Sunday, October 31, 2010

Application of Thermal Energy Transfer

Uses of good conductors of heat

-Cooking utensils, kettles, saucepans and boilers are usually made of aluminum or stainless steel where direct heating is involved.

-Soldering iron rods are made of iron with the tip made of copper because copper is a much better conductor of heat than iron.

-Heat exchangers such as those used in a laundry enable precious fuel to be saved

Uses of bad conductors of heat(insulators)

Insulators are very useful if we want to minimize heat flow or heat loss. Some applications of insulators are:

-Handles of saucepans, kettles, teapots, irons and soldering iron rods are made of wood or plastics which are very poor conductors of heat. In this way, the hot utensil or iron can picked up without scalding our hand.

-Table mats are usually made of cork so that hot kitchenware can be placed on them without damaging the table-top.

-Sawdust is used to cover ice blocks because of its good insulating property.

-Wooden ladles are very useful for stirring or scooping hot soup and also for scooping rice that has just been cooked.

-Wooden blankets or clothes are used to keep people warm on cold and chilly days

Common Applications of convection

1)Electric Kettles - The heating coil will be placed at the bottom of the kettle to aid transfer of thermal energy in water by convection. When the kettle is being heated up, the water around the heating coil will be heated up and it expands, causing it to be less dense . The heated water would then rise while the cooler regions in the upper part of the kettle would then descend to replace the heated water.

2) Heater - Household hot water system is designed based on the convection in liquids.

a) Water is heated in the boiler. The hot water wexpands and become less dense. It then rises and flows to the upper half of the cylinder.

b)To replace the hot water, cold water from the cisterm falls into the lower half of the cylinder and then it will eventually flow into the boiler due to the pressure difference.

c) The overflow pipe is attached to the top half of the cylinder just in case the temperature of the water gets too high and causes a large expansion of water. If the water expands a lot, it go into the overflow pipe which leads back to the cistern.

d) The hot water tap which is led from the overflow pipe must be lower than the cistern so that the pressure difference between the cistern and the tap causes the water to flow out of the tap.

3) Air conditioner - It is always installed near to the celing of a room to facilitate setting up of convection currents. The rotary fan inside an air conditioner releases cool dry air into the room. As cool air is denser, it sinks. The warm air below would then rise and is drawn into the air conditioner where it is cooled. This way, the air is recirculated and the temperature of the air would eventually fall to the desired value.

4)Refrigerators - It works the same way as the air conditioner. The freezing unit is usually placed at the top to cool the air and facilitate setting up of convection currents. The convection currents inside the refrigeration cabinet help cool the contents inside the refridgerator.

Common Applications of Radiation

1) Teapots - Teapots have shiny surfaces which are bad emitters of radiation. It enables them to keep tea warm for a longer time than black teapots. The shiny surface are also bad absorbers of radiation, thus, enabling them to keep cold liquids cool for a longer time than black containers.

2) Greenhouses - During the day, infrared radiation from the Sun passes through the glass roof of the greenhouse. This warms up the soil and plants in the greenhouse. As the contents in the greenhouse get warm, they emit infrared radiation also.
The infrared radiation that the contents in the greenhouse emit is slightly different from the infrared radiation emitted by the Sun. The infrared radiation from the contents in the greenhouse are not able to pass through the glass roof. Therefore, the infrared radiation by both the Sun and the contents in the greenhouse gets trapped. The amount of infrared radiation would then slowly increase over time, causing the temperature in the greenhouse to increase.

3) Vacuum flasks - It is designed to keep liquids hot by minimising heat loss in four possible ways, namely conduction, convection, radiation and evaporation.

-The cork is usually made of an insulator of heat.

-The trapped air above the liquid is minimal as air is a very poor conductor of heat.

- Conduction and convection through the sides of the flask are prevented by the vacuum between the double-glass walls of the flask.

- To minimise heat loss through radiation, the walls of the glass are silvered so as to reflect heat back into the hot liquid. Convection and evaporation can only occur when the cork is removed during use. Heat loss by radiation is harder to stop as radiant heat can pass through a vacuum.


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