Unit I – Basic Principles - Syllabus

• Importance of heat transfer in chemical engineering operations
• Modes of heat transfer
• Mean Temperature Difference

Importance of heat transfer in Chemical Engineering

• Importance of heat transfer in chemical engineering
• Almost all processes involve production or absorption of energy in the form of heat
• Laws governing transfer of heat and types of heat transfer
• Types of equipment

Heat Transfer

• What is heat transfer?
• Movement of heat from one place to another; also known as heat flow, heat exchange, transfer of thermal energy
• Nature of heat flow
• When two objects at different temperatures are brought into thermal contact, heat flows from the object at the higher temperature to that at the lower temperature.
• The net flow is always in the direction of temperature decrease
• Modes of heat transfer: Conduction, Convection, Radiation

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Heat Conduction

• Conduction is the transfer of heat through materials by the direct contact of matter.
• Dense metals like copper and aluminum are very good thermal conductors.

Heat Conduction

• A thermal insulator is a material that conducts heat poorly.
• Heat flows very slowly through the plastic so that the temperature of your hand does not rise very much.

Heat Conduction

• Styrofoam gets its insulating ability by trapping spaces of air in bubbles.
• Solids usually are better heat conductors than liquids, and liquids are better conductors than gases.

Heat Conduction

• The ability to conduct heat often depends more on the structure of a material than on the material itself.
• Solid glass is a thermal conductor when it is formed into a beaker or cup.
• When glass is spun into fine fibers, the trapped air makes a thermal insulator.

Thermal Conductivity

• The thermal conductivity of a material describes how well the material conducts heat.

Thermal �Conductivity

• Heat conduction in solids and liquids works by transferring energy through bonds between atoms or molecules.

Heat Conduction Equation

P_H = κ A (T₂ -T₁)

L

Area of cross section (m²)

Length (m)

Thermal conductivity

(watts/m^oC)

Heat flow

(watts)

Temperature

difference (^oC)

Variables for conduction

Calculate Heat Transfer

• A copper bar connects two beakers of water at different temperatures.
• One beaker is at 100°C and the other is at 0°C.
• The bar has a cross section area of 0.0004 m² and is one-half meter (0.5 m) long.
• How many watts of heat are conducted through the bar from the hot beaker to the cold beaker?
• The thermal conductivity of copper is 401 W/m°C.

Conduction

• If a temperature gradient exists in a continuous substance, heat can flow unaccompanied by any observable motion of matter. Heat flow of this kind is called conduction.
• Metallic solids: conduction results from motion of unbound electrons
• Solids of poor electrical conductivity and Liquids: transport of momentum of individual molecules along the temperature gradient
• Gases: heat is diffused from hotter regions to colder regions due to random motion of molecules

Convection

• When a current or macroscopic particle of fluid crosses a specific surface, such as the boundary of a control volume, it carries with it a definite quantity of enthalpy. Such a flow of enthalpy is called a convective flow.
• Types of convection: Natural and forced convection
• Natural convection: Convection currents are the result of buoyancy forces generated by differences in density, which in turn caused by temperature gradients in the fluid mass.
• Forced convection: Convection currents are set in motion by the action of a mechanical device such as a pump or agitator; flow independent of gradients of density

Radiation

• Radiation: Transfer of energy through space by electromagnetic waves.
• If radiation is passing through empty space, it is not transmitted into heat or any other form of energy nor is it diverted from its path.
• If however, matter appears in its path, the radiation will be transmitted, reflected or absorbed
• Fused quartz transmits practically all the radiation that strikes it.
• A polished opaque surface or mirror will absorb most of the radiation impinging on it
• A black or matte surface will absorb most of the radiation received by it and will transform such absorbed energy quantitatively into heat.