Thermal Design of Shell & Tube

General Introduction to the Thermal Design of Shell & Tube Heat Exchangers Steve Noe Calgary February 26, 2003

Objectives: Provide an overview on the general specification requirements for shell & tube type heat exchangers. Our discussions will include: • General Heat Exchanger specifications • TEMA Types • Bu Bund ndle le & Tu Tube be la layo yout ut pa para rame mete ters rs

• Operational Variables • Des Design ign Lim Limits its – pres pressur sure e drop drops s & vel veloci ocitie ties s • Optimi Optimizat zation ion of of geometr geometry y – baffle baffle types types / shel shelll type type / streams streams placements

Objectives: Provide an overview on the general specification requirements for shell & tube type heat exchangers. Our discussions will include: • General Heat Exchanger specifications • TEMA Types • Bu Bund ndle le & Tu Tube be la layo yout ut pa para rame mete ters rs

• Operational Variables • Des Design ign Lim Limits its – pres pressur sure e drop drops s & vel veloci ocitie ties s • Optimi Optimizat zation ion of of geometr geometry y – baffle baffle types types / shel shelll type type / streams streams placements

TEMA Types

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TEMA – Tubular Exchangers Manufacture’s Association – Recommended practices for

the thermal and mechanical design of shell & tube type heat exchangers – Design & Manufacturing

tolerances

Exchanger Specifications Front Head Types TEMA A – A – The head can be bolted directly to the tubesheet or to a mating shell flange. The bolted flat cover permits access to the tubes without interfering with the piping connections. connections. TEMA B – B – Similar to the A type head, the B head is bolted directly to the tubesheet or to a mating shell flange. Since the cover is welded to the head cylinder, the head must be removed to gain access to the tubes and nozzle to piping connections to be unbolted. Several different types of covers. The advantage with this design is that it provides the lowest cost head closure. TEMA C – C – The head cylinder cylinder and tubesheet tubesheet are integral. The shell is bolted bolted to the back side of the tubesheet which allows for bundle removal. Use for hazardous tubeside fluids, or when frequent shellside cleaning in necessary. TEMA N – N – With this construction, the head cylinder and shell cylinder are welded to the tubesheet. The flat bolted cover permits access to the tubes without interfering with the piping connections. connections. Used with fixed tubesheets with hazardous fluids on the tubeside. TEMA D – D – Is a high pressure enclosure. The channel barrel and the tubesheet are usually usually forged. The cover cover may be bolted to the end of the head cylinder or inserted into the cylinder and held by a system of shear key rings.

Exchanger Specifications •

Shell Types –

E-type – Is the most common of the shell types. The one shell pass with the entrance and exit nozzles at opposite ends is the ideal arrangement for excellent performance. Used with a single tube pass temperature crosses can be avoided.

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F-type – A longitudinal baffle divides the shell into two passes. Both the inlet and exit tube side nozzles are placed at the same end of the exchanger. The F-shell is typically used when a temperature cross exists that would otherwise force the design into multiple shells in series. They are not recommended with removable bundles. A tight seal is required at the long baffle as the unit will not perform as designed should there be fluid leakage across the longitudinal baffle. The amount of heat transferred is greater than for an E-shell, but the shell side pressure drop is also higher and there is some thermal leakage, radiate heat transfer, across the long plate.

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G-type- The entrance and exit nozzles are placed in the middle of the shell. The shell side fluid splits into two with the two halves flowing in opposite directions around a partial longitudinal baffle. It is a hybrid 2-pass shell.

Exchanger Specifications • Shell Types: – H-type – The H type is similar to the G-type, but with two inlet, two

exit nozzles, and two partial longitudinal baffles. The flow is double split or in four directions. It is used when there is limited available pressure drop on the shell side. The arrangement is quite often used for shell side thermosiphon applications or shell side condensers with low allowable pressure drop. – J-type – It has one nozzle in the middle used for entrance or exit. It

also has two nozzles at opposite ends for exit or entry. Either way, the flow is divided with each half of the total fluid flowing through half the shell. It is used when there is limited available pressure drop on the shell side. It can not be used when the tube side temperature crossed the temperature on the shell side. The J type is quite often used for shell side condensers.

