Environmental Heat Exchanger (EHX)

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General Description

This is a simple method of representing heat transfer between the unit and the environment. The user may set the final temperature in the unit, or an energy loss or gain to the unit and the EHX sub-model will calculate and display the variable 'HeatFlow' based on the user defined heat transfer or final temperature:

  • A positive value denotes heat flows into the unit;
  • A negative value denotes heat loss out of the unit.

The feed and product temperatures are also displayed for user information.

Notes:

  1. The EHX sub-model is solved sequentially with any other sub-models that may be enabled in the Evaluation Block (EB) in a unit. Therefore, the user must be careful to ensure that the order of evaluation in the EB will produce the desired results.
  2. The final temperature of the material from the unit may NOT be the same as the temperature of the material from the EHX sub-model if there are other sub-models enabled after the EHX.

Video Links

The following video is part of the Tutorial showing users how to insert and configure a EHX:

Heat Transfer Models

The method of transferring the energy between the unit and the environment is defined by the selection chosen in the 'Model' field. There are a number of options. Some of these options are valid for both Steady State and Dynamic EHX blocks, and some will only be valid in one or the other:

  • The term EHX, or Environmental Heat Exchanger, is used for steady state projects or in units with no content in Dynamic projects.
  • The term CEHX, or Content Environmental Heat Exchanger, is used in units with content in Dynamic modelling. Only available in Build 137 or later.
  • If the heading has EHX and CEHX then the method is valid for both types of environmental heat exchanger, otherwise it is only valid for the type stated.

None (EHX & CEHX)

This is equivalent to turning the EHX model off - there is no heat transfer between the unit and the environment.

LossPerQm (EHX)

The user is able to specify a heat loss per unit of mass flowing through the unit operation. A positive number indicates heat transferred to the environment. A negative value denotes heat flowing from the environment to the unit operation.

This is normally used in pipes to emulate heat exchange with the environment as the material flows through the pipe.

Product Temp (EHX & CEHX)

The user specifies the final temperature of the material exiting from or inside the unit. This is very useful where the user knows the final temperature from a unit, or where user wishes to control the temperature in a unit.

It may also be used in the exit pipe from a unit that has a PID unit controlling the exit temperature: If the PID measures the Inlet (Ti) temperature of the pipe, the user may then use the EHX to ensure that the temperature out of the pipe (To) remains constant and equal to the PID set point, while the PID settles down.

Temp Drop (EHX)

The user specifies the required temperature drop between the material entering and exiting the sub-model:

  • A positive drop means that the temperature will drop;
  • A negative drop means that the temperature will rise across the unit.

Note that the Temp Change method below provides similar functionality.

Temp Change (EHX)

The user specifies the required temperature change for the material entering and exiting the sub-model:

  • A positive change means that the temperature will rise;
  • A negative change will give a temperature drop across the unit.

This method is only available in SysCAD 9.3

Loss to Ambient (EHX)

This method is normally used for heat transfer between material in pipes and the environment. SysCAD calculates the overall heat transfer coefficient from various user-defined parameters and estimates the final product temperature based on the assumption that the environment temperature remains constant.

Loss to Ambient 2 (EHX & CEHX)

This method is normally used for heat transfer between material in a tank or container and the environment. SysCAD calculates the heat loss based on user specified Reference temperature (Feed or Product), heat transfer coefficient (HTC) and Area. This calculation is similar to the process unit Simple Heater.

Loss to Ambient 3 (EHX & CEHX)

This method is similar to the method above, Loss to Ambient 2, except that the user only specifies a heat loss factor instead of both a heat transfer coefficient (HTC) and an Area. SysCAD then calculates the heat loss based on the user specified Reference Temperature and Heat Loss Factor.

This method is only available in SysCAD 9.3

SuperHeat Loss (EHX)

This method allows the user to lose some of the heat from a superheated stream - normally superheated steam. The user may specify the approach to the saturated temperature. (The saturated temperature is calculated at the unit pressure). If this approach temperature = 0, the stream exiting the sub-model will be at the saturated temperature.

This method is only available in SysCAD 9.3

Fixed Heat Flow (EHX & CEHX)

The user specifies a fixed heat flow to or from the unit:

  • A negative flow is for heat flowing out of the unit;
  • A positive flow is for heat flowing into the unit.

In SysCAD 9.3, the user can limit the temperature in a number of ways such as limiting the temperature drop or rise, or specifying a minimum or maximum temperature. The model will not add more or remove more heat than requested but may add less or remove less heat if one of the temperature limits is reached.

  • If the feed temperature is already below the minimum temperature, then no heat will be removed.
  • If the feed temperature is already above the maximum temperature, then no heat will be added.

