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 submodel 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:
 The EHX submodel is solved sequentially with any other submodels 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.
 The final temperature of the material from the unit may NOT be the same as the temperature of the material from the EHX submodel if there are other submodels 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 both EHX and CEHX are allowed, then the method is valid for both types of environmental heat exchanger, otherwise it is only valid for the type stated.
Heat Transfer Models  EHX  CEHX  Description 

None  N/A  N/A  This is equivalent to turning the EHX model off  there is no heat transfer between the unit and the environment. 
LossPerQm  Yes  Yes  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. 
Product Temp  Yes  Yes  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  Yes  No  The user specifies the required temperature drop between the material entering and exiting the submodel:
Note that the Temp Change method below provides similar functionality. 
Temp Change  Yes  No  The user specifies the required temperature change for the material entering and exiting the submodel:

Loss to Ambient  Yes  No  ***** Method will be discontinued. Use 'Loss to Ambient 4' instead! ***** This method is normally used for heat transfer between material in pipes and the environment. SysCAD calculates the overall heat transfer coefficient from various userdefined parameters and estimates the final product temperature based on the assumption that the environment temperature remains constant. 
Loss to Ambient 2  Yes  Yes  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  Yes  Yes  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. 
Loss to Ambient 4  Yes  No  (Available in most recent version of Build138 and later, this is the corrected method for Loss to Ambient). This method is normally used for heat transfer between material in pipes and the environment. SysCAD calculates the overall heat transfer coefficient from various userdefined parameters and estimates the final product temperature based on the assumption that the environment temperature remains constant. Temperature change in the pipe is taken into account to calculate LMTD. 
Loss to Ambient 5  Yes  No  (Available in Build139 and later). This method is normally used for heat loss calculations from a pipe that has scale on the inside and insulation on the outside and explicitly includes radiation heat loss. SysCAD calculates the outer layer surface temperature. If there is insulation then the outer layer is the surface temperature of the insulation, if there is no insulation, then it is the temperature of the outer surface of the pipe. 
SuperHeat Loss  Yes  No  This method is normally used for heat transfer between material in pipes and the environment. SysCAD calculates the overall heat transfer coefficient from various userdefined parameters and estimates the final product temperature based on the assumption that the environment temperature remains constant. 
Fixed Heat Flow  Yes  Yes  The user specifies a fixed heat flow to or from the unit:

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:
 [math]\displaystyle{ \mathbf{\mathit{Q=\dot{m}\int Cp dT}} }[/math]
 [math]\displaystyle{ \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.
Method  Notes  Equation  Terms 

Loss to Ambient 
The equations are used to determine the UA value in the heat transfer calculation. NOTE: This method will be discontinued in future updates, contains conversion factor error for conductivity, please use Loss to Ambient4 for most recent build of Build138 and later. 


Loss to Ambient 2 
This equation is used to calculate UA. 


Loss to Ambient 3 
The UA is specified by the user. 


Loss to Ambient 4 
The equations are used to determine the UA value in the heat transfer calculation.



Loss to Ambient 5 
Available in Build139 or later. The equations are used to determine the outer layer surface temperature (T_{s}).