Aspen Hetran – Exchanger Specifications •

Shell Types: –

K-type – This type of shell is referred to as a kettle pool boiler. The tube bundle is far smaller then the kettle diameter. A weir normally exists beyond the tube bundle to maintain a liquid level over the tube bundle. The enlarged space void of tubes above the bundle serves as a disengagement space, allowing separation of the liquid and vapor. Kettles are frequently used under distillation columns to provide vapor reflux and energy back to the column for distillation.

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X-type – Flow is distributed along the entire length of the bundle and flows across the bundle perpendicular to the tubes. The distribution is accomplished by multiple nozzles along the shell or via open areas at the top & bottom of the shell. Support plate type baffles are used to support tubes. With this shell arrangement, the shell side pressure drop is minimized.

Exchanger Specifications • Rear Head Types – Non-removable bundle types – TEMA L – The head can be bolted directly to the tubesheet or to a

mating shell flange. The bolted flat cover permits access to the tubes without interfering with the piping connections. – TEMA M – Similar to the L type head, the B head is bolted directly to the tubesheet or to a mating shell flange. Since the cover is welded to the head cylinder, the head must be removed to gain access to the tubes and nozzle piping connections to be unbolted. Several different types of covers. The advantage with this design is that it provides the lowest cost head closure. – TEMA N – With this construction, the head cylinder and shell cylinder are welded to the tubesheet. The flat bolted cover permits access to the tubes without interfering with the piping connections. The cost of construction is slightly higher than that of a B type head.


Rear Head Types – Floating heads with Removable bundles –

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TEMA P – An outside packed joint is provided which allows the rear tubesheet/cylinder to float independent of the shell. The rear cover is generally a flat bolted cover but a formed cover welded to the cylinder can also be used. Due to the packed joint configuration, design pressure and temperature are generally limited to 250 psi and 400 F, respectively. TEMA S – The S type assembly consists of an internal floating rear tubesheet. The tubesheet is sandwiched between a split ring backing flange assembly bolted to the rear head cover. A separate shell cover is bolted to the rear shell flange. The split ring/rear head cover assembly must be removed from the rear prior to removing the tube bundle. TEMA T – The T type, with a internal floating rear tubesheeet, is similar to the S type. The primary difference being that the rear head cover is bolted directly to the rear tubesheet and can be pulled out with the bundle through the shell. TEMA W – An outside lantern ring acts as the rear head seal. The rear head cover is bolted to the lantern ring assembly. Generally the W type arrangement can be utilized at higher pressures than the P type rear head, up to 400 psi. TEMA U – The tubes are bent in U shape which as as the rear head. A very lost cost alternative to a fabricated rear head. The disadvantage is that the cleaning of the inside of the tubes is difficult.


Tubesheets –

Single tubesheets -- The tubesheets act as the interface plate to separate the shell and tube sides fluids of the exchanger. The tubes are attached to the tubesheets by roller expansion and/or welding. The tie rods are bolted to the shell side of the front tubesheet and are used to facilitate the assembly of the tube bundle.

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Double tubesheets -- are used when it is extremely important to avoid any leakage between the shell and tube side fluids. Double tubesheets shorten the length of the tube which is in contact with the shell side fluid and therefore reduces the effective surface area. They also affect the location of the shell side nozzles and possibly the adjacent baffle spacing. The gap type double tubesheet has two separate tubesheet plates separated by a space, usually about 150 mm (6 in.). The integral type double tubesheet is similar but is made from a single piece of material by machining out a honeycomb pattern midway inside the plate. If any tube joint leaks develop, a sensor between the tubesheets will notify the plant operations.

Exchanger Specifications • Tubing - Bare – Plain or bare tubes are most commonly used in shell and tube

heat exchangers. They are the lowest cost and easiest to install. Plain tubes come in straight lengths or bent to form u-tubes. Plain tubes are either formed from strips and welded or drawn/seamless. The seamless tubes have better structural integrity, but are more expensive. – Tubes come in several standard outside diameters from ¼ in to 2 inches, and from wall thickness of 0.022 inches to 0.109. The most common tube sizes are 0.75 in or 1 in. Tubes come in many different materials. The tube material selection affects the extent of corrosion, erosion, fouling, vibration, thermal stresses, and the resistance to heat transfer. – Tubes must be able to withstand the internal and external pressure. The tubes must also be able to withstand the thermal stresses due to the differential expansion of the shell and tube sides, and the corrosive nature of the fluids. The thermal conductivity of the tubing affects the tube wall resistance to heat transfer.


