Types Of Heat Transfer

 

Managing the temperature in a building always involves controlling heat transfer. Heat transfer or heat exchange is the transition of thermal energy from hotter object or area to cooler object or area as described by the law of thermodynamics or the Clausius statement. Whenever there is a difference in temperature across a given area (for example, inside and outside a building), heat naturally flows from warm areas to colder area. Regardless of direction it seeks a balance until the temperature is the same across the whole area. In the summer or hot climate area, high heat from roofs and walls travels from outside to inside and in winter, heat flows from the inside of a building to the outside. Heat transfer can never be stopped, it can only be slowed or reflect away. The purpose of insulation is to impede this natural process. In hot weather, you want to stop heat from being transferred into the building, and in cold weather you want to stop it from being transferred out.


Radiant Heat Barrier is used as insulation against radiant heat transfer and can be used to its greatest advantage in instances where radiation is the predominant means of heat transfer. It is  the bright aluminum surface of all Radiant Heat Barrier products that gives it is insulating properties.    


Radiant Heat Barrier is an excellent reflector of all long wave radiant heat that strikes it - reflecting up to 95% of all radiant heat. Radiant Heat Barrier will reradiate only 5% of heat in which it comes in close physical contact, compared with the approximate 90% reradiation of ordinary building materials, at the same temperature.


Heat is transferred in three distinct ways which is by conduction, convection or radiation, or all of which may be occurring at any given time. By using our knowledge of the three methods of transferring heat, we can see how to save on our cooling and heating energy bills.

 

 

 

Conduction


Conduction is the transfer of the heat through a solid object, liquid or gas. In order for the heat to be conducted, there should be physical contact between particles and some temperature difference. When one part of an object is heated, the molecules within it vibrate against one another, begin to move faster and more vigorously, when these molecules hit other molecules within the object they cause heat to be transferred through the entire object. Substances that conduct heat readily are called conductors, while substances that don't conduct heat readily are called insulators. Metal (e.g. copper, platinum, gold iron, etc) is a good conductor of thermal energy, aluminum is a good insulator. Wood falls somewhere in between. Conduction is greater in solids, where molecules are in constant contact.


As density decreases so does conduction. Therefore, fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas, fewer collision between atoms means less conduction. Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that the density of the gas is such that molecules of the gas may be expected to collide with each other before they transfer heat from one surface to another. After this point density, conductivity increases only slightly with increasing pressure and density. In liquids (except liquid metals) and gases, the molecules are further apart, giving lower chance of molecules colliding and passing on the thermal energy. 


The handle on a cast iron skillet gets hot as heat is transferred from the bottom by means of conduction. Therefore, thermal conductivity is the measure of the speed of heat flow passed from particle to particle. The rate of heat flow through a specific material will be influenced by the difference of temperature and its thermal conductivity.



One way that heat can escape from your house is by conduction through the walls, ceilings, and windows. Most walls and ceilings contain material to stop this heat conduction called insulation. There are many types of insulation, depending on the location in the house and the degree of insulation that is desired. One popular measure of the effectiveness of insulation is called the "R" factor. The R factor is a measure of the resistance of the insulation to the transfer of heat. The higher the R factor, the more resistance, and the less heat that is transferred. The materials and labor to install insulation cost money, so depending on the size and location of the house, there is an R factor that is the most cost efficient. Most older homes were built when the cost of energy was lower and the insulation was more expensive for the same R factor rating. Therefore, it may save you both energy and money to invest in more insulation in your walls, roof or ceilings.

 
The same concept of insulation applies to windows. Typically, the "insulation" that is used is air, since regular insulation is a bit hard to see through. The air is trapped between two panes of glass. These double paned insulated windows significantly decrease the loss of heat through the windows.

