CAPITULO IV: RESULTADOS
4.2. Discusión de los resultados
The Heat Exchanger page in the Dynamics tab for most vessel unit operations in UniSim Design contains the options use to configure heat transfer method within the unit operation.
There are three options to choose from:
• None radio button option indicates that there is no energy stream or heat exchanger in the vessel. The Heat Exchanger page is blank and you do not have to specify an energy stream for the unit operation to solve.
Figure 1.10
Figure 1.11
The new strip chart is automatically named:
objectname-DLn where:
objectname = name of the object n = an integer number that increases
each time a new strip chart with the same objectname is created
Refer to Section 11.7 - Databook in the UniSim Design User Guide for more information.
1-22 Common Views
• Duty radio button option indicates that there is an energy stream in the vessel. The Heat Exchanger page contains the UniSim Design standard heater or cooler parameters and you have to specify an energy stream for the unit operation to solve.
• Tube Bundle radio button option indicates that there is heat exchanger in the vessel and enables you to simulate a kettle reboiler or chiller. The Heat Exchanger page contains the parameters used to configure a heat exchanger and you have to specify material streams of the heat exchanger for the unit operation to solve.
Duty Radio Button
When you select the Duty radio button the following options are available.
Heater Type Group
In the Heater Type group, there are two heating methods available to the general vessel operation:
• Vessel Heater
The Tube Bundle option is only available in Dynamics mode.
The Tube Bundle option is only available for the following unit operations: Separator, Three Phase Separator, Condenser, and Reboiler.
Figure 1.12
specified or calculated in the SP field is applied to the vessel’s holdup.
where:
Q = total heat applied to the holdup
QTotal = duty calculated from the duty source
If you select the Liquid Heater radio button, the duty applied to the vessel depends on the liquid level in the tank. You must specify the heater height in the Top of Heater and Bottom of Heater cells that appear with Heater Height as % Vessel Volume group.
The heater height is expressed as a percentage of the liquid level in the vessel operation. The default values are 5% for the Top of the Heater and 0% for the Bottom of the Heater. These values are used to scale the amount of duty that is applied to the vessel contents.
where:
L = liquid percent level (%) T = top of heater (%) B = bottom of heater (%)
The Percent Heat Applied can be calculated as follows:
Q = QTotal (1.1)
Percent Heat Applied Q QTotal
---×100%
=
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It is shown that the percent of heat applied to the vessel’s holdup directly varies with the surface area of liquid contacting the heater.
Duty Source/Source Group
In the Duty Source/Source group, you can choose whether UniSim Design calculates the duty applied to the vessel from a direct energy source or from a utility source.
• If you select the Direct Q radio button, the Direct Q group appears, and you can directly specify the duty applied to the holdup in the SP field.
Figure 1.13
Figure 1.14
0 20 40 60 80 100
0 20 40 60 80 100
Liquid Percent Level, L
Percent of Heat Applied
Percent Heat Applied for a Liquid Heater
B T
in the Direct Q group.
• If you select the Utility radio button, the Utility Properties group appears, and you can specify the flow of the utility fluid.
The duty is then calculated using the local overall heat transfer coefficient, the inlet fluid conditions, and the process conditions. The calculated duty is then displayed in the SP field or the Heat Flow field. For more
information regarding how the utility option calculates duty, refer to Chapter 12 - Logical Operations.
If you select the Heating radio button, the duty shown in the SP field or Heat Flow field is added to the holdup. If you select the Cooling radio button, the duty shown in the SP field or Heat Flow field is subtracted from the holdup.
Object Description
SP The heat flow value in this cell is the same value specified in the Duty field of the Parameters page on the Design tab. Any changes made in this cell is reflected on the Duty field of the Parameters page on the Design tab.
Min. Available Allows you to specify the minimum amount of heat flow.
Max. Available Allows you to specify the maximum amount of heat flow.
Figure 1.15
1-26 Common Views
Tube Bundle Radio Button
When you select the Tube Bundle radio button, the following options are available.
The Tube Bundle option allows you to configure a shell tube heat exchanger (for example, kettle reboiler or kettle chiller).
• In the kettle reboiler, the process fluid is typically on the shell side and the process fluid is fed into a liquid "pool"
which is heated by a number of tubes. A weir limits the amount of liquid in the pool. The liquid overflow is placed under level control and provides the main liquid product.
The vapor is circulated back to the vessel.
• In the kettle chiller, the process fluid is typically on the tube side with a refrigerant on the shell side. The
refrigerant is typically pure and cools by evaporation. The setup is similar to the reboiler except that there is no weir or level control.
