1. REGENERATIVE HEAT EXCHANGERS
B51GU – Heat Transfer and Heat Exchangers
REGENERATIVE HEAT EXCHANGERS
Vanee
GROUP 2:
AYESHA SHAIK KHAJA MOHIDIN | H
KALAIVANEE SIVASAMY | H

2. i. WHAT IS A HEAT EXCHANGER?
A heat exchanger or HEX is a device in which heat (thermal energy) is transferred from a warmer fluid to a colder fluid without fluid mixing.
The heat transfer mechanisms involved are convection and conduction.
Vanee

3. II. HOW MANY KINDS OF HEAT EXCHANGERS?
There are Two Main Kinds of Heat Exchangers;
Recuperators - A wall separates hot and cold fluids and heat is transferred by a combination of convection and conduction.
Examples;
Shell and Tube HEX
Compact HEX
Plate and Frame HEX
Regenerators - Hot and cold fluid pass alternately over an exchanger core or “matrix”. 
Fixed Bed
Rotary
Micro-Scale
"Rothemule" Regenerators.
Vanee

4. II.I CLASSIFICATION OF HEAT EXCHANGERS
Vanee

5. III. WHAT ARE REGENERATIVE HEAT EXCHANGERS?
Regenerative Heat Exchangers A.K.A. Regenerators
The exchanger core or “matrix” serves as a heat storage device.  
Hot fluid passes through the matrix and gives up heat to the storage material. 
The hot fluid is switched off and a cold fluid passes through the same matrix and the heat is regenerated from the matrix to the cold fluid.
Two main types of regenerative heat exchangers:
Fixed Bed Regenerators
Rotary Regenerators
Ayesha

6. IV. REGENERATOR WORKING MECHANISM
A regenerative heat exchanger or regenerator is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid.
This happens cyclically or repetitively thereby regenerating the cold fluid with heat.
Ayesha

7. IV.I Biological Analogy to a Regenerator:
The nose and throat = regenerative heat exchanger when breathing.
INHALATION: Cool air is warmed through the throat.
EXHALATION: Warmed air deposits heat onto nasal passage.
Nasal turbinates are used to warm and moisturize the air flowing into the nose (much like a hygroscopic rotary regenerative heat exchanger).
Ayesha

8. IV.II Regenerator Working Principles:
Inside the regenerator;
The hot fluid is brought into contact with the heat storage medium.
For a time period, the hot fluid flows through the porous material, heating it up.
Then the flow is halted (the heat is entrained in the heat exchange medium).
The hot fluid is displaced with the cold fluid (flowing in the opposite direction) which absorbs the heat that has been given to the heat storage medium.
This sequence is repeated in each chamber, without any mixing of fluids.
Ayesha

9. V. INVENTION AND HISTORICAL USAGE OF REGENERATORS
Regenerative heating was invented in the Industrial Revolution.
The Stirling engine was a heat engine invented by Robert Stirling in 1816, where the regenerator is a key component.
Regenerators evolved for usage in the glass and steel making industries, as well as chemical and petrochemical.
Ayesha

10. VI. Regenerator Cross-Section Analysis
Components of a Regenerator:
Heat Exchange Chambers
Rotary Gas Valves
Seals
Flow Configuration:
The two fluids (hot and cold) flow through the same channel intermittently, in opposite directs and at different times, exchanging the stored heat without mixing.
Operation Modes: (time-dependant)
Intermittent operation
Semi-continuous operation
Ayesha

11. VII. Material Type and Geometry in a Regenerator
HEAT STORAGE MEDIUM: Heat Exchange Chamber (porous storage material)
INSULATED CASINGS: containing porous heat exchange material.
Geometries and Types of Heat Exchange Materials:
For large regenerators; Bricks, Molded Bricks Packing, Baffle Bricks, Gravel
For small regenerators; Spherical Particles, Honeycombs, Wire Bundles, Monolith, Woven-Screen or Raschig Rings
The selection of the material type and geometry will depend mainly on the factors of regenerator size, dust sensitivity, pressure drop, and heat transfer coefficient.
Ayesha

