Shell-and-Tube Heat Exchangers

Introduction to Heat ExchangerA heat exchanger can be defined as any device that transfers heat from one fluid to another or from or to a fluid and the environment. Whereas in direct contact heat exchangers, there is no intervening surface between fluidsIn indirect contact heat exchangers, the customary definition pertains to a device that is employed in the transfer of heat between two fluids or between a surface and a fluid

Why shell-and-tube?CEC survey: S&T accounted for 85% of new exchangers supplied to oil-refining, chemical, petrochemical and power companies in leading European countries. Why?Can be designed for almost any duty with a very wide range of temperatures and pressuresCan be built in many materialsMany suppliersRepair can be by non-specialistsDesign methods and mechanical codes have been established from many years of experience

Scope of shell-and-tubeMaximum pressure Shell 300 bar (4500 psia) Tube 1400 bar (20000 psia)Temperature range Maximum 600oC (1100oF) or even 650oC Minimum -100oC (-150oF)Fluids Subject to materials Available in a wide range of materialsSize per unit 100 - 10000 ft2 (10 - 1000 m2)Can be extended with special designs/materials

ConstructionBundle of tubes in large cylindrical shellBaffles used both to support the tubes and to direct into multiple cross flowHeaders Types ShellTubesBaffle

Baffle Type and GeometryBaffles support the tubes for structural rigidity, thus prevent tube vibration and saggingThey also divert the flow across the tube bundle to obtain a higher heat transfer coefficientBaffles can be transverse or longitudinalTransverse baffles are plate type or rod typePlate bafflessingle and double segmental most commonbaffle spacing is critical (optimum between 0.4 and 0.6 of the shell diameter)triple and no-tubes-in-window segmental baffles for low pressure drop applications

Figure 8.8 Plate Baffle Types

Figure 8.8 Plate Baffle Types (continued)

Avoiding vibration (cont.) Inlet support bafflesDouble-segmental bafflesNo tubes in the window - with intermediate support bafflesTubesWindows with no tubesIntermediate baffles

RODbafflesTend to be about 10% more expensive for the same shell diameter

TEMA terminologyLetters given for the front end, shell and rear end typesExchanger given three letter designationShellFront endstationary head typeRear endhead type

Front head typeA-type is standard for dirty tube sideB-type for clean tube side duties. Use if possible since cheap and simple.

BChannel and removable coverBonnet (integral cover)A

More front-end head typesC-type with removable shell for hazardous tube-side fluids, heavy bundles or services that need frequent shell-side cleaningN-type for fixed for hazardous fluids on shell sideD-type or welded to tube sheet bonnet for high pressure (over 150 bar)BND

Basic ComponentsShell TypesFront and rear head types and shell types are standardized by TEMA, identified by alphabetic characters (Fig. 8.2)E-shell is the most commoncheap and simple configurationone-shell pass and one- or multiple-tubepassesif one-tube pass, nominal counterflow is achievedmost common for single-phase shell fluid applicationsF-shell used when there are two tube passes and pure counterflow is desiredlongitudinal baffle results in two-shell passesunits in series, each shell pass represents one unithigher pressure drop than that for E-shell

Shell Types (continued)J-shell has divided flowfor low pressure drop applicationsnormally, single nozzle for shell-fluid at tubecenter, two nozzles near tube endswhen used for condensing the shell fluid, two inlets for shell-side vapor and one central outlet for condensate (figure)X-shell has cross flowcentral shell-fluid entry and exitno baffles are usedvery low pressure dropused for vacuum condensers and low-pressure gasesG-shell and H-shell are single- and double-split flow

Shell Types (continued)G-shell and H-shell are single- and double-split flowG-shell has a horizontal baffle with endsremoved, central shell-fluid entry and exitH-shell is similar, but with two baffles,and two nozzles at the entry and exit

Tube Bundle Types (rear head types)Main objectives in design are to accommodate thermal expansion and allow easy cleaning (or to provide the least expensive construction)U-tube configuration (Fig. 8.4)allows independent expansion of tubes and shell (unlimited thermal expansion)only one tube sheet is needed (least expensive construction)tube-side cannot be mechanically cleanedeven number of tube passesindividual tubes cannot be replaced (except those in the outer row)

Tube Bundle Types (continued)Fixed tube sheet configuration (Fig. 8.5)allows mechanical cleaning of inside of tubes but not outside because shell is welded to the tube sheetslow-costlimited thermal expansionindividual tubes replaceablePull-through floating head (Fig. 8.6)allows the tube sheet to float move with thermal expansionthe tube bundle can be removed easily for cleaning suitable for heavily fouling applications

Figure 8.2TEMAs Standard Shell, Front-end and Rear-end Types

ExampleBESBonnet front end, single shell pass and split backing ring floating head

1-2 shell and tube Heat Ex.

Shell-side flow

Tubes and Tube PassesA large number of tube passes are used to increase fluid velocity and heat transfer coefficient, and to minimize foulingTube wall thickness is standardized in terms of the Birmingham Wire Gauge (BWG) of the tube (Tables 8.1 & 8.2)Small tube diameters for larger area/volume ratios, but limited for in-tube cleaningLarger tube diameters suitable for condensers and boilersFins used on the outside of tubes when low heat transfer coefficient fluid is present on the shell-sideLonger tubes fewer tubes, fewer holes drilled, smaller shell diameter, lower cost. However limitations due to several factors result in 1/5 1/15 shell-diameter-to-tube-length ratio

Tube layoutsTriangular layouts give more tubes in a given shellSquare layouts give cleaning lanes with close pitchpitchTriangular30oRotatedtriangular60oSquare90oRotatedsquare45o

What is this?

FoulingShell and tubes can handle fouling but it can be reduced bykeeping velocities sufficiently high to avoid depositsavoiding stagnant regions where dirt will collectavoiding hot spots where coking or scaling might occuravoiding cold spots where liquids might freeze or where corrosive products may condense for gases

TEMA is the Tubular Exchanger Manufacturers Association. Their Standards are almost universally accepted.

CopyrightHyprotech UK Ltd holds the copyright to these lectures. Lecturers have permission to use the slides and other documents in their lectures and in handouts to students provided that they give full acknowledgement to Hyprotech. The information must not be incorporated into any publication without the written permission of Hyprotech.

85 per cent is a higher figure than in the pie chart in lecture 1. The difference is that the above figure is for the limited range of industries shown while the pie chart is for all industrial applications.The grids are offset so that it takes a set of 4 grids to support a given tube on the top, bottom left and right.Message: use B unless you want to clean inside the tubes frequently.Message, use 30 or 600 layouts unless you need to clean the shell side mechanically.The students should be able to identify this as a BJM (we are guessing at the M because we cannot see the back end).If you specify a very high fouling resistance, the clean exchanger will over perform when fist installed. The operators will then throttle back on the flow of the service stream thus helping to promote fouling. Then the designer can say, I told you it has a high fouling.