Tubing – Enhanced –

Tubes are also available with externally enhanced surfaces such as low fins. These type tubes enhance the heat transfer as well as provide more surface area per unit length for a given shell size. They are generally used with a change of phase. You do pay a premium for the enhanced surface tubes. They normally become economical to use when the ratio of the tube side to shell side film coefficient is 3:1. They should not be used with fluids that have high surface tensions. Low fins are most effective in pure cross flow (X shells, NTIW, and segmental baffles). In longitudinal flow, such as triple segmental baffles, rod or strip baffles, the fin valleys are not effectively penetrated by the flow. In such situations pressure drop increases due to the fins acting as a rough surface. While such an increase is not reflected in heat transfer. Use of low fins in such cases is questionable. Common fin densities by tubing material (density shown in fins/inch): Carbon Steel

19

Nickel Alloy 600 (Inconel)

28

Stainless Steel

16, 28

Nickel Alloy 800

28

Copper

19, 26

Hastelloy

30

Copper-Nickel 90/10

16, 19, 26

Titanium

30

Copper-Nickel 70/30

19, 26

Admiralty

19, 26

Nickel Carbon Alloy 201

19

Aluminum-Brass Alloy 687

19

Nickel Alloy 400 (Monel)

28


Twisted Tape Inserts –

Twisted tape inserts are used with single phase fluids that are normally in laminar or transition flow regimes. They can increase the heat transfer by 2 to 3 times with a pressure drop increase between 4 to 10 times. They are relatively inexpensive and correlations available are quite reliable.

Exchanger Specifications • Tube Pitch – Tubes are normally spaced a standard distance from one another.

The center-center distance between any two adjacent tubes is called the tube pitch. The ratio of the tube pitch to the tube diameter varies from 1.1 to 1.5, with the most common being 1.25. TEMA controls the minimum pitch. It will vary by TEMA class, tube layout type, and size of tubes. – The tube pitch will affect the shell side heat transfer coefficient,

shell side pressure drop, number of tubes for a given shell size, and capability of mechanically cleaning the outside surface of the tubes. The larger the pitch the lower the film coefficient, and the lower the pressure drop.


Tube Pattern –

The tube pattern is defined with respect to the flow direction. There are four different pattern options. They are listed below in order from the layout producing the highest film coefficient and highest pressure drop to the lowest.

Exchanger Specifications • Tube Layout –

The number of tubes in a shell depend upon the tube diameter, pattern, pitch, type of bundle, outer tube limit, type of shell, number of passes, placement of impingement plates, location of nozzles, location of tie-rods, etc.

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There are several possible ways to layout tubes for four or more passes. The primary effect on the thermal design is due to the different number of tubes, which are possible for each type. The Quadrant layout has the advantage of usually (but certainly not always) giving the highest tube count. It is the required layout for all U-tube designs of four or more passes. The tube side nozzles must be offset from the centerline when using quadrant layout. The Mixed layout has the advantage of keeping the tube side nozzles on the centerline. It often gives a tube count close to quadrant and sometimes exceeds it. A Ribbon layout nearly always gives a layout with fewer tubes than quadrant or mixed layout. It is used for an odd number of tube passes. It is also the layout preferred for X-type shells. The primary advantage of ribbon layout is the more gradual change in operating temperature of adjacent tubes from top to bottom of the tubesheet. This can be especially important when there is a large change in temperature on the tube side, which might cause significant thermal stresses in mixed and especially quadrant layouts.


Tube Layout types

Quadrant

Mixed

Ribbon


Tube Layout – deviation in number of tubes per tube side pass –

For multipass layouts, it is desirable to have close to the same number of tubes in each pass when there is no change of phase on the tube side. However, for most layouts of more than two passes, this would require removing tubes which would otherwise fit within the outer tube limit. Since it is preferable to maximize the surface area within a given shell and minimize the possible shell side bypassing, a reasonable deviation in tubes per pass is usually acceptable (5% or less deviation). It is recommended that you avoid large deviations since this gives significantly different velocities in some passes and wastefully increases the pressure drop due to additional expansion and contraction losses. Most computer programs base the tube side calculations on an average number of tubes per pass, so that such aberrations are not reflected in the thermal design.

