Shell and Tube Heat Exchanger

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General Description

The Shell and Tube Heat Exchanger is used to transfer energy from one stream to another. It is primarily used to transfer latent heat from a condensing vapour stream, typically steam, to the tube side stream.

The unit may also be used for sensible heat exchange between two streams without any phase changes. If this unit is used in this mode, the HOT stream must be connected to the SHELL side of the heat exchanger.

Finally, the unit can also be used to transfer latent heat from an evaporating liquid stream, typically water, to the tube side stream.

There are two operational modes for the Shell and Tube Heat Exchanger:

1. As a stand-alone unit, or
2. As part of a Flash Train - when condensing.

The operational mode is decided by the overall configuration of the flowsheet in which the unit is located. If the heat exchanger is correctly connected to other units such as Barometric Condensers, Heat Exchangers and Flash Tanks, the model may become part of the entire Flash Train. The user does not have to specify that the unit is part of the Flash Train, SysCAD will do this automatically. Refer to Flash Train for the rules and theory governing this behaviour.

For condensing and evaporating modes, H2O is the usual component that is condensed or evaporated, however alternate components can be selected (example NH3) by selecting the component in the VLE page.

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.

Diagram

The diagram shows the default drawing of the shell and tube 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 vapour (typically 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, they may choose a different drawing from a pull down menu.

Inputs and Outputs

 Label RequiredOptional InputOutput Number of Connections Description Min Max. Tube_In 1 Required In 1 20 The liquid or slurry feed to the unit. Note: In the case where users wish to use this unit for Liq/Liq or Liq/Gas, Gas/Gas heat exchange, the COLD side must be connected to this point. Tube_Out Required Out 1 1 The liquid or slurry outlet. Note: In the case where users wish to use this unit for Liq/Liq or Liq/Gas, Gas/Gas heat exchange, this is the COLD side outlet. Shell_In 1 Required In 1 20 For Fully Condensing Mode, this is the steam inlet. For Fully Evaporating Mode, this is the liquid to be evaporated. Note: In the case where users wish to use this unit for Liq/Liq or Liq/Gas, Gas/Gas heat exchange, the HOT side must be connected to this point. Shell_Out Required Out 1 1 For Fully Condensing Mode, this is the condensate outlet. For the Fully Evaporating Mode, this is the vapour outlet. Note: In the case where users wish to use this unit for Liq/Liq or Liq/Gas, Gas/Gas heat exchange, this is the HOT side outlet. Vent Optional Out 0 1 Optional vent for non-condensable and excess steam.

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 'Shell In' will flow straight out of 'Shell Out' with no change in temperature or phase;
• All streams connected to 'Tube In' will flow straight out of 'Tube Out' with no change in temperature;
• No energy exchange will occur between the Shell and Tube.

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,

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

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:

$\mathbf{\mathit{Q=m(H_{in}-H_{out})}}$
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 LMTD of the unit. This is then used to calculate the heat transfer between the two streams.

Flowchart

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

Data Sections

The default access window consists of several sections:

1. ShellTube1 tab - This first tab contains general information relating to the unit.
2. Results page - This second tab contains result fields for the shell and tube sides, and for the Flash Train Macro model, if the unit is part of a flash train.
3. VLE - Only visible if the unit is in either Fully Condensing or Fully Evaporating mode.
4. EHX - Optional tab, only visible if the EnvironHX is enabled on the first page.
5. QVent - The pages related to the vent will contain data on the flow from the vent. Note: The vent will only have flow if specified by the user.
6. Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
7. Links tab, contains a summary table for all the input and output streams.
8. Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Shell and Tube Heat Exchanger Page

Unit Type: ShellTube1 - The first tab page in the access window will have this name.

Tag (Long/short)

