1. A Presentation on HEAT EXCHANGER DESIGN
BY:
Prateek Mall
Roll no
3rd year

2. 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 .

3. 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.

4. CLASSIFICATION OF HEAT EXCHANGER
Basic Classification
Regenerative Type
Recuperative Type
Classification Based On Fluid Flow
Liquid/Liquid
Liquid/Gas
Gas/Gas

5. 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

6. 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

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

8. 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.

9. PLATE HEAT EXCHANGER
This type of heat exchanger are categorized in following types:-
Plate & Frame Heat Exchanger
Spiral Heat Exchanger

10. 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.

12. 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

13. 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 > Tubular Heat Exchanger
Up to 4 HTU in case of Plate heat exchanger

14. 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

15. 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

16. Heat Exchanger T, 0C P, N/cm² Q, l/h
Plate heat exchanger ,00,00
Double pipe no limit
Shell and tube no limit

17. Pressure Drop Heat Recovery Conservation of energy- very important
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

18. 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

19. 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

20. 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:

21. Assuming an adiabatic tip, the fin efficiency is

22. A Methodology for Heat Exchanger Design Calculations
- The Log Mean Temperature Difference (LMTD) Method -
A form of Newton’s Law of Cooling may be applied to heat exchangers by
using a log-mean value of the temperature difference between the two fluids:
Evaluation of depends on the heat exchanger type.
Counter-Flow Heat Exchanger:

23. 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:

24. 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.

25. Assuming no l/v phase change and constant specific heats,
Negligible or no change in
Negligible or no change in

26. Heat exchangers are designed by the usual equation:
q = U*A*LMTD"
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.

27. 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