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HEAT TRANSFER

Heat is a form of energy resulting from molecular vibration of any substance. Heat transfer from one  substance to other by three methods.

          (a) Conduction            (b) Convection            (c) Radiation

In Conduction method, heat transfer in solid substances by molecular contact with each other  (or) Heat transfer between Two materials in Physical contact.

In Convection method heat transfer within static fluid and leads to Buoyancy (by physical movement of differential Density molecules) which causes Natural circulation.

In Radiation method heat transfer by wave. Heat energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum.

LATENT HEAT

The amount of heat required to change a unit mass of a substance from one state to another state at constant temperature. 

SPECIFIC HEAT

The amount of heat required to change the temperature of a unit mass of a substance 

through 1 degree centigrade.

SENSIBLE HEAT

The amount of heat required to change the temperature of a substance.

INTRODUCTION TO TEMPERATURE

Temperature is the measurement of heat intensity or hotness or coldness of any substance. And there are many kinds of scales to measure the temperature in,

                Centigrade  - ° C

                Fahrenheit   - ° F

                Kelvin          - ° K

HEAT EXCHANGER  ≠   HEAT CHANGER  

Heat Exchanger

PRINCIPLE

A heat exchanger is a device that transfers heat energy from one fluid to another. 

Many different types of heat exchangers are used in the power plant.

Terminologies used depends on applications are, Condenser, Heater, Cooler, Chiller, Steam Generator, Evaporator, Vaporiser etc. 

Classification:

Fluid contact: Direct contact heat exchanger

                               Surface contact heat exchanger

Direct Contact Heat Exchanger:

Direct contact heat exchangers transfer heat from one fluid to another by direct contact of the fluids.

Two types of direct contact heat exchangers are:

                Evaporative 

                Complete mixing heat exchangers

In an Evaporative heat exchanger, air is brought into direct contact with the water to be cooled. Since the air is drier and cooler than the water (Ta is less than Tw), the air will accept (absorb) some of the water vapor. The heat energy Q carried away from the water (in the form of vapor) cools the water and heats the air. The process is convective heat transfer because there is motion through the fluid.

Example:

                 Cooling Towers

               Distillation (Fractional) Column

Surface Contact Heat Exchanger:

Surface heat exchangers transfer heat from one fluid through a surface to another fluid at a lower temperature. Most surface heat exchangers used in power plants are the shell and tube type. For example, Feed water heaters, Condensers, and Lube oil coolers are shell and tube surface heat exchangers.

 Tubular type:

                Shell & Tube

Tube arrangement: Straight tube, U tube, Bowed tube, Coiled tube

Position: Vertical, Horizontal

No of Pass (Flow arrangement): single pass, Multiple pass

Single pass:  Here the fluid flows once through the heat exchanger along its length.

Multi pass:  Here the fluid flows through the heat exchanger along its length and then it’s reversed again through the whole length. In multi pass arrangements the flow can pass along the length of heat exchanger in multiple times. Heat transfer is more as the number of passes are increases.

Flow Direction: Counter flow, Parallel flow, Cross flow

                         

Counter flow heat exchanger, in which the two fluids flow parallel but in opposite directions. This type of flow arrangement allows the largest change in temperature of both fluids and is therefore most efficient.

Parallel flow heat exchangers, in which the two fluids flow parallel to each other in same direction. This is less efficient than counter flow but does provide more uniform wall temperatures.

Cross flow heat exchangers, in which the two fluids flow at right angles to each other. The efficiency is intermediate between counter flow and parallel flow heat exchangers. 

Plate type:

Plate Type

 Plate Type Heat Exchanger

Shell & Tube

Shell & Tube Heat Exchanger

In straight tube heat exchangers, the tubes are arranged in a bundle and enclosed by the shell.  Straight tubes allow easy mechanical cleaning of tube interiors.  Straight bundles can also be easily removed for replacement.  Normally, straight tube bundles have one floating head.

U-tube bundles have two advantages: The expansion problem is simplified,  half as many tube to tube sheet joints are needed. Since the tubes are U-shaped, only one tube sheet is needed. This tube sheet is rigidly attached to the shell. The tubes are free to move back and forth within the shell when heated and cooled. Exceptionally long U-tubes are supported in the shell. A disadvantage of U-tube bundles is that it is difficult to mechanically clean the tubes' interior and exterior surfaces.

Coil tube heat exchangers have one or more coils of tubing installed inside a shell. Since expansion is absorbed in the coiled tube like a spring, seals or gaskets are not needed between the floating head and the shell. Coil tube heat exchangers are therefore suitable for high pressure. These heat exchangers are usually small and often used for boiler water sampling coolers

A bowed tube bundle can be solidly bolted to the shell at each end. The bow in the tubes allows for thermal expansion. An important advantage of bowed tubes is that when the tubes cool, any scale buildup on the inside or outside of the tubes will crack off as the tubes bend.

Finned Tube

Spiral Tube

Spiral Tube Heat Exchangers – Spiral heat exchangers are very basic in their structure. They consist of two separate spiral chambers as shown in the schematic below. These two chambers house hot and cold liquids separated from each other by spiral metal sheet. Heat transfer coefficients on both sides are high. The hot and cold fluid flows are counter current to each other all the way through the exchangers. These factors lead to much lower surface area requirements than shell and tube exchangers. These exchangers can be used for highly viscous fluids at low, medium pressures.

About me

 

I am being a Mechanical engineer with 40 years of experience in maintenance of production line and Power plant machines. During my services, I had come across many different kinds of problems and breakdowns which leads to partial or total shutdown of the plant. A suitable & safe method had to be adopted to fix the deficiency and put back the machine into operational. 

In my entire tenure, I had taken up many challenging tasks and successfully solved the issue. To expedite the maintenance, it is very much essential to understand the technology of machine. 

The idea of this website is to share my experience with young generation of Mechanical Engineers. 

Here is a moral story of an Old man's Experience.

A giant ship engine failed. The ship’s owners tried one expert after another, but none of them could figure but how to fix the engine.

Then they brought in an old man who had been fixing ships since he was a young. He carried a large bag of tools with him, and when he arrived, he immediately went to work. He inspected the engine very carefully, top to bottom.

Two of the ship’s owners were there, watching this man, hoping he would know what to do. After looking things over, the old man reached into his bag and pulled out a small hammer. He gently tapped something. Instantly, the engine lurched into life. He carefully put his hammer away. The engine was fixed!

A week later, the owners received a bill from the old man for ten thousand dollars.

“What?!” the owners exclaimed. “He hardly did anything!”

So they wrote the old man a note saying, “Please send us an itemized bill.

The man sent a bill that read:

Tapping with a hammer………………….. $ 2.00

Knowing where to tap…………………….. $ 9,998.00

Effort is important, but knowing where to make an effort makes all the difference!

Good Luck