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 pressures Can be built in many materials Many suppliers Repair can be by non-specialists Design methods and mechanical codes have been established from many years of experience

Maximum pressure Shell 300 bar (4500 psia) Tube 1400 bar (20000 psia) Temperature range Maximum 600oC (1100oF) or even 650oC Minimum -100oC (-150oF)

Construction: Bundle of tubes in large cylindrical shell Baffles used both to support the tubes and to direct into multiple cross flow Gaps or clearances must be left between the baffle and the shell and between the tubes and the baffle to enable assembly.

Tube Layouts:Triangular, Rotated Triangular, Square and Rotated Square

Triangular layouts give more tubes in a given shell Square layouts give cleaning lanes with close pitch

TEMA Terminology: Letters given for the front end (ABCND), shell (EFGJHX) and rear end types (Fixed tube sheet, U tube and floating head type). Use fixed tube sheet if DT below 50oC, otherwise use other types to allow for differential thermal expansion Exchanger given three letter designation

U tube: Allow bundle removal and mechanical cleaning on the shell side It is simple in design but it is difficult to clean the tube side round the bend.

Allocation of fluids Put dirty stream on the tube side - easier to clean inside the tubes Put high pressure stream in the tubes to avoid thick, expensive shell When special materials required for one stream, put that one in the tubes to avoid expensive shell Cross flow gives higher coefficients than in plane tubes, hence put fluid with lowest coefficient on the shell side If no obvious benefit, try streams both ways and see which gives best design

Shell and tubes can handle fouling but it can be reduced by keeping velocities sufficiently high to avoid deposits avoiding stagnant regions where dirt will collect avoiding hot spots where coking or scaling might occur avoiding cold spots where liquids might freeze or where corrosive products may condense for gases

Avoiding Vibrations:Inlet support baffles - partial baffles in first few tube rows under the nozzles Double segmental baffles - approximately halve cross flow velocity but also reduce heat transfer coefficients Patent tube-support devices No tubes in the window (with intermediate support baffles) J-Shell - velocity is halved for same baffle spacing as an E shell but decreased heat transfer coefficients

Tube side enhancement for heat transfer:Spiral wound wire and twisted tape Increase tube side heat transfer coefficient but at the cost of larger pressure drop (although exchanger can be reconfigured to allow for higher pressure drop) In some circumstances, they can significantly reduce fouling. In others they may make things worse Can be retrofitted

Problems of Conventional S & TZigzag path on shell side leads to Poor use of shell-side pressure drop Possible vibration from cross flow Dead spots Poor heat transfer Allows fouling Recirculation zones Poor thermal effectiveness, e

DOUBLE PIPE HE

Hair pin: union of two legs hairpin construction is preferred because it requires less space Packing & gland: The packing and gland provides sealing to the annulus and support the inner pipe. Return bend: The opposite ends are joined by a U-bend through welded joints. Support lugs: Support lugs may be fitted at these ends to hold the inner pipe position. Flange: The outer pipes are joined by flanges at the return ends in order that the assembly may be opened or dismantled for cleaning and maintenance. Union joint: For joining the inner tube with U-bend.

Counter current max. heat transfer within minimum area due to more L.M.T.D Co-current Used for viscous fluids & gives lesser value of L.M.T.D Co & counter current gives same value of L.M.T.D if one of the fluid stream is isothermal (e.g steam) Series-parallel arrangement This configuration is used when value of pressure exceeds its limits (500psig shell side and 500 psig tube side) .pressure drop problem can be solved by: Reversing the location of streams By-passing one of the fluid streams Dividing of stream at higher pressure drop( series-parallel arrag.)

