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, 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 two operational modes for the Shell and Tube Heat Exchanger, a) as a stand-alone unit or b) as part of a Flash Train. 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 governing this behaviour.

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. 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 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	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	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	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	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	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.

Model Theory

The unit is based on traditional heat exchanger theory¹,

_{File:Models-Heat-Exchanger-image002.gif}

[math]\displaystyle{ \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

_{File:Models-Heat-Exchanger-image004.gif}[math]\displaystyle{ \mathbf{\mathit{\boldsymbol{\Delta}T_{LM}}} }[/math] - Log Mean Temperature Difference (LMTD)

for counter current flow

_{File:Models-Heat-Exchanger-image006.gif}

[math]\displaystyle{ \mathbf{\mathit{\boldsymbol{\Delta}T_{LM}\frac{(t_1^H-t_2^C)-(t_2^H-t_1^C)}{ln(\frac{t_1^H-t_2^C}{t_2^H-t_1^C})}}} }[/math]

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.

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

_{File:Models-Heat-Exchanger-image008.gif}

[math]\displaystyle{ \mathbf{\mathit{Q=m(H_{in}-H_{out})}} }[/math]

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 LTMD 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 sections and variable names are described in detail in the following tables. The default heat exchanger access window may consist of 4 sections. This number may increase or decrease, based on user configuration.

The default access window consists of two sections,

a) The first tab has the same name as the model tag, contains general information relating to the unit.

b) HX tab

c) Optional tab Environmental Heat Exchanger (EHX)

a) The last section Audit contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

First Section- Tag Name

Tag / Symbol	Input or Calc	Description
Common First Data Section
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.

Second Section- HX

Tag / Symbol	Input or Calc	Description
RqdOpMode	Input	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. 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. 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. 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.
ActOpMode	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.
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.
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.
UALMTD	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.
Duty	Calc	The calculated duty of the heat exchanger
TheorArea	Calc	The calculated theoretical Area based on duty. It suggests a heater size so that duty = theoretical duty.User may use this for the new heater area and see if the results will be better.
LMTDFact	Input	The LMTD factor of the heat exchanger. This is usually 100%.
SuctionP	Calc	The pressure at the steam inlet of the heat exchanger. I.e. this is the pressure of the vapour from the Flash Tank, less any configured pressure drops in the connecting pipes.
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
Note: EXTRA FIELDS ARE VISIBLE IF SHELL AND TUBE HX IS PART OF A FLASH TRAIN. THESE ARE DESCRIBED BELOW. FOR FURTHER INFORMATION PLEASE REFER TO Flash Train.
FlashTrain	Calc	A unique tag assigned to the flash train by SysCAD. Each unit in the flash train will have the same tag in this block.
FlashTrainEqp	Calc	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
IgnoreAreaLimit	Tick Box	If this box is ticked, then SysCAD will fully condense ALL the steam that enters the unit, regardless of the physical heater limitation. This option is usefully for users not caring about heater sizes but would cause a problem for users trying to get values for heater design. So use this option with caution.
NonCondVentFrac	Input	User can specify the fraction of "other gases" going to the vent.
QmVentRqd	Input	The required amount of steam which is lost to the vent. Note this amount refers to STEAM only and does not include the non-condensable, that is, other gases.
QmSteamCond	Calc	The amount of steam condensed.

Tag.HX.Pri.	The following variables define the primary, or tube, side of the unit.
Mode	Calc		The unit determines by which mode the primary stream will transfer heat and displays the mode here.
		Sensible	Heat transfer with no phase change.
		Condensing	Heat transfer will involve change from vapour to liquid phase.
Qm	Calc		The mass flow through the primary side of the heat exchanger
Cp	Calc		The specific heat of the primary stream
Ti	Calc		The temperature of the primary stream entering the unit.
To	Calc		The temperature of the primary stream leaving the heat exchanger.
Pi	Calc		The pressure of the primary stream entering the unit.
Po	Calc		The pressure of the primary stream leaving the unit.
dT	Calc		The difference in temperature between the entering and leaving streams.
SatT	Calc		The saturated temperature of the primary stream.
SatP	Calc		The saturated pressure of the primary stream.
Duty	Calc		The duty calculated using Primary side values.
Tag.HX.Sec	The following variables define the secondary, or shell, side of the unit.
Mode	Calc		The unit determines by which mode the secondary stream will transfer heat and displays the mode here.
		Sensible	Heat transfer with no phase change.
		Condensing	Heat transfer will involve change from vapour to liquid phase.
Cp	Calc		The specific heat of the secondary stream
T	Calc		The temperature of the secondary stream entering the unit.
Ti	Calc		The temperature of the secondary stream entering the unit.
To	Calc		The temperature of the secondary stream leaving the unit.
Pi	Calc		The pressure of the secondary stream entering the unit.
Po	Calc		The pressure of the secondary stream leaving the heat exchanger.
dT	Calc		The difference in temperature between the entering and leaving streams.
SatT	Calc		The saturated temperature of the secondary stream.
SatP	Calc		The saturated pressure of the secondary stream.
Duty	Calc		The duty calculated using secondary side values.

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 pipe for a description of the variables.

Hints and Comments

Operation Mode selection and Shortcomings:

The Shell&Tube heat exchange has been designed mainly to operate under the Fully condensing mode. Although other operation methods are available, they only work under certain configurations. For example, if the Shell and Tube Heat Exchanger is used to exchange heat between two liquid streams, then the following rule 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.

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.

Workaround and Recommendation:

For operations of Liq/Liq, Liq/Gas and Gas/Gas heat exchange, use the Heat exchanger instead of the Shell & Tube Heat Exchange.

General Configuration Hints:

a) Ensure that the HTC and Area are correct. If these variables are not configured, the heat exchanger will not operate as expected.

b) The Environmental Heat Transfer occurs between the shell side fluid and the environment.

c) The user MUST specify FullyCondensing if the unit is required to condense the steam entering the tubes.

d) If the heat exchanger is disabled, but the EHX is enabled, then the shell side stream will still transfer heat to the environment.

e) 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.

f) 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) The intended usage of the heat exchanger model in ProBal mode is:

• 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, 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.hlp)

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 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:

There is a known HX LMTD display error, which occurs when the HX is in Fully condensing mode with superheated steam as its feed. Under these conditions, Duty displayed does not equal to UA*LMTD.

The reason for this is that SysCAD calculates the LMTD using only Saturated conditions, for it being the main source of energy, a simple hand calc on the LMTD will confirm this. However, if the user calculates the heater duty using UA*LMTD only, the portion of heat from the desuperheating of steam will be omitted. The significance of this error will depend on the degree of superheat of the steam.

In summary:

- the Duties displayed on the HX are correct

- the LMTD display is generally correct, except when steam being fed is superheated

- the difference in (duty - UA*LMTD) is duty for desuperheating of steam