Input / Calc

Description/Calculated Variables / Options

Requirements

EnvironHX Tick Box Environmental Heat Exchanger (EHX) is enabled here. If this is 'On' then the associated page, EHX will become visible and may be configured.
Note: The user does not have to configure an environmental heat exchange, even if this block is checked.
If Heat Loss is enabled, then less heat will be available for heat transfer with the tube side.
In the case of flash train configuration, more steam is required to satisfy the heater requirements.
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. However, if the Environmental Heat Exchanger is enabled, then this will still function.
OpMode Inoperative This disables the heat exchanger. There will be no heat exchange between the two streams.
Liquor/Liquor Energy transfer will occur via sensible heat exchange only. Note: The HOT side must be connected to the "Shell_in" Inlet. Please see Hints and Comments .
Liquor/Gas Energy transfer will occur via sensible heat exchange only, thus, gas phase will not condense. Note: The HOT side must be connected to the "shell_in" Inlet, regardless if it is the gas phase or not. Please see Hints and Comments .
Gas/Gas Energy transfer will occur via sensible heat exchange only. Neither stream will condense. Note: The HOT side must be connected to the "Shell_in" Inlet. Please see Hints and Comments .
Fully Condensing The stream connected to the "Shell_in" should consist of steam. The unit will attempt to condense all of the steam at the saturation temperature. If there is an excess of steam the unit will send this steam to the vent, regardless of if there is a vent stream configured.
Fully Evaporating The stream connected to the "Shell_in" inlet should consist of colder Liquid to be fully evaporated, the stream connected to the "tube_in" should be consist of hotter liquid to provide the evaporation energy. The unit will attempt to evaporate all of the liquid at the saturation temperature. If the heat exchanger area provided is not big enough for the evaporation, SysCAD will display the required area under the field "TheorArea". If the "Tube_in" stream is not able to provide enough energy to fully evaporated the "Shell_in" Stream, thus resulting in temperature cross over, the unit operation will stop working and display a RED status.

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.

ApproachDT

Input

The minimum temperature difference allowed between either inlet stream and the opposing outlet stream. For example, if the tube side inlet temperature was 20 deg C, and the ApproachDT was 10 deg C, then the shell side outlet temperature would be limited to a minimum of 30 deg C. This field is NOT visible if Fully Condensing is the Operating Mode.

U * A

Calc

U * A = HTC * Area

LMTD

Calc

The calculated Log Mean Temperature Difference. Please see Log Mean Temperature Difference (LMTD) Discussion if superheated steam is condensing on the Shell side.

TheoreticalDuty / TheorDuty

Calc

The calculated theoretical duty. User should check if this is the same as the Duty. A discrepancy between the two normally suggests that the heater is not sized to handle its given flow. (Please see Log Mean Temperature Difference (LMTD) Discussion if superheated steam is condensing on the Shell side.)

Duty

Calc

The calculated duty of the heat exchanger

TheoreticalArea / TheorArea

Calc

The calculated theoretical Area based on duty. Ideally the heater should be sized so that duty = theoretical duty. The user may use this for the new heater area and to check the results.

LMTDFact

Input

The LMTD factor of the heat exchanger. This is usually 100%, but can vary for different heater configurations.

CondenseAll Tick Box If this box is ticked, then SysCAD will fully condense ALL the vapour that enters the unit, regardless of the physical heater limitation. This option is useful if the user is not concerned about the heater size, but it will cause a problem for users trying to get values for heater design. It may also lead to temperature crossover as all the vapour is condensed and the energy is transferred across to the tube side stream (HINT: if the user wishes to prevent temperature crossover then enter a large area rather than using this CondenseAll option). So use this option with caution. This option is only visible if the Fully Condensing mode is selected. (It cannot be enabled in Flash train mode.)

NonCondVentFrac

Input

User can specify the fraction of non-condensable gases sent to the vent. This option is only visible if the Fully Condensing mode is selected.

VentMassFlowReqd / VentQmReqd

Input

The required amount of steam which is lost to the vent.
Notes

1. This amount refers to STEAM only and does not include any non-condensable or other gases.
2. The steam is vented at the same conditions (temperature and pressure) as the incoming shell stream(s).

CondensedMassFlow / CondensedQm

Calc

The amount of steam condensed. This option is only visible if the Fully Condensing mode is selected.

QmEvaporated

Calc

The amount of steam evaporated. This option is only visible if the Fully Evaporating mode is selected.

HX Page

(Second tab page)

Tag (Long/short)

Input / Calc

Description/Calculated Variables / Options

Primary - Tube (Hot) Side

Pri...

Mode
(Based on OpMode and flows)

Sensible Heat transfer with no phase change.
Condensing Heat transfer will involve change from vapour to liquid phase.
Evaporating Heat transfer will involve change from liquid to vapour 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 (Cold) side

Mode
(Based on OpMode)
Sensible Heat transfer with no phase change.
Condensing Heat transfer will involve change from vapour to liquid phase.
Evaporating Heat transfer will involve change from liquid to vapour 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.