Advantages of DPHE Compactness Very high heat transfer coefficients on both sides of the exchanger Close approach temperatures in counter-current flow Ease of maintenance. Heat transfer area can be added or subtracted without complete dismantling the equipment. High pressure ranges (30 MPa shell side , 140 MPa tube side) High temperatures range (600 C) Ease of inspection on both sides Ease of cleaning Low cost No Local overheating and possibility of stagnant zones is also reduced Fouling tendency is less low pressure loss Used for small applicationsLimitations: It is not as cost effective as most shell and tube exchangers It requires special gaskets Limited volumetric capacity FoulingFouling : formation of a scale or a deposit on a heat transfer surface is called foulingTypes of fouling: Precipitation fouling ( due to dissolved salts of Ca & Mg ) Particulate fouling( due to suspended particles ) Corrosion fouling Chemical reaction fouling (due to deposits formed by chemical reactions) Bio fouling ( due to the attachment of bio chemical species ) Solidification fouling ( due to sub cooling of fluids )

shell & tube heat exchangers are: designed to withstand the greatest temperature and pressure condition Ideal for large scale applications Commonly used in petrochemical industry where dangerous substances are present (protective shell) Consists of very bulky or heavy construction, baffles are used to increase mixing Subject to water hammer and corrosion High pressure loses

Factors influencing fluid input in shell or tube side: Flow rates : Cannot be considered because water side flow rate is not given Flow areas : Higher mass flow rate stream is passed through greater flow area which cannot be considered because we don't know which stream is of higher flow rate Tube side fluid : As we know that water causes a lot of fouling and corrosion hence we will take water in the tube side in this way it would cause lesser damage to the heat exchanger. Pipe side/annulus side : Benzene will be taken on annulus side Flow arrangement : Counter current flow is selected because it reduces the required surface area

1/Uo=1/ho +(Ao/Am)x(ro-ri/kw)+Ao/Ai(1/hi) Am = (OD-ID) / ln (OD/ID) Flanged joints are used for connecting pipes and instruments to vessels, for manhole covers, and for removable vessel heads when ease of access is required.Types of flange, and selectionSeveral different types of flange are used for various applications. The principal types used in the process industries are:1. Welding-neck flanges.2. Slip-on flanges, hub and plate types.3. Lap-joint flanges.4. Screwed flanges.5. Blank, or blind, flanges.

Welding-neck flanges, Figure 13.33a: have a long tapered hub between the flange ring and the welded joint. This gradual transition of the section reduces the discontinuity stresses between the flange and branch, and increases the strength of the flange assembly. Welding-neck flanges are suitable for extreme service conditions; where the flange is likely to be subjected to temperature, shear and vibration loads. They will normally be specified for the connections and nozzles on process vessels and process equipment.

Slip-on flanges, Figure 13.33b: slip over the pipe or nozzle and are welded externally, and usually also internally. The end of the pipe is set back from 0 to 2.0 mm. The strength of a slip-on flange is from one-third to two-thirds that of the corresponding standard welding-neck flange. Slip-on flanges are cheaper than welding-neck flanges and are easier to align, but have poor resistance to shock and vibration loads. Slip-on flanges are generally used for pipe work. Figure 13.33b shows a forged flange with a hub; for light duties slip-on flanges can be cut from plate.

Lap-joint flanges, Figure 13.33c: are used for piped work. They are economical when used with expensive alloy pipe, such as stainless steel, as the flange can be made from inexpensive carbon steel. Usually a short lapped nozzle is welded to the pipe, but with some schedules of pipe the lap can be formed on the pipe itself, and this will give a cheap method of pipe assembly. Lap-joint flanges are sometimes known as Van-stone flanges.

Screwed flanges, Figure 13.33d: are used to connect screwed fittings to flanges. They are also sometimes used for alloy pipe which is difficult to weld satisfactorily.

Blind flanges (blank flanges): are flat plates, used to blank off flange connections, and as covers for manholes and inspection ports.

Gaskets are used to make a leak-tight joint between two surfaces. It is impractical to use flanges required to make a satisfactory seal under pressure without a gasket. Gaskets are made from semi-plastic materials; which will deform and flow under load to fill the surface irregularities between the flange faces, yet retain sufficient elasticity to take up the changes in the flange alignment that occur under load.The following factors must be considered when selecting a gasket material:1. The process conditions: pressure, temperature, corrosive nature of the process fluid.2. Whether repeated assembly and disassembly of the joint is required.3. The type of flange and flange face

Vegetable fibre and synthetic rubber gaskets can be used at temperatures of up to 100C. Solid polyfluorocarbon (Teflon) and compressed asbestos gaskets can be used to a maximum temperature of about 260 C. Metal-reinforced gaskets can be used up to around 450C.