Кыргызча жаны ыр коломун болуп кичине козумдун бат

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  1. Кыргызча жаны ыр коломун болуп кичине козумдун батсан ичине cлушать
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  79. If a single tube pass is used and provided there are more than three baffles, then near counter-current flow is achieved. In order to allow the fluid to flow backwards and forwards across the tubes part of the baffle is cut away. Both ASME and BS5500 are widely used and accepted throughout the world but some countries insist that their own national codes are used. A U-tube unit can be used to overcome thermal expansion problems and allow the bundle to be removed for cleaning.
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  81. G-type shells and H shells are normally specified only for horizontal thermosyphon reboilers. Considering each header and shell type in turn: A-Type front header This type of header is easy to repair and replace.
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  83. Crazy Games - In order to use these most effectively the exchanger should be designed for their use.
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  85. Regardless of the type of industry the exchanger is to be used in there are a number of common features see. A shell and tube exchanger consists of a number of tubes mounted inside a cylindrical shell. Two fluids can exchange heat, one fluid flows over the outside of the tubes while the second fluid flows through the tubes. It is sometimes referred to as the Stationary Header. The remainder of this section concentrates on exchangers that are covered by the TEMA Standard. Shell and tube geometric terminology 1 Stationary Front Head—Channel 20 Slip-on Backing Flange 2 Stationary Front Head—Bonnet 21 Floating Tubesheet Skirt 3 Stationary Front Head Flange 22 Floating Tubesheet Skirt 4 Channel Cover 23 Packing Box Flange 5 Stationary Head Nozzle 24 Packing 6 Stationary Tubesheet 25 Packing Follower Ring 7 Tubes 26 Lantern Ring 8 Shell 27 Tie Rods and Spacers 9 Shell Cover 28 Transverse Baffles or Support Plates 10 Shell Flange—Stationary Head End 29 Impingement Baffle or Plate 11 Shell Flange—Rear Head End 30 Longitudinal Baffle 12 Shell Nozzle 31 Pass Partition 13 Shell Cover Flange 32 Vent Connection 14 Expansion Joint 33 Drain Connection 15 Floating Tubesheet 34 Instrument Connection 16 Floating Head Cover 35 Support Saddle 17 Floating Head Flange 36 Lifting Lug 18 Floating Head Backing Device 37 Support Bracket 19 Split Shear Ring Tema Designations The popularity of shell and tube exchangers has resulted in a standard nomenclature being developed for their designation and use by the Tubular Exchanger Manufactures Association TEMA. This nomenclature is defined in terms letters and diagrams. The first letter describes the front header type, the second letter the shell type and the third letter the rear header type. Fixed Tubesheet Exchanger L, M, and N Type Rear Headers In a fixed tubesheet exchanger, the tubesheet is welded to the shell. This results in a simple and economical construction and the tube bores can be cleaned mechanically or chemically. However, the outside surfaces of the tubes are inaccessible except to chemical cleaning. If large temperature differences exist between the shell and tube materials, it may be necessary to incorporate an expansion bellows in the shell, to eliminate excessive stresses caused by expansion. Such bellows are often a source of weakness and failure in operation. In circumstances where the consequences of failure are particularly grave U-Tube or Floating Header units are normally used. This is the cheapest of all removable bundle designs, but is generally slightly more expensive than a fixed tubesheet design at low pressures. U-Tube Exchangers In a U-Tube exchanger any of the front header types may be used and the rear header is normally a M-Type. The U-tubes permit unlimited thermal expansion, the tube bundle can be removed for cleaning and small bundle to shell clearances can be achieved. However, since internal cleaning of the tubes by mechanical means is difficult, it is normal only to use this type where the tube side fluids are clean. Floating Head Exchanger P, S, T and W Type Rear Headers In this type of exchanger the tubesheet at the Rear Header end is not welded to the shell but allowed to move or float. The tubesheet at the Front Header tube side fluid inlet end is of a larger diameter than the shell and is sealed in a similar manner to that used in the fixed tubesheet design. The tubesheet at the rear header end of the shell is of slightly smaller diameter than the shell, allowing the bundle to be pulled through the shell. The use of a floating head means that thermal expansion can be allowed for and the tube bundle can be removed for cleaning. There are several rear header types that can be used but the S-Type Rear Head is the most popular. A floating head exchanger is suitable for the rigorous duties associated with high temperatures and pressures but is more expensive typically of order of 25% for carbon steel construction than the equivalent fixed tubesheet exchanger. Considering each header and shell type in turn: A-Type front header This type of header is easy to repair and replace. It also gives access to the tubes for cleaning or repair without having to disturb the pipe work. It does however have two seals one between the tube sheet and header and the other between the header and the end plate. This increases the risk of