Calculation example for the VERD module
A pentane flow of 7200 kg/h is to be evaporated in the tubes of a horizontal evaporator. Heating medium is saturated steam at 5 bar.
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Tube side |
Shell side |
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Medium: |
n-pentane |
saturated steam |
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Pressure: |
5 bar |
5 bar |
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Boiling temp. |
92.59 °C |
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Mass flow: |
7200 kg/h |
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Only standard tube and shell diameters shall be used.
Properties of the medium to be evaporated shall be entered via 'Free Input'.
· Select side of evaporation: Tube side evaporation
· Select tube material: Steel
· Select medium: "Free input" tube side and 'Steam condensation' shell side
· Tube dimension: 20 x 2 Shell-Bu 12 Bend R. 26
· Tube pitch: 26 x 60°
· Select "Free input / design" for the shell diameter. This means the VERD module chooses a standard shell from its list. This selection means that the steam velocity in the tubes does not exceed the desired velocity of 30 m/s in the tubes when at completely evaporating the pentane).
· Installation position: horizontal
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Tube-side:
Shell-side: The boiling temperature is determined by the vapor pressure equation, the mass flow by the energy balance.
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TIP: Conversion of units
A click on the unit or a double click on the input field the conversion of units is activated.
Enter the values in the unit you have at hand e.g. mass flow in kg/h. The value is immediately converted in all other displayed units. The unit with the activated checkbox is displayed on the mask and will be used for the documentation.
Please confirm your input with 'ENTER'. After having entered all data, please click 'OK'. The program switches to the SDAT module where you can enter the physical properties of pentane.
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Enter the properties for the boiling liquid in the upper part of the mask; the properties for the saturated vapour and the heat of evaporation in the lower part.
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Required properties: · Critical pressure · Molecular weight
of the boiling liquid: · Density · Specific heat capacity · Dynamic or kinematic viscosity · Thermal conductivity · Surface tension
of the steam: · Density · Specific heat capacity · Dynamic or kinematic viscosity · Thermal conductivity
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Reference values at a reduced pressure of 0.1: · Heat of evaporation · Density of boiling liquid · Density of saturated vapor · Surface tension |
After having entered all required properties, the program switches automatically to the VERD input mask.
Place the cursor on a field of the reference data. The reference heat flux q0 and the reference heat transfer coefficient a0 of n-pentane can be taken from a table, (blue question mark ?).
The reference properties at Pc/10 (10% of critical pressure) are only required for evaporation in horizontal tubes. With water, ammonia, carbon dioxide or when you are using our properties program PROPER the reference data are entered automatically.
Please note:
If you want to vaporize a medium of which there is no reference heat transfer coefficient in the table, you can calculate this reference heat transfer coefficient with the 'HABA' module. Start the HABA module by selecting 'ATLAS' / 'chapter' and selecting HABA in the listing.
The values for the reference heat flux and the reference heat transfer coefficient, have been taken from a table called Heat transfer coefficients α0 at tube-side evaporation, which can be called up by pressing the blue question mark of the input field
As soon as all reference data have been entered the evaporator is being calculated. Under the presupposition
that the velocity in the tubes does not exceed 30 m/s with complete evaporation of the pentane (see. Basic data) the program has chosen a shell (219.1 x 4.5 mm) and determined the number of tubes (37 tubes).
The theoretical heat transfer area has been calculated and by using the overdesign of 15% the final bundle length and the actual area are calculated.
The overdesign is the ratio of final and required heat exchanger area. The overdesign was set as 15% (s. Enter system data).
With boiling, the heat transfer depends on many influencing values, e.g. surface characteristics and surface roughness of the heating area. That is why the uncertainty in calculation of the heat exchanger is quite high. We recommend using a sufficiently high overdesign of at least 15%.
If you have doubtful physical properties of the evaporating medium or if you do not know the characteristics of the surface of the heating area, you should use a higher overdesign.
If you enter the overdesign, the final bundle results in an unmanageable value. Overwrite the final bundle length with the value of the manufacturer, e.g. 1.2 m. The evaporator is recalculated with the new length and the overdesign is now determined as 15.65 %.
You can now change the evaporator, e.g. if you need a bigger shell and more tubes are required.
Overwrite the number of tubes with 40 for
example. The program now looks for a shell in which at least
40 tubes fit. The program selects the next bigger one (273 x 6.3 mm) with 61
tubes. You can also select another shell by pressing the "Basic data"
button and selecting a new shell in the shell selection box.
VERD creates automatically a tubesheet for the evaporator. You can display, edit, save as DXF file or print the tube sheet in the SPIE module. The tube sheet can also be saved into a database and loaded into the WTS KOND and VERD program (see SPIE manual).