12. VIII. Mathematical Modelling of Regenerative Heat Exchangers
The design and performance of a regenerator include thermal characteristics and resistance characteristics. It is therefore necessary to calculate the pressure drop and heat transfer.
The heat transfer rate in a regenerator is a function of a number of independent parameters, such as;
sphere diameter,
height of the fixed bed,
diameter of the rotary heat wheel,
gas and air velocities,
reversal time,
and temperatures of the cooled and preheated gas.
Ayesha

13. IX. Fixed Bed Regenerators
The transfer of heat takes place between the fluid and solid phases and the transfer can be either steady state or transient.
The solid phase can be in several forms:
randomly dumped packings
ordered mono-sized particles
monolithic blocks of various geometrical shapes
 The fixed bed geometry itself is normally cylindrical and the flow of the fluid through the bed is parallel to the axis of the cylinder.
Vanee

14. IX.I Fluid Flow in Fixed Bed Regenerators
The transfer of heat takes place between alternating hot fluid & cold fluid and a fixed solid phase in the regenerator.
Vanee

15. IX.I.I HOT AND COLD FLOW IN A FIXED BED REGENERATOR
Vanee

16. IX.I.II PARALLEL FIXED BED REGENERATORS
Vanee

17. IX.II Matrix and Checkers in Fixed Bed Regenerators
The matrix or checkers are stationary and what changes is the intermittent flowing of hot & cold fluid through the heat exchanger.
Common materials used as matrix/checker:
ceramic
bricks
silica-alumina (for High Temp operations)
beds of basalt or flint chips (Low Temp operations)
gravel
The key factor is to maximize heat transfer area for heat exchange to happen between the flowing fluids and matrix.
Vanee

18. IX.II Matrix and Checkers in Fixed Bed Regenerators
Vanee

19. x. ROTARY REGENERATORS
Ayesha

20. X.I ROTARY REGENERATORS DEFINED
In rotary regenerative heat exchangers (also referred to as Ljungstrom air preheaters or heat wheels), the heat transfer is facilitated by storing heat in a cylindrical rotating porous media, and by alternating the flow of hot and cold gases through the regenerator. 
There are two kinds of rotary regenerators;
Pure Heat Recovery - specifically non-hygroscopic heat transfer and recovery
Heat and Humidity Transfer - transfer humidity (hygroscopic) whilst maintaining the ability to transfer heat from the outlet gas to the inlet gas.
The ability to recover both thermal (sensible) heat and humidity (latent) energy is what makes rotary heat exchangers highly efficient. They typically have efficiencies of 70-90% with a pressure drop of under 200 Pa being normal.
Ayesha

21. X.I ROTARY REGENERATOR WORKING EXAMPLE
Ayesha

22. X.II ROTARY REGENERATOR WORKING PRINCIPLES
In a rotary regenerator, cylindrical packing rotates about the axis of a cylinder between a pair of gas seals.
Hot and cold fluid flow simultaneously through ducting on either side of the gas seals and then intermittently through the rotating packing.
Ayesha

23. X.III ROTARY REGENERATOR CONSTRUCTION
Ayesha

24. XI. ADVANTAGES AND DISADVANTAGES OF REGENERATORS
Cost
Size
Maintenance
Environmental
Efficiency (relatively high efficiency)
Disadvantages:
Seal failure and Cross-Mixing
Complexity (and added cost)
Pressure Drop
Vanee

25. XII. APPLICATIONS AND LIMITATIONS OF REGENERATORS
Regenerators are utilized mainly in gas/gas heat recovery applications in power stations, production factories, oil and gas rigs and other energy intensive industries.
They are particularly applied in;
Air conditioning systems
Space Heating
Industrial applications for waste heat recovery (WHR) opportunities include;
Waste Heat Recovery Systems
Glass Manufacturing
Cement Manufacturing
Aluminium Production
THE FUTURE?
Ayesha

26. Thank you for listening 
QUESTIONS?
Vanee and Ayesha
Thank you for listening 