Notes for "Loss to Ambient 4"
 The pipe may have a layer of insulation on it.
 The pipe and insulation are assumed to have uniform properties and constant diameter along the length.
 The area used for heat transfer calculations is based on the inside diameter of the pipe.
 The thickness of the insulation layer may be set to 0 and the insulation will be ignored in the calculations.
 Radiation convection coefficient may be used to approximate radiation heat transfer from the outer surface. Typical values for insulated pipes are between 5 and 10 W/m^2.K. It may be set to 0 and will then be ignored in calculations. This approximation for radiative heat loss is not valid for hot uninsulated pipes and radiative losses will need to be calculated explicitly for those cases.
Data Sections
A description of the variables on the EHX tab page is given here.
Tag (Long/Short)  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 submodel or material in the content of the unit.  
TempChange  The user may specify a required temperature change across the submodel. A positive value will give an temperature rise, while a negative value will result in a temperature drop. Only available in flow evaluation blocks (not content evaluation blocks).  
Loss to Ambient  This method will be discontinued in future updates, contains conversion factor error for conductivity, please use Loss to Ambient4 for most recent build of Build138 and later SysCAD calculates the overall heat transfer coefficient from various userdefined 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 Ambient 2  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 Ambient 3  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.  
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 in flow evaluation blocks (not content evaluation blocks).  
FixedHeatFlow  The user specifies a fixed heat loss/gain.  
Loss to Ambient 4  (Available in most recent version of Build138 and later, this is the corrected Loss to Ambient method.) SysCAD calculates the overall heat transfer coefficient from various userdefined parameters and estimates the final product temperature based on the assumption that the environment temperature remains constant. This method is normally used for heat transfer between material in pipes and the environment, temperature change in the pipe is taken into account to calculate LMTD. Only available in flow evaluation blocks (not content evaluation blocks).  
Loss to Ambient 5  (Available in Build139 and later.) SysCAD calculates the heat transfer from various userdefined parameters and estimates the final product temperature based on the specified reference temperature. This method is normally used for heat transfer between material in pipes and the environment. Only available in flow evaluation blocks (not content evaluation blocks).  
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 Change (TempChange) method.  
DeltaT_Reqd / dT_Reqd  Input  The required temperature change across the submodel. SysCAD will calculate the heat loss/gain required to produce the required temperature change. 
Loss to Ambient method. (This method will be discontinued in future updates, contains conversion factor error for conductivity, please use Loss to Ambient4 for most recent build of Build138 and later)  
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 1.0e6 W/m.K (NOTE: This is out by factor of 1000) 
Ins_Cond  Input  The conductivity (ki) of the insulation material. Minimum value is set at 1.0e6 W/m.K (NOTE: This is out by factor of 1000) 
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  Feed  Use the Feed temperature to the submodel as the reference temperature in the heat loss calculation. 
Product  Use the Product temperature from the submodel 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.  
RefTemp  Feed  Use the Feed temperature to the submodel as the reference temperature in the heat loss calculation. 
Product  Use the Product temperature from the submodel 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. 
Loss to Ambient 4 method. (Available in most recent version of Build138 and later, this is the corrected Loss to Ambient method.)  
Diameter / 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.Thickness  Input  Pipe / vessel wall thickness. 
Pipe.Cond  Input  The conductivity (kp) of the pipe / vessel. Minimum value is set at 1.0e6 W/m.K 
Ins.Thickness  Input  Thickness of the insulation used (wi) NB This can be set to 0 and insulation will be ignored. 
Ins.Cond  Input  The conductivity (ki) of the insulation material. Minimum value is set at 1.0e6 W/m.K 
HTC.Stream  Input  Heat transfer coefficient (h1) of the fluid flowing inside the pipe/vessel. 
HTC.EnvAir  Input  Heat transfer rate due to Air flow in the environment (h2). 
HTC.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 Ambient 5 method. (Available in Build139 and later)  
Diameter / Diam  Input  Inside diameter (ID) of the pipe / vessel. Straight Cylinder is assumed. Minimum ID is set at 1 μm. 
Length  Input  Length (L) of the pipe / vessel. Straight Cylinder is assumed. Minimum length is set at 1mm. 
RefTemp  Feed  Reference temperature used for the calculation is the feed temperature. (Ti) 
Product  Reference temperature used for the calculation is the product temperature. (Ti)  
Mean  Reference temperature used for the calculation is the average of Feed and Product temperature. (Ti)  
AmbTOverride  Input  The ambient temperature override. This overrides the ambient temperature specified in the Plant Model  Environment Tab 
Ambient_T  Display  The ambient temperature (To). The ambient temperature is specified in the Plant Model  Environment Tab 
Scale.Thickness  Input  The build up of material inside the pipe, thus reducing the ID of pipe. For a clean pipe, this value = 0. 
Pipe.Thickness  Input  Pipe / vessel wall thickness. 
Ins.Thickness  Input  Thickness of the insulation used, NB This can be set to 0 and insulation will be ignored. 
Scale.Cond  Input  Thermal conductivity (ka) of the scale build up material. Minimum value is set at 1.0e5 W/m.K 
Pipe.Cond  Input  Thermal conductivity (kb) of the pipe / vessel. Minimum value is set at 1.0e5 W/m.K 
Ins.Cond  Input  Thermal conductivity (kc) of the insulation material. Minimum value is set at 1.0e5 W/m.K 
HTC.Stream  Input  Convection heat transfer coefficient (hi) to fluid inside the pipe/vessel. 
HTC.EnvAir  Input  Convection heat transfer coefficient (ho) to the environment (ho). 
Emissivity  Input  The ratio of the thermal radiation from a surface to the radiation from an ideal black surface. This value should be less than 1. 
Area  Calc  The heat transfer area. ([math]\displaystyle{ 2 \pi r_2 L }[/math]) see Loss to Ambient 5 
Pipe.Flux  Calc  Rate of energy transfer / unit area (pipe ID), see Q_{c}' in Loss to Ambient 5 
Pipe.HTC  Calc  Overall heat transfer coefficient from stream to ambient based on convection and conduction only. 
Pipe.HTCeff  Calc  Effective overall heat transfer coefficient from stream to ambient including convection, conduction and radiation. 
PipeOD.Temperature / T  Calc  Temperature at pipe outside diameter, see T_{pos} in Loss to Ambient 5 
Surface.Temperature / T  Calc  Temperature at outside surface, see T_{s} in Loss to Ambient 5 
HeatFlow / T  Calc  Heat flow from pipe. NB heat flow from pipe to ambient is negative in sign. 
Feed.Temperature / T  Calc  Temperature of flow into pipe. 
Prod.Temperature / T  Calc  Temperature of flow out of pipe after heat transfer. 
MeanTemperature  Calc  Average temperature between feed and product temperatures. 
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)  
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  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 submodel (Product Temperature  Feed Temperature). 
DeltaT_Rate  Calc  The rate of temperature change across the submodel 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:
 Temperature difference for heat transfer is based only on the RefTemp (submodel 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 submodel, use this method with caution or consider an alternative heat loss calculation method.
 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.
 HeatFlow = HTC*Area*(Ambient_T  RefTemp) (ie. heat losses are reported as a negative heat flow). SysCAD prevents temperature crossover 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 submodel. 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 (373.15°C).  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
 For EHX used in a Pipe note that any pressure drop is applied before the EHX.
 If EHX is used in combination with a Reaction Block (RB), the two submodels will be evaluated in sequence. If the reactions are very endothermic/exothermic, it may cause the Reaction Block product temperature to reach the project limits before applying the separate EHX, resulting in an incorrect result. To avoid this, please use the Heat Exchange option of the reaction file (RHX) instead of the EHX, as the RHX is evaluated simultaneously with the reactions.
 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.