Heat Exchangers Design

Heat Exchangers Design

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 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 finger tips, although there are lots of study material and lectures available, to obtain good job for an engineer it is first and foremost topic which make him stand in front line. Showing interest on 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 traveling between the area of 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 contain this energy which is stored and come into picture when there is driving force which make to travel and that is know to be a temperature difference, this measuring parameter is chosen by use because the temperature is directly related to the heat quantity.

shell and tube heat exchanger tube sheet CAD diagram
Tube sheet
In a shell and tube heat exchanger, the ‘tube pitch’ is defined as the shortest center to center 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 is facilitate highest heat transfer rate than other 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 cross flow area depends upon baffle spacing, clearance and pitch. Shell side heat transfer co-efficient in case of square pitch as compared to the triangular pitch under 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.
shell and tube heat exchanger baffles cross section CAD diagram
Baffle cross section
Baffles are the inter connectors of the tubes and shell of the heat exchangers. The shell side fluid will pass through the direction depending on the baffles arrangements. Rate of heat transfer also will depend on the baffles spacing. Different types of baffles are used as disk & ring, segmental and orifice. Transverse baffle as 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. For given 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.
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 & number of tubes and operated at 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 & number of tubes and operated at the same liquid flow rate.
                      Clearance between shell & baffles and between tube & baffles should be minimum to avoid by-passing of the fluid, but it should be permit the removal of tube bundle. Baffles are supported independently of the tubes by tie rods and positioned by spacers. Tie rods are fixed at one end in the tube sheet by making blind holes and the minimum number of tie rods is 4 with at least 10 mm diameter.
Design of shell and tube heat exchanger using CAD

       
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 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. High specific heat of water makes it a widely used coolant in heat exchangers.

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 co-efficient 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 counter current, LMTD correction factor is used. The various types of baffles used in heat exchanger are 

1. Segmental

2. Disc and doughnut

3. Orifice

Shell side pressure drop depends on

1. Mass velocity of shell side fluid 
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

Tube rolling
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:

Comparatively less heating surface
Considerable space requirement
Prone to leakage
High maintenance cost

In normal practice the outlet temperature of 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 are 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 heat exchanger:
1.Viscosity
2. Fouling
3. Corrosiveness
4. Pressure.
Gas to Gas heat exchanger

 
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