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.

There are two operational modes for the 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 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. Note that for a heat exchanger to be included in a flash train, the steam line must be connected to the 'Secondary In'.

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 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.
PriIn	In	1	20	The Process Stream to the unit.
PriOut	Out	1	1	The Process Stream outlet
SecIn	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	Out	1	1	The cooling or heating fluid outlet.
PVent	Out	0	1	Not working
SVent	Out	0	1	Not working

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.

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, 6^th Edition, 1984.

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 three sections,

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

b) The second tab HX.

c) Optional tab Vapour Liquid Equilibrium (VLE)

d) Optional tab Environmental Heat Exchanger (EHX)

a) Audit, fully described in Audit Section. 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 / Calc	Options	Description
RqdOpMode	Input	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	The stream connected to the SecIn 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.
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.
Duty	Calc		The calculated duty of the heat exchanger
LMTDFact	Input		The LMTD factor of the heat exchanger. This is usually 100%.
QmVentRqd	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.
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 heat exchanger.
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		Calculated rate of heat transfer.
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.
Qm	Calc		The mass flow through the secondary side of the heat exchanger
Cp	Calc		The specific heat of the secondary stream
Ti	Calc		The temperature of the secondary stream entering the unit.
To	Calc		The temperature of the secondary stream leaving the heat exchanger.
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.

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

General Configuration Hints:

a) If a heat exchanger is to be included in a Flash Train, the steam must be connected to the secondary side and the unit operation mode must be set to FullyCondensing.

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

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

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

e) The unit has a problem with excess vapour in the unit. Vent lines are not to be used until the problem has been fixed.

f) 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 (T_o) 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 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 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 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.