A Presentation on HEAT EXCHANGER DESIGN

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A Presentation on HEAT EXCHANGER DESIGN
BY Prateek
Mall Roll no.-0812851024 3rd year
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WHAT ARE HEAT EXCHANGERS?

• Heat exchangers are one of the most common
  pieces of equipment found in all plants.
• Heat Exchangers are components that allow the
  transfer of heat from one fluid (liquid or gas)
  to another fluid.
• In a heat exchanger there is no direct contact
  between the two fluids. The heat is transferred
  from the hot fluid to the metal isolating the two
  fluids and then to the cooler fluid.
• The mechanical design of a heat exchanger
  depends on the operating pressure and temperature
  .

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APPLICATION OF HEAT EXCHANGERS

• Heat exchange is used every where around the
  human and
• its surroundings.
• Heat exchangers are used in many industries, some
  of
• which include
• Waste water treatment,
• Refrigeration systems,
• Wine-brewery industry,
• Petroleum industry,
• In aircraft industry to make the aircraft cool
  during the flights.

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CLASSIFICATION OF HEAT EXCHANGER

• Basic Classification
• Regenerative Type
• Recuperative Type
• Classification Based On Fluid Flow
• Liquid/Liquid
• Liquid/Gas
• Gas/Gas

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• Classification by flow arrangements
• Concurrent Flow in same direction
• Thermodynamically poor
• High thermal stresses since large
• temperature difference at inlet
• Counter current- flow opposite to each other
• Thermodynamically superior
• Minimum thermal stresses
• Maximum heat recovery
• Least heat transfer area
• Cross flow- Flow perpendicular to each other
• In between the above
• Space is important

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TUBULAR HEAT EXCHANGER

• This type of heat exchanger are categorized in
  following types-
• Double Pipes heat Exchanger
• Shell Tube Heat Exchanger
• Spiral Tube Heat Exchanger

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DOUBLE-PIPE HEAT EXCHANGER
Simplest type has one tube inside another - inner
tube may have longitudinal fins on the outside
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SHELL AND TUBE HEAT EXCHANGER

• Shell and tube heat exchangers consist of a
  series of tubes. One set of these tubes contains
  the fluid that must be either heated or cooled.
  The second fluid runs over the tubes that are
  being heated or cooled so that it can either
  provide the heat or absorb the heat required.
• A set of tubes is called the tube bundle and can
  be made up of several types of tubes plain,
  longitudinally finned.

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PLATE HEAT EXCHANGER

• This type of heat exchanger are categorized in
  following types-
• Plate Frame Heat Exchanger
• Spiral Heat Exchanger

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PLATE FRAME HEAT EXCHANGER

• A plate type heat exchanger consists of plates
  instead of tubes to separate the hot and cold
  fluids.
• The hot and cold fluids alternate between each of
  the plates. Baffles direct the flow of fluid
  between plates.
• Because each of the plates has a very large
  surface area, the plates provide each of the
  fluids with an extremely large heat transfer
  area.
• Therefore a plate type heat exchanger, as
  compared to a similarly sized tube and shell heat
  exchanger, is capable of transferring much more
  heat.
• This is due to the larger area the plates provide
  over tubes.

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SELECTION OF HEAT EXCHANGERS

• Terminal Temperatures
• Types of Fluids
• Properties of Both Fluids
• Flow Arrangement
• Operating Pressure and Temperature
• Pressure Drop
• Heat Recovery
• Fouling
• Ease of Inspection, Cleaning, Repair
  Maintenance
• Materials of Construction
• Cost of Heat Exchanger

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Terminal Temperatures

• Performance of Heat Exchanger depends on terminal
  temperatures
• Heat Transfer Units (HTU) defined as ratio of
• Temperature of one fluid
• Mean temperature difference between the
  fluids
• Plate heat exchanger gt Tubular Heat Exchanger
• Up to 4 HTU in case of Plate heat exchanger

