SHELL AND TUBE HEAT EXCHANGER

Shell-and-Tube Exchangers.

• This exchanger, is generally built of a bundle of round tubes mounted in a cylindrical shell with the tube axis parallel to that of the shell.
• One fluid flows inside the tubes, the other flows across and along the tubes.

• A variety of different internal constructions are used in shell-and-tube exchangers, depending on the desired heat transfer and pressure drop performance and the methods employed to reduce thermal stresses, to prevent leakages, to provide for ease of cleaning, to contain operating pressures and temperatures, to control corrosion, to accommodate highly asymmetric flows, and so on.
• The three most common types of shell-and-tube exchangers are

1. Fixed tubesheet design
2. U-tube design
3. Floating-head type.

The major components of this exchanger are tubes (or tube bundle), shell, frontend head, rear-end head, baffles, and tubesheets

Tubes.

• Round tubes in various shapes are used in shell-and-tube exchangers.
• Most common are the tube bundles with straight and U-tubes used exchangers.
• In most applications, tubes have single walls, but when working with radioactive,reactive, or toxic fluids and potable water, double-wall tubing is used.
• In most applications, tubes are bare, but when gas or low-heat-transfer coefficient liquid is used on the shell side, low-height fins (low fins) are used on the shell side.
• Tubes are drawn, extruded, or welded, and they are made from metals, plastics, and ceramics, depending on the applications.

Shells

• The shell is a container for the shell fluid.
• Usually, it is cylindrical in shape with a circular cross section, although shells of different shapes are used.
• Although the tubes may have single or multiple passes, there is one pass on the shell side.
• To increase the mean temperature difference and hence exchanger effectiveness, a pure counterflow arrangement is desirable for a two-tube-pass exchanger.  

Nozzles

• The entrance and exit ports for the shell and tube fluids, referred to as nozzles
• These are pipes of constant cross section welded to the shell and channels.
• They are used to distribute or collect the fluid uniformly on the shell and tube sides.

 Front- and Rear-End Heads. 

• These are used for entrance and exit of the tube fluid.
• The front-end head is stationary, while the rear-end head could be either stationary (allowing for no tube thermal expansion) or floating, depending on the thermal stresses between the tubes and shell.
• The major criteria for selection of the front-end head are cost, maintenance and inspection, hazard due to mixing of shell and tube fluids, and leakage to ambient and operating pressures.
• The major criteria for selection of the rear-end head are the allowance for thermal stresses, a provision to remove the tube bundle for cleaning the shell side, prevention of mixing of tube and shell fluids, and sealing any leakage path for the shell fluid to ambient.

Baffles

• Baffles may be classified as transverse and longitudinal types.
• The purpose of longitudinal baffles is to control the overall flow direction of the shell fluid such that a desired overall flow arrangement of the two fluid streams is achieved.
• Single- and double-segmental baffles are used most frequently due to their ability to assist maximum heat transfer (due to a high-shell-side heat transfer coefficient) for a given pressure drop in a minimum amount of space.
• Triple and no-tubes-in-window segmental baffles are used for low-pressure-drop applications.
• The choice of baffle type, spacing, and cut is determined largely by flow rate, desired heat transfer rate, allowable pressure drop, tube support, and flow-induced vibrations.

Tubesheets

• These are used to hold tubes at the ends.
• A tubesheet is generally a round metal plate with holes drilled through for the desired tube pattern, holes for the tie rods (which are used to space and hold plate baffles), grooves for the gaskets, and bolt holes for flanging to the shell and channel.
• To prevent leakage of the shell fluid at thetubesheet through a clearance between the tube hole and tube, the tube-to-tubesheet joints are made by many methods, such as expanding the tubes, rolling the tubes, hydraulic expansion of tubes, explosive welding of tubes, stuffing of the joints, or welding or brazing of tubes to the tubesheet.
• The leak-free tube-to-tubesheet joint made by the conventional rolling process.

TUBULAR HEAT EXCHANGER

Tubular heat exchanger

• These exchangers are generally built of circular tubes, although elliptical, rectangular, or round/flat twisted tubes.
• Tubular exchangers can be designed for high pressures relative to the environment and high-pressure differences between the fluids.
• Tubular exchangers are used primarily for liquid-to-liquid and liquid-to-phase change (condensing or evaporating) heat transfer applications.
• They are used for gas-to-liquid and gas-to-gas heat transfer applications primarily when the operating temperature and/or pressure is very high or fouling is a severe problem on at least one fluid side and no other types of exchangers would work.
• These exchangers may be classified as

1. Shell-and tube heat exchangers
2. Double-pipe heat exchangers
3. Spiral tube heat exchangers

Classification of the heat exchanger by flow arrangement

Classification of the heat exchanger according to flow arrangement

• The heat exchanger can be classified according to flow arrangement as follows

Parallel flow heat exchanger

• In a parallel flow (also referred to as cocurrent or cocurrent parallel stream) exchanger, the fluid streams enter together at one end, flow parallel to each other in the same direction, and leave together at the other end

• In a parallel flow exchanger, a large temperature difference between inlet temperatures of hot and cold fluids exists at the inlet side, which may induce high thermal stresses in the exchanger wall at the inlet.

