Simple Heat Exchanger

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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

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
Label	Required Optional	Input Output	Min	Max	Description
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 heater uses the following equation to calculate the duty required or the final temperature of the stream.

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

where

Q - Rate of Heat Transfer

[math]\displaystyle{ \mathbf{\mathit{\dot{m}}} }[/math] - mass flow rate

Cp - heat capacity

[math]\displaystyle{ \mathbf{\mathit{\boldsymbol{\Delta}T}} }[/math] - temperature difference

Other Side Flow Calculation

Based on user specified temperature requirement, SysCAD will calculate the required mass flow for the other feed stream (the one which is NOT used as the SideDefinition). This can be simply calculated or passed through the secondary feed stream as a Demand flow. The calculated value can be used as a setpoint for an external controller.

Optional Side Calculations

HXSizeCalc: Simple Exchanger Design Calcs:

The side calculation is based on the actual duty of the Simple Heat Exchanger. The 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]\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

[math]\displaystyle{ \mathbf{\mathit{\boldsymbol{\Delta}T_{LM} = \cfrac{\Delta T_2 -\Delta T_1}{ln \left( \cfrac{\Delta T_2}{\Delta T_1} \right) }}} }[/math] - Log Mean Temperature Difference (LMTD)

For Counter Current Flow [math]\displaystyle{ \; \Delta T_2 = T_{H_{in}} - T_{C_{out}} \quad }[/math] and [math]\displaystyle{ \; \Delta T_1 = T_{H_{out}} - T_{C_{in}} }[/math]

based on the above equation, variables are: HTC, area. User can specify values for one of these variables, SysCAD will calculate the other term using the Simple Heat Exchanger duty and calculated LMTD.

Model Configuration Example

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

User will first specify which side should be calculated first by setting the "Side Definition" on the first Tab.
User specifies Flow, and (T or Duty) for this first side. The Duty or T is calculated using Equation 1 in Model Theory.
Based on the user input stream temperatures, SysCAD determines which side (primary or secondary) is the HOT side.
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.
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.
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

Assumptions and Limitations

The simple heat exchanger cannot handle phase changes. Thus, it will not be suitable for steam condensing calculations.
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.
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.
No reactions are allowed in the heater.
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,

SimpleHX tab - This first tab contains general information relating to the unit.
Calc tab - Optional second tab which allows the user to perform some sizing and/or flow calculations.
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, 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.