leakage and the cost of the header over a B-Type Front Header. Y-Type front header Strictly speaking this is not a TEMA designated type but is generally recognized. It can be used as a front or rear header and is used when the exchanger is to be used in a pipe line. It is cheaper than other types of headers as it reduces piping costs. It is mainly used with single tube pass units although with suitable partitioning any odd number of passes can be allowed. F-Type shell This is generally used when pure countercurrent flow is required in a two tube side pass unit. This is achieved by having two shells side passes—the two passes being separated by a longitudinal baffle. The main problem with this type of unit is thermal and hydraulic leakage across this longitudinal baffle unless special precautions are taken. J-Type shell This tends to be used when the maximum allowable pressure drop is exceeded in an E-Type Shell even when double segmental baffles are used. It is also used when tube vibration is a problem. The divided flow on the shellside reduces the flow velocities over the tubes and hence reduces the pressure drop and the likelihood of tube vibration. When there are two inlet nozzles and one outlet nozzle this is sometimes referred to as an I-Type Shell. L-Type rear header This type of header is for use with fixed tubesheets only, since the tubesheet is welded to the shell and access to the outside of the tubes is not possible. The main advantages of this type of header are that access can be gained to the inside of the tubes without having to remove any pipework and the bundle to shell clearances are small. The main disadvantage is that a bellows or an expansion roll are required to allow for large thermal expansions and this limits the permitted operating temperature and pressure. M-Type rear header This type of header is similar to the L-Type Rear Header but it is slightly cheaper. However, the header has to be removed to gain access to the inside of the tubes. Again, special measures have to be taken to cope with large thermal expansions and this limits the permitted operating temperature and pressure. In practice it is not a low cost design, because the shell has to be rolled to small tolerances for the packing to be effective. S-Type rear header This is a floating rear header with backing device. It is the most expensive of the floating head types but does allow the bundle to be removed and unlimited thermal expansion is possible. It also has smaller shell to bundle clearances than the other floating head types. However, it is difficult to dismantle for bundle pulling and the shell diameter and bundle to shell clearances are larger than for fixed head type exchangers. T-Type rear header This is a pull through floating head. It is cheaper and easier to remove the bundle than with the S-Type Rear Header, but still allows for unlimited thermal expansion. It does, however, have the largest bundle to shell clearance of all the floating head types and is more expensive than fixed header and U-tube types. U-tube This is the cheapest of all removable bundle designs, but is generally slightly more expensive than a fixed tubesheet design at low pressures. However, it permits unlimited thermal expansion, allows the bundle to be removed to clean the outside of the tubes, has the tightest bundle to shell clearances and is the simplest design. A disadvantage of the U-tube design is that it cannot normally have pure counterflow unless an F-Type Shell is used. Also, U-tube designs are limited to even numbers of tube passes. The square layouts are required where it is necessary to get at the tube surface for mechanical cleaning. The triangular arrangement allows more tubes in a given space. The tube pitch is the shortest center-to-center distance between tubes. Since a square layout is used for cleaning purposes, a minimum gap of 6. Baffle types Baffles are installed on the shell side to give a higher heat-transfer rate due to increased turbulence and to support the tubes thus reducing the chance of damage due to vibration. There are a number of different baffle types, which support the tubes and promote flow across the tubes. The center-to-center distance between baffles is called the baffle-pitch and this can be adjusted to vary the crossflow velocity. In practice the baffle pitch is not normally greater than a distance equal to the inside diameter of the shell or closer than a distance equal to one-fifth the diameter or 50. In order to allow the fluid to flow backwards and forwards across the tubes part of the baffle is cut away. The height of this part is referred to as the baffle-cut and is measured as a percentage of the shell diameter, e. The size of the baffle-cut or baffle window needs to be considered along with the baffle pitch. It is normal to size the baffle-cut and baffle pitch to approximately equalize the velocities through the window and in crossflow, respectively. There are two main types of baffle which give longitudinal flow: Tube inserts These are normally wire wound inserts or twisted tapes. They are normally used with medium to high viscosity fluids to improve heat transfer by increasing turbulence. There is also some evidence that they reduce fouling. In order to use these most effectively the exchanger should be designed for their use. This usually entails increasing the shell diameter, reducing the tube length and the number of tubeside passes in order to allow for the increased pressure loss characteristics of the devices. To a large extent these often