Counterflow heat exchanger

• In a counterflow or countercurrent exchanger, the two fluids flow parallel to each other but in opposite directions within the core.

• The counterflow arrangement is thermodynamically superior to any other flow arrangement.
• It is the most efficient flow arrangement, producing the highest temperature change in each fluid compared to any other two-fluid flow arrangements for a given overall thermal conductance (UA), fluid flow rates (actually, fluid heat capacity rates), and fluid inlet temperatures.

• The maximum temperature difference across the exchanger wall thickness (between the wall surfaces exposed on the hot and cold fluid sides) either at the hot- or cold-fluid end is the lowest, and produce minimum thermal stresses in the wall for an equivalent performance compared to any other flow arrangements.

Crossflow heat exchanger

• In this type of exchanger, the two fluids flow in directions normal to each other.
• In a crossflow arrangement, mixing of either fluid stream may or may not occur, depending on the design.

CLASSIFICATION OF HEAT EXCHANGERS

Classification of heat exchangers

• The heat exchanger is classified on the basis of the following criteria

1. According to flow arrangements of fluid
2. According to construction
3. According to the heat transfer process

Classification of the Heat exchanger according to flow arrangements of fluid

1. Parallel flow heat exchanger
2. Counterflow heat exchanger
3. Crossflow heat exchanger

Classification of the Heat exchanger according to construction

•  Tubular heat exchangers
1. Shell-and tube heat exchangers
2. Double-pipe heat exchangers
3. Spiral tube heat exchangers

• Plate-type heat exchangers
• Extended surface heat exchangers
• Regenerative heat exchangers

Classification of the Heat exchanger according to the heat transfer process

1. Direct contact type or mixers
2. Transfer type or Recuperators
3. Storage type or Regenerative

What is Henry’s law and Raoult’s law

Henry’s law

It states that partial pressure of the solute gas is proportional to a mole fraction of that component in liquid phase but proportionality constant is H

p_A = H* x_A=Y_A . P

       where

• p_A = equilibrium partial pressure of pure liquid 'A'
• H = Henry's law constant for A in specific solvents
• x_A= mole fraction of A in the liquid phase
• Y_A = mole fraction of A in the vapor phase
• P = equilibrium pressure
• Henry's law is valid when x_A is close to zero that is for a dilute solution of 'A'

Raoult’s law

It states that the equilibrium partial pressure of components 'A' is equal to the product of vapor pressure and mole fraction of 'A' in the liquid phase.

         Mathematically

p_A = P_A⁰* x_A=Y_A . P

      where

• P_A⁰ = vapor pressure of pure liquid A
• Raoult's law is valid when x_A is close to 1 that is when the liquid phase is almost pure

DIFFERENCE BETWEEN UNIT OPERATION AND UNIT PROCESS

Difference between Unit operation and Unit process

• The chemical process is a combination of unit processes and Unit operation. Hence the Difference between Unit operation and Unit process as follows.

Unit process

The unit process involves a chemical change or sometimes it referred to as chemical changes along with physical change leading to the synthesis of various useful product

• It also provides basic information regarding the reaction temperature and pressure, the extent of chemical conversions and yield of product of the reaction, nature of reaction whether endothermic or exothermic, type of catalyst used.
• Example: hydrogenation, oxidation, nitration, etc

Unit operations

The operations carried out in the chemical process industry involving physical changes in the materials handled or in the system under consideration are called Unit operations

• Hence unit operations involve the physical separation of the products obtained during various unit processes.
• It is very important in chemical industries for separation of various products formed during the reaction.
• Individual operations have common techniques and are based on the same principles.

Above figure shows the unit process flow diagram for the production of benzene from toluene via various unit operations which are carried out in reactor, gas separator and still

Types of unit operations are as follows

1. Mechanical operations:-Example:- size reduction, conveying, filtration
2. Fluid flow operations:- In these operations, pressure difference act as a driving force.
3. Heat transfer:- In these operations, temperature difference act as a driving force. Example:- Evaporation
4. Mass transfer:- In these operations, the concentration difference act as a driving force. Example:-Distillation

WHAT IS MOLALITY

Molality

Molality is defined as the mole of the solute dissolved in one kilogram of solute.

Example: -
A solution of caustic soda contains 20 % NaOH by weight of a solution. The density of the solution is 1.196kg/lit. Find the molality
Solution ⇒

Given: - 20 % NaOH in the caustic soda solution
Basis: - 100 kg of solution

Formula:-

As the Solution contains 20 % of NaOH

∴ Weight of NaOH = 100 * 0.2 = 20 kg.................(1)

Weight of water (solvent ) = 100 - 20 = 80 kg..............(2)

As we know