Exchanger Specifications – Tube Layout • Tie-rods and Sealing Strips – The tie-rods and spacers are necessary to keep the baffles and

tube support plates at their appropriate locations. Their number and diameter vary with the shell diameter. The number of tubes in an exchangers will be reduced due to the space occupied by the tierods and spacers. Usually each set of a tie-rod and spacer requires the removal of four to six tubes around it. The TEMA standards provide guidance as to the minimum number required. – Sealing strips are used to reduce bypassing of the shell side flow around the bundle between the shell ID and the outer most tubes. They are installed in pairs on the baffles usually by welding. In fixed tubesheet (L, M, & N rear heads) and U-tube heat exchangers the clearance between shell ID and the outer tube limit is comparatively small. Therefore sealing strips are seldom needed for these types. In inside floating head (S & T rear heads), outside packed floating head (P rear head), and floating tubesheet (W rear head) heat exchangers, the potential for bypassing is much greater. In these cases sealing strips are generally required. Generally, one pair of seals strips is used for every 6 rows of tubes in cross-flow.

Exchanger Specifications – Tube Layout • Tie-rods and Sealing Strips

Seal strips are added to prevent bypassing of the flow around the tube bundle. Tie rods are used to facilitate assembly and to position the baffles.


Baffles types Baffles are used to direct the shell side flows so that the fluid velocity is increased to a point to maintain a high heat transfer coefficient as well as to minimize fouling. In horizontal exchangers the baffles also aid in supporting the tube bundle to prevent the tubes from sagging or vibrating. – Single segmental provides the best thermal performance but also the highest pressure drop. The multi-segmental types decrease pressure drop significantly with a corresponding reduction in heat transfer coefficient. The Rod and Strip types will provide the lowest pressure drop but with a significant reduction in performance. –

Single Segmental

No Tubes in Window

Double Segmental

Rod

Triple Segmental

Full Support

Strip

Exchanger Specifications - Baffle Types •

Single Segmental –

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The segmental baffles are the most commonly used. They also give the highest heat transfer coefficient. The clearance between the baffle hole and the tube, and the baffle OD and the shell is dictated by TEMA. The baffles are made out of round plates with segments removed. Both the baffle cut and the baffle spacing will influence the shell side velocity. The baffle cuts will vary from 15-45% of the shell diameter. A cut of 20-25% will normally provide the highest film coefficient for a given pressure drop. The baffle cuts for single segmental baffles are oriented 180 deg from one another. This allows fluid to flow over the bundle in cross-flow and make contact with the tubes several times in an exchanger. The baffle orientation applies to the direction of the baffle cut in segmental baffles. It is very dependent on the shell side application for vertical heat exchangers; the orientation has no effect . It may affect the number of tubes in a multipass vertical heat exchanger. For horizontal heat exchangers it is far more important. The baffle cut applies to segmental baffles and specifies the size of the baffle window as a percent of the shell I.D. Single segmental baffles cuts vary between 15% to 45%. Greater than 45% is not practical because it does not provide for enough overlap of the baffles. Less than 15% is not practical, because it results in a high pressure drop through the baffle window with relatively little gain in heat transfer (poor pressure drop to heat transfer conversion). Generally, where baffling the flow is necessary, the best baffle cut is around 25%. For double and triple segmental baffles, the baffle cut generally pertains to the most central baffle window.

Exchanger Specifications – Baffle types •

No Tubes in Window –

There is also a single segmental baffles where no tubes are placed where the baffle cut creates the baffle window (NTIW). These baffles are normally used to resolve vibration problems. They provide support for the tubes at each baffle segment. Intermediate support plates are often used with no tubes in the window baffles. They provide additional tube support and very little additional pressure drop. The baffle cut for NTIW is normally limited to approximately 15%. Otherwise, the sacrifice in the number of tubes/shell would be too costly.

The advantage of a NTIW arrangement is that the tube unsupported span can be minimized to avoid vibration. Additional intermediate support plates can be also added as shown.

Exchanger Specifications – Baffle types •

Double / Triple Segmental and Rod / Strip –

Double segmental baffles are an excellent selection for moderately reducing the shell side pressure drop. Double segmental baffles create more of a parallel type flow than a cross flow that exists with single segmental baffles. There should exist at least a two row overlap between adjacent baffle segments.

