For a variety of industrial services where large heat transfer surfaces are required, shell and tube heat exchangers are commonly used. These heat exchange equipments can be fabricated from a wide range of materials of construction.
A shell and tube heat exchanger consists of a number of parallel tubes, ends of which are fixed in the tube sheets and the entire tube bundle is enclosed in a close fitting cylindrical shell. In this exchanger, heat transfer surface is the one that is offered by tubes. One of the fluid flows through the tubes, while the outer fluid flows through the space created between tubes and shell, i.e., outside the tubes. These 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 the hot fluid to the cold fluid. The fluid flowing through the tubes is called the tube fluid/tube side fluid, whereas the fluid flowing outside the tube is called the shell fluid/shell side fluid.
If none of the fluids condenses or evaporates, the unit is known as heat exchanger. When one of the fluids condenses, then the unit is known as condenser or as 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 as cooler, evaporator, etc. based on a primary purpose for which they are used.
Shell :
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 upto 60 cm diameter or fabricated from plates. The minimum thickness of shell made of carbon steel varies from 5 mm to 11 mm depending upon the diameter.
Tubes :
Standard heat exchanger tubes used in many industrial processes may be of various sizes and lengths. The outside diameter of tubes vary from 6 mm to 40 mm. The tubes with outside diameters of 19 mm and 25 mm are very common. The tube lengths used are 0.5, 2.5, 3, 4, 5 and 6 meters. 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 tube 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 a nozzle, then flows through the tubes in parallel flow. It may flow in one pass, i.e., once through or in more than one pass, i.e., many times. In general, an even number of the tube side passes are used.
Tube pitch :
The shortest centre-to-centre distance between the adjacent tubes is called as the tube pitch.
Clearance :
The shortest distance between two tubes is called as the clearance.
The minimum pitch is 1.25 times the outside diameter of tube. The clearance should not be less than 0.25 times the outside diameter of tube, the minimum clearance being 4.76 mm.
The tubes are commonly laid out either on a square pitch or on an equilateral triangular pitch as shown in Fig. The advantage of a square pitch arrangement is that it permits external cleaning of the tubes and causes a low pressure drop on the shell side fluid. If the fluids are very clean, a triangular pitch arrangement is used. With a triangular pitch arrangement, a larger number of tubes can be accommodated in a given shell diameter than with a square pitch arrangement and usually creates a large turbulence in the shell side fluid.
Fig. Square and Triangular pitch
Baffles :
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 supports for the tubes and dampers against vibration. The baffles cause the fluid to flow through the shell at right angles to the axes of the tubes. To avoid bypassing of the shell side fluid the clearance between the baffles and shell, and the baffles and tubes must be minimum.
The centre-to-centre 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 ring, orifice, etc. The segmental baffles are most commonly used. 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 as 25 % cut segmental baffle. 25 % cut segmental baffle is the optimum one giving good heat transfer rates without an excessive pressure drop. The baffle thickness usually ranges from 3 mm to 6 mm. Fig. shows a segmental baffle.
Fig. Segmental baffle detail
Tie rods are used to hold the baffles in place, with spacers to position / locate the baffles. Tie rods are fixed at one end in the tube sheet by making blind holes. Usually 4 to 6 tie rods with atleast 10 mm diameter are necessary.
Tube sheet :
It is essentially a flat circular plate with a provision for making gasketed joint, around a pheriphery. A large number of holes are drilled in the tube sheet according to the pitch requirements.
Fig. (i) Tube sheet triangular pitch of tubes
(ii) Tube sheet square pitch of tubes
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 Passes :
With the help of passes (i.e., 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 a fluid to travel, without increasing the length of the exchanger, that leads to high heat transfer rates.
The passes on the shell side are : single pass, two pass, single split pass. The passes on the tube side are : one, two, four, six upto twelve. Passes on the tube side are formed by partitions placed in the shell cover and channels.
When we use a single pass partition on the tube side, the tube side fluid flows twice through the heat exchanger. In this case the pass partition divides the tubes equally in two sections. It is provided in the channel so that inlet and outlet connections for the tube side fluid are provided on the same channel. Fig. shows a channel with a pass partition incorporated in a shell and tube heat exchanger.
Multipass construction decreases the cross section of the fluid path that 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 losses.
Fig. Channel of heat exchanger with pass partition