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Design concept of Steam Generators and Thermal calculations
Design
This program does not perform any mechanical strength calculations of any part of the steam generator.
In the steam generator, the primary fluid is transported through tubes while its heat is transferred from the primary fluid to the secondary fluid in the shell.
Inside the shell, the secondary fluid temperature increases to its saturation temperature and steam is produced. This steam exits the steam generator to participate
in heating, humidifying or other progresses before condensing and returning to the steam generator as feedwater.
This program is built to make thermal design of Steam to Steam generators, where the primary fluid is steam and secondary fluid enters the unit as feed water.
The program has two steam generator arrangements: Horizontal and Vertical.
The steam generator components are contained within a cylindrical vessel.
The primary fluid (steam) enters the inlet plenum of the steam generator at a pressure available
in the particular project, flows inside the U-tubes and transfers heat to the water on the shell
side. All arrangements assume 2-pass tubeside. Material of the components of the steam generator, except tubes, either carbon steel or stainless steel and results in negligible effect
on thermal calculations.
Horizontal Arrangement:
The horizontal tank of the Steam Generator usually has a Kettle type shape, but some manufacturers build
the tank using seamless or welded pipe and two elliptical heads or dishes.
The horizontal tube bundle is submerged in a pool of water at the base of the
oversize tank. The tank also is known as Kettle reboiler.
The height of the tube bundle is usually 40-60% of the tank/shell ID.
The submergence of the tube bundle is assured by an overflow weir or level switches/controllers at height of typically 2-3 in. from the upper surface of topmost tubes.
The advantage of this system that it is: suitable for vacuum operation and high vaporization rate up to about
80% of the feed.
Vertical Arrangement:
Vertical Steam Generator is basically a vertical shell and tube heat exchanger, where vaporization happens in the shell and heating steam is flowing in the tubes.
The shell can be the same nominal size as a tube bundle or oversized, depending of thermal requirements.
Vertical tube bundle is installed with a tubesheet located at the bottom of the shell and attached to the shell's body flange.
There, a heating steam flows up to the tube bundle and comes back down as condensate.
The feed water supply connection is on the bottom of the shell and the outlet connection is on the top as shown on the program schematic.
The vertical tube bundle is covered by the feed water 2-3 in. above the upper surface of the topmost U-bend. The water level in the shell is maintained by flow switches/controllers.
Tube bundle:
This program uses the U-tube bundle design which consists
of tubes which are bent in the form of a „U‟ and rolled back into the tube sheet.
U-tube bundle type is most common and practical for the steam generator application.
The advantages of this design are:
-Relatively low cost.
-Less labor during manufacturing process.
-The design requiree only one tube sheet, and consequently less labor in the manufacturing of the tube sheet and rolling tubes into one tube sheet. Hence, there is less possibility of leaks through the tube to tube sheet joints.
-Easy removal of the bundle from shell.
The disadvantages are:
-Manufacturer needs to have the equipment for bending different sizes of tube, bending radiuses and tube materials or
it has to order the U-bent tubes from outside suppliers.
-The bent area becomes the weakest point in the u-bent tube. This area should be thoroughly checked for the thickness of the bend, due to the tendency for tube wall to thin out during a bending process. Furthermore, the process changes the appearance of the bends, possibly causing knuckles or other deficiencies which may negatively impact the functioning of the generator
-More complicated tube cleaning process than in the straight tube design.
The tubes are supported by full baffles/supports.
The span between the adjacent full baffles/supports (in our case unsupported tube length) is limited by TEMA requirements, table RCB-4.52.
The program’s default setting of the span is 12 in. Max. tube bundle length is limited to 240 in. and min. length is limited to 18 in.
The common tube outside diameters and wall thickness gages are incorporated in the program and can be selected from the dropdown boxes.
The program uses the tube materials that are common for steam generator application.
The program selects a tube layout according to thermal calculations. Tube layouts for all bundle sizes have 2 -pass arrangement.
