Endothermic Reactions

An Endothermic Reaction is one that needs to absorb some form of energy (normally heat) from its environment or surroundings to carry out the process, causing a depletion of energy capacity of the surroundings.
A chemical reaction where the energy content of the products is more than that of the reactants; heat is taken in by the system.

Example: Solid Barium Hydroxide Octahydrate and Ammonium Thiocyanate are mixed in a beaker and the beaker is placed inside another beaker containing water. The mixture is stirred and the two solids react resulting in absorption of heat from the surrounding, i.e water in another beaker, resulting in decreasing in temperature of water kept in outer beaker.

Capillary-porous materials

Capillary-Porous Materials are porous structures made of materials like Steel, Aluminum, Nickel or Copper in various ranges of pore sizes. Fibrous materials, like ceramics, have also been used widely.
The main disadvantage of ceramic fibres is that, they have little stiffness and usually require a continuous support by a metal mesh. More recently, interest has turned to carbon fibres as a wick material. Carbon fibre filaments have many fine longitudinal grooves on their surface, high capillary pressures, and are chemically stable. A number of heat pipes that have been successfully constructed using carbon fibre wicks seem to show a greater heat transport capability. The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. It must also be able to distribute the liquid around the evaporator section, to any area, where heat is likely to be received by the heat pipe.

Example: Working of the capillary-porous material inside the Heat-Pipe - Inside the container is a liquid under its own pressure, that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. Animation depicts the concept.

Corona Discharge

Corona Discharge is an electrical discharge around a conductor in a gas, that occurs, when the electric field around the conductor exceeds the value required to ionize the gas, but is insufficient to cause a spark.

Example: A phenomenon called 'glow or corona discharge' maybe sometimes observed in high-tension power lines in damp weather.

Stirling Effect

Stirling Effect is the expansion of a gas when heated, followed by the compression of the gas when cooled.
In 1816, a Scottish Reverend, Robert Stirling won a patent on a hot air engine that converted heat to work (or vice versa) through repeated compression and expansion of the working fluid at different temperature levels. The concept was not used as for cooling until 1834, when John Hershel used a closed cycle Stirling engine for making ice.

Example: In Stirling refrigeration cycle, the piston compresses and expands the working fluid (Helium) and it is driven by an AC linear motor. The displacer shuttles this gas back and forth from the cold side to the warm side of the cooler. During expansion, heat is absorbed at the cold side; and during compression, heat is rejected at the hot side.

Nernst-Ettinghausen Effect

No description available

Piezoelectric Fan

A Piezoelectric Fan consists of blades made of stainless steel, brass or even Mylar. Attached to the blades is the magic ingredient -- a patch of piezoelectric ceramic material. The piezoelectric material deforms in the presence of a voltage field. The shape and size of the material determines the way it bends. Positive and negative electrical voltages affect the material differently. As a positive voltage is applied, the ceramic expands, causing the blade to move in one direction. A negative electrical voltage can cause the ceramic material to contract and move the blade back in the opposite direction. This results in circulation of air, which is used to cool objects.

Example: The cramped interiors of laptop computers contain empty spaces which are large enough to accommodate a piezoelectric fan. The fan dramatically reduces the interior temperatures of laptop computers.

Peltier Effect

Peltier Effect is the production or absorption of heat at the junction of two metals, on the passage of a current. Heat is produced or absorbed depending on the direction and amount of current flow.
The change in temperature of either junction of a thermocouple when a current is maintained in the thermocouple, and after allowance is made for a temperature change due to resistance.

Example: Thermoelectric coolers are solid-state heat pumps that operate on the Peltier effect, the theory that there is a heating or cooling effect when electric current passes through two conductors. A voltage applied to the free ends of two dissimilar materials creates a temperature difference. With this temperature difference, Peltier cooling will cause heat to move from one end to the other. A typical thermoelectric cooler will consist of an array of p- and n- type semiconductor elements that act as the two dissimilar conductors.