Model Theory

The equations used in the three Loss to Ambient Methods are as follows:

Given the flow conditions and the heat transfer data, total heat flow between the stream and the environment and the final stream outlet temperature are estimated from the following two equations:

  1. [math]\mathbf{\mathit{Q=m\int Cp dT}}[/math]
  2. [math]\mathbf{\mathit{Q=UA*\Delta T}}[/math]
  • The UA value is calculated differently for each of the methods, as described in the sections below.
  • ΔT = (Ambient_T - Ref_T). The ambient (environmental) temperature is assumed to remain constant.
  • The temperature of the material in the unit that is used for the calculation (Ref_T) may either be the Feed or the Product Temperature (for methods 2 and 3), depending on which value the user has selected.

Loss to Ambient

This method is normally used to calculate the heat transfer between a pipe and the ambient conditions. The following equations are used to determine the UA value in the heat transfer calculation:

[math]\mathbf{\mathit{UA=\frac{1}{\frac{1}{h_1A_1}+\frac{w_p}{k_pA_2}+\frac{w_i}{k_iA_3}+\frac{1}{(h_2+0.95h_r)A_4}}}}[/math]
[math]\mathbf{\mathit{A_1=2\pi rL}}[/math]
[math]\mathbf{\mathit{A_2=\left[A_1+2\pi \left(r_1+w_p\right)L\right]/2}}[/math]
[math]\mathbf{\mathit{A_3=\left[A_4+2\pi \left(r_1+w_p\right)L\right]/2}}[/math]
[math]\mathbf{\mathit{A_4=2\pi \left(r+w_p+w_i\right)L}}[/math]
r = Inside pipe diameter (ID) / 2
L = length of the straight pipe.
wp = Pipe / vessel wall thickness. (wp = OD - ID)
wi = insulation thickness
kp = The thermal conductivity of the pipe / vessel
ki = The thermal conductivity of the insulation material
h1 = Heat transfer film coefficient of the fluid flowing inside the pipe/vessel.
h2 = Heat transfer film coefficient for the outside surface of the pipe/vessel to the environment.
hr = Heat transfer film coefficient estimate for thermal radiation effects.

Loss to Ambient 2

In this case the following simple equation is used to calculate UA:

[math]\mathbf{\mathit{UA=HTC * A}}[/math]
where
HTC = Overall Heat Transfer Coefficient.
Area = The total area available for heat transfer between the unit and the environment.

Loss to Ambient 3

In this case UA = the user defined Heat Loss Factor.

(This method is only available in SysCAD 9.3)

Data Sections

A description of the variables on the EHX tab page is given here.

