Heat Exchanger

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General Description

The Heat Exchanger is general purpose unit used to transfer energy from one stream to another. It is primarily used for sensible heat exchange between two fluids without any phase changes. As well, it can also be used to transfer latent heat from a condensing vapour stream, currently only steam, to a liquor stream. The liquor stream may contain liquids and solids.

This unit is mainly used in the Stand Alone mode. If the user wishes to have a heat exchanger as part of a Flash Train, then it is recommended to use either the Shell and Tube Heat Exchanger or the Shell and Tube Heat Exchanger 2. This documentation will only discuss the variables for a 'stand alone' heat exchanger.

An Environmental Heat Loss may be included in the unit. This allows the user to specify a heat loss, or gain, between the unit and the environment.

Notes:

  1. The Boiler & Combustion Project, which is distributed with SysCAD in the Examples folder, demonstrates the use of this model.
  2. If the heat exchanger is correctly connected to other units such as Flash Tanks, the model may become part of the entire Flash Train. However, this is NOT recommended, as the Shell and Tube Heat Exchanger or the Shell and Tube Heat Exchanger 2 will be more suitable in this application. If the Heat Exchanger is included in a flash train, the steam line must be connected to the 'Secondary In'.

Diagram

Models-Heat-Exchanger-image001.gif

The diagram shows the default drawing of the Heat Exchanger, with the required connecting streams. The user may also connect a vent stream to the unit. This is optional and allows non-condensable and excess steam to be removed from the unit.

The physical location of the connections is not important; the user may connect the streams to any position on the drawing. When the user inserts a heat exchanger into a flowsheet, he may choose a different drawing from a pull down menu.

Inputs and Outputs

Label Required
Optional
Input
Output
Number of Connections Description
Min Max.

PriIn

1 Required

In

1

20

The Process Stream to the unit.

PriOut

Required

Out

1

1

The Process Stream outlet

SecIn

1 Required

In

1

20

The cooling or heating fluid. If the unit is to be part of a Flash Train, the steam inlet must be connected to this inlet.

SecOut

Required

Out

1

1

The cooling or heating fluid outlet.

PVent

Optional

Out

0

1

The Vent stream from the Primary Side. Note: Only one vent stream can be connected to the unit, either to the Primary or the secondary side, but NOT both.

SVent

Optional

Out

0

1

The Vent stream from the Secondary Side. Note: Only one vent stream can be connected to the unit, either to the Primary or the secondary side, but NOT both.


Note: Incoming streams to the same connection label are perfectly mixed before any unit operations are performed.

Behaviour when Model is OFF

If the user disables the unit, by un-ticking the On tick box, then the following actions occur:

  • All streams connected to 'Primary' inlet will flow straight out of 'Primary' outlet with no change in temperature;
  • All streams connected to 'Secondary' inlet will flow straight out of 'Secondary' outlet with no change in temperature;
  • No energy exchange will occur between the Primary and Secondary streams.

So basically, the unit will be 'bypassed' without the user having to change any connections.


Model Theory

The unit is based on traditional heat exchanger theory1,

[math]\mathbf{\mathit{Q=UA\boldsymbol{\Delta}T_{LM}}}[/math]
where
Q - Rate of Heat Transfer
U - Overall coefficient of Heat Transfer
A - Area available for Heat Transfer
[math]\mathbf{\mathit{\boldsymbol{\Delta}T_{LM} = \frac{\Delta T_2 -\Delta T_1}{ln(\frac{\Delta T_2}{\Delta T_1})}}}[/math] - Log Mean Temperature Difference (LMTD)
For Counter Current Flow [math] \Delta T_2 = T_{H_{in}} - T_{C_{out}} [/math] and [math] \Delta T_1 = T_{H_{out}} - T_{C_{in}} [/math]

Notes:

  1. If the flow through the heat exchanger is not completely counter current, then user must input a LMTD correction factor to correct for the different flow configuration. These correction factors are available in most references on Heat Transfer theory, and should be available from specific heat exchanger suppliers.
  2. If the heat exchanger has Superheated Steam condensing on the shell side, then the LMTD method will produce small inaccuracies, please see Log Mean Temperature Difference (LMTD) Discussion.


Heat transfer to the individual streams is calculated using the following equation:

[math]\mathbf{\mathit{Q=m(H_{in}-H_{out})}}[/math]
where
Q - Rate of Heat Transfer
m - Mass flow of the stream
Hin - Enthalpy of entering stream
Hout - Enthalpy of leaving stream

Using the stream enthalpies in the heat transfer calculations ensures that the variation of specific heat with temperature is taken into consideration.