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Properties of Both Fluids

• Heat Transfer Calculations
• Pumping Calculations
• Viscosity
• Low viscosity- Plate heat exchanger
• High viscosity- Scraped surface heat exchanger
• Thermal conductivity
• Density
• Specific heat
• Thermal diffusivity

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Operating Pressure and Temperature

• Mechanical Design
• Operating Pressure
• Operating Temperature
• Problems of high operating temperature and
  pressure
• Vibration
• Fatigue
• Thermal stresses, etc.
• Plate heat exchanger free from such problems
  however plate thickness and gasket material limit
  its application

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• Heat Exchanger T, 0C P, N/cm² Q, l/h
• Plate heat exchanger 260 21 50,00,00
• Double pipe 540 70 no limit
• Shell and tube 540 105 no limit

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Pressure Drop

• Important for
• Pumping Cost - proportional to pressure drop
• Heat Transfer Rate - proportional to pressure
  drop

Heat Recovery

• Conservation of energy- very important
• Recovery of heat from used/waste process streams
• Less than 50 in tubular heat exchangers
• Up to 95 in plate heat exchanger

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Fouling

• Deposition of solid material- poor conductor of
  heat
• Decreases heat transfer
• Decreases flow rate
• Lead to corrosion
• Loss of valuable materials
• Affects the design and size of the unit
• Affects the production runs
• Factors affecting fouling
• Velocity- High velocity less fouling
• Shearing force Turbulence
• Laminar layer thickness Residence time
• Surface temperature important for heat
  sensitive liquids
• - small temperature difference required
• Bulk fluid temperature more fouling in less
  bulk temperature
• Composition

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Materials of Construction

• Material of construction depends on
• Properties of the fluids such as heat
  sensitivity, fouling, corrosivity,
• Operating temperature and pressure
• Welding ease
• Availability
• Conformance to all applicable laws, codes and
  insurance requirements
• Cost
• Materials
• Stainless steel Carbon steel Graphite
• Aluminum Titanium Hastalloy
• Gaskets
• Nitryl rubber Butyl rubber
• Teflon Compressed asbestos fibers

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Overall Heat Transfer Coefficient

• An essential requirement for heat exchanger
  design or performance calculations.

• Contributing factors include convection and
  conduction associated with the
• two fluids and the intermediate solid, as
  well as the potential use of fins on both
• sides and the effects of time-dependent
  surface fouling.

• With subscripts c and h used to designate the
  hot and cold fluids, respectively,
• the most general expression for the overall
  coefficient is

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Assuming an adiabatic tip, the fin efficiency is
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A Methodology for Heat Exchanger Design
Calculations - The Log Mean Temperature
Difference (LMTD) Method -

• A form of Newtons Law of Cooling may be
  applied to heat exchangers by
• using a log-mean value of the temperature
  difference between the two fluids

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• Parallel-Flow Heat Exchanger

• Note that Tc,o can not exceed Th,o for a PF HX,
  but can do so for a CF HX.

• For equivalent values of UA and inlet
  temperatures,

• Shell-and-Tube and Cross-Flow Heat Exchangers

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NTU METHOD
The Number of Transfer Units (NTU) Method is
used to calculate the rate of heat transfer in
heat exchangers (especially counter current
exchangers) when there is insufficient
information to calculate the Log-Mean Temperature
Difference(LMTD).

• Assume negligible heat transfer between the
  exchanger and its surroundings
• and negligible potential and kinetic energy
  changes for each fluid.

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• Assuming no l/v phase change and constant
  specific heats,

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• Heat exchangers are designed by the usual
  equation
• q UALMTD"
• wherein
• U is the overall heat-transfer coefficient,
• A is the area of the heat-exchange surface, and
• LMTD is the Log Mean Temperature Difference.

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Conclusions

• General heat exchanger selection situation
  involves minimising cost subject to a long list
  of possible constraints
• In general, robustness is a very important factor
  - shell-and-tube exchangers may not be the most
  efficient, but they score highly in this category