HEAT TRANSFERCHE 3004

Lt Col G. Junior VirgoExt. 3263*

In our daily activities we often experience temperature changes of one form or another.

Examples are

refrigerators, ovens and stoves in our homes

automotive radiators, and electric fans. *

Heat: Energy in transition under the driving force of a temperature gradient

Driving force: Bodies or systems at different reference points interact or are made to interact causing flow in one direction.

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Temperature gradient: from hot to cold or cold to hot

The difference in temperature is the driving force that governs this phenomenon and the process is called HEAT TRANSFER.*

The transfer of heat is integral to most chemical and engineering processes

Heat absorbed or emitted

Fluids heated or cooled

Not only explains the mechanisms involved but also, the prediction of the applicable rates and magnitudes

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All heat transfer must obey the first and second laws of thermodynamics

The transfer of heat at the desired rate

is the fundamental principle upon

which this entire course is based

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Processes concerned with furnaces, evaporators,distillation plants, dryers and reactor vessels.

Three means by which heat transfer occurs: conduction, convection radiation

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heat transfer at the intermolecular level of a substance

energetic interactions between adjacent particles.

high energy is associated with high molecular activity hence heat applied to a particular point of a substance is passed on to other molecules in the immediate vicinity when they collide with each other.

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Regions with greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions

The rate at which the heat is conducted from one point to another is directly dependent on the temperature gradient between the points.

Fouriers Law holds true

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Convective heat transfer The redistribution of energy due partly to conduction and fluid motion a combination of conduction within the boundary layer immediately adjacent to the heat source and the motion of the bulk of the fluid brought about by the displacement of high-energy molecules due to fluid motion. *

Natural or free convection

the fluid motion results from the changing thermodynamic properties of the fluid due to the effects of the temperature difference (eg. density gradient, thermal conductivity, viscosity & thermal expansion)

Forced convection.

the motion is influenced by some external force (usually mechanical)

Newtons Law of Cooling*

The net transfer of heat via electromagnetic radiation between two opposite surfaces.

Materials at temperatures above absolute zero will absorb, reflect and emit radiant heat.

Heat incident on the body excite subatomic particles to higher energy states*

On returning to their more stable states the energy is released as radiation.

Radiation is the only means of heat transfer that does not require a transporting medium and is the principal means by which heat from the sun reaches the earth.

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Heat transfer applications are as diverse as the application of devices that ensure comfort and a high quality of life in our daily existence

Usually a combination of one or more of the three modes mentioned above is involved.

The human body is one of the most efficient heat transfer devices in operation in our daily lives.

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Organs involved are the lungs, skins and kidneys.

We are totally dependent on electricity from power plants, refrigerators, stoves and air conditioning system.Heat exchangers are simply equipment consisting of two or more fluids at different temperatures that are physically separated by a wall to prevent fluid/material mixing. *

Depending on the inlet and outlet temperatures of the fluid of main concern a heat exchanger can be employed as either a heater or a cooler.

The equipment design and orientation on a process plant depends largely on the nature of the fluids to be heated or cooled. Hence, we might employ double pipe (simplest form), shell and tube, spiral, and plate heat exchangers. *

H/Es may be categorized as double pass or multipass types with or without baffles and fixed or floating heads.

There are, also, contact heaters that combine say, steam directly with the fluid to be heated. Ensures maximum heat transfer but high levels of dilution will result.*

A means of transferring heat between media and is the main unit operation of crystallization processes.

Concentration of process streams is the major application of evaporation, which is usually accompanied by cooling by way of the latent heat of vapourization.*

Heat transfer equipment in industrial plants varies from simple electrical fans for cooling to huge assemblies of pipes and conduits in arrangements called heat exchangers,

Cooling fans are used in cooling towers to cool high volumes of heating fluids (usually water) that are used in heat exchangers to cool a particular product. *

Cooling fans also used in electronic circuitry where the heating effects of electric current accounts for a very high heat per unit area in those continually decreasing components.Furnaces are equipment used to transfer heat directly to materials, for example in the calcining of ceramics, cement clinker or hydrated alumina in rotary kilns or fluid flash calciners. *

Furnaces were the earliest wide scale applications of heat exchange equipment by mankind during the fabrication of tools and weapons directly after the storage.Boilers are heat transfer equipment used primarily in power plants to produce high-pressure steam to drive turbines. *

Evaporators are vessels that concentrate solutions by boiling and evaporation.

Variations include multiple effect evaporators connected in series to flash tanks in an intricate arrangement that ensures the recovery and reuse of flashed steam.

Other equipment include heating elements used in water heaters, and solar panels

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