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Triple segmental baffles are used infrequently primarily because of their high cost. Hetran’s triple segmental baffles differ from the triple segmental baffles shown in the TEMA standards. They are used when a significant reduction in the shell side pressure drop is required.

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Rod baffles are licensed through Phillips Petroleum Company. Only a limited number of fabricators are licensed to use them. Each tube is supported at four circumferential points at each baffle. The flow is parallel to the bundle. They provide an excellent remedy when vibration or restrictive pressure drop on the shell side is a problem. They are only used with square patterns.

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Strip or Nest type baffles are used with the triangular pattern. Each tube rests in a vshaped cradle and is supported at each baffle. The strips are usually about 25 mm (1 in.) wide and 3 mm (0.125 in.) thick. The strips are placed between every tube row. Intersecting strips can be notched to fit together or stacked and tack welded. The strips are welded to a circular ring. They provide the same benefits as the Rod baffles with the square layouts.


Baffle cut orientation –

For a single phase fluid in a horizontal shell, the preferable baffle orientation horizontal, although vertical and rotated are also acceptable. The choice will not affect the performance, but it will affect the number of tubes in a multipass heat exchanger. The horizontal cut has the advantage of limiting stratification of multicomponent mixtures, which might separate at low velocities. The rotated cut is rarely used. Its only advantage is for a removable bundle with multiple tube passes and rotated square layout. In this case the number of tubes can be increased by using a rotated cut, since the pass partition lane can be smaller and still maintain the cleaning paths all the way across the bundle. (From the tubesheet, the layout appears square instead of rotated square.) For horizontal shell side condensers, the orientation should always be vertical, so that the condensate can freely flow at the bottom of the heat exchanger. These baffles are frequently notched at the bottom to improve drainage. For shell side pool boiling, the cut (if using a segmental baffle) should be vertical. For shell side forced circulation vaporization, the cut should be horizontal in order to minimize the separation of liquid and vapor. For double and triple segmental baffles, the preferred baffle orientation is vertical. This provides better support for the tube bundle than a horizontal cut which would leave the topmost baffle unsupported by the shell. However this can be overcome by leaving a small strip connecting the topmost segment with the bottom most segment around the baffle window between the O.T.L. and the baffle O.D.

Exchanger Specifications • Impingement Plates – Impingement plates are most commonly located underneath the

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inlet shell side nozzle between the nozzle opening and the tube bundle. They can also be located in a nozzle dome above the tube bundle. They are used to prevent damage to the tubes due from the impact of the incoming fluid. A secondary benefit is better flow distribution of the fluid over the tube bundle. TEMA recommends that inlet impingement protection be installed under the following conditions: When the rho*V2 through the inlet nozzle exceeds 2232 kg/(m*s2) or 1500 lb/(ft*s2) for non-corrosive, non-abrasive, single phase fluids When the rho*V2 through the inlet nozzle exceeds 744 kg/(m*s2) or 500 lb/(ft*s2) for corrosive or abrasive liquids When there is a nominally saturated vapor When there is a corrosive gas When there is two phase flow at the inlet


Impingment plate type –

To accommodate an impingement plate on the bundle, tubes will normally be removed under the inlet nozzle so that the shell entrance area equals the crosssectional area of the nozzle. This is approximately equal to removing any tubes within a distance of 1/4 the nozzle diameter under the center of the nozzle. An impingement plate that is usually circular, un-perforated, equal in diameter to the inside diameter of the nozzle, and approximately 3 mm or 1/8 in. thick. An alternative is to put a plate in a nozzle dome, which means suspending the impingement plate in an enlarged nozzle neck, which may be a dome or a cone. Both types have their advantages and disadvantages. If the plate is on the bundle, the flow is more widely distributed, and there is neither the expense for the enlarged nozzle neck nor the increased potential of fabrication problems when cutting a large hole in the shell (as can often happen with vapor inlet nozzles). However, since tubes are removed, it may require larger diameter shell, tubesheets, flanges, etc. Especially in cases where the tubesheets and/or shell are made of alloy and the inlet nozzle is not large, the impingement plate in the nozzle dome may be significantly less expensive. For some special applications, the plate may be perforated. The primary advantage being that the perforations will help reduce the velocity into the bundle. The main concern with perforated plates is that flow through the holes could cause localized erosion for certain tube materials. Other arrangements are sometimes used to either further reduce the velocity or protect the bundle by other means.


