Heat Transfer

Everybody will agree that good thermal insulation of one house is key for comfort and for saving on the heating bill. Yet, there are many concepts behind those two words of thermal insulation. In this first installment, we look at the physics of thermal insulation. In the second installment, we will look at the parameters used to measure and compare the quality of thermal insulation solutions.  

The goal of thermal insulation is to maintain a constant and comfortable temperature in one house by preventing heat transfer between indoor and outdoor climate. In cold months, thermal insulation will prevent the indoor heat to escape outdoor. In hot months, it will slow down the entrance of outside heat in the house. 

Heat is defined as thermal energy possessed by a substance. In general, to detect the presence of heat possessed by an object that is by measuring the temperature of the object. If the temperature is high then the heat contained by the object is very large, and vice versa if the temperature is low then the heat contained in a bit. The size of the required heat of an object (substance) depends on three factors

 

1.      mass of substance

2.      Type of substance (heat type)

3.      Temperature changes

So that mathematically can be formulated

Q = m.c. (t₂ - t₁)

where:

Q is the heat required (J)

m is the mass of the object (kg)

c is the heat type (J / Kg^oC)

(t2-t1) is the change in temperature (^oC)

Heat can be divided into 2 types

• Heat used to raise the temperature 

• Heat is used to change the form (latent heat), which is used in the equation there are two kinds of latent heat Q = Q = mU and mL. With U is the heat of vapor (J / kg) and L is the heat of melting (J / kg)

Heat transfer

Heat can move from one place to another. How do I heat it move? Heat can be transferred in three ways, namely conduction or delivery, or flow convection, and radiation or emission.

1.    Conduction 

     

     Heat conduction is a very intuitive concept: it represents the spontaneous   transfer of heat through matter, in order to even out temperature differences. This explains why a metal bench or chair "feels" cold, while a wooden one "feels" warm. Metal is a very good heat conductor and dissipate body heat through its mass. Wood on the contrary is a poor heat conductor. For the same reason, a metal spoon in boiling water quickly becomes hot, while a wooden one stays cool at the other extremity. Materials used for thermal insulation must have a very small heat conductivity to be efficient.

Conduction is the transfer of heat through a substance without an accompanying transfer of particles of the substance. Based on the heat conductivity, the object can be divided into two, namely:

• Conductor : Conductor is a substance that has good heat conductivity. Example: iron, steel, copper, aluminum, etc.

•  Isolator :  An isolator is