Heat Transfer Notes

Heat Transfer Types

• Four types of heat transfer: conduction, convection, radiation, evaporation, and condensation.

• Examples are very important.

Conduction

• Transfer of energy directly from direct contact of hot and cold molecules.

• Examples:

  • Pot on a stove: heat from the burner to the pot, then to the water.

  • Ventilator heater (hospital setting): plate of heater touches the plate of the device on top, transferring heat to warm the water.

  • Touching cold metal: body heat transfers to the metal, making it feel cold.

• High thermal conductivity metals quickly draw heat away from the skin, creating a feeling of cold.

Convection

• Involves fluid molecules at different temperatures mixing.

• Examples:

  • Infant incubator: air is warmed in one location and circulated to carry heat.

  • Forced air heating in houses: air molecules mix and warm the air.

  • Convective oven: molecules heat up and circulate around the food.

Radiation

• Heat transfer occurs without direct physical contact.

• Examples:

  • Sunlight: radiant heat warms objects without direct contact.

  • Radiant heat warmer for babies: heat comes from above without direct contact.

  • Kerosene or electric heater: heat radiates across the room.

Vaporization

• According to the first law of thermodynamics, energy must come from the surroundings.

• Bulk storage of oxygen: Compressed liquid oxygen is exposed to ambient temperatures and vaporized into gaseous form.

Evaporation

• Heat is taken from the air surrounding the liquid, cooling the air.

• Example: Sweating - liquid sweat evaporates and cools the skin.

Condensation

• The opposite of evaporation; heat is given back to the surrounding air, warming it.

• Example: Refrigerator - Food cools as energy is passed through the walls of the refrigerator into the pipes, warming the refrigerant which is carried into the atmosphere.

Heat Transfer in Our Environment

• Heat always moves from hot objects to colder ones.

• Materials impact heat transfer.

  • Conduction: Good conductors (copper, silver, iron, steel) vs poor conductors (wood, styrofoam, paper, air).

  • Convection: Up and down movement of gases and liquids; warmer rises, cooler falls, creating a convection current.

  • Radiation: Electromagnetic waves transfer heat to objects.

Melting and Freezing

• Melting: Changing from a solid to a liquid; it takes heat.

• Latent heat of fusion: The number of calories required to change from a solid to liquid without changes in temperature.

  • 1 gram of solid into a liquid without changing temperature.

• Freezing: Changing from a liquid to a solid

• Requires a large amounts of externally applied energy

• Freezing returns energy to its surroundings.

Sublimation

• Transition from a solid to a vapor without becoming a liquid.

• Example: Dry ice - it sublimates from its solid form into a gas CO_2 without first melting and becoming a liquid.

Vaporization (Liquid to Vapor)

• Two forms:

  • Boiling

  • Evaporation

Boiling

• A liquid can also change into a gas at temperatures lower than its boiling point and called evaporation.

• Liquid is the temp at which the vapor pressure exceeds atmospheric pressure

  • Boiling occurs earlier with lower atmospheric pressure and at lower temperatures.

Evaporation

• A liquid can also change into a gas at temperatures lower than its boiling point through a process called evaporation.

• Water is a good example of that.

Humidity

• The amount of water vapor in the atmosphere involving the kinetic energy of molecules in the air.

Absolute Humidity

• The actual weight or amount or weight of the water vapor in the air.

• Can be measured by weighing the water vapor extracted from air.

• Fully saturated or partially saturated

• Air is fully saturated with the water vapor at 37 degrees Celsius and 760 millimeters of mercury, which is 43.8 milligrams per liter.

• Percent saturated * water vapor content (43.8).

Relative Humidity

• Ratio of the actual water vapor content to its saturated capacity at a given temperature.

• RH = \frac{Content}{Capacity} * 100