Shell and Tube Heat Exchanger 2

From SysCAD Documentation

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General Description

The Shell and Tube Heat Exchanger 2 is used to transfer energy from one stream to another. It is primarily 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. The unit may also be used for sensible heat exchange between two fluids without any phase changes.

There are three operational modes for the Shell and Tube Heat Exchanger 2.

Sensible HX - used for simple heat exchange operations between the two sides, with no phase change.
Condensing (Demand) - used as part of the Flash Train, where steam is being supplied by a connecting Flash tank. Steam supplied will be condensed.
Condensing (Live Steam) or Condensing (Stand Alone) - used where steam is being added with fixed flows, such as from a feeder or another unit operation with user defined flowrates. Steam supplied will be condensed.

If the heat exchanger is inserted as part of a Flash Train, see Flash Train for a description of the theory and variables. This documentation will only discuss the variables for a 'stand alone' heat exchanger.

An Environmental Heat Loss may be included in the unit when it is operating in Condensing or LiveSteam modes. This allows the user to specify a heat loss, or gain, between the unit and the environment.

A key difference with the Shell and Tube Heat Exchanger is that a reaction block (RB) can be included.

Diagram

The diagram shows the default drawing of the shell and tube heat exchanger 2, with the required connecting streams. The user may also connect a vent stream to the unit. This is optional and allows non-condensible 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	Input /Output	No. of Connections		Description
		Min	Max.
Tube_In	In	1	10	The liquid or slurry feed to the unit. Reactions are allowed in the Tube side.
Tube_Out	Out	1	1	The liquid or slurry outlet.
Shell_In	In	1	10	The steam inlet.
Shell_Out	Out	1	1	The condensate out.
Vent	Out	0	1	Optional shell side vent for non-condensible and excess steam.

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

Model Theory

General 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 $\Delta T_1 = T_{H_{out}} - T_{C_{in}}$

Note:

If the heat exchanger has Superheated Steam condensing on the shell side, then this 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

H_in - Enthalpy of entering stream

H_out - 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.

Autoclave Option

Selecting the Autoclave option: Calculates MTD based on difference between condensing temperature and primary (tube) outlet temperature. This simulates a fully mixed autoclave with internal steam heating. The advantage of this over the present Tank heat exchange element is that it can be act as part of a Flash Train.

The Autoclave option is available for Condensing and Live Steam operations. Note that in Live Steam operation, area is ignored, all available steam will be condensed to the the limit where the incoming primary stream is heated to the steam condensing temperature. Steam flow should be controlled externally in this case.

Including Reactions

Selecting the Use.RMTD option: (Reactive Mean Temperature Difference). If there are significant reactions along a heat exchanger section, then the temperature profile may be different to that used in calculation LMTD (Log Mean Temperature Difference). The effective mean temperature difference is also different, though in most cases only by a small percentage. Though for example, if very large amounts of heat were generated by reactions, the heat transfer direction could even reverse.

The RMTD option implements an alternative calculation of MTD, accounting for the reaction heat. In some cases (the example just given) this may give numerical errors. Use the LMTD in that case. If the LMTD and RMTD are dramatically different, this is an indication that the reactions have a significant influence on overall heat transfer.

Data Sections

The default access window consists of several sections:

The first tab contains general information relating to the unit.
RB - Optional tab, only visible if the Reactions are enabled.
ShellSide - Optional tab, only visible if the user selects Tube Detail for the layout.
TubeSide - Optional tab, only visible if the user selects Tube Detail for the layout.
Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
Links tab, only visible in SysCAD 9.2, contains a summary table for all the input and output streams.
Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Class: ShellTube2 - The first tab page in the access window will have this name.

