Basics of Heat Exchangers

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi

The story of a First Thermodynamic Device?!?!?!

HEAT EXCHANGER

Human Need of Power is Responsible for Its Innovation!

Heat Exchanger Made Power Generation Viable!!!

A first Step towards Scientific living Style

A True Mediator !!!

• Fire is a discovery rather than an invention.

• Homo erectus probably discovered fire by accident.

• Fire was most likely given to man as a 'gift from the heavens' when a bolt of lightning struck a tree or a bush, suddenly starting it on fire.

• The flaming touch and the campfire probably constituted early man's first use of 'artificial' lighting.

• As early as 400,000 BC, fire was kindled in the caves of Peking man.

• Prehistoric man, used primitive lamps to illuminate his cave.

• Various Oils were used as fuels.

Invention of FIRE, FLAME and TORCH

Sharing of Skills

The First Civilized Food Processing !!!!!

Fire Can only Heat Solids !!!!!

A Search for Ubuntu Device ….

• A generous, hospitable, friendly, caring and compassionate.

• They share what they have/get/earn……..

• A person with ubuntu is open and available to others…….

• Various religions identified them as Mediators.

• Business Mediators.

• Energy Mediators – A first step in Civilization and Development…

EARLIEST TYPES OF HX : COOKING

• Primitive humans may first have savoured roast meat by chance, when the flesh of a beast killed in a forest fire was found to be more palatable and easier to chew and digest than the customary raw meat.

• They probably did not deliberately cook food, though, until long after they had learned to use fire for light and warmth.

• It has been speculated that Peking man roasted meats, but no clear evidence supports the theory.

• During Palaeolithic Period, Aurignacian people of southern France apparantly began to steam their food over hot embers by wrapping it in wet leaves.

• Crude procedures

– as toasting wild grains on flat rocks and using shells, skulls,

– or hollowed stones to heat liquids.

• Introduction of pottery during the Neolithic Period.

• A paste, toasted to crustiness when dropped on a hot stone, made the first bread.

Heat Exchangers Enhance the Utility of Fire ….

Can they do so beyond stomach ????

The Aelopile

• In 130BC. Hero, a Greek mathematician and scientist is credited with inventing the first practical application of steam power, the aelopile.

Branca's Steam Turbine

• In 1629, Giovanni Branca, of the Italian town of Loretto, described, in a work' published at Rome, a number of ingenious contrivances.

The Savery Engine

Thomas Savery, July 2, 1698, patented the design of the first engine which had the most important advance in actual construction. A working model was submitted to the Royal Society of London.

Newcomen Engine

The original Thomas Newcomen engine was invented in 1712.

James Watt’s Engine

James Watt radically improved Newcomen's engine (1769) by condensing the steam outside the cylinder .

No Recognition to The Heat Exchanger !!!?!?!?

Onset of Heat Exchangers

The Plain Cylinder Boiler:

The Scientific Development of HXs

The Cornish Boiler

The Scotch Boiler

The Scientific Engineering !!!!!!

Progress in Rankine Cycle

Year 1907 1919 1938 1950 1958 1959 1966 1973 1975

MW 5 20 30 60 120 200 500 660 1300

p,MPa 1.3 1.4 4.1 6.2 10.3 16.2 15.9 15.9 24.1

Th oC 260 316 454 482 538 566 566 565 538

Tr oC -- -- -- -- 538 538 566 565 538

FHW -- 2 3 4 6 6 7 8 8

Pc,kPa 13.5 5.1 4.5 3.4 3.7 3.7 4.4 5.4 5.1

,% -- ~17 27.6 30.5 35.6 37.5 39.8 39.5 40

A Train of External HXs in A Power Plant

Consequence of An Internally Efficient Power Plant

Impact of Cycle Improvement on Capability of Fire

Stockholm 1920The Ljungström Air Preheater

Economic Impact of the Landmark

• The use of a Ljungström Air Preheater in a modern power plant saves a considerable quantity of fuel.

• So much that the cost of the preheater is generally recovered after only a few months.

• It has been estimated that the total world-wide fuel savings resulting from all Ljungström Air Preheaters which have been in service is equivalent to 4,500,000,000 tons of oil.

• An estimate shows that the Ljungström Air Preheaters in operation annually saves about $30 Billion US.

• The distribution of thermal power capacity in which Ljungström Air Preheaters are installed over the world is shown in the table below.

Heat Exchanger : An Effective Landlord

• Creates a housing for both donor and Receiver.• How to accommodate both in a single housing?• Space Sharing & Time sharing • Space sharing: Donor and Receiver are present always.• Develop partition(s) in the house(HX).• Time Sharing : Donor And Mediator for sometime and

Mediator and Receiver for sometime : Repeat!• Central Limit Theorem : It is impossible to have time and

space sharing in one system.• Time Sharing : Regenerators• Space Sharing : Recuperators• Shell & Tube HXs

Design Considerations for Heat Exchangers

• When preparing to design a heat exchanger, do you ever wonder where to start? 

• You've done it before, but you hate that feeling of getting half way through the design and realizing that you forgot to consider one important element. 

• The thought process involved is just as important as the calculations involved. 

• Let's try to map out a heat exchanger design strategy. 

• We'll do so with a series of questions followed by information to help you answer the questions

Is there a phase change involved in my system?

• A quick look at the boiling points compared with the entrance and exit temperatures will help you answer this question.

How many "zones" are involved in my system?

Various Simple Zones

What are the physical properties of the streams involved?

• Get the physical properties for each zone separately to ensure accuracy, but in some cases it is acceptable to use an average value.   

• Physical properties that you will want to collect for each phase of each stream will include:  heat capacity, viscosity, thermal conductivity, density, and latent heat (for phase changes). 

• These are in addition to the boiling points of the streams at their respective pressures.

What are the allowable pressure drops and velocities in the exchanger?

• Pressure drops are very important in exchanger design (especially for gases). 

• The pressure drop and velocities must be limited.  • The velocity is directly proportional to the heat transfer

coefficient which is motivation to keep it high, while erosion and material limits are motivation to keep the velocity low. 

• Typical liquid velocities are 1-3 m/s.  • Typical gas velocities are 15-30 m/s. 

What is the estimated area of the exchanger?

• Unfortunately, this is where the real fun begins in heat exchanger design! 

• You'll need to find estimates for the heat transfer coefficients that you'll be dealing with. 

• Once you've estimated the overall heat transfer coefficient, use the equation Q = UoATlm to get your preliminary area

estimate. 

• Remember to use the above equation to get an area for each zone, then add them together.

What geometric configuration is right for my exchanger?

• Now that you have an area estimate, it's time to find a geometry that meets your needs. 

• Once you've selected a shell diameter, tubesheet layout, baffle and tube spacing, etc., it's time to check your velocity and pressure drop requirements to see if they're being met. 

• Experienced designers will usually combine these steps and actually obtain a tube size that meets the velocity and pressure drop requirements and then proceed. 

• If your pressure drop requirements are low, avoid using four or more tube passes as this will drastically increase your pressure drop. 

• Now you have a geometry selected that meets all of your needs.• Now that I have a geometry in mind, what is the actual overall heat transfer

coefficient?• What is the actual area of the exchanger using the 'actual' heat transfer

coefficient?