Phase Transitions(Heat Pipe)

A Heat Pipe is a simple device that can quickly transfer heat from one point to another. They are often referred to as the "superconductors" of heat, as they possess an extra ordinary heat transfer capacity and rate, with almost no heat loss. Inside the container is a liquid under its own pressure, that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. Heat pipes have an effective thermal conductivity many thousands of times that of copper.

Air-Flow -Impingement Jets, etc

"Air impingement cooling" calls for jets of air to be directed, or "impinged," perpendicularly against a product´s surface at a rate of several thousand feet per minute. These columns of high-pressure air interrupt the boundary layer, changing the air from stagnant laminar sublayers to transitional moving currents and finally to turbulent cells of moving air. This action creates high and low pressure areas that assist in the movement of the air, reducing the thickness of the boundary layers and increasing heat transfer rates.

Fluid Spray

Fluid Spray is basically a jet of air or liquid. When this jet is sprayed on a hot object, the heat is absorbed by it, in turn cooling the object.

Example: Using a stream of water to cool the burnt area. Here the heat from the burnt area is absorbed by the stream of water, which in turn cools the burnt area.

Thermoacoustic Effect

Thermoacoustic Effect states that, a sound wave heats and cools small parcels of gas along the length of its propagation. This effect is used in refrigeration.

Example: Consider a high amplitude sound wave in a tube. As the sound wave travels back and forth in the tube, the gas compresses and expands (that's what a sound wave is). When the gas compresses it heats up, and when it expands it cools off. A stack of plates is placed inside the tube such that they are at the initial temperature of the gas. The sound wave compresses and heats the gas. As the gas slows to turn around and expand, the gas close to the plate gives up heat to the plate. The gas cools slightly and the plate below the hot gas warms slightly. The gas then moves, expands, and cools off, becoming colder than the plate. As the gas slows to turn to move from plate to another, the cool gas takes heat from the previous plate, heating slightly and leaving the plate below the gas cooler than it was. This continues till the last plate and this is the concept that is used in thermoacoustic refrigeration.

Transpiration Cooling(Evaporation)

Transpiration Cooling is the act or process of excreting water in the form of vapor, resulting in cooling the surface from where evaporation occurs.

Example: As water is evaporated from the leaves into the atmosphere, a transpiration pull is set up and this induces movement of water up the stem from the roots and the absorption of water from the soil. Thus evaporation of water during transpiration produces a cooling effect on the leaves of a plant.

Conduction

Conduction is the transfer of heat or electricity through a substance, resulting from a difference in temperature between different parts of the substance (in the case of heat), or from a difference in electric potential (in the case of electricity).

Example: Furnace wall of boiler plant, made up of fire bricks, insulators and steel casing results in temperature gradient from inner fire brick surface to outer steel casing surface resulting in flow of heat from inner surface to outer surface, during which temperature of 500 degree centigrade at inner surface reduces or cools to 20 degree centigrade, at outer steel casing surface.

Convection

Convection is heat transfer in gases or liquids, by the circulation of currents from one region to another. It involves the transportation of heat through fluids, (either liquids or gases) which results in cooling of an object.

Example: Convective heat transfer is what cools the car engine by circulating water through the hot parts. The animation illustrates the same.

Radiation

Radiation is transfer of heat energy as electromagnetic waves. As a result, the object emitting radiation gets cooled inturn.

Example: At night times, earth cools by emitting infrared radiation to space.

Fluidized Bed

A fluidized bed is a bed of solid particles with a stream of air or gas passing upward through the particles at a rate great enough to set them in motion. As the air travels through the particle bed, it imparts unique properties to the bed, which behaves as a liquid. It is possible to propagate wave motion, which creates the potential for improved mixing. This process technology is used to achieve uniform process conditions. Thus, the fluid bed can be used to dry the wet product, to cool hot objects, agglomerate particles, improve flow properties, instantize the product, or produce coated particles for controlled release or taste masking.

Example: In fluid bed cooling, cold gas (usually ambient or conditioned air) is used. Conditioning of the gas may be required to achieve sufficient product cooling in an economically sized plant and to prevent pick up of volatiles (usually moisture). Heat may also be removed by cooling surfaces immersed in the fluidized layer.