The distance between the centers of two adjacent rows of tubes along a pass partition center line depends on tube OD:
½” OD - 1”
5/8”OD-1½”
¾”OD-1½”
1”OD-2½”
In order to fit as many tubes as possible inside the baffle/support for a certain size of the bundle
the distance between two adjacent tube rows (see above) is kept as small as practically possible and the outer tube limit is calculated as shown below:
OTL,in= Baffle /Support diameter-1/8”
The minimum distance between two adjacent tube rows for all tube diameters is based on industry bending standards. For 1/2" OD it is 1 1/4" center to center of U-bend and for 3/4" OD it is 1 3/4"
crisscross arrangement of the U-tube installation. It will help to insert the U-tube into first rows along the pass partition.
Nozzles:
The program assumes that the steam generator nozzles (shell and head) include: flanges of 150# rating and standard schedule pipes.
All pressure drop and dimension calculations are based on the above assumption.
Thermal Calculations
Since the velocity of the liquid flowing over the bundle during vaporization is very low, maximum boiling film coefficient for water is set to 1000 and maximum flux is set to 30000 BTU/(hr)(ft2). Maximum condensing film coefficient for steam in tubes is 1500. The program calculates the required heat transfer area using maximum flux as the limit, but also gives two overdesign values with the flux correction and without the correction. This is the user decision: which way to follow. The safest way is to use the heat transfer area with the flux correction.
Input:
The program is written to design steam to steam Steam Generators. It means that the heating media is saturated steam and the heated media is feed water which during the process is converted to saturated steam.
The program has two design arrangements for steam generators: horizontal and vertical.
The user should choose the arrangement based on such factors as space limits, required capacities, and available budget. Vertical units are cheaper relative to the same horizontal tube bundle size of same tube OD, but they are very limited in tube length. This program has limited the vertical bundle length to maximum 36” straight tube length. This restriction limits the capacity of vertical units. This restriction is based on the opinion that the vertical bundles with the steam supply at the bottom of the bundle are more susceptible to premature steam condensation in the supply leg of the bundle, which leads to a such undesirable events as hammering that can destroy tubes in the bundle and produce a loud noise during operation cycle. Furthermore, the premature condensation will dramatically decrease the heat transfer rate of the steam generator. The short vertical tube bundles that within this restriction work well and have a long enough lifespan time.
The input area of the page is divided in four parts- : General input:
Project name and user name are optional. Steam generator arrangement: default setting is Horizontal. It can be changed to Vertical from the drop box. Cold, shell/kettle side which includes :
Entering water temperature, F: input limited from 32 F to 210 F. default setting is 40 F. The input must be an integer
Required steam flow rate, Lb/Hr, capacity of the unit: input must be an integer. The text box must be filled.
Required shell steam pressure PSIG: input must be an integer and limited from 1 to 120 PSIG. The text box must be filled.
Available pressure drop, PSI: input must be an integer and the default setting is 1 PSI.
Fouling factor, ft2-hr-F/BTU: default setting is 0.000. User can select other options from the drop box.
Selection of fouling factor depends on the expected time between the cleaning of heat exchangers.
TEMA gives some idea of fouling factor values. Hot, tube side which includes :
Available tube steam pressure PSIG: input must be an integer and limited from 1 to 180 PSIG.The text box must be filled.
Available pressure drop, PSI: input must be an integer and default setting is 5 PSI.
Fouling factor, ft2-hr-F/BTU: default setting is 0.000. User can select other options from the drop box. Construction:
The construction part deals with the construction details of the steam generators which can influence the thermal design results :
Tube OD, is selected from the drop box from ½” to 1” OD, the most common tube sizes that are used to build steam generator tube bundles. The default setting is ½” OD.
Tube material is selected from the drop box. The selection consists of Copper,Cu-Ni 90/10, Stainless Steel,Carbon Steel,Admiralty and Titanium.
The default Setting is Copper. The material does not reference to the specific specification of the type of material (For example: ASME sect. II Part D or other codes).
The tube wall thickness drop box includes standard tube wall thicknesses common in heat exchanger industry. The default setting is 0.035”.
Other drop boxes as Kettle/Shell Inside Dia, Bundle nom Dia, Support thickness and Shell/Tube Inlet/Outlet connection sizes have default setting displayed in the boxes. User can select other sizes or run the program with default values and the program will select and process the values dictated by the calculations. The user, after the first run with default values, can optimize the result by selecting different values in the drop boxes and run the program again.