In the case of one of the streams condensing the heat transfer is based on the assumption that the vapour is condensed at the saturation temperature. The condensate leaves the unit at this temperature, i.e. there is no further cooling of the liquid. If the vapour enters the unit above the saturation temperature, it will be cooled to the saturation temperature and then condensed.

The unit uses an iterative technique to determine the LTMD of the unit. This is then used to calculate the heat transfer between the two streams.

Assumptions

  1. The overall heat transfer coefficient remains constant throughout the unit.
  2. The condensate is not supercooled.
  3. The flows through the heat exchanger are essentially counter current.

Reference

Perry, R.H., Perry's Chemical Engineers' Handbook, McGraw Hill Inc, 6th Edition, 1984.

Flowchart

The following shows the sequence of events if sub model options are switched on. See next heading for more information.


Models-Heat-Exchanger-image010.gif

Data Sections

The default access window consists of several sections,

  1. HeatExchanger1 tab - This first tab contains general information relating to the unit.
  2. HX tab - This second tab contains data relating to the 'Primary' and 'Secondary' side of the Heat Exchanger.
  3. Vapour Liquid Equilibrium (VLE) tab - Optional tab. Contains information relating to 'Vapour Liquids Equilibrium'.
  4. Environmental Heat Exchanger (EHX) - Optional tab. Contains information relating to the heat transfer to the environement.
  5. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  6. Links tab, contains a summary table for all the input and output streams.
  7. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.


Heat Exchanger Page

Unit Type: HeatExchanger1 - The first tab page in the access window will have this name.

Tag (Long/Short) Input / Calc Description

Common Data on First Tab Page

Requirements

PriDPOn Tick Box This can be used to switch on pressure drop across the primary side of the heat exchanger.
SecDPOn Tick Box This can be used to switch on pressure drop across the secondary side of the heat exchanger.
EnvironHX Tick Box This can be used to switch on Environmental Heat Exchanger (EHX). Note: The user does not have to configure an environmental heat exchange, even if this block is checked.

Heat Exchange (HX)

On Tick Box This is used to enable or disable the unit. If the unit is disabled, then there is no heat transfer between the two streams.
OpMode Inoperative This disables the heat exchanger. There will be no heat exchange between the two streams.
Liquor/Liquor The unit will expect both streams to consist of liquor (liquids only, or liquids and solids). Energy transfer will occur via sensible heat exchange only. If one of the streams does consist of vapours, the vapour will NOT condense.
Liquor/Gas The unit expects the stream that is connected to the SecIn to consist of gas. However, the vapours will NOT condense.
Gas/Gas The unit expects both streams to consist of gases only. Neither stream will condense.
Fully Condensing If this unit is required as part of a Flash Train then the Flashed Steam must be connected to the SecIn. The unit will attempt to condense all of the vapour at the saturation temperature. If there is an excess of vapour the unit will send the excess to the vent, regardless of if there is a vent stream configured.
OpMode.Actual Output This variable will show the last operating mode before the solver is started. The unit then determines which of the above modes the exchanger will emulate and displays the mode here.
HTC Input The required overall Heat Transfer Coefficient of the heat exchanger.
Area Input The area available for heat transfer.
U * A Calc The product of the above two numbers.
LMTD Calc The calculated Log Mean Temperature Difference.
Duty Calc The calculated duty of the heat exchanger
LMTDFact Input The LMTD factor of the heat exchanger. This is usually 100%.
VentMassFlowReqd / VentQmReqd Input The required amount of non-condensable or steam which is lost to the vent. Note The unit calculates the amount of steam that can be condensed and sends the excess steam to the vent. If this number is greater than the required steam loss, the unit will send the calculated amount of steam to the vent, and not the user required flow.

Primary (Tube side)

Mode
(Based on OpMode)
Sensible Heat transfer with no phase change.
Condensing Heat transfer will involve change from vapour to liquid phase.
MassFlow / Qm Display The mass flow through the primary side of the heat exchanger
Cp Display The specific heat of the primary stream
TemperatureIn / Ti Calc The temperature of the primary stream entering the unit.
TemperatureOut / To Calc The temperature of the primary stream leaving the heat exchanger.
DeltaT / dT Calc The difference in temperature between the entering and leaving streams.
PressureIn / Pi Calc The pressure of the primary stream entering the unit.
PressureOut / Po Calc The pressure of the primary stream leaving the heat exchanger.
SatT@P Calc The saturated temperature of the primary stream at stream pressure.
SatP@T Calc The saturated pressure of the primary stream at stream temperature.
SatPP@T Calc The saturated partial pressure of the primary stream at stream temperature.
PPFrac Calc The partial pressure fraction.
Duty Calc Calculated rate of heat transfer.

