Simple Heat Exchanger

From SysCAD Documentation
Jump to navigation Jump to search

Navigation: Main Page -> Models -> Energy Transfer Models


General Description

The simple heat exchanger is a generic "heat exchanger" model without regards to its size. It allows the user to specify the product temperature, temperature rise, temperature drop or duty for one side of the heat exchanger. It then transfers energy across from side to the other to meet the stated requirements without there being any limits due to heat transfer area, heat transfer coefficient, or temperature cross-over.


Diagram

Simple Heat Exchanger diagram.png

The diagram shows the default drawing of the Simple Heat Exchanger, with the required connecting streams. The unit will not operate unless all of the above streams are connected.

The physical location of the connections is not important; the user may connect the streams to any position on the drawing.

Inputs and Outputs

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

Primary In (PriIn)

1 Required

In

1

10

Primary feed to heat exchanger.

Secondary In (SecIn)

1 Required

In

1

10

Secondary feed to heat exchanger.

Primary Out (PriOut)

Required

Out

1

1

Primary output stream from heat exchanger.

Secondary Out (SecOut)

Required

Out

1

1

Secondary output stream from heat exchanger.

Behaviour when Model is OFF

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

  • The Primary Product stream will be the same as Primary Feed stream with no change in temperature;
  • The Secondary Product stream will be the same as Secondary Feed stream with no change in temperature;

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

Model Theory

Heater Duty or Final Temperature:

The Simple Heat Exchanger uses the following equation to calculate the duty required or the final temperature of the relevant stream.

(1) [math]\mathbf{\mathit{Q=\dot{m}\int Cp\boldsymbol{\Delta}T}}[/math]

where
Q - Rate of Heat Transfer
[math]\mathbf{\mathit{\dot{m}}}[/math] - mass flow rate
Cp - heat capacity
[math]\mathbf{\mathit{\boldsymbol{\Delta}T}}[/math] - temperature difference

Simple Heat Exchanger Design Calcs:

The Simple Heat Exchanger also contains some side calculations for HTC or Area. These calculations are for user information only, thus they have no affect on the mass and energy balance of the model. The basic equation used for these calculations is:

(2) [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]

NOTE: These side calculations are based on the actual duty of the simple heat exchanger.

Model Configuration Example

The simple heat exchanger is designed to exchange heat between two streams, Hot Side and Cold Side.

  1. User will first specify which side should be calculated first by setting the "Side Definition" on the first Tab.
  2. User specifies Flow, and (T or Duty) for this first side. The Duty or T is calculated using Equation 1 in Model Theory.
  3. Based on the user input stream temperatures, SysCAD determines which side (primary or secondary) is the HOT side.
    SimpleHX Example1.png
  4. The duty from the first side can be used to calculate the condition for the second side. The duty will stay the same, so user can vary (Flow and/or T) for the second side to achieve the same duty.
  5. Flow for second side can be optionally adjusted using Demand.
    • This only works if the feed comes from a feeder directly and the feeder has Demand.On selected.
  6. Temperature out for second side can be directly specified or based on approach temperature to the first side.
    • If the "Side Definition" equals the hot side, then the Approach Temperature = Approach Hot Feed temperature
    • If the "Side Definition" equals the cold side, then the Approach Temperature = Approach cold Feed temperature
    SimpleHX Example2.png

Assumptions and Limitations

  1. The simple heat exchanger cannot handle phase changes. Thus, it will not be suitable for steam condensing calculations.
  2. The simple heat exchanger does not prevent physically unrealistic heat transfer (such as temperature cross-over) from occuring, although the user may get warning messages.
  3. Apart from when the model is off, the only instance when heat exchange is prevented from occurring is when one side (but not both) has no flow.
  4. No reactions are allowed in the heater.
  5. When using the Approach Method for "other Side", the approach temperature should be > 0. If users sets this to 0, a temperature crossover warning will be shown.

Data Sections

The default access window consists of several sections,

  1. SimpleHX tab - This first tab contains general information relating to the unit.
  2. Calc tab - Optional second tab which allows the user to perform some sizing and/or flow calculations.
  3. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  4. Links tab, contains a summary table for all the input and output streams.
  5. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Simple Heat Exchanger Page

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

Tag (Long/Short) Input / Calc Description/Calculated Variables / Options
Common Data on First Tab Page