Shell Entrance/Exit Variations –

It is often necessary to remove tubes from underneath the nozzle to reduce pressure drop at the entrance, to facilitate the entry of the fluid into the bundle, or to reduce the velocity to limit the probability of erosion or vibration.

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One option, is to remove tubes within the nozzle projection. This will eliminate any tubes, which would extend beyond the lowest part of the nozzle cylinder. In many cases, using this option will have no effect since nozzles, which are relatively small in comparison to the shell diameter (say smaller than 1/4 the shell diameter) will not extend to the first row of tubes anyway. A distributor belt with a full layout is the most effective way to reduce entrance velocities, but it is usually the most expensive. Another option is to remove tubes so that the shell entrance area equals the nozzle entrance area. When you remove tubes so that the shell entrance area equals the inlet nozzle area, the tube layout is the same as when installing an impingement plate on the bundle. This is usually a very effective way of decreasing entrance velocities.


Shell Entrance/Exit types –

The bundle entrance velocity must be controlled to prevent tube vibration and erosion. It is possible to increase the bundle entrance flow area by removing tubes at the entrance or providing a special distribution device. TEMA has specific recommendations on the maximum mass flow rates recommended for various applications. When these are exceeded, the program will provide a warning to this effect.

Exchanger Specifications • Nozzles –

Nozzles provide the ports of entry and exit for the shell side and tube side fluids. Nozzle sizes should be carefully selected. You don’t want to select too small a nozzle for you run the risk of taking too much of a pressure drop, vibration or erosion problems underneath the nozzle, and possible fluid mal-distribution. However, you can’t specify too large a nozzle size for then you may have fabrication problems. The criteria used for the nozzle size is to insure that the nozzle can be constructed yet will not take too high of a percentage of the available pressure drop.

• Nozzle Orientation –

The logical orientation of the nozzles follows the laws of nature, that is, fluids being cooled should enter the top and exit the bottom, and fluids being heated should enter the bottom and exit the top.


Nozzles - Vents & Drains –

The vent connection is provided to allow all the air in an exchanger to leave the exchanger as it gradually fills with fluid at startup. It also helps to equalize the pressure when a liquid is drained at shutdown. It is also important to be able to vent inert gases that may accumulate in condenser during normal operation. Should the inerts begin to accumulate, then the condensing film coefficient will collapse.

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The drain connection is provided to drain the liquid at the shutdown for maintenance, or should the process fluids change. Any liquid allowed to remain will quickly cause it to foul and corrode.

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Typical sizes for drains or vents range from 1 to 3 inches.


Flanges –

Body flanges (rings) are used to bolt two adjacent sections of a exchanger together . The joint is made leak proof by using gaskets.

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There are three basic types of flanges. The integral type - where the hub and the flange are a continuous piece either by originally manufactured or made so by a full penetration weld. These type are the most costly.

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The loose type – these either do not have any attachment to the shell or the attachment is not considered integral with the shell

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The optional type – these are attached to the vessel wall or nozzle such that the assembly acts as an integral unit. These differ from the integral type in not having any hub or n having a very small hub. The vessel wall acts as the hub.


Expansion Joints –

Expansion joints are used to eliminate the excessive stresses caused by the differential thermal expansion between the shell and the tube bundle due to cyclic temperature conditions in a fixed tubesheet exchanger. The requirement for one is determined according to the TEMA standards. It is necessary to have the mean metal temperatures calculated from a thermal program to make an accurate calculation. A mechanical design must be done to determine the requirement for an expansion joint.

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The common types of expansion joints are flanged only, flanged and flued, un-reinforced bellows, and reinforced bellows.


Supports –

Heat exchangers can be in either a vertical or horizontal position. In a horizontal position, saddle supports are used. In a vertical exchanger cylindrical skirts, brackets, or columns are used for supporting the exchanger.

Operational Variables •

Operational Variables –

The user of a computer design program must make selection of all constructional and operational input items required for design. Sometimes input items are dictated by others, but these should be critically examined and changed if needed. Contradictory demands and limitations will often exist. It is up to the user to select a compromise which will produce the best overall results. The following sections will provide a general idea of what to watch when specifying input data.