depend on each other. For instance, the allocation of a dirty fluid to the shellside directly affects the selection of exchanger tube layout. Never allocate hazardous fluids such they are contained by anything other than conventional bolted and gasketted—or welded—joints. Shell selection E-type shells are the most common. If a single tube pass is used and provided there are more than three baffles, then near counter-current flow is achieved. If two or more tube passes are used, then it is not possible to obtain pure countercurrent flow and the log mean temperature difference must be corrected to allow for combined cocurrent and countercurrent flow using an F-factor. G-type shells and H shells are normally specified only for horizontal thermosyphon reboilers. J shells and X-type shells should be selected if the allowable DP cannot be accommodated in a reasonable E-type design. For services requiring multiple shells with removable bundles, F-type shells can offer significant savings and should always be considered provided they are not prohibited by customer specifications Front header selection The A-type front header is the standard for dirty tubeside fluids and the B-type is the standard for clean tubeside fluids. The A-type is also preferred by many operators regardless of the cleanliness of the tubeside fluid in case access to the tubes is required. Do not use other types unless the following considerations apply. A C-type head with removable shell should be considered for hazardous tubeside fluids, heavy bundles or services requiring frequent shellside cleaning. The N-type head is used when hazardous fluids are on the tubeside. A D-type head or a B-type head welded to the tubesheet is used for high pressure applications. Y-type heads are only normally used for single tube-pass exchangers when they are installed in line with a pipeline. Rear header selection For normal service a Fixed Header L, M, N-types can be used provided that there is no overstressing due to differential expansion and the shellside will not require mechanical cleaning. If thermal expansion is likely a fixed header with a bellows can be used provided that the shellside fluid is not hazardous, the shellside pressure does not exceed 35 bar 500 psia and the shellside will not require mechanical cleaning. A U-tube unit can be used to overcome thermal expansion problems and allow the bundle to be removed for cleaning. However, countercurrent flow can only be achieved by using an F-type shell and mechanical cleaning of the tubeside can be difficult. An S-type floating head should be used when thermal expansion needs to be allowed for and access to both sides of the exchanger is required from cleaning. Other rear head types would not normally be considered except for the special cases. Impingement protection is always required for gases which are corrosive or abrasive, saturated vapors and two phases mixtures. Thermal Design The thermal design of a shell and tube exchanger is an iterative process which is normally carried out using computer programs from organizations such as the Heat transfer and Fluid Flow Service HTFS or Heat Transfer Research Incorporated HTRI. However, it is important that the engineer understands the logic behind the calculation. In order to calculate the heat transfer coefficients and pressure drops, initial decisions must be made on the sides the fluids are allocated, the front and rear header type, shell type, baffle type, tube diameter and tube layout. The tube length, shell diameter, baffle pitch and number of tube passes are also selected and these are normally the main items that are altered during each iteration in order to maximize the overall heat transfer within specified allowable pressure drops. Mechanical Design The mechanical design of a shell and tube heat exchanger provides information on items such as shell thickness, flange thickness, etc. These are calculated using a pressure vessel design code such as the Boiler and Pressure Vessel code from ASME American Society of Mechanical Engineers and the British Master Pressure Vessel Standard, BS 5500. ASME is the most commonly used code for heat exchangers and is in 11 sections. Section VIII Confined Pressure Vessels of the code is the most applicable to heat exchangers but Sections II—Materials and Section V—Non Destructive Testing are also relevant. Both ASME and BS5500 are widely used and accepted throughout the world but some countries insist that their own national codes are used. In order to try and simplify this the International Standards Organization is now attempting to develop a new internationally recognized code but it is likely to be a some time before this is accepted. REFERENCES TEMA Seventh Edition. Boiler and Pressure Vessel code, ASME American Society of Mechanical Engineers. British Master Pressure Vessel Standard, BS 5500.
  86. This is primarily thanks to first person shooters such as, and. Gusto protection is always required for gases which are corrosive or abrasive, saturated vapors and two phases mixtures. It also has smaller shell to bundle clearances than the other floating head types. We do our best to make the site as fast and reliable as possible. Later our have met off. The ocean-side city of Newport Beach is only minutes away. TTY service is also available by dialing 212-504-4115. However, since internal cleaning of the tubes by mechanical means is difficult, it is normal only to use this type where the tube side fluids are piece.
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