Tag/Symbol Input/Calc Description
Model None No heat transfer between the unit and the environment
LossPerQm The user is able to specify a heat loss per unit of mass flowing through the unit. A positive number indicates heat transferred to the environment. A negative value denotes heat flowing from the environment to the unit. Only available in flow evaluation blocks (not content evaluation blocks).
ProductTemp The user may specify the final temperature of the material exiting from the sub-model or material in the content of the unit.
TempDrop The user may specify the required temperature drop. Not available in SysCAD 9.3. Only available in flow evaluation blocks (not content evaluation blocks).
TempChange The user may specify a required temperature change across the sub-model. A positive value will give an temperature rise, while a negative value will result in a temperature drop. Only available flow evaluation blocks in SysCAD 9.3 (not content evaluation blocks).
Loss to Ambient SysCAD calculates the overall heat transfer coefficient from various user-defined parameters and estimates the final product temperature based on LMTD. The environmental temperature is assumed to remain constant. Only available in flow evaluation blocks (not content evaluation blocks).
Loss to Ambient2 SysCAD calculates the overall heat loss based on user specified HTC and Area and the temperature difference between the Feed and environmental temperatures. The environmental temperature is assumed to remain constant.
Loss to Ambient3 SysCAD calculates the heat loss to the Environment based on the user specified Reference temperature and a Heat Loss Factor. The environmental temperature is assumed to remain constant. Only available in SysCAD 9.3.
SuperHeat Loss The user specifies an approach temperature to the saturated temperature at the unit pressure. (The species used for saturation calculations is specified in the field SaturationCmp on the 'Species' page of Plant Model - see Plant Model - Species). Only available flow evaluation blocks in SysCAD 9.3 (not content evaluation blocks).
FixedHeatFlow The user specifies a fixed heat loss/gain.
Loss per Mass Flow (LossPerQm) method.
LossPerQm Input The required Energy change per unit mass, +ve number for heat loss, -ve number for heat gain.
Product Temperature (ProductTemp) method.
TemperatureReqd / T_Reqd Input The required product temperature. SysCAD will calculate the heat loss/gain required to maintain the unit at the required temperature.
DeltaT_RateLimit Input The maximum rate of change of temperature with time. Only shown for content evaluation blocks.
Temperature Drop (TempDrop) method. Not available in SysCAD 9.3.
TempDrop Input The required product temperature drop. SysCAD will calculate the heat loss for the required temperature drop.
Temperature Change (TempChange) method. Only available in SysCAD 9.3.
DeltaT_Reqd / dT_Reqd Input The required temperature change across the sub-model. SysCAD will calculate the heat loss/gain required to produce the required temperature change.
Loss to Ambient method.
Diam Input Inside diameter (ID) of the pipe / vessel. Straight Cylinder is assumed. Minimum ID is set at 1 mm.
Length Input Length (L) of the pipe / vessel. Straight Cylinder is assumed. Minimum length is set at 1mm.
AmbTOverride Input The ambient temperature override. This overrides the ambient temperature specified in the Plant Model - Environment_Tab
Ambient_T Display The ambient temperature. The ambient temperature is specified in the Plant Model - Environment_Tab
Pipe_Thick Input Pipe / vessel wall thickness. (wp = OD ID)
Ins_Thick Input Thickness of the insulation used (wi)
Pipe_Cond Input The conductivity (kp) of the pipe / vessel. Minimum value is set at 1.0e-6 W/m.K
Ins_Cond Input The conductivity (ki) of the insulation material. Minimum value is set at 1.0e-6 W/m.K
CnvHTC_Liq_Pipe Input Heat transfer coefficient (h1) of the fluid flowing inside the pipe/vessel.
CnvHTC_Lag_Air Input Heat transfer rate due to Air flow in the environment (h2).
CnvHTC_Radiation Input Heat transfer rate due to radiation (hr).
U*A / UA Calc Calculated overall heat transfer based on equation given in the model theory.
Power Calc Calculated Power.
LMTD Calc Calculated Log Mean Temperature Difference.
Loss to Ambient2 method.
RefTemp
(Only available in SysCAD 9.3)
Feed Use the Feed temperature to the sub-model as the reference temperature in the heat loss calculation.
Product Use the Product temperature from the sub-model as the reference temperature in the heat loss calculation.
AmbTOverride Input The ambient temperature override. This overrides the ambient temperature specified in the Plant Model - Environment_Tab
Ambient_T Display The ambient temperature. The ambient temperature is specified in the Plant Model - Environment_Tab
HTC Input The Overall Heat Transfer coefficient.
Area Input The Heat Transfer area.
Loss to Ambient3 method. Only available in SysCAD 9.3.
RefTemp Feed Use the Feed temperature to the sub-model as the reference temperature in the heat loss calculation.
Product Use the Product temperature from the sub-model as the reference temperature in the heat loss calculation.
AmbTOverride Input The ambient temperature override. This overrides the ambient temperature specified in the Plant Model - Environment_Tab
Ambient_T Display The ambient temperature. The ambient temperature is specified in the Plant Model - Environment_Tab
HeatLossFactor Input The Heat Loss factor used to calculate the heat flow between the material in the unit and the environment.
SuperHeat Loss method. (The species used for saturation calculations is specified in the field SaturationCmp on the 'Species' page of Plant Model - see Plant Model - Species) Only available in SysCAD 9.3.
ApproachT Input The approach temperature to the Saturated temperature at the unit pressure.
Fixed Heat Flow method.
HeatFlowReqd Input The required Energy change, -ve number for heat loss, +ve number for heat gain.
TempLimitMethod
(Only available in SysCAD 9.3)
None No temperature limits.
Temp Drop User specifies a maximum temperature drop.
Temp Rise User specifies a maximum temperature rise.
Temp Drop and Rise User specifies a maximum temperature drop and a maximum temperature rise.
Minimum Temp User specifies a minimum product temperature.
Maximum Temp User specifies a maximum product temperature.
Min and Max Temp User specifies minimum and maximum product temperatures.
MaxTempDrop / MaxTDrop Input Only available if the Temp Drop or Temp Drop and Rise Temp Limit Methods are chosen. The maximum temperature drop (decrease) from the feed temperature to the product temperature.
Note: Must be a positive number (>= 0). If a negative number is specified, this will be converted to a positive number upon running.
MaxTempRise / MaxTRise Input Only available if the Temp Rise or Temp Drop and Rise Temp Limit Methods are chosen. The maximum temperature rise (increase) from the feed temperature to the product temperature.
Note: Must be a positive number (>= 0). If a negative number is specified, this will be converted to a positive number upon running.
MinTemperature / MinT Input Only available if the Minimum Temp or Min and Max Temp Temp Limit Methods are chosen. The minimum product temperature.
MaxTemperature / MaxT Input Only available if the Maximum Temp or Min and Max Temp Temp Limit Methods are chosen. The maximum product temperature.
Note: If using the Min and Max Temp method, the maximum temperature must be greater than or equal to the minimum temperature. If a maximum temperature less than the minimum temperature is specified, then the maximum will be set equal to the minimum upon running.
TrackTempLimit Tick Box Only available if one of the Temp Limit Methods are chosen. If disabled, the user will not receive warning messages if one of the temperature limits is reached, preventing the user specified heat flow to be added or removed.
LimitState Display Only displayed if one of the Temp Limit Methods are chosen. Displays the status of the temperature limit test/s.
The following is applicable for all methods.
HeatFlow Calc The calculated heat flowrate.
Feed.Temperature / Feed.T Calc The temperature of material before heat exchange has taken place. Not shown if Model=None.
Prod.Temperature / Prod.T Calc The temperature of material after heat exchange has taken place. Not shown if Model=None.
DeltaT / dT Calc The temperature change across the sub-model (Product Temperature - Feed Temperature).
DeltaT_Rate Calc The rate of temperature change across the sub-model DeltaT/Timestep. Only shown in content evaluation blocks.
MassFlow / Qm Calc The mass flow of material entering the sub model. Only shown in flow evaluation blocks (not content evaluation blocks).
Mass Calc The mass of material in the content of the unit. Only shown in content evaluation blocks.