Figure 1.16
The Tube Bundle option is only available in Dynamics mode.
If you had an energy stream attached to the unit operation, UniSim Design automatically disconnects the energy stream when you switch to the Tube Bundle option.
a heat exchanger has been attached to the unit operation.
The following table lists and describes the options available to configure the heat exchanger:
Figure 1.17
Object Description Parameters group
Tube Volume cell Allows you to specify the volume of the tubes in the heat exchanger.
Vessel Liquid U cell Allows you to specify the heat transfer rate of the liquid in the shell.
Vessel Vapor U cell Allows you to specify the heat transfer rate of the vapour in the shell.
Tube Liquid U cell Allows you to specify the heat transfer rate of the liquid in the tube.
Tube Vapor U cell Allows you to specify the heat transfer rate of the vapour in the tube.
Heat Transfer Area
cell Allows you to specify the total heat transfer area between the fluid in the shell and the fluid in the tube.
Bundle Top Height
cell Allows you to specify the location of the top tube/
bundle based on the height from the bottom of the shell.
Bundle Bottom
Height cell Allows you to specify the location of the bottom tube/bundle based on the height from the bottom of the shell.
Specs group
Tube Dp cell Allows you to specify the pressure drop within the tubes. You have to select the associate checkbox in order to specify the pressure drop.
Tube K cell Allows you to specify the pressure flow
relationship value within the tubes. You have to select the associate checkbox in order to specify the pressure flow relationship value.
1-28 Common Views
Tube UA Reference
Flow cell Allows you to set a reference point that uses UniSim Design to calculate a more realistic UA value. If no reference point is set then UA is fixed.
UA is the product of overall heat transfer multiply with overall heat transfer area, and depends on the flow rate.
If a value is specified for the Reference Flow, the heat transfer coefficient is proportional to
the . The equation below is used to determine the actual UA:
Reference flows generally help to stabilize the system when you do shut downs and startups as well.
Minimum Flow Scale
Factor cell The ratio of mass flow at time t to reference mass flow is also known as flow scaled factor. The minimum flow scaled factor is the lowest value which the ratio is anticipated at low flow regions.
This value can be expressed in a positive value or negative value.
• A positive value ensures that some heat transfer still takes place at very low flows.
• A negative value ignores heat transfer at very low flows.
A negative minimum flow scale factor is often used in shut downs if you are not interested in the results or run into problems shutting down the heat exchanger.
If the Minimum Flow Scale Factor is specified, the actual UA is calculated using the
ratio if the ratio is greater than the Min Flow Scale Factor. Otherwise the Min Flow Scale Factor is used.
Calculate K button Allows you to calculate the K value based on the heat exchanger specifications.
Shell Dp cell Allows you to specify the pressure drop within the shell.
Summary group
Actual UA cell Displays the calculated UA in Dynamics mode.
Shell Liq. Percent
Level cell Displays the calculated liquid level in the shell at percentage value.
Tube Liq. Volume
Percent cell Allows you to specify in percentage value the volume of liquid in the tube.
Shell Duty cell Displays the calculated duty value in the shell.
Object Description
mass flow ratio)0
Aactual UAspecified mass flowcurrent mass flowreference mass flow---⎠reference⎞0
HoldUp View
The HoldUp view displays the calculated results on the heat exchanger in the following tabs:
• General. Displays the phase, accumulation, moles, volume, duty and holdup pressure of the heat exchanger.
Select the Active Phase Flip Check checkbox to enable UniSim Design to check if there is a phase flip between Liquid 1 (light liquid) and Liquid 2 (heavy liquid) during simulation and generate a warning message whenever the phase flip occur. If the checkbox is clear, UniSim Design generates a warning only on the first time the phase flip occur.
Use Tube Trivial Level and Fraction Calc. radio button
Allows you to select the volume percent level variable for the vessel fraction calculation.
This option uses a variable that is independent of the vessel shape or orientation.
Use Tube Normal Level and Fraction Calc. radio button
Allows you to select the liquid percent level variable for the vessel fraction calculation.
This option uses a variable that is dependant of the vessel shape and orientation.
ViewTubeHoldUp
button Allows you to access the tube HoldUp property view.
Figure 1.18
Object Description
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• Nozzles. Allows you to modify nozzle configuration attached to the heat exchanger.
• Efficiencies. Allows you to modify the efficiency of the recycle, feed nozzle, and product nozzle of the heat exchanger.
Figure 1.19
Figure 1.20
density, and molecular weight of the holdup in the heat exchanger.
• Compositions. Displays the composition of the holdup in the heat exchanger.
Figure 1.21
Figure 1.22
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