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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: