

Dependencies

To use this parameter, set the parameters Flow arrangement to . Shell and tube.

The type of mixing of the heat transfer fluids in each duct. Mixing in this context is the lateral movement of the heat transfer fluid as it travels along the duct to the outlet. The flows remain separate from each other. Non-mixing flows are often found in channels with plates, baffles, or fins. This characteristic affects the efficiency of the heat exchanger: unmixed flows are most efficient and mixed flows are less efficient.

Dependencies

To use this parameter, set the parameter Flow arrangement to . Shell and tube.

The number of heat transfer units at each anchor point in the heat exchanger efficiency search table. The table is two-dimensional and the independent coordinates are the number of heat transfer units and the heat capacity factor. The block interpolates and extrapolates the reference points to determine the efficiency at any value of the number of transfer units. Interpolation is done using a linear function and extrapolation is done to the nearest value.

The specified numbers must be greater than zero and monotonically increasing from left to right. The dimensionality of this vector must correspond to the number of rows in the table Effectiveness table, E(NTU,CR). If the table has rows and columns, the vector for the number of carry units must be the length of elements.

Dependencies

To use this parameter, set the Flow arrangement parameters to Generic - effectiveness table.

The heat capacity coefficient values corresponding to the reference points in the heat exchanger efficiency table. The table is two-dimensional and the independent coordinates are the number of heat transfer units and the heat capacity factor. The block interpolates and extrapolates the reference points to determine the efficiency at any value of the heat capacity coefficient. Interpolation is done using a linear function, and extrapolation is done to the nearest value.

The coefficients must be positive and strictly increasing from left to right. The dimensionality of the vector should correspond to the number of columns in the table Effectiveness table, E(NTU,CR). If the table has rows and columns, the vector of heat capacity coefficients should be the length of elements.

The heat capacity coefficient is the ratio of the minimum and maximum values of the flux heat capacity.

Dependencies

To use this parameter, set the parameter Flow arrangement to . Generic - effectiveness table.

Values of heat exchanger efficiency in reference points of a two-dimensional table specified by coordinates: number of heat transfer units and heat capacity coefficient. The block interpolates and extrapolates the table values to determine the efficiency at arbitrary combinations of these parameters. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The efficiency values must be non-negative. They should be ordered by rows in the order of increasing number of transfer units (from top to bottom), and by columns - in the order of increasing heat capacity coefficient (from left to right). The number of rows should correspond to the dimensionality of the vector Number of heat transfer units vector, NTU, and the number of columns to the dimensionality of the vector Thermal capacity ratio vector, CR.

Dependencies

To use this parameter, set the Flow arrangement parameter to the value of Generic - effectiveness table.

The minimum cross-sectional area of the channel through which the heat transfer fluid flows, between the inlet and outlet. If it is a set of channels, tubes, slots or grooves, the parameters value is defined as the sum of the smallest areas at the point of minimum flow area.

The effective internal diameter of the channel at the cross-section with the smallest area. For non-circular channels, the hydraulic diameter is the equivalent diameter of a circle with an area equal to the area of the existing channel. Its value is equal to the ratio of the minimum cross-sectional area of the channel to one quarter of its total perimeter.

If the channel is given by a set of channels, pipes, slots or grooves, the total perimeter is equal to the sum of the perimeters of all elements. If the channel is a circular pipe, its hydraulic diameter is equal to its actual diameter.

Total volume of heat transfer medium contained in the gas channel.

Reynolds number value corresponding to the lower boundary of the transition zone between laminar and turbulent flow regimes. Above this value, inertial forces begin to dominate, resulting in a transition from laminar to turbulent flow. The value by default corresponds to a round pipe with a smooth inner surface.

The Reynolds number value corresponding to the upper boundary of the transition zone between laminar and turbulent flow regimes. Below this value, viscous forces begin to dominate, resulting in a transition from turbulent to laminar flow. The By default value corresponds to a round pipe with a smooth inner surface.

Parameters allows you to select one of the models for calculating pressure losses due to viscous friction. The parameter defines which expressions will be used in the calculation of losses and which block parameters should be set as input. Details of the calculations depending on the selected parameterization are given in the block Heat Exchanger Interface (G).

The total loss coefficient that takes into account all hydraulic resistance to flow in the channel, including wall friction losses (major losses) and localised resistance due to bends, elbows and other geometry changes (minor losses).

The loss coefficient is an empirical dimensionless quantity widely used to describe pressure losses due to viscous friction. It can be calculated from experimental data or, in some cases, obtained from technical documentation.

Dependencies

To use this parameter, set the parameter Pressure loss model to . Pressure loss coefficient.

Mathematical model for heat transfer between the heat transfer medium and the wall. The choice of model determines which expressions to apply and which parameters to specify for heat transfer calculations.

For more details see the block E-NTU Heat Transfer.

