Shell & Tube
Shell and Tube Heat Exchanger is the most common of the various types of unfired heat transfer equipment. It is the only type which can be considered for large surface areas having pressure greater than 30 bar and temperatures greater than 260.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. The fluids can be single or two phase and can flow in a parallel or a cross/counter flow arrangement. The shell and tube exchanger consists of four major parts: Front Header: this is where the fluid enters the tube side of the exchanger. Rear Header: this is where the tube side fluid leaves the exchanger or where it is returned to the front header in exchangers with multiple passes. Tube bundle: this consists of the tubes, tube sheets, baffles and tie rods etc. to hold the bundle. Shell: this contains the tube bundle.
Tubular Exchanger Manufactures AssociationInc
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, known as TEMA, which is intended to supplement the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1.TEMA presents three mechanical standards which specify design, fabrication and materials of shell and tube heat exchangers as follow: Class R: petroleum and related processing applications. Class C: commercial and general process applications. Class B: chemical process service.
Types of shell and tube heat exchangers
There are three types of shell and tube heat exchangers:
• Fixed tube sheet exchangers
• U-tube exchangers
• Floating header exchangersHeader and Shell Types:
Arrangement of tubes
The selection of tube diameter is a compromise taking into account the fouling nature of the fluids, the space available and the cost. Tubes of 19.05 and 25.4 mm outside diameter are the most widely used, but small units with clean fluids may use tubes as small as 6.35 mm outside diameter.The maximum tube length for removable bundle exchangers may be restricted to about 9 m. The maximum tube length for fixed tubesheet exchangers is less important but may be limited to about 15 m.The tubes are laid out in triangular or square patterns in the tube sheets. 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. The tube spacing is given by the tube pitch/tube diameter ratio, which is normally 1.25 or 1.33. Since a square layout is used for cleaning purposes, a minimum gap of 6.35 mm (0.25 in) is allowed between tubes.
Arrangement of Baffles
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 three baffle types, which support the tubes and promote flow across the tubes shows:
This plate is about 6 mm thick, flat or curved, with dimensions greater than the nozzle bore.To protect the tubes below the shell-side inlet nozzle from damage due to solid particles or liquid droplets entrained in the shell-side fluid, an impingement plate may be required.
Tie rods, Spacers
Tie rods and spacers hold the bundle and locate the baffles in their correct positions. Tie rods are screwed into the stationary tubesheet and extend the length of the bundle up to the last baffle. All the rods have spacer tubes fitted over them, each spacer being tube or pipe with an inside diameter greater than that of tie rod diameter, and a length equal to the required baffle spacing. Undesirable shell side leakage paths, such as the gaps due to pass partitions or gaps between bundle and shell, shall be blocked by tie rods and spacers.
The methods of Heat Transfer Enhancement
Thermal & Mechanical Design
The thermal design of a shell and tube exchanger is an iterative process which is done using computer programs such as the Heat transfer and Fluid Flow Service (HTFS) or Heat Transfer Research Incorporated (HTRI). In order to calculate the heat transfer coefficients and pressure drops, initial decisions must be made on the sides the fluids, 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 the main items during each iteration in order to maximize the overall heat transfer within specified allowable pressure drops. The mechanical design of a shell and tube exchanger is done using computer programs such as PV Elite which provides information on items such as shell thickness, flange thickness,nozzle thickness, etc. but stresses should not exceed allowable stresses.