1-Classification of Heat Exchangers & Selection Criteria

Heat Exchanger

Heat Exchanger:  A device whose primary purpose is the transfer of thermal energy between two fluids is named as heat exchanger, which is abbreviated as ‘HX’.  The transfer of heat is usually accomplished by means of a device known as a heat exchanger. Common applications of heat exchangers include boilers, coolers, cooling water heat exchangers, evaporators, and condensers.  The basic design of a heat exchanger normally has two fluids of different temperatures separated by some conducting medium. The most common design has one fluid flowing through metal tubes and the other fluid flowing around the tubes. On either side of the tube, heat is transferred by convection and Heat is transferred through the tube wall by conduction.

Classification of Heat Exchangers:

 They can be classified based on different aspects of their construction and operation. The different systems of classification are, 1) Classification according to transfer process 2) Classification according to Degree of surface compactness 3) Classification according to Pass arrangement 4) Classification based on Flow arrangement 5) Classification based on Geometry of construction 6) Classification based on Number of fluids 7) Classification based on Heat transfer mechanism (Phase change).

Classification according to transfer process :

In a Direct contact type heat exchanger, heat is transferred through direct contact between hot and cold fluids. For example, Spray towers and cooling towers. The Indirect contact type heat exchanger, firstly the heat is transferred from the hot fluid to an impervious surface and then to cold fluid. These type of heat exchangers are also called surface heat exchangers. For example, Direct transfer (Recuperators, e.g. shell & tube heat HX), storage type (Thermal regenerator) and fluidized bed heat exchangers.

Direct contact type heat exchanger :

Cooling Tower

Spray Tower

Indirect contact type heat exchanger :

Shell & tube heat exchanger

Fluidized Bed heat exchanger

Classification according to Degree of surface compactness:

 The compactness of a heat exchanger is defined as the ratio of heat transfer area and heat exchanger volume. This is also called area density. a. Compact Heat exchanger (area density greater than 700 m2/m3 or 213 ft2/ft3 or hydraulic diameter ≤ ¼ inch) for gas stream. And (area density greater than 400 m2/m3 or 122 ft2/ft3 for liquid stream). b. Non-compact Heat exchanger (area density less than 700 m2/m3) for gas stream. And (area density less than 400 m2/m3 for liquid stream).

Classification according to Pass arrangement :

1) Single pass arrangement 2) Multiple pass arrangement

Classification based on Flow arrangement:

 The choice of a particular flow arrangement depends upon the required heat exchanger effectiveness, fluid flow paths, packing envelope, allowable thermal stresses, temperature levels and other design criteria.  Heat exchangers may be classified according to their flow arrangement. 1) Parallel-flow 2) Counter-flow 3) Cross-flow 4) Divided-flow 5) Split-flow

 In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side.  In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is most efficient, in that it can transfer the most heat.  In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.  In a divided-flow heat exchanger, the shell fluid in this case is divided and tube fluid is multi-pass.  In a split-flow heat exchanger, the arrangement differs from the divided-flow in only one respect that is the shell side fluid recombines after passing over the tubes.

Split Flow Heat Exchanger

Classification based on Geometry of construction :

 The heat exchangers classify based on geometry of construction are, 1. Tubular Heat Exchangers 2. Plate Heat Exchangers 3. Extended surface Heat Exchangers 4. Regenerative Heat Exchangers

Double pipe Heat Exchanger

Spiral Heat Exchanger

Shell & Tube Heat Exchanger

Circular Fins Heat Exchanger

Plate Heat Exchanger:

 Gasketed Plate  Spiral Plate  Lamella

 Limited to below 25 bar and 250ºC  Plate heat exchangers have three main types : gasketed ,spiral and lamella heat exchangers.  The most common of the platetype heat exchangers is the gasketed plate heat exchanger

Gasketed Plate Heat Exchanger:

 The most common of the plate-type heat exchangers is the gasketed plate heat exchanger

Spiral Plate Heat exchanger :

Ideal flow conditions and the smallest possible heating surface

Lamella Consisting of cylindrical shell having a number heat transfering lamellas. Similar to tubular heat exchanger

Advantages Plate heat exchangers yield heat transfer rates three to five times greater than other types of heat exchangers. The design of the plate heat exchanger allows to add or remove plates to optimize performance, or to allow for cleaning, service, or maintenance. Plate exchangers offer the highest efficiency mechanism for heat transfer available in industry.

Disadvantages Plate exchangers are limited when high pressures, high temperatures, or aggressive fluids are present. Because of this problem these type of heat exchangers have only been used in small, low pressure applications such as on oil coolers for engines.

Rotary regenerative heat exchanger :

The rotary regenerative heat exchanger, commonly known as the air pre-heater or gas reheater. In a rotary heat exchanger heat is transferred from a hot gas to a cold one via a rotating cylinder of densely packed metal sheets, called elements. These elements are packed in containers and slowly rotate through one gas stream and into the other. A hot gas flows over the surface of the metallic elements, raising their temperature. As the rotor turns, at around 1RPM, the heated elements move into the cool gas stream, increasing its temperature accordingly.

Rotary regenerative heat exchanger

Extended Surface Heat Exchangers:

 Plate Fin Heat Exchangers  Tube Fin Heat Exchangers

Plate Fin Heat Exchangers: For gas to gas applications. Widely used in energy recovery, process industry, refrigeration and air coditioning systems.