Tag / Symbol	Input / Calc	Description/Calculated Variables / Options
Common First Data Section
Requirements
On	Tick Box	Switches the Heat Exchanger on/off
OpMode	Sensible HX	This will allow sensible heat exchange between two streams. No phase change will take place automatically.
	Condensing (Demand)	This is for use in a flash train configuration where the steam to the Shell side is supplied by a Flash tank. In this configuration, the steam input to the Shell and Tube HX is in demand mode and the steam will be fully condensed (excluding vent amount).
	Condensing (Live Steam)	This is method should be selected if the Shell will be supplied by a Live Steam Feed. The user will specify or control the steam flow to the HX. Note that in this mode the area is ignored, all available steam will be condensed and the incoming primary stream will be heated until either all the energy from the condensing steam is consumed or the temperature equals the incoming steam temperature. Steam flow should be controlled externally in this case. If Excess Steam is fed to the heat exchanger then this could lead to an energy imbalance. The user will receive a warning if this occurs.
	Condensing (Stand-alone)	This method is identical to the Condensing (Live Steam) method described above.
Reactions	List box	Switches the Reactions on/off. Reactions occur on the tube side only.
Autoclave	Tick Box	If this is enabled the unit will use temperature difference rather than LMTD in heater calculations. Calculates MTD based on difference between condensing temperature and primary (tube) outlet temperature. This simulates a fully mixed autoclave with internal steam heating. This is only available if OpMode is one of the Condensing modes.
Use.RMTD	Tick Box	This enables an alternative calculation of MTD, accounting for the reaction heat. See theory above. This is only visible if Reactions are 'On'.
Reaction.Ht	Calc	This field displays the energy from the reactions. This is only visible if Reactions are enabled.
IgnoreSteamFlow	Tick Box	Adjust steam feed rate to match duty and report excess or low flow conditions. When the flash train model has converged correctly, the steam flow rate will be matched and an error is not reported. This is only available if OpMode is Condensing (Demand).
Flow.Damping	Input	In solving macro models, there may be situations when the solver overshoots and fails to converge. This is because the demand from the model is interacting with supply from Flash Tanks. If the unit model steam flow and pressure seems to be oscillating about, then put in a value of 0.8 for the flow damping. This will stabilize the oscillations and allow the model to converge. It will slow the overall convergence rate if used when unnecessary, so only turn this on (change the default value of 0.0) if flash trains are not converging. Only available if OpMode is Condensing (Demand).
EnvironHX	Tick Box	Switches the environmental heat exchanger sub model on/off, used for additional heat losses. This is only available if OpMode is one of the Condensing modes.
Model	Only available if EnvironHX has been selected.
	None	No environmental heat loss.
	FixedHeatFlow	Fixed Environmental heat loss for the unit operation.
Env.Heat.Loss	Input/Calc	Only shown if EnvironHX has been selected. The amount of heat lost to the environment.
Layout	Overall	User specifies the overall heater area and HTC
Layout	TubeDetail	User can specify the tube side in more detail. Refer to the Tubeside Tab for details.
HTC	Input/Calc	If the Overall method is selected then this field allows the user to specify the heat transfer coefficient. If the TubeDetail method is selected then this field shows the calculated heat transfer coefficient (the HTC is specified on the ShellSide/TubeSide tabs).
Area	Input/Calc	If the Overall method is selected then this field allows the user to specify the heat transfer area. If the TubeDetail method is selected then this field shows the calculated heat transfer area (based on user specified information on the TubeSide tab).
LMTDFact	Input	User specified LMTD factor
QmVentRqd	Input	User specified amount of steam to be vented
Calculation Modes (Only shown if Tube Detail is selected for Layout)
TSHxCalc	Lumped
TSHxCalc	Detail
TSdPCalc	Fixed	User specified pressure drop for the tubeside.
TSdPCalc	Full
TS.Scaling	Tick Box	Allows scaling for the Tubeside

Results
U*A	Calc	The heat exchanger UA.
TheorDuty	Calc	The theoretical duty of the heat exchanger.
LMTD	Calc	The calculated log mean temperature difference. 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.
RMTD	Calc	The Reactive Mean Temperature Difference - this method of calculating the log mean temperature difference includes the temperature change due to the reactions.
TheorArea	Calc	The calculated heat exchanger area required to handle the duty.
Duty	Calc	The calculated Heater Duty
Tot.Cond.Duty	Calc	The total duty from the condensing steam.

Tube Side
Tube.In.T	Calc	Tube in temperature.
Tube.Out.T	Calc	Tube out temperature.
Tube.In.P	Calc	Tube in pressure.
Tube.Out.P	Calc	Tube out pressure.
Tube.Qm	Calc	The mass flowrate going through the tube side..

Shell Side
Shell.In.T	Calc	Shell in temperature.
Shell.Out.T	Calc	Shell out temperature.
Shell.In.P	Calc	Shell in pressure.
Shell.Out.P	Calc	Shell out pressure.
Shell.Qm	Calc	The mass flowrate going through the Shell side.
Vent.Qm	Calc	The mass flowrate going through the vent.
Vent.NonCond.Qm	Calc	The mass flowrate of non condensible going through the vent.
Condensate.Demand	Calc	This is only shown if the OpMode selected is Condensing (Demand). This is the amount of steam which is passed back to the flash train.
LastMassFlow	Calc	This is only shown if the OpMode selected is Condensing (Demand). This is the last mass flowrate through the shell side. This is used in combination with Flow.Damping.
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.
SuctionP	Calc	The pressure on the inlet to the shell side of the heat exchanger.
QRqd	Calc	The calculated mass flow of steam required by the heat exchanger.
QCond	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

ShellSide Tab

This tab page is only shown if Tube Detail is selected for Layout.

