Embedded Heat Exchanger (HX)

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Navigation: Models ➔ Sub-Models ➔ Embedded Heat Exchanger (HX)

Related Links: Evaporator


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

Embedded HX.png

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]\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]

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]\mathbf{\mathit{Q=m(H_{in}-H_{out})}}[/math]
where
Q - Rate of Heat Transfer
m - Mass flow of the stream
Hin - Enthalpy of entering stream
Hout - 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, 6th 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
NumofHeater Input Visible when Embedded Heater / cooler is used. Specifies the number of embedded heat exchangers in the evaporator.
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.
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).
CondensingMethod
(Only visible when Embedded Heater is used)
All The Steam addition is manually specified by the user. All steam will condense.
All (Calc Demand) The steam demand is calculated base 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 Embedded Cooler is used)
All The component defined in the VLE Tab will fully evaporate.
All (Calc Demand) The demand is calculated base on the user specified Heater 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.
DutyReqd Input The required fixed heat flow to be exchanged between the two streams. Only visible when Fixed Heat Flow method is used.
HTC Input/Calc Visible with method set to any of the demand methods, allows the user to specify the heat transfer coefficient.
Area Input/Calc Visible with method set to any of the demand methods, allows the user to specify the heat transfer area per heater.
Heater / Cooler Results
Duty Calc The calculated Heat Exchanger Duty
TotalArea Calc The total Area used (HX Area * number of HX).
U*A / UA Calc The heat exchanger UA.
LMTD Calc The calculated log mean temperature difference.
TheoreticalDuty / TheorDuty Calc The theoretical duty of the heat exchanger.
HeatFlow Calc The total heat exchanged.
Calculated Vapour/Liquid Flow Demand
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 mass imbalance in the unit operation. This will warn the user if the amount of flow supplied exceeds the heat exchanger requirements. Most common when the heater is not in Demand General or Demand FlashTrain mode and the steam flow has not been set up to auto adjust.
Tube Side (Recycle Stream)
Tube.TemperatureIn / Tube.Ti Calc Tube in temperature.
Tube.TemperatureOut / Tube.To Calc Tube out temperature.
Tube.PressureIn / Tube.Pi Calc Tube in pressure.
Tube.MassFlow / Tube.Qm Calc The mass flowrate going through the tube side..
Shell Side (Condensing / Evaporating)
Shell.TemperatureIn / Shell.Ti Calc Shell in temperature.
Shell.TemperatureOut / Shell.To Calc Shell out temperature.
Shell.PressureIn / Shell.Pi Calc Shell in pressure.
Shell.MassFlow / Shell.Qm Calc The mass flowrate going through the Shell side.
Shell.DmdQm Calc Only visible if part of a flash train. 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 Only visible if part of a flash train. 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)).