Requirements

On Tick Box If this option is deselected, the heater will not be operational and thus inlet conditions = outlet conditions for both Primary and Secondary streams.
SideDefinition Primary The temperature or duty requirements can be specified for the primary side only.
Secondary The temperature or duty requirements can be specified for the secondary side only.
Primary/Secondary side requirements
(the following selections will be applied to the Primary OR Secondary side, depending on the SideDefinition chosen)
Method FixedDuty This allows the user to specify the heat exchange duty. The outlet temperature will be calculated from this.
ProductT This allows the user to specify the required outlet temperature. The heat exchange duty is calculated from this.
TemperatureDrop This allows the user to specify the required temperature drop across the heat exchanger. The heat exchange duty is calculated from this.
TemperatureRise This allows the user to specify the required temperature rise across the heat exchanger. The heat exchange duty is calculated from this.
DutyReqd Input This field is only visible if FixedDuty is chosen for Method. The required duty.
Note: a positive duty is used for heating (rise in temperature), while a negative duty is used for cooling.
TemperatureReqd / T_Reqd Input This field is only visible if ProductT is chosen for Method. The required product temperature.
TempDropReqd / TDropReqd Input This field is only visible if TemperatureDrop is chosen for Method. The required temperature drop across the heat exchanger.
Note: a negative drop can be used to define a temperature rise.
TempRiseReqd / TRiseReqd Input This field is only visible if TemperatureRise is chosen for Method. The required temperature rise across the heat exchanger.
Note: a negative rise can be used to define a temperature drop.

Options

Other.CalcFlow Tick Box If this option is selected, the Calc tab page will appear with some options for calculating the required flow on the other side of the heat exchanger (ie. NOT the SideDefinition side).
HXSizeCalc Tick Box If this option is selected, the Calc tab page will appear with some options for calculating either the area or the heat transfer coefficient (HTC) based on the actual duty of the heat exchanger.
TrackOneSideFlow Tick Box If this option is selected, warning messages will be given if one side of the heat exchanger (but not both) has no flow and hence no heat exchange can occur.

Results

Duty Calc The actual duty of the Heat Exchanger. This will always be a positive number.
LMTD Calc The actual LMTD (Log Mean Temperature Difference) of the Heat Exchanger.
HotSide Feedback Only visible in SysCAD 138 or newer. Returns the Hot side (primary or secondary) based on feed temperature. If both primary and secondary feed temperatures are the same, then this will return "unknown".
Approach.HotFeed Calc Only visible in SysCAD 138 or newer. Displays the temperature difference between Hot Side In and Cold Side Out. (Hot Side Ti - Cold Side To).
Approach.ColdFeed Calc Only visible in SysCAD 138 or newer. Displays the temperature difference between Hot Side Out and Cold Side In. (Hot Side To - Cold Side Ti)
Results (Primary - Hot/Cold Side)
Pri...
MassFlow / Qm Calc The mass flow rate of the primary stream.
TemperatureIn / Ti Calc The inlet temperature of the primary stream.
TemperatureOut / To Calc The outlet temperature of the primary stream.
dT Calc The (outlet-inlet) temperature difference of the primary stream.
TempDrop / TDrop Calc The (inlet-outlet) temperature difference of the primary stream.
PressureIn / Pi Calc The inlet pressure of the primary stream.
PressureOut / Po Calc The outlet pressure of the primary stream.
Duty Calc The duty of the primary side of the heat exchanger. A negative duty means cooling occurred (temperature drop) while a positive duty means heating occurred (temperature rise).
Results (Secondary - Hot/Cold Side)
Sec...
MassFlow / Qm Calc The mass flow rate of the secondary stream.
TemperatureIn / Ti Calc The inlet temperature of the secondary stream.
TemperatureOut / To Calc The outlet temperature of the secondary stream.
dT Calc The (outlet-inlet) temperature difference of the secondary stream.
TempDrop / TDrop Calc The (inlet-outlet) temperature difference of the secondary stream.
PresssureIn / Pi Calc The inlet pressure of the secondary stream.
PressureOut / Po Calc The outlet pressure of the secondary stream.
Duty Calc The duty of the secondary side of the heat exchanger. A negative duty means cooling occurred (temperature drop) while a positive duty means heating occurred (temperature rise).

Calc

This page is only visible if the Other.CalcFlow or HXSizeCalc options are chosen on the first tab page.

