For an engineer and designer heat transfer equipments are the most important equipments or the devices which he comes through during his career, for that a good theoretical knowledge on the concept of heat transfer is required and well ideology on the equipment selection, it working principle will help to design for a particular purpose and even to operate them in safe and economical mode. It is a vast topic which has its importance in every field of engineering and science.
From the basic concept, it will help you to understand and gain the subject at your fingertips, although there are lots of study material and lectures available, to obtain a good job for an engineer it is first and a foremost topic which makes him stand in front line. Showing interest in heat transfer one will automatically learn all the topics related which are useful in the area of software design using latest software's platform, research and development, logical calculation. These are made easy by the heat transfer because it is application oriented as you can feel, measure and sense the heat flow and mechanisms where your brain can understand when you can see some not just studying by imagination.
From the basic concept, it will help you to understand and gain the subject at your fingertips, although there are lots of study material and lectures available, to obtain a good job for an engineer it is first and a foremost topic which makes him stand in front line. Showing interest in heat transfer one will automatically learn all the topics related which are useful in the area of software design using latest software's platform, research and development, logical calculation. These are made easy by the heat transfer because it is application oriented as you can feel, measure and sense the heat flow and mechanisms where your brain can understand when you can see some not just studying by imagination.
A energy which can be observed to be travelling between the area of the hot and cold surface will be considered as heat, this word heat is termed as heat transfer when we study the process of this transformation of the energy. Every object contains this energy which is stored and come into picture when there is driving force which makes to travel and that is known to be a temperature difference, this measuring parameter is chosen by use because the temperature is directly related to the heat quantity.
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| Tube sheet |
In a shell and tube heat exchanger, the ‘tube pitch’ is defined as the shortest centre to centre distance between adjacent tubes. There are different types of tube arrangement in a heat exchanger as triangular pitch, square pitch, diagonal square pitch etc. in which triangular pitch arrangement facilitates highest heat transfer rate than another arrangement. Width and depth of grooves in the tube sheet holes normally are 1/8 and 1/64 inch respectively. In a multipass shell and tube heat exchanger, the shell side crossflow area depends upon baffle spacing, clearance and pitch. Shell side heat transfer coefficient in case of the square pitch as compared to the triangular pitch under the similar condition of fluid flow and tube size is less. Minimum tube pitch recommended for shell and tube heat exchangers is about 1.25 times the outside diameter of the tube. 6.5mm is the minimum recommended ligament for square pitch arrangement in heat exchangers.
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| Baffle cross section |
Baffles are the interconnectors of the tubes and shell of the heat exchangers. The shell side fluid will pass through the direction depending on the baffles arrangements. The rate of heat transfer also will depend on the baffles spacing. Different types of baffles are used as disk & ring, segmental and orifice. A transverse baffle as the segmental baffle, disk type baffle & helical type baffle etc. The shell side pressure drop in a shell and tube heat exchanger is maximum for orifice baffle. Forgiven number of passes, pitch and tube diameter, the maximum number of tubes that can be accommodated in a shell of tripled inside diameter will be considerably more than 9 times. Copper is the best tube material from thermal conductivity point of view. In a heat exchanger, shell side fluid velocity can be changed by changing the tube layout and pitch. Baffles are provided in a shell and tube heat exchanger to increase the turbulence and velocity of the shell side fluid. Minimum baffle spacing recommended in a shell and tube heat exchanger is equal to 5 cm or 0.2 times of shell diameter. In a multipass shell and tube heat exchanger, the baffles on shell side are primarily provided for creating turbulence.
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| A plate and frame heat exchanger |
Design of a shell and tube heat exchanger
Minimum cleaning lane of 6.5 mm is provided when tubes are on a square pitch. Minimum tube sheet thickness (in which tubes are fixed) is equal to the O.D. of the tube up to 15mm tube diameter; and for > 15 mm tube diameter, tube sheet thickness is smaller than tube diameter. O.D. of the tube is 6 to 40 mm while the tube lengths used are 0.5, 2.5, 3.0, 4.0, 5.0, 6 meters.The average in the tubes of a 1- 4 heat exchanger is 4 Times that in 1-1 heat exchanger having the same size & the number of tubes and operated at a same liquid flow rate.Tube side pressure drop in a 1-2 heat exchanger (for turbulent flow of fluids through the tubes) is about 1/4 times, that in a 1-1 heat exchanger having the same size & the number of tubes and operated at the same liquid flow rate.
The clearance between shell & baffles and between tube & baffles should be minimized to avoid by-passing of the fluid, but it should permit the removal of the tube bundle. Baffles are supported independently of the tubes by tie rods and positioned by spacers. Tie rods are fixed at one end of the tube sheet by making blind holes and the minimum number of tie rods is 4 with at least 10 mm diameter.
In case of a shell and tube heat exchanger, the minimum shell thickness for carbon steel (inclusive of corrosion allowance) depends on shell diameter and is in the range of 5 – 11mm.The ratio of tube length to shell diameter in case of the liquid shell and tube heat exchanger ranges from 4 to 8. Steam is preferred to be used as a heating medium in heat exchangers, because of its high latent heat. The high specific heat of water makes it a widely used coolant in heat exchangers.
The ratio of tube length to shell diameter for a shell and tube heat exchanger is 4:1 to 8:1 for liquid-liquid exchanger and< 4:1 for gas-gas exchangers. In a shell and tube heat exchanger, the overall heat transfer coefficient is proportional to the tube side (volumetric flow rate)0.8. This is valid, only when the ratio of the tube side film resistance to the total resistance is almost equal to 1. In a multipass shell and tube heat exchanger, the problem of differential expansion between the shell and tube passes is taken care of by using a U-bend or floating head tube sheet.
For multipass shell and tube heat exchanger, when the flow is a mixed one that is co-current and countercurrent, LMTD correction factor is used. The various types of baffles used in the heat exchanger are
2. Baffle spacing
3. Shell diameter
4. Tube pitch and diameter
5. Density and viscosity of the shell side fluid.
In shell and tube heat exchanger the tubes are generally connected to the tube sheet by the following two methods
Ferrule connection
Ferrule connection
Because of the abundance and high heat capacity, water is used as coolant in heat exchange equipments.
The main disadvantages of concentric pipe heat exchangers are:
Considerable space requirement
Prone to leakage
High maintenance cost
The main disadvantages of concentric pipe heat exchangers are:
Considerable space requirement
Prone to leakage
High maintenance cost
In normal practice, the outlet temperature of the water is not allowed to reach much higher than 500C to avoid excessive corrosion. Because of the flexibility possible in baffle arrangements, extremely large or small volume of fluids is best routed through the shell side of a shell and tube heat exchanger. The advantage of square-pitch arrangements over the triangular pitch in case of heat exchanger tubes are:
1.Easily accessible for external cleaning
2.Low-pressure drop
The following characteristics of the fluid are to be considered while decreasing its route in the heat exchanger:
1.Viscosity
2. Fouling
3. Corrosiveness
4. Pressure.
Read Also:
Double pipe heat exchanger
Heat Transfer Formulas
1.Easily accessible for external cleaning
2.Low-pressure drop
The following characteristics of the fluid are to be considered while decreasing its route in the heat exchanger:
1.Viscosity
2. Fouling
3. Corrosiveness
4. Pressure.
