For a variety of industrial services where large heat transfer surfaces are required, shell and tube heat exchanger is commonly used. These heat exchanger equipment can be fabricated from a wide range of construction materials.
A shell and tube heat exchanger consists of several parallel tubes, the ends of which are mounted in the tube sheets, and the entire tube bundle is enclosed in a close-fitting cylindrical shell. In this exchanger, the heat transfer surface is the one that is offered by tubes. One of the fluids flows through the tubes and is called the tube side fluid while the outer fluid flows through the space created between tubes and shell, i.e. outside the tubes, and is called the shell side fluid. Two fluids are in thermal contact but are physically separated by a metal wall of the tubes. Heat flows through the metal wall of the tubes from a hot fluid to a cold fluid.
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If none of the fluids condenses or evaporates the unit is known as a heat exchanger. When one of the fluids condenses it is known as a condenser or a heater depending on whether the primary purpose of the unit is to condense one fluid or to heat the other. Similarly, such units may be called cooler, evaporation, etc. Based on the primary purpose for which they are employed.
Shell in the heat exchanger
It is usually a cylindrical casing through which one of the fluids flows in one or more passes. Shell is commonly made of carbon steel. It may be cut to the required length from a standard pipe up to 60 cm in diameter or fabricated from a plate. The minimum thickness of a shell made of carbon steel varies from 5 mm to 11 mm depending upon the diameter.
Tubes in the heat exchanger
Standard heat exchanger tubes used in many industrial processes may be of various sizes and lengths. The outside diameter of tubes varies from 6 mm to 40 mm. The tubes with an outside diameter of 19 mm to 25 mm are very common. The tube lengths used are 0.5, 2.5, 3, 4, 5, and 6 meters.Â
The wall thickness of tubes is usually expressed in terms of Birmingham Wire Gauge (BWG). It depends upon the material of construction and diameter. For 19 or 25 mm outside diameter tubes of mild steel, 10 or 12 BWG is common. The tubes that are placed in a tube bundle inside the shell are either rolled or welded to the tube sheet. The tube-side fluid first enters a header (bonnet) or channel through nozzles, then flows through tubes in parallel flows. It may flow in one pass or in more than one pass. In general, an even number of tube side passes are used.
Tube pitch of heat exchanger
The shortest center-to-center distance between the adjacent tubes is called the tube pitch.
Heat exchanger Clearance
The shortest distance between two tubes is called the clearance.
The minimum pitch is 1.25 times the outside diameter of the tube. The clearance should not be less than 0.25 times the outside diameter of the tube, the minimum clearance being 4.76 mm.
The tubes are commonly laid out either on a square pitch or on an equilateral triangular cleaning of the tubes and cause a low-pressure drop on the shell side fluid. If the fluids are very clean, a triangular pitch is used. The triangular pitch arrangement incorporates a large number of tubes in a given shell diameter than with a square pitch arrangement incorporates a larger number of tubes in a given shell diameter than with a square pitch and usually creates large turbulence in the shell side fluid.
BafflesÂ
Baffles are commonly employed within the shell of a heat exchanger to increase the rate of heat transfer by increasing the velocity and turbulence of the shell side fluid and also as structural support for the tubes and dampers against vibration. The baffles cause the fluids to flow through at right angles to the axes of the tubes. To avoid bypassing the shell side the clearance between the baffles and shell, and the baffles and tubes must be minimal.
The center-to-center distance between adjacent baffles is known as baffle spacing or baffle pitch. The baffle spacing should not be greater than the inside diameter of the shell and should not be less than one-fifth of the inside diameter of the shell. The optimum baffle spacing is 0.3 to 0.50 times the shell diameter.
Various transverse baffles used are segmental, disc and rings, orifice, etc. The segmental baffles are most commonly used. The segmental baffle is a drilled circular disk of sheet metal with one side cut away. When the height of the baffle is 75% of the inside diameter of the shell it is called a 25% cut segmental baffle. A 25% cut segmental baffle is the optimum one giving good heat transfer rates without excessive pressure drop. The baffle thickness usually ranges from 3 mm to 6 mm. Fig. shows a segmental baffle.
Tie rods are used to hold the baffles in place, with spacers to position/locate the baffles. Tie rods are fixed at one end of the tube sheet by making blind holes. Usually, 4 to 6 tie rods with at least 10 mm diameter are necessary.
Tube sheet
It is essentially a flat circular plate with a provision for making gasketed joints, around a periphery. A large number of holes are drilled in the tube sheet according to the pitch requirements.
Tube sheet thickness ranges from 6 mm to 25.4 mm for tube outside diameter of 6 mm to 40 mm.
Shell Side and Tube Side PassesÂ
With the help of passes (flow paths), we can change the direction of flow in the shell and tubes. Passes are generally used to obtain higher velocities and longer paths for fluid to travel, without increasing the length of the exchanger, which leads to high heat transfer rates.
The passes on the shell side are single, two passes, and single split passes. The passes on the tube side are one, two, four, six up to twelve. Passes on the tube side are formed by partitions placed in the shell cover and channels.
When we use single-pass partitions on the tube side, the tube side fluid flows twice through the heat exchanger. In this case, the pass partition divides the tubes equally into two sections. It is provided in the channel so that the inlet and outlet for tube side fluid are provided on the same channel. Fig shows the incorporated pass partition in the shell and tube heat exchanger.
Multipass construction decreases the cross-section of the fluid path which increases the fluid velocity which in turn increases the heat transfer coefficients. But these have certain disadvantages such as more complicated constructions and high friction lossesÂ
Take these Notes is, Orginal Sources:Â Unit Operations-II, KA Gavhane