Tag (Long/Short) Input / Calc Description/Calculated Variables / Options
Other (Primary/Secondary) side flow calculation (this section is only visible if the Other.CalcFlow option is chosen on the first tab page)
(the following selections will be applied to the Primary OR Secondary side, depending on the SideDefinition chosen)
This section uses the actual duty of the heat exchanger to calculate the required flow of the 'other' side of the heat exchanger based on the following requirements:
DemandConnection None (Manual) The required flow (DemandQm) will be calculated but not used by the model. It is up to the user to use an external controller to fetch this value.
General Demand The required flow (DemandQm) will be passed back through the feed streams using the General Demand functionality.
Method ProductT This allows the user to specify the required outlet temperature. Using the actual heat exchange duty, this is used to determine the required flow of the other side of the heat exchanger.
TemperatureDrop This allows the user to specify the required temperature drop across the heat exchanger. Using the actual heat exchange duty, this is used to determine the required flow of the other side of the heat exchanger.
TemperatureRise This allows the user to specify the required temperature rise across the heat exchanger. Using the actual heat exchange duty, this is used to determine the required flow of the other side of the heat exchanger.
Approach (This method is only available in SysCAD 138 or newer). This method allows the user to specify the outlet temperature (other side) based on an approach Temperature to one of the inlet stream to the heat exchange. The inlet stream used is set by Simple HX tab - SideDefinition.

For example, if the Side definition used is "Primary", then the Approach T will be relative to the "Primary" inlet Temperature.

  • If "Primary" used is the hot side, then T_out (cold_out) = T_in (Hot in) - Approach temperature. In this situation, the approach T = Approach.HotFeed (displayed on the SimpleHX Tab).
  • If "Primary" used is the cold side, then T_out (Hot_out) = T_in (Cold In) + Approach temperature. In this situation, the approach T = Approach.ColdFeed (displayed on the SimpleHX Tab).
  • See Model Configuration Example for more information.
TemperatureReqd / T_Reqd Input This field is only visible if ProductT is chosen for Method. The required product temperature.
TempDropReqd / TDropReqd Input This field is only visible if TemperatureDrop is chosen for Method. The required temperature drop across the heat exchanger.
Note: a negative drop can be used to define a temperature rise.
TempRiseReqd / TRiseReqd Input This field is only visible if TemperatureRise is chosen for Method. The required temperature rise across the heat exchanger.
Note: a negative rise can be used to define a temperature drop.
ApproachReqd / Approach Input Only available in SysCAD 138 or newer. This field is only visible if Approach is chosen for Method. The required approach temperature. This value is usually a positive number.
DemandDamping Input There may be situations when the solver overshoots and fails to converge. If the unit model flow and temperature seems to be oscillating about, then adding damping 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 the model is not converging.
QmTarget Tick Box If this option is ticked, then the required flow (DemandQm) will be considered a target and the user will not receive any warning messages if the ActualQm does not equal DemandQm.
 
ActualApproach Calc Only visible in SysCAD 138 or newer. This field displays the actual approach temperature achieved.
TargetTemperature / TargetT Calc The actual Target Temperature. This is determined based on the actual feed temperature and the preceding temperature requirement. If this side is being heated, the TargetT can not be less than the feed temperature. If this side is being cooled, the TargetT can not be greater than the feed temperature.
ActualTemperature / ActualT Calc The actual outlet temperature. This will be the same as shown on the first tab page and is just shown here for comparison.
DemandMassFlow / DemandQm Calc The required mass flow in order to achieve the Target Temperature.
ActualMassFlow / ActualQm Calc The actual mass flow. This will be the same as shown on the first tab page and is just shown here for comparison.
DemandQmError / DemandQmErr Calc The difference between the required mass flow and the actual mass flow (DemandQm - ActualQm).
DemandQmRelError / DemandQmRelErr Calc The relative difference between the required mass flow and the actual mass flow (DemandQm - ActualQm) / DemandQm.
HX Sizing Calculation (this section is only visible if the HXSizeCalc option is chosen on the first tab page)
ModelDuty Calc This is the actual duty for the simple heat exchanger (thus duty calculated in the first tab page). It is used for all calculations.
ModelLMTD Calc The actual Log Mean Temperature Difference for the simple heat exchanger.
CalculateWhat HTC This allows the user to calculate the required HTC based on actual Duty and actual LMTD and specifying Area.
Area This allows the user to calculate the required Area based on actual Duty and actual LMTD and specifying HTC.
HTC Input/Calc Heat Transfer Coefficient - This field can be an input or a calculated variable depending on the method selected.
Area Input/Calc Heat Transfer Area - This field can be an input or a calculated variable depending on the method selected.
U*A / UA Calc HTC x Area

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 

DLL:

HeatExchange.dll

Units/Links

Heat Transfer: Simple Heat Exchanger

or

Group:

Energy Transfer

Units/Links

Heat Transfer: Simple Heat Exchanger

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


Insert into Project

 

Insert Unit

Heat Transfer

Simple Heat Exchanger

See Insert Unit for general information on inserting units.