Fin Types in Plate-Fin Heat Exchangers:

PLAIN A sheet of metal with corrugated fins at right angles to the plates

PERFORATED A plain fin constructed from perforated material

SERRATED Made by simultaneously folding and cutting alternative sections of fins. These fins are also known as the lanced or multi-entry pattern.

HERRINGBONE Made by displacing the fins sideways at regular intervals to produce a zig-zag effect.

Tube Fin Heat Exchangers: For gas to liquid heat exchangers. Used as condersers in electric power plant, as oil coolers in power plants, as ir cooled exchangers in process and power industires.

Finned Tubes:

Tubular Heat exchanger: They are so widely used because the technology is well established for making precision metal tubes capable of containing high pressures in a variety of materials. There is no limit to the range of pressures and temperatures that can be accommodated.

Tubular Heat exchanger

Shell & Tube Heat Exchanger

Double Pipe Heat Exchanger

Shell & Tube Heat Exchanger:

 These are the most commonly used heat exchangers in oil refineries and other large chemical processes.  These are used when a process requires large amounts of fluid to be heated or cooled.  These provide transfer of heat efficiently.  These use baffles on the shell-side fluid to accomplished mixing or turbulence.  Tube : strong, thermally conductive, corrosion resistant, high quality  Outer shell : durable, highly Strong  Inner tube : having effective combination of durability, corrosion resistant and thermally conductive.

Shell & Tube Heat exchangers

U- Tube Heat exchangers

Fixed Tube Heat exchangers

Floating Head Heat exchangers

U - Tube Heat Exchanger: U-Tube heat exchanger consisting of straight length tubes bent into a U-shape surrounded by a shell.

Advantages: Boath initial and maintenance costs are reduced by reducing the number of joints. They have enlarged shell sections for vapor-liquid separation.

Disadvantages:They have drawbacks like inability to replace individual tubes except in the outer row and inability to clean around the bend.

Fixed Tube Heat Exchanger:  These have straight tubes that are secured at both ends to tube sheets welded to the shell.  They are the most economical type design.  They have very popular version as the heads can be removed to clean the inside of the tubes.  Cleaning the outside surface of the tubes is impossible as these are inside the fixed part.  Chemical cleaning can be used.

Floating Head Heat Exchanger  In floating head heat exchanger, one tube end is free to float within the shell and the other is fixed relative to the shell.  A floating head is excellent for applications where the difference in temperature between the hot and cold fluid causes unacceptable stresses in the axial direction of the shell and tubes.  The floating head can move, so it provides the possibility to expand in the axial direction.  Easy inspection,cleaning or maintenance.

Double Pipe Heat Exchanger:  They consist of one pipe concentrically located inside a second, larger one.  Cold and hot liquid respectively flows in the gap of inner pipe and sleeve pipe. Structure is simple and heat transmission is large.  They utilize true counter-current flow which maximizes the temperature differences between the shell side and tube side fluids.  It is the most efficient design and require less surface area.

 Advantages:

They can operates in true counter current flow permitting extreme temperature cross.  Ideal for wide temperature ranges.  Provides shorter deliveries than shell and tube due to standardization of design and construction.

Classification based on Number of fluids:

Two fluid Heat exchangers Three fluid Heat exchangers Multi fluid Heat exchangers

Classification based on Heat transfer mechanism (Phase Change): 

    

Heat transfer mechanism such as single phase or two phase convection can be used for the classification of heat exchangers as follows, Single phase convection on both sides Single phase convection on one side and two phase convection on other side Two phase convection on both sides Combined convective and radiative heat transfer Commonly phase change heat exchangers are, a) Reboilers (Evaporator) b) Condensers (Total or partial condenser)

Reboiler:  Reboiler to generate vapor to drive fractional distillation.  Types of Reboilers a. Kettle Reboilers b. Forced Recirculation Reboilers c. Thermosiphon Reboilers

Kettle Reboilers:

Kettle Type Reboilers: Advantages

Disadvantages

 Insensitive to hydrodynamics  High heat fluxes are possible  Can handle high vaporization  Simple piping  Unlimited area

 All the dirt collects and non volatiles accumulate  Shell side is difficult to clean  Difficult to determine the degree of mixing  Oversize shell is expensive

Thermosiphon Reboiler:

Thermosiphon Reboiler: It operate using natural circulation with process flow on the shell side The process flow on the tube or shell side in vertical units. It does not require a pump for recirculation In thermosiphon reboiler, sensible heat transfer followed by nucleate boiling.

Forced Recirculation Reboilers:

Forced Recirculation Reboilers:  These reboiler types have two

mechanisms of heat transfer: sensible heat transfer followed by nucleate boiling.  Process flow is typically on the tube side of a standard exchanger in the vertical position.

Condensers Condensers

b) Water Cooled Condensers

   

b) Air Cooled Condensers

Horizontal shell and tube Condensers Vertical shell and tube Condensers Shell and coil Condensers Double pipe Condensers

BASIC CRITERIAS FOR THE SELECTION OF HEAT EXCHANGERS:

Generally speaking, the selection process begins with the specification of the environment in which the heat exchanger will work. This includes the process and location parameters such as,  Working temperature ranges  Working pressures  Amount of heat transfer  Types of fluids & Their flow rates  Available space  Weight limitation  Corrosion  Fouling of the fluids