Maximum Permissible Pressure Drop and Flow Velocity –

Convective heat transfer of all types increases with flow velocity and hence with pressure drop. High flow velocity suppresses fouling. For these reasons it is advantages to use as high flow velocities as feasible. The resulting heat exchanger will be less expensive, compact, but the higher pressure drop will require higher pumping costs. There will be obvious limitations to both the velocity and pressure drop.

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The pressure drop must be in a sound relation to the operating pressure. The following can be used as a general guide for selecting allowable pressure drops: Pressure vacuum operation 15 psi – 30 psi > 30 psi

Allowable Pressure Drop 0.1 of operating pressure 0.15 – 0.3 of operating pressure 15 psi


Limits of Flow Velocity –

Flow velocity limits are dictated by erosion or tube vibration prevention, regardless of pressure drop. TEMA standards specify the limit for flow within the tube bundle in terms of v = (4000/density)**0.5 ft/s. For cooling water this results in approximately 8 ft/s.

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Tube side maximum velocity is usually dependent on the tube material with 8 ft/s being a reasonable value for carbon steel and copper alloys. You can safely increase this value by 50% for stainless steel, and there is no velocity limit for titanium. At the low velocity extreme, the minimum of 3 ft/s should try to be maintained to prevent severe fouling of water.

Operational Variables • Fluid Allocation Shell or Tube Side In some cases the fluid allocation to either tube or shell-side is obvious. It is dictated by cleaning, pressure, corrosion, or material selection requirements. In other cases the allocation of the streams must be carefully considered for optimum performance. – Pressure drop is much easier to adjust by shell and baffle type selection than on the tube side, and extremely low pressure drop can be obtained in X-shells. The stream with low or sensitive pressure drop requirements may have to be tried in both options. High pressure drop is best utilized on the shell side. – High pressure and corrosive fluids are best placed on the tube side, which eliminates expensive shells. If corrosion, can’t be eliminated but only slowed down by material selection, the corrodible component must be periodically replaced. This eliminates fixed tube sheet designs if the baffles are also affected. – Viscous fluids are preferably placed on the shell side, where the induced turbulence will result in higher heat transfer. The use of low finned tubes is often considered, but usually limited to Reynolds Numbers greater than 1000. –


Fouling and Cleaning –

Fouling will affect not only the exchanger size and its cost , but also the cost of cleaning, which can be very substantial when plant down-time is considered. Careful attention should be paid to design elements which can mitigate fouling and/or promote the ease of cleaning. Area allocated to fouling should adhere to the following guidelines: •

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1) The area associated with fouling should not exceed 40%. If higher fouling is required, then consider allocating two exchangers in parallel, one of which can be shutdown for cleaning while the other is in operation. 2) At the low end, the area attributed to fouling should be at least 10% in order to provide an adequate safety factor.

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The tube side is much easier to clean mechanically than the shell-side, as no removal of the bundle is necessary. Therefore, fluids requiring regular cleaning should be placed on the tube-side in a horizontal position . For cooling tower water, tube-side velocities should be 6 ft/s (2 m/s) and under no circumstances be below 3 ft/s (1m/s).

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Shell side cleaning by chemical means is increasingly accepted. If mechanical cleaning on the shell-side is required, removable bundle construction is used with square or rotated square layouts used. Baffle spacing to shell diameter ratio of ½, with baffle cuts of 25% will produce a uniform flow which is recommended.

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Low profile finned tubes have shown to have an anti-fouling tendency. The edges of the fins help to break-up the fouling layers and promote their removal.

Operational Variables • Tube Vibration Control – Particular combinations of high velocity and the length of

unsupported tube span may conspire under certain conditions to excite the tubes into a flow-induced vibration. Designs where tube vibration is indicated can lead to tube damage, a situation that must be corrected. – There are normally several corrective measures that can correct the problem. The judgment of the heat exchanger designer is normally necessary. Any changes made to the constructional elements will affect the thermo-hydraulic performance, and consequently require that they be rerun through the design calculations. – Any one or combinations of remedial actions are possible and multiple solutions are usually required to decide on the best design. – Acoustic vibration can develop if the shell fluid is gas only. If the probability of such vibrations is indicated, the most effective remedy is to install “detuning baffles”.

Thermal Design of Shell & Tube

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