Flash Train Macro Model

Note: Extra fields are visible if the unit is part of a Flash Train. These fields are described below. Please refer to Flash Train.

VapourMassFlowReqd / VQmReqd Calc The calculated mass flow of steam required by the heat exchanger.
CondMassFlow / CondQm Calc The amount of steam condensed by the heat exchanger.
MinSatPress Calc The minimum saturated pressure of steam that could satisfy the heat requirements on the tube side of the unit.
FlashTrain Display A unique tag assigned to the flash train by SysCAD. Each unit in the flash train will have the same tag in this block.
FlashTearBlock Display Displays the name of the tear block that is part of the Flash Train.
FlashTrainEqp List This contains a list of all of the equipment tags in this flash train. For example, if this is Heat Exchanger 1 in the above diagram, then the list would be as follows:

Heat_Exchanger_1

Flash_Tank_2

Vent Stream

The user can specify an amount of steam to be lost from the shell without contacting the tubes, QmVentRqd. If a vent stream is connected the steam will be sent to this link. If the user has not connected a vent stream, this amount of steam will be shown in the tab 'Last Vent' (however, this is not recommended - if the user expects steam to vent then it is good practice to connect a vent stream to the unit) If the amount of steam sent to the unit is less than the user defined amount, QmVentRqd, the unit will flag the user that the specified conditions cannot be met.

Any non-incondensable gases in the steam will also be vented via the vent stream.

If excess steam is sent to the shell and tube heat exchanger, i.e. it is not all condensed in the unit, then the excess steam will report to the condensate line. This is true even if the user has connected a vent stream to the unit.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 DLL: HeatXch1.dll → Units/Links → Heat Transfer: Shell & Tube Heat Exchanger(1) or Group: Energy Transfer → Units/Links → Heat Transfer: Shell & Tube Heat Exchanger(1)

Insert into Project

 Insert Unit → Heat Transfer → Shell & Tube Heat Exchanger(1)

See Insert Unit for general information on inserting units.

Operation Mode selection and Shortcomings

The Shell&Tube heat exchange has been designed to operate under the Fully Condensing mode but can also be used for general Liq/Lig, Liq/Gas and Gas/Gas heat exchange. However, if the Shell and Tube Heat Exchanger is used to exchange heat between two liquid streams (or other combinations), then the following rules must be followed:

1. The Hot Side must be connected to the "Shell_in" Connection point
2. The Cold Side must be connected to the "Tube_In" Connection point.

A problem will arise when the temperatures of HOT and COLD side are unknown or interchangeable due to operation, when this is the case, the Shell and Tube Heat Exchanger will NOT operate correctly. If the hot and cold sides are interchangeable in different scenarios, then the Heat Exchanger model may be more appropriate.

General Configuration Hints

1. NOTE: For Liquor/Liquor, Liquor/Gas and Gas/Gas operation modes, the hot side MUST be connected to the Shell_In and the cold side MUST be connected to the Tube_In.
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 (EHX) occurs between the shell side fluid and the environment. If heat loss is enabled, then less heat is available for heat transfer with the tube side.
4. The user MUST specify FullyCondensing if the unit is required to condense the steam entering the tubes.
5. If the heat exchanger is disabled, but the EHX is enabled, then the shell side stream will still transfer heat to the environment.
6. If the unit has a bad mass and energy balance this could be caused by an excess of steam flowing to the unit. The portion of the steam that is not condensed will be vented, whether a vent stream is configured or not. The amount will be displayed in the LastVent section.
7. If the user specifies an amount of steam to be vented, and the unit calculates a larger amount of excess steam, the unit will vent the calculated amount of steam and flag the user that there is a problem with the unit.

Hints on using a Shell & Tube Heat Exchanger

a) Some of the intended uses of the heat exchanger model in ProBal mode are:

• 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).
• 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, in fully condensing mode, it behaves differently in that you can not "set" any flows to the HX, the flow is governed by the size of the HX. As part of the flash train, the fully condensing HX will calculate the steam amount required and "demand" this of the preceding units.

b) In ProBal mode, if you are using a "stand-alone" heater, you must control the steam flow by setting the Steam flowrate - typically a Qm_Rqd through a controller. Changing the pressure or temperature of the Steam in the feed would not change the steam flowrate without the use of some form of control logic.

c) A pressure drop can be specified in the pipes between flash tanks and/or heat exchangers. This may be required for a flash train arrangement.

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.