Simple Heater

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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

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]\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

Feed Flow Calculations

Available from Build 139.33457. Based on user specified temperature requirement, SysCAD will calculate the required mass flow for the feed stream. This can be simply calculated or passed through the feed stream as a Demand flow. The calculated value can be used as a setpoint for an external controller.

Optional Side Calculations

1. SteamCalc: Steam requirements side calculation: (Available from Build 139.)

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]\displaystyle{ \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.

2. 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]\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 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)

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. FeedCalc tab - Available from Build 139.33457. This optional tab allows the user to calculate the feed mass flow requirement based on an outlet temperature requirement. Only visible if the FeedCalc option is enabled.
4. Steam tab - Available from Build 139. 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.
5. 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.
6. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
7. Links tab, contains a summary table for all the input and output streams.
8. 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 UnitType or SubClass.
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	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	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.
FeedCalc	Tick Box	Available from Build 139.33457. This field is only visible if Fixed Duty or Duty Tag is chosen for Method. This option will add the FeedCalc tab for calculations of required feed flow to meet temperature requirement based on the actual duty of the heater.
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	Available from Build 139. 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	Available from Build 139. Only visible if SteamCalc and HXSizeCalc options are 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	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.

FeedCalc

Available from Build 139.33457. This page is only visible if the FeedCalc option is ticked on the first tab page.

Tag (Long/Short)	Input / Calc	Description/Calculated Variables / Options
FC...
Flow Requirement Calculation to achieve a target Temperature (This section uses the actual duty of the heater to calculate the required flow of the feed stream 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	Product Temperature	This allows the user to specify the required outlet temperature. Using the actual heater duty, this is used to determine the required flow of the feed stream.
	Temperature Change	This method allows the user to specify the required temperature change across the heater. Using the actual heater duty, this is used to determine the required flow of the feed stream.
TemperatureReqd / T_Reqd	Input	This field is only visible if Product Temperature is chosen for Method. The required product temperature.
DeltaT_Reqd / dT_Reqd	Input	This field is only visible if Temperature Change is chosen for Method. The required temperature change across the heater. A positive value indicates a temperature rise across the heater while a negative value indicates a temperature drop across the heater.
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 (MassFlow.Reqd) will be considered a target and the user will not receive any warning messages if MassFlow.Actual does not equal MassFlow.Reqd.
Results
TemperatureIn / Ti	Calc	The temperature of the feed stream. If the Temperature Drop, Temperature Rise or Temperature Change methods are used, this value is used to determine the required outlet temperature.
TargetTemperature / TargetT	Calc	The calculated Target Temperature. This is determined based on the preceding temperature requirement and may also depend on the actual feed temperature (TemperatureIn), as well as other limiting factors such as project temperature.
OutletT.Reqd	Calc	The calculated Target Temperature. This is determined based on the preceding temperature requirement and may also depend on the actual feed temperature (TemperatureIn).
OutletT.Actual	Calc	The actual outlet temperature. This will be the same as shown on the first tab page and is just shown here for comparison.
OutletT.Error	Calc	The difference between the required temperature and the actual temperature (OutletT.Reqd - OutletT.Actual).
OutletT.RelError / RelErr	Calc	The relative difference between the required temperature and the actual temperature (OutletT.Reqd - OutletT.Actual) / OutletT.Reqd (calculated in K).
Calculated Feed Flow Demand
MassFlow.Reqd	Calc	The required mass flow in order to achieve the Target Temperature.
MassFlow.Actual	Calc	The actual mass flow. This will be the same as shown on the first tab page and is just shown here for comparison.
MassFlow.Error	Calc	The difference between the required mass flow and the actual mass flow (MassFlow.Reqd - MassFlow.Actual).
MassFlow.RelError / RelErr	Calc	The relative difference between the required mass flow and the actual mass flow (MassFlow.Reqd - MassFlow.Actual) / MassFlow.Reqd.

Steam

Available from Build 139. 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

Add to Configuration File

Sort either by DLL or Group:

	DLL:	HeatExchange.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 Flowsheet

Insert Unit

→

Heat Transfer

→

Simple Heater

See Insert Unit for general information on inserting units.

Example Project

Demo Ammonia Project