Tag / Symbol	Input / Calc	Description/Calculated Variables / Options
Requirements
ShellSideHTC	Input	Shell side heat transfer coefficient.
Results
Qm	Calc	Mass Flow going through Shell Side.
Sat.T	Calc	Saturation Temperature.
Cond.T	Calc	Condensing Temperature.
SatP	Calc	Saturated pressure.

TubeSide Tab

This tab page is only shown if Tube Detail is selected for Layout.

Tag / Symbol	Input / Calc	Description/Calculated Variables / Options
Requirements
Tube.conductivity	Input	Tube side conductivity.
NumberTubes	Input	User specified number of tubes used to calculate the Heat Transfer area.
TubeLength	Input	User specified tube length used to calculate the Heat Transfer area.
TubeID	Input	User specified inner diameter used to calculate the Heat Transfer area.
TubeOD	Input	User specified outside diameter used to calculate the Heat Transfer area.
TubeSidePasses	Input	Tube side passes.
EntranceCc	Input	The contraction coefficient for entrance losses.
ExitCc	Input	The contraction coefficient for exit losses.
HeatTransfer
TubeSideHTC	Input	Tube side heat transfer coefficient.
Visible with TS.Scaling selected.
Scale.Thick	Input	tubeside scale thickness.
Scale.Cond	Input	tubeside scale conductivity.

Results
TS.Vel	Calc	Tube side velocity.
TS.G	Calc	The average mass flow velocity.
Cp	Calc	Only shown if OpMode is Condensing(Demand). This is the Specific Heat.
TS.Rht	Calc	Only shown if OpMode is Condensing(Demand). This is the specific reaction heat (Reaction.Ht/mass flow).
In
Tin	Calc	Tube side temperature in
TSin.Re	Calc	Tube side inlet Reynolds number
TSin.ft	Calc	Tube side inlet friction factor.
TSin.dPdL	Calc	Tube side pressure drop per unit length.
Out
Tout	Calc	Tube side temperature out
TSout.Re	Calc	Tube side outlet Reynolds number
TSout.ft	Calc	Tube side outlet friction factor.
TSout.dPdL	Calc	Tube side pressure drop per unit length.
Total
TS.dPIO	Calc	Tube side pressure drop
TS.dP	Input	Tube side pressure drop required

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(2)
or	Group:	Energy Transfer	→	Units/Links	→	Heat Transfer: Shell & Tube(2)

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

Insert into Project

Insert Unit

→

Heat Transfer

→

Shell & Tube(2)

See Insert Unit for general information on inserting units.

Hints and Comments

Operation Mode selection and Shortcomings

The Shell&Tube Heat Exchanger 2 has been designed to operate under the Live Steam/Condensing modes but can also be used for general liquid/liquid, liquid/gas and gas/gas heat exchange using the Sensible HX mode. When the Shell and Tube Heat Exchanger 2 is used in one of the two condensing modes (Live Steam/Condensing), then the following rules must be followed:

The Hot Side must be connected to the "Shell_in" Connection point
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 2 will NOT operate. There will be no heat exchange and reactions will not proceed. If the hot and cold sides are interchangeable in different scenarios, then the Heat Exchanger model may be more appropriate.

General Configuration Hints

NOTE: For Live Steam/Condensing operation modes, the hot side MUST be connected to the Shell_In and the cold side MUST be connected to the Tube_In.
Ensure that the HTC and Area are correct. If these variables are not configured, the heat exchanger will not operate as expected.
The Environmental Heat Transfer (EHX) occurs between the shell side fluid and the environment.
The user MUST use Live Steam or Condensing mode if the unit is required to condense the steam entering the tubes.
If the heat exchanger is disabled, but the EHX is enabled, then the shell side stream will still transfer heat to the environment.
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.
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 (T_o).

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 T_o.

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.

Shell and Tube Heat Exchanger 2

From SysCAD Documentation

Contents

General Description

Diagram

Inputs and Outputs

Model Theory

General Theory

Autoclave Option

Including Reactions

Data Sections

Requirements

Results

Tube Side

Shell Side

Flash Train Macro Model

ShellSide Tab

TubeSide Tab

Vent Stream

Adding this Model to a Project

Hints and Comments

Operation Mode selection and Shortcomings

General Configuration Hints

Hints on using a Shell & Tube Heat Exchanger

Comments on Heat Balance around the Heater

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