If the Support thickness drop box has a default value, the program will select the thickness of the support based on TEMA recommendations (TABLE CB-4.41 ) .
The 'assumed bundle length' text box input value is limited to between 18 in. to 240 in. tube bundel straight length. The user should tentatively select the tube bundle straight length. The selection is usually based on initial information such as available room for installation, etc. The text box must be filled.
The input must be an integer. The 'span between adjacent supports' text box: default setting is 12 in. The user can input a different value. The input must be an integer. If the input distance ill exceeds TEME recommendations (TABLE RCB-4.52 ), a warning will appear on the screen.
Optimization:
The user may try to optimize results of the program's run by changing the following: bundle length, bundle nominal diameter, shell/kettle inside diameter, nozzle sizes, distance between supports, etc.
Overdesign of bundle % in ‘Results’ table shows the percentage ratio of the selected tube bundle heat transfer area to the area required by calculations minus 100%. If the bundle is underdesigned the value will appear with the sign “-“. It is up to the user how close he/she can bring the selected tube bundle area to the calculated heat transfer area using different variables as bundle length, bundle nom. Dia , or tube OD.
Usually, it is a good practice to keep tube bundle overdesign at 25%, or less if a budget consideration plays a role in the design (e.g. market dictates low price for the unit).
Although allowable pressure drops on both sides in the program have default values, it is important to evaluate them as per the system requirement where the unit. Values that are too low may increase the unit size and the price, those that are too high not reflect the system requirements and will create additional pressure drop in the system that may jeopardize the system performance. If the calculated pressure drop in the unit exceeds allowable,- a warning will appear on the screen: ‘Tubeside pressure drop exceeds allowable pressure drop.’ or ‘Shellside pressure drop exceeds allowable pressure drop.’
To fix this problem for both sides the user may start with increasing inlet and outlet nozzles sizes.
For the tubeside :
1. Reduce total tube length by decreasing the tube bundle length.
2. Increase shell dia.
3. Lessen the tube wall thickness.Go to lower gauge
4. Change tube OD.
For shellside: increase of shell/kettle diameter.
The program in run time may display some warnings. Most of them are self-explanatory.
Some of them refer to ῥV2 entrance and exit value, where:
ῥ - density of fluid entering or exiting shell or head pounds per cubic foot
V2 –velocity of fluid in power 2, feet per second.
In order to protect the tube bundle against impinging fluid this value has to be kept under:
1500 for entering feed water, 4000 for outlet steam from shell/kettle and 6000 for inlet steam in head/tube side as per TEMA RCB-4.6
The best way to fix this problem is to increase the nozzle size or install impingement protection on inlet.
The warning:'Product steam velocity exceeds allowable. Possibility of entrapment.' refers to excessive moisture in the steam in the shell/kettle where the velocity of steam exceeds the critical velocity for this particular condition of steam. In this case drops of moisture are carried out from the steam generator to the system. To eliminate the problem you can try to increase the size of the outlet nozzle and /or the diameter of the shell/kettle.
In case of a vertical arrangement, the program will suggest an enlarged top portion of the shell to reduce steam velocity in the shell, if user decides to increase “Kettle/Shell ID” in the response to the warning.
The schematic of the unit will reflect it. Flow induced vibration:
The program checks several parameters defined by TEMA standard:
-Natural frequency of each tube.
-Acoustic frequency.
-Vortex Shedding Frequency.
-Turbulent Buffeting Frequency .
-Estimate of Critical Flow Velocity.
When these parameters exceed allowable the program displays the following warnings: Crossflow velocity in shell exceeds critical crossflow velocity. Possibility of vibration
MAX. TURBULENT BUFFETING AMPLITUDE EXCEEDS RECOMENDED.
Possibility OF ACOUSTIC RESONANCE. TEMA CONDITION A PARAMETER.
Possibility OF ACOUSTIC RESONANCE. TEMA CONDITION B PARAMETER.
Possibility OF ACOUSTIC RESONANCE. TEMA CONDITION C PARAMETER.
You can try changing a span between supports, tube OD, tube thickness, bundle diameter or length, nozzle sizes, or some fluid conditions if it is possible to fix the fluid induced vibration problems.