Simple Heater

From SysCAD Documentation
Jump to navigation Jump to search

Navigation: Models ➔ Energy Transfer Models ➔ Simple Heater


General Description

The simple heater is generic "heat exchanger" model without regards to its heating media or heater size. It is only designed to give estimates of

1) Heater duty required by specifying the stream outlet temperature or

2) Stream Outlet temperature by specifying the heater duty.

Note: The Calcine & Fuel Project, which is distributed with SysCAD in the Examples folder, demonstrates the use of this model.

Diagram

Models-Simple-Heater-image001.gif

The diagram shows the default drawing of the Simple Heater, 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.
Input Required In 1 1 Input stream to heater.
Output Required Out 1 1 Output stream from heater.

Behaviour when Model is OFF

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

  • The Feed stream will flow straight out of the Product 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]\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

Optional Side Calculations

1. HXSizeCalc: Simple Heater Design Calcs:

The side calculation is based on the actual duty of the Simple Heater. 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]\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} = \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]\; \Delta T_2 = T_{H_{in}} - T_{C_{out}} \quad [/math] and [math] \; \Delta T_1 = T_{H_{out}} - T_{C_{in}} [/math]
  • based on the above equation, variables are: HTC, area and LMTD. User can specify values for two of these variables, SysCAD will calculate the third term using the Simple Heater duty.
  • If the UseSteamCalcLMTD tickbox is checked, the LMTD calculated by the SteamCalc side calculator will be used for the heater size calculation. (Only available in build139 or later and only visible when the SteamCalc option is in use)

2. SteamCalc: Steam requirements side calculation: (only available in Build139 or later)

The side calculation is based on the actual duty of the simple heater. The calculations are for user information only, thus they have no affect on the mass and energy balance of the model.
Should only be used if the Simple Heater is being used in the heater mode.
The basic equation used for these calculations is:
(3) [math]\mathbf{\mathit{Q=\dot{m}(H_{in}-H_{out})}}[/math]
  • based on the above equation, user can specify the steam properties and SysCAD calculate the flowrate using the heater duty.
  • This will also calculate the LMTD based on the steam properties and simple heater inlet and outlet temperatures.

Assumptions and Limitations

  1. The simple heater cannot handle phase changes. Thus, it will not be suitable for steam condensing calculations.
  2. The simple heater has only one inlet and one outlet, if multiple stream inlet/outlets are required, the user will have to add in tie unit operations for mixing and splitting.
  3. No reactions are allowed in the heater.

Data Sections

The default access window consists of several sections,

  1. SimpleHeater tab - This first tab contains general information relating to the unit.
  2. VLE - Only visible if the Saturated T method is selected. Allows the user to set the component that the saturation temperature is based on.
  3. Steam tab - Only available in Build 139 or later. This optional tab allows the user to calculate the steam requirement for the unit based on the actual duty. Only visible if the SteamCalc option is enabled.
  4. HXCalc tab - This optional tab allows the user to calculate design data for the unit based on the actual duty. Only visible if the HXSizeCalc option is enabled.
  5. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  6. Links tab, contains a summary table for all the input and output streams.
  7. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Simple Heater Page

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

Tag (Long/Short) Input / Calc Description/Calculated Variables / Options
Tag Display This name tag may be modified with the change tag option.
Condition Display OK if no errors/warnings, otherwise lists errors/warnings.
ConditionCount Display The current number of errors/warnings. If condition is OK, returns 0.
GeneralDescription / GenDesc Display This is an automatically generated description for the unit. If the user has entered text in the 'EqpDesc' field on the Info tab (see below), this will be displayed here.
If this field is blank, then SysCAD will display the unit class ID.

Requirements

On Tick Box If this option is deselected, the heater will not be operational and thus inlet conditions = outlet conditions.
Method Fixed Duty This allows the user to specify the heater duty. The outlet temperature will be calculated from this.
Duty Tag This allows the user to set a tag that the unit will 'read' to set the duty. This is used to transfer energy from another unit operation.
Product Temperature This allows the user to specify the required outlet temperature. The heater duty is calculated from this.
Saturated T This method is only available in SysCAD 9.3 Build 137 or later. The outlet temperature will be equal to the saturation temperature for the component selected on the VLE tab page. The heater duty is calculated from this.
TemperatureDrop This allows the user to specify the required temperature drop across the heater. The heater duty is calculated from this.
TemperatureRise This allows the user to specify the required temperature rise across the heater. The heater duty is calculated from this.
DutyReqd Input This field is only visible if Fixed Duty 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.
DutyTag Input This field is only visible if Duty Tag is chosen for Method. The required SysCAD Tag for the transferred duty, e.g. Boiler.HeatRqd1 (kW) or P1.EHX.HeatFlow (kW).
DutyReqd.Meas Calc Only available in Build 137 or later. This field is only visible if Duty Tag is chosen for Method. The current value of the SysCAD Tag for the transferred duty.
DutyFactor Input This field is only visible if Duty Tag is chosen for Method. The duty factor. To change from heating to cooling or vice versa, use -1.
TemperatureReqd / T_Reqd Input This field is only visible if Product Temperature 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 heater.
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 heater.
Note: a negative rise can be used to define a temperature drop.