Notes on using EHX Loss to Ambient2 method:

  1. Temperature difference for heat transfer is based only on the RefTemp (sub-model Feed or Product temperature) and environmental temperatures and does not include the final product temperature from the unit - if there are large temperature changes, significant heats of reaction, flashing or condensation occurring after the EHX sub-model, use this method with caution or consider an alternative heat loss calculation method.
  2. If the RefTemp temperature is less than the ambient temperature, then it is possible to have a heat gain and the product temperature will be higher than the feed temperature.
  3. HeatFlow = HTC*Area*(Ambient_T - RefTemp) (ie. heat losses are reported as a negative heat flow). SysCAD prevents temperature cross-over if the calculated heat flow is very large due to a large HTC and/or area. Hence, if RefTemp is greater than Ambient_T, then the product temperature will be greater than or equal to the Ambient Temperature. Similarly, if RefTemp is less than Ambient_T, then the product temperature will be less than or equal to the Ambient Temperature. In either of these cases the actual HeatFlow reported will be less than the HeatFlow calculated by the equation given above.

Error Messages associated with EHX

This section contains a list of Error Messages that you may get when using the EHX sub-model. It includes reasons why you may see these errors and also hints as to how you can fix the error.

This is not a comprehensive list of all of the errors that are possible, but it is meant to assist when you are debugging a SysCAD project.

Error Message Possible Causes Hints and Fixes
Product temperature at Limit The unit contains liquid water and the required Product temperature exceeds the critical temperature of water (373.15C). 1) Decrease the required EHX product temperature below the critical temperature of water.
2) Remove any liquid water from the stream before entering the EHX block, e.g add a reaction block and convert the water to steam.
The required Product temperature exceeds the Maximum Project temperature. You can view this value in the Plant Model Access window - Environment Tab 1) Decrease the required EHX product temperature below the maximum Project temperature
2) Increase the Maximum Project temperature - you must exit the project and edit the Project Configuration file to do this. Please see General Configuration
The required Product temperature is lower than the Minimum Project temperature. You can view this value in the Plant Model Access window - Environment Tab 1) Increase the required EHX product temperature above the minimum Project temperature
2) Decrease the Minimum Project temperature - you must exit the project and edit the Project Configuration file to do this. Please see General Configuration

Hints and Comments

  1. For EHX used in a Pipe note that any pressure drop is applied before the EHX.
  2. If used in combination with a Reaction Block (RB) and if the heat exchange should occur with the reactions, then the Heat Exchange option of the reaction file is more appropriate as the heat exchange is calculated simultaneously with the reactions. Without this approach the reactions may go to temperature limits before applying the separate EHX resulting in an incorrect result.
  3. Care should be taken with the Fixed Heat Flow option - this makes it difficult to make overall changes to flows in the model. In particular if you have pipes as part of a Flash Train macro model, you should avoid using Fixed Heat Flow since this can lead to instability in solving the Flash Train model. Use one of the environmental models or Loss Per Qm so that changes in mass flow will be reflected in changes in heat loss.