Secondary (Shell side)

Mode
(Based on OpMode)
Sensible Heat transfer with no phase change.
Condensing Heat transfer will involve change from vapour to liquid phase.
MassFlow / Qm Display The mass flow through the secondary side of the heat exchanger
Cp Display The specific heat of the secondary stream
TemperatureIn / Ti Display The temperature of the secondary stream entering the unit.
TemperatureOut / To Calc The temperature of the secondary stream leaving the heat exchanger.
DeltaT / dT Calc The difference in temperature between the entering and leaving streams.
PressureIn / Pi Calc The pressure of the secondary stream entering the unit.
PressureOut / Po Calc The pressure of the secondary stream leaving the heat exchanger.
SatT@P Calc The saturated temperature of the secondary stream at stream pressure.
SatP@T Calc The saturated pressure of the secondary stream at stream temperature.
SatPP@T Calc The saturated partial pressure of the secondary stream at stream temperature.
PPFrac Calc The partial pressure fraction.
Duty Calc Calculated rate of heat transfer.

Last Vent and Qm Sections

These sections show the physical properties of the stream flowing out of the vent. This stream may have flow whether the vent stream is connected to the unit or not. If no vent stream is connected and there is flow in the vent, it will be 'lost' to atmosphere.

The user either specifies this stream, or it is the non-condensable or excess steam as calculated by the unit. If the user specifies a vent stream and the unit calculates a larger vent stream, the larger number will be vented. In this case the unit will flag the user that the specified conditions cannot be met.

The variables shown on these pages are identical to those shown for any normal pipe. Please refer to the Material Flow Properties for a pipe for a description of the variables.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 

DLL:

HeatXch1.dll

Units/Links

Heat Transfer: Heat Exchanger(1)

or

Group:

Energy Transfer

Units/Links

Heat Transfer: Heat Exchanger(1)

See Project Configuration for more information on adding models to the configuration file.


Insert into Project

 

Insert Unit

Heat Transfer

Heat Exchanger(1)

See Insert Unit for general information on inserting units.

Hints and Comments

General Configuration Hints:

  1. We do not recommend using the heat exchanger as part of a Flash Train, as it may introduce instability to the macro model. However, if you do want to use it for this application, the steam MUST be connected to the secondary side and the unit operation mode must be set to FullyCondensing.
  2. Ensure that the HTC and Area are correct. If these variables are not configured, the heat exchanger will not operate as expected.
  3. The Environmental Heat Transfer occurs between the shell side fluid and the environment.
  4. The user MUST specify FullyCondensing if the unit is required to condense the steam entering the unit.
  5. The unit has a problem with excess vapour in the unit. Vent lines are not to be used until the problem has been fixed.
  6. Make sure there are only ONE (1) PriOut and ONE (1) SecOut stream connected to the model.

Hints on using a Heat Exchanger:

a) What we normally use our heat exchanger for in ProBal mode is one of the following:

  • To help size the heat exchanger area with known flowrates. - In this case, we will specify the Steam flowrate, Steam properties such as T and P, HTC and have a "controller" calculate the HX area to achieve a certain cooling water outlet temperature (To) or LMTD.
  • To find out the steam requirements based on known heat exchanger size and steam properties. - In this case, we will input HX HTC & area, as well as the steam properties (T & P), then through a controller, work out the steam flowrate to achieve a certain cooling water outlet temperature To.
  • As part of the Flash Train (Note that when the HX is set up as part of a flash train, it behaves differently in that you can not "set" any flows to the HX, the flow is governed by the size of the HX.)
This is what the heat exchanger meant to do in ProBal mode without you having to add in bits of pgm codes to do other clever things. (The pgm is a built-in language much like Visual basic is for Excel. It is used to extend the functionality of the model. For more help on the pgm language, please refer to pgm help)

b) In ProBal mode, if you are using a stand-alone heater, you must control the steam flow by setting the Steam flowrate - Qm_Rqd either through a controller or via the PGM for variable flows. Changing the pressure or temperature of the Steam in the feeder unit would not change the steam flowrate without the use of a pgm file.

c) The pressure you set at the feeder is the pressure of the steam supply, likewise with the temperature. This is not to be confused with the pressure drop exerted by the pipes and valves. If you want to simulate a pressure drop in a line (this will include any bends and valves in the pipe) you may do so in the pipe model - under the first tab page there is a field called Press_Mode. We normally do not use this in a stand alone heat exchanger as we can achieve the required flowrate by setting the Qm_Rqd. However, it would be of use if the heat exchanger is part of a</span> Flash Train, since the flash train does take into account the pressure network when working out the flash pressures etc.

Comments on Heat Balance around the Heater:

The HX LMTD will contain a small error when the HX is in Fully condensing mode with superheated steam as its feed. Please see Log Mean Temperature Difference (LMTD) Discussion for further information.