Options

HXSizeCalc Tick Box This option will add the HXCalc tab for calculations of 1) Area 2) heat transfer coefficient (HTC) or 3) log mean temperature difference (LMTD) based on the actual duty of the heater.
SteamCalc Tick Box Only available in Build 139 or later. This option will add the Steam tab for calculations of required amount of saturated or superheated steam based on the actual duty of the heater. User should only tick this if the simple heater is configured as heater.
UseSteamCalcLMTD Tick Box Only available in Build 139 or later and only visible if SteamCalc option is in ticked. This option will use the LMTD calculated by the SteamCalc (Steam tab) for HXSizeCalc calculations (HXCalc tab).

Results

Duty Calc The actual Duty of the Heater.
HeatFlow Calc Only visible in Build 137 or later. The heat flow to the heater. This will be +ve when heat is added and -ve when heat is removed.
MassFlow / Qm Calc The mass flow rate.
TemperatureIn / Ti Calc The inlet temperature.
TemperatureOut / To Calc The outlet temperature.
DeltaT / dT Calc The change in temperature (To-Ti).
PressureIn / Pi Calc The inlet pressure.
PressureChange / dP Calc The change in pressure.
PressureOut / Po Calc The outlet pressure.
DensityIn / Rhoi Calc The inlet density.
DensityOut / Rhoo Calc The outlet density.
VapourFracIn / Vfi Calc The inlet vapour fraction.
VapourFracOut / Vfo Calc The outlet vapour fraction.
VapourFrac / Vf Calc The outlet vapour fraction.

Steam

Only available in Build 139 or later. This page is only visible if the SteamCalc option is ticked on the first tab page.

Tag (Long/Short) Input / Calc Description/Calculated Variables / Options
Steam Requirement Calculation
SteamSpecification/SteamSpec Saturated T The steam requirement will be based on saturated steam at a user specified temperature.
Saturated P The steam requirement will be based on saturated steam at a user specified pressure.
SuperHeat T,P The steam requirement will be based on superheated steam at a user specified temperature and pressure.
SuperHeat dT,P The steam requirement will be based on superheated steam at a user specified superheat and pressure.
TemperatureReqd/T_Reqd Input/Calc Steam temperature requirement - This field can be an input or a calculated variable depending on the method selected. Please make sure this is higher than the simple heater outlet temperature.
PressureReqd/P_Reqd Input/Calc Steam pressure requirement - This field can be an input or a calculated variable depending on the method selected.
SuperHeatReqd/P_Reqd Input Only visible if SteamSpecification = SuperHeat dT,P. Steam superheat requirement - The number of degrees of superheat above the saturation temperature.
HeatLossFrac Input The fraction of heat loss from the steam. As this increases, the amount of steam required for a fixed duty will increase.
Results
SuperHeat Calc This is the actual duty for the simple heater (thus duty calculated on the first tab page). It is used for all calculations.
DutyReqd Calc This is the required duty of the steam condensation. This is calculated from the actual duty for the simple heater (thus duty calculated on the first tab page) but taking into account any heat losses based on the user specified HeatLossFrac. It should be equal to SteamMassFlowReqd*(SteamEnthalpy-CondEnthalpy).
SteamMassFlowReqd/QmReqd Calc The calculated mass flow of the steam required to meet the calculated DutyReqd.
SteamEnthalpy Calc The specific enthalpy of the steam based on the user specifications.
CondEnthalpy Calc The specific enthalpy of condensed steam (water) based on the user specifications.
Steam.SatT Calc The temperature of the steam at saturated temperature. This is the condensate out temperature, used for LMTD calculations.
In.T Calc The temperature of the simple heater inlet stream, used for LMTD calculations.
Out.T Calc The temperature of the simple heater inlet stream, used for LMTD calculations.
LMTD Calc Log Mean Temperature Difference. This value can be used in the Heater Size Calculations. This value is only valid if the steam temperature is higher than the simple heater outlet temperature.

HXCalc

This page is only visible if the HXSizeCalc option is ticked on the first tab page.

Tag (Long/Short) Input / Calc Description/Calculated Variables / Options
HX Sizing Calculations
ModelDuty Calc This is the actual duty for the Simple Heater (thus duty calculated in the first tab page). It is used for all calculations.
CalculateWhat HTC This allows the user to calculate the required HTC based on Actual Simple Heater Duty and specifying Area and LMTD.
Area This allows the user to calculate the required Area based on Actual Simple Heater Duty and specifying HTC and LMTD.
LMTD This allows the user to calculate the required LMTD based on Actual Simple Heater Duty and specifying HTC and Area.
In Build139 and later, only visible if "UseSteamCalcLMTD" option (SimpleHeater tab) is NOT selected.
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
LMTD Input/Calc Log Mean Temperature Difference - This field can be an input or a calculated variable depending on the method selected.
In Build139 and later, if options "SteamCalc" and "UseSteamCalcLMTD" (SimpleHeater tab) are selected, then the LMTD calculated by the SteamCalc (steam tab) will be displayed here.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

  DLL:
HeatExchange.dll
Units/Links Heat Transfer: Simple Heater
DLL:(Build136 or earlier)
HeatXCh1.dll
Units/Links Heat Transfer: Simple Heater
or Group:
Energy Transfer
Units/Links Heat Transfer: Simple Heater

See Model Selection for more information on adding models to the configuration file.


Insert into Project

  Insert Unit Heat Transfer Simple Heater

See Insert Unit for general information on inserting units.

Example Project

Demo Ammonia Project