Effective surface area used in heat transfer between the heat transfer medium and the wall. The effective surface area is the sum of the primary and secondary surface areas, the area where the wall is exposed to the fluid, and the fin area, if used. The fin surface area is usually calculated from the fin efficiency factor.

Heat transfer coefficient for convection between gas and wall. Resistance caused by deposits is accounted for separately in the parameters Fouling factor.

Dependencies

To use this parameter, set the Heat transfer coefficient model parameters to . Constant heat transfer coefficient.

Thermal resistance due to deposits that form over time on exposed wall surfaces. Deposits, because they create a new solid layer between the heat transfer medium and the wall through which heat must pass, add additional thermal resistance to the heat transfer path. The deposits grow slowly and the resistance caused by them is accordingly assumed to be constant during the simulation.

The mass flow rate below which numerical smoothing is applied. This is to avoid discontinuities when the flow is stagnant.

Lower limit for the heat transfer coefficient between gas and wall. If the calculation gives a lower heat transfer coefficient, the value Minimum fluid-wall heat transfer coefficient replaces the calculated value.

The characteristic length travelled during heat transfer between the gas and the wall. This length is taken into account in the calculation of the hydraulic diameter, on which the heat transfer coefficient and the Reynolds number in the tabulated heat transfer parameterizations depend.

Dependencies

To use this parameter, set the parameters Heat transfer coefficient model to the value of Tabulated data - Colburn factor vs. Reynolds number or Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Constant value of Nusselt number for laminar flows. The Nusselt number is required to calculate the heat transfer coefficient between the heat transfer medium and the wall. The value by default corresponds to a cylindrical pipe.

Dependencies

To use this parameter, set the parameter Heat transfer coefficient model to the value of Correlation for flow inside tubes.

Reynolds number at each reference point of the Colburn factor lookup table. The block performs inter- and extrapolation of the table values to determine the Colburn factor at any Reynolds number. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The values of Reynolds numbers must be greater than zero and monotonically increasing from left to right. They can cover laminar, transient and turbulent regimes. The dimensionality of this vector should correspond to the dimensionality of the vector Colburn factor vector for the calculation of tabulated reference points.

Dependencies

To use this parameter, set the Heat transfer coefficient model parameters to Tabulated data - Colburn factor vs. Reynolds number.

Colburn factor at each reference point of the Reynolds number lookup table. The block interpolates and extrapolates the table values to determine the Reynolds number at any Colburn factor. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The Colburn factor values must not be negative and must line up from left to right in ascending order of the corresponding Reynolds numbers. The dimensionality of this vector should correspond to the dimensionality of the vector Reynolds number vector for Colburn factor for the calculation of tabulated reference points.

Dependencies

To use this parameter, set the Heat transfer coefficient model parameter to Tabulated data - Colburn factor vs. Reynolds number.

Reynolds number at each anchor point of the Nusselt number lookup table. The table is two-parameter, where Reynolds and Prandtl numbers are used as independent coordinates. The block performs inter- and extrapolation of the table values to determine the Nusselt number at any Reynolds number. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The values of Reynolds numbers must be greater than zero and monotonically increasing from left to right. They can cover laminar, transient and turbulent regimes. The dimensionality of this vector should correspond to the number of rows in the table Nusselt number table, Nu(Re,Pr). If the table has rows and columns, the Reynolds number vector must be of length elements.

Dependencies

To use this parameter, set the parameter Heat transfer coefficient model to Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Prandtl number at each anchor point of the Nusselt number lookup table. The table is two-parameter, where Reynolds and Prandtl numbers are used as independent coordinates. The block performs inter- and extrapolation of the table values to determine the Nusselt number at any Prandtl number. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The values of Prandtl numbers must be greater than zero and monotonically increasing from left to right. They can cover laminar, transient and turbulent regimes. The dimensionality of this vector should correspond to the number of columns in the table Nusselt number table, Nu(Re,Pr). If the table has rows and columns, the Prandtl number vector must be of length elements.

Dependencies

To use this parameter, set the parameter Heat transfer coefficient model to Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Nusselt number at each reference point of the Reynolds-Prandtl number search table. The block interpolates and extrapolates the table values to determine the Nusselt number at any pair of Reynolds-Prandtl numbers. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value. By determining the Nusselt number, the table provides the data for the calculation from which the heat transfer coefficient between the fluid and the wall is determined.

The Nusselt number must be greater than zero. Each value should be arranged from top to bottom in order of increasing Reynolds numbers and from left to right in order of increasing Prandtl numbers. The number of rows should be equal to the dimensionality of the vector Reynolds number vector for Nusselt number, and the number of columns should be equal to the dimensionality of the vector Prandtl number vector for Nusselt number.

Dependencies

To use this parameter, set the parameter Heat transfer coefficient model to the value of Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

The total distance the flow must travel between ports. In multi-pass shell-and-tube heat exchangers, the total distance is the sum of all the passes through the shell. In tube bundles, corrugated plates and other ducts where the flow is divided into parallel branches, it is the distance travelled per branch. The longer the flow path, the greater the basic pressure loss due to viscous friction against the walls.

Dependencies

To use this parameter, set the parameters Pressure loss model to the value of Correlation for flow inside tubes or Tabulated data - Darcy friction factor vs. Reynolds number.

Total local pressure losses expressed in length. The length of the straight duct results in equivalent losses equal to the sum of the existing local resistances of branches, tees and connections. The greater the equivalent length, the higher the pressure losses due to local resistances.

Dependencies

To use this parameter, set the parameter Pressure loss model to . Correlation for flow inside tubes.

The average height of the roughnesses on the wall surface that result in viscous friction losses. The greater the average height, the rougher the wall and the greater the pressure loss due to viscous friction. The surface roughness value is used to derive the Darcy friction coefficient from the Haaland relationship.

Dependencies

To use this parameter, set the parameter Pressure loss model to the value of Correlation for flow inside tubes.

Pressure loss correction for laminar flow. This parameter is called the shape coefficient and can be used to obtain the Darcy friction coefficient for laminar pressure loss calculations. The By default value corresponds to cylindrical pipes.

Some additional shape coefficients for non-circular cross sections can be determined from analytical solutions of the Navier-Stokes equations. A duct with a square cross-section has a shape factor of 56, a duct with a rectangular cross-section with an aspect ratio of 2:1 has a shape factor of 62, and a coaxial pipe has a shape factor of 96. A thin duct between parallel plates also has a shape factor of 96.

Dependencies

To use this parameter, set the parameter Pressure loss model to the value of Correlation for flow inside tubes.

Reynolds number at each reference point of the Darcy friction coefficient search table. The block interpolates and extrapolates the table values to determine the Darcy friction coefficient at any Reynolds number. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The values of Reynolds numbers must be greater than zero and monotonically increasing from left to right. They can cover laminar, transient and turbulent regimes. The dimensionality of this vector should correspond to the dimensionality of the vector Darcy friction factor vector for the calculation of tabulated reference points.

Dependencies

To use this parameter, set the Pressure loss model parameters to Tabulated data - Darcy friction factor vs. Reynolds number.

Darcy friction coefficient at each reference point in the Reynolds number lookup table. The block interpolates and extrapolates the table values to determine the Darcy friction coefficient at any Reynolds number. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The values of the Darcy friction coefficient shall not be negative and shall line up from left to right in ascending order of the corresponding Reynolds numbers. The dimensionality of this vector should correspond to the dimensionality of the vector Reynolds number vector for Darcy friction factor for the calculation of tabulated reference points.

Dependencies

To use this parameter, set the Pressure loss model parameter to Tabulated data - Darcy friction factor vs. Reynolds number.

Reynolds number at each reference point of the Euler number search table. The block performs inter- and extrapolation of the table values to determine the Euler number at any Reynolds number. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The values of the Reynolds numbers must be greater than zero and monotonically increasing from left to right. They can cover laminar, transient and turbulent regimes. The dimensionality of this vector should correspond to the dimensionality of the vector Euler number vector for the calculation of tabulated reference points.

Dependencies

To use this parameter, set the Pressure loss model parameters to Tabulated data - Euler number vs. Reynolds number.

Euler number at each reference point of the Reynolds number search table. The block interpolates and extrapolates the table values to determine the Reynolds number at any Euler number. Interpolation is performed using a linear function, and extrapolation is performed to the nearest value.

The values of the Darcy friction coefficient shall not be negative and shall line up from left to right in ascending order of the corresponding Reynolds numbers. The dimensionality of this vector should correspond to the dimensionality of the vector Reynolds number vector for Euler number for the calculation of tabulated reference points.

Dependencies

To use this parameter, set the Pressure loss model parameters to Tabulated data - Euler number vs. Reynolds number.

Effective surface area used in heat transfer between the heat transfer medium and the wall. The effective surface area is the sum of the primary and secondary surface areas, the area where the wall is exposed to the fluid, and the fin area, if used. The fin surface area is usually calculated from the fin efficiency factor.

Thermal resistance due to deposits that form over time on exposed wall surfaces. Deposits, because they create a new solid layer between the heat transfer medium and the wall through which heat must pass, add additional thermal resistance to the heat transfer path. The deposits grow slowly and the resistance caused by them is accordingly assumed to be constant during the simulation.

Lower limit for the heat transfer coefficient between the heat transfer medium and the wall. If the calculation gives a lower heat transfer coefficient, the value Minimum fluid-wall heat transfer coefficient replaces the calculated value.

Gas temperature in the channel at the beginning of the simulation.

Gas pressure in the channel at the beginning of the simulation.