 

  

 

 

Convection

 

Convection is the transfer of heat energy between a solid surface and the nearby movement of a liquid or gas (such as air). As fluid motion goes more quickly the convective heat transfer increases. The presence of bulk motion of fluid enhances the heat transfer between the solid surface and the fluid. By this mode, heat is transferred when a heated air/gas or liquid moves from place to another, carrying its heat with it. The rate of heat flow will depend on the temperature of the moving gas or liquid and on its rate of flow.


Circulatory air motion due to warmer air rising and cooler air falling is a common mechanism by which thermal energy is transferred. Inside of a wall air removes heat from a hot exterior wall, then circulates to the older interior wall where it loses the heat. An open chimney flue provides a good example of convective heat loss during the winter. Warm air will rise up the chimney and cold air will fall down into the home. The greater the temperature difference between the inside and outside of the dome, and the larger the openings in the home, the easier it is for air to move and the greater losses you will have due to convection.


The main fluid available to transfer heat by convection in your home is air. Air can come in and out of your home in many different places: cracks and holes in the home, gaps and voids in ceilings, walls, floor, around your windows, cracks under the doors, places where wires or pipes come into the house, and even from the electrical outlets.


Convection also occurs if air can circulate through the insulation - if insulation is to be effective, it must prevent air from flowing easily through it. Properly applied insulation reduces convective heat loss by resisting and minimizing air movement.


When air is heated, it expands, and therefore becomes less dense, so it rises. The rising warm air displaces cooler air, which sinks. When the motion is constant, it's called a convective loop. Woodstoves and windows cause convective loops by heating or cooling  (respectively) the air closest to them. Even in homes with airtight walls and ceilings, convective loops can occur inside poorly insulated wall cavities, too, degrading the performance of the insulation.


 

 

Radiation

 

Heat energy is transmitted in the form of light, as infrared radiation or another form of electromagnetic waves. No medium is necessary for radiation to occur, for it is transferred through electromagnetic waves; radiation works even in and through a perfect vacuum (like) space) or air. Radiant heat transfer occurs between objects that are not touching. This is a direct transfer of heat from one object to another, without heating the air in between. The sun heating the earth is an example of radiant heat transfer. The sun warms the earth without warming the space between the sun and the earth. An example of radiant heat transfer is found in a typical attic during the hot climate or summer. The sun radiates heat to the roof, which in turn radiates heat down toward the ceiling. If the insulation covering the ceiling does not effectively resist radiant heat transfer, then the ceiling will become increasingly warm - radiate heat down into the home - and the home will be uncomfortable. Properly applied insulation arrests radiant heat transfer.


This energy emanates from a hot body and can travel freely only through completely transparent media. Radiation cannot pass through a solid object like plywood roof sheathing. The atmosphere, glass and translucent materials pass a significant amount of radiant heat, which can be absorbed when it falls on a surface. When the sun shines on asphalt shingles or roof, heat is transferred to the plywood sheathing by conduction. After the plywood has been warmed by conduction, it can radiate heat into the attic. It is a well known fact that light-coloured or whiny surfaces reflect more radiant heat than clack or dark surfaces, therefore the former will be heated more slowly.


As  was previously stated, heat will flow in any direction where a temperature difference occurs. Therefore all areas which separate the interior of a house or building from the outside or which separate heated spaces from unheated (or air conditioned) spaces need to have the amount of heat they are subjected to reduced. Obviously an important step in the creation of an energy efficient house or building is to control heat loss or gain, which accounts for 75% of the total energy loss of a home.


Radiant heat barrier, aluminum foil with a low-emissivity (low-e) surface. Although radiant heat barrier have a few applications in residential construction - they are sometimes integrated with roof sheathing - they are rarely cost-effective when compared to conventional insulation options.

 

No matter what you do, some heat energy will radiate away from your house on a cold night. The lower the outside temperature of your house, the less energy that will be radiated. Therefore, the insulation you use to prevent conduction will also cut down on the losses from radiation.

144006 Visitors144006 Visitors144006 Visitors144006 Visitors144006 Visitors144006 Visitors144006 Visitors