Embedded Heat Exchanger (HX)

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Introduction

The Embedded Heat Exchanger is used in the Evaporator models. It allows heating or cooling of Evaporator contents with external media (heating or cooling fluid).

It is enabled by connecting the HX_Shell streams, show in the picture below.

Diagram

To enable the embedded heater, the HX_Shell connections (to and from) must be made, these are highlighted in red in the above picture.

Behaviour when Model is OFF

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

• All streams connected to the 'Shell' inlet will flow out of the 'Shell' outlet with no temperature or phase change;
• All streams connected to the 'Tube' inlet will flow out of the 'Tube' outlet with no temperature or phase change;
• No energy exchange will occur.

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

Model Theory

The unit is based on traditional heat exchanger theory¹,

[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]

Notes:

1. 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.
2. If the heat exchanger has Superheated Steam condensing on the shell side, then the LMTD method will produce small inaccuracies, please see Log Mean Temperature Difference (LMTD) Discussion.

Heat transfer to the individual streams is calculated using the following equation:

[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 LMTD of the unit. This is then used to calculate the heat transfer between the two streams.

Reference

1. Perry, R.H., Perry's Chemical Engineers' Handbook, McGraw Hill Inc, 6^th Edition, 1984.

HX Page

Tag (Long/Short)	Input / Calc	Description/Calculated Variables / Options
Heater / Cooler Requirements
On	Tick Box	Switches the Embedded Heat Exchanger on/off
HeaterCount / NumofHeaters	Input	Specifies the number of embedded heat exchangers in the evaporator. This can also be changed on the first tab page.
Type	Fixed Heat Flow	This will exchange a fixed amount of heat between two streams. No phase change will take place automatically.
	Sensible	This will allow sensible heat exchange between two streams. No phase change will take place. (not currently implemented)
	Condensing	Only visible when Embedded Heater is used. This is used to condense the steam. Steam must be connected to Embedded HX Shell (inlet).
	Evaporating	Only visible when Embedded Cooler is used. This is used to evaporate the water. Water (or another component with Predefined Flashing Data) must be connected to Embedded HX Shell (inlet).
DutyReqd	Input	Only visible when Fixed Heat Flow method is used. The required fixed heat flow to be exchanged between the two streams.
CondensingMethod (Only visible when Type = Condensing)	All	The Steam addition is manually specified by the user. All steam will condense.
	All (Calc Demand)	The steam demand is calculated based on the user specified Heater HTC and Area (UA). User needs to set the steam flow based on the calculated amount.
	All-Demand General	The steam supply comes from (directly or indirectly) a Feeder with Demand.on selected.
	All-Demand Flashtrain	The steam supply comes from another process unit in Flash train mode (usually the vapour from the preceding evaporation stage)
EvaporatingMethod (Only visible when Type = Evaporating)	All	The component defined on the VLE Tab will fully evaporate.
	All (Calc Demand)	The demand is calculated based on the user specified Cooler HTC and Area (UA). User needs to set the flow based on the calculated amount.
	All-Demand General	The flow supply comes from (directly or indirectly) a Feeder with Demand.on selected.
UseOldDemandCalc	Tickbox	Visible with method set to any of the demand methods. Allows users to continue to use old demand calculations for backward compatibility. It is recommended to leave this unticked.
HTC	Input	Visible with method set to any of the demand methods. The user specified Heat Transfer Coefficient (HTC).
Area	Input	Visible with method set to any of the demand methods. The user specified heat transfer area per heat exchanger.
Heater / Cooler Results
Duty	Calc	The calculated Heat Exchanger Duty
TotalArea	Calc	Visible with method set to any of the demand methods. The total Area used (HX Area * number of HX).
UA	Calc	Visible with method set to any of the demand methods. The heat exchanger UA.
LMTD	Calc	The calculated log mean temperature difference.
TheoreticalDuty / TheorDuty	Calc	Visible with method set to any of the demand methods. The theoretical duty of the heat exchanger.
HeatFlow	Calc	The net heat flow to the unit. This should be zero as heat is merely exchanged (no external energy is added or removed).
Calculated Vapour/Liquid Flow Demand This group of tags is only visible when one of the demand methods is chosen.
DemandFlow.Reqd	Calc	The calculated demand flow to meet the Heat Exchanger requirements. When the HX is not in Demand General or Demand FlashTrain mode, use a General controller or SetTag Controller to get this value to set the Embedded HX Shell (in) flowrate.
DemandFlow.Actual	Calc	The actual demand flow to Heater Exchanger .
DemandFlow.Error	Calc	The difference between the required and actual vapour/liquid flows, (DemandFlow.Reqd - DemandFlow.Actual). This will warn the user if the amount of flow supplied doesn't match the heat exchanger requirements. Most common when the heater is not in Demand General or Demand FlashTrain mode and the steam flow is not controlled.
DemandFlow.RelErr	Calc	The relative difference between the required and actual vapour/liquid flows, (DemandFlow.Reqd - DemandFlow.Actual)/DemandFlow.Reqd.
Tube Side (Recycle Stream)
Tube.TemperatureIn / Ti	Calc	Tube in temperature.
Tube.TemperatureOut / To	Calc	Tube out temperature.
Tube.PressureIn / Pi	Calc	Tube in pressure.
Tube.MassFlow / Qm	Calc	The mass flowrate going through the tube side.
Shell Side (Condensing / Evaporating)
Shell.TemperatureIn / Ti	Calc	Shell in temperature.
Shell.TemperatureOut / To	Calc	Shell out temperature.
Shell.PressureIn / Pi	Calc	Shell in pressure.
Shell.MassFlow / Qm	Calc	The mass flowrate going through the Shell side.
Shell.DmdQm	Calc	Visible with method set to any of the demand methods. This is the amount of steam needed from connected flash train units (flash tank or steam feeders), not including any fixed amount of steam input.
Shell.FxdQm	Calc	Visible with method set to any of the demand methods. This is the amount of steam added in manually, thus not from connected flash train units. Note this portion of steam addition is not included in the pressure calculation of the flash train group.
Flash Train Macro Model Note: Extra fields are visible if the unit is part of a Flash Train. These fields are described below. Please refer to Flash Train.
VapourMassFlowReqd / VQmReqd	Calc	The calculated mass flow of steam required by the Embedded Heater Condenser.
CondMassFlow / CondQm	Calc	The amount of steam condensed by the Embedded Heater Condenser.
MinSatPress	Calc	The minimum saturated pressure of steam that could satisfy the heating requirements of the Embedded Heater Condenser.
FlashTrain	Display	A unique tag assigned to the flash train by SysCAD. Each unit in the flash train will have the same tag in this block.
FlashTearBlock	Display	Displays the name of the tear block that is part of the Flash Train.
FlashTrainEqp	List	This contains a list of all of the equipment tags in this flash train. the list might be as follows: FT_EVAPORATOR_2_HX FT_EVAPORATOR_3_Evap
PAdvBase	Input	Additional damping or acceleration for calculated pressure change (DP) for the iteration. DP = DP * PAdvBase when PAdvExtra=0.
PAdvExtra	Input	Additional damping or acceleration for calculated pressure change (DP) for the iteration. DP = DP * (PAdvBase + PAdvExtra*(VQmReqd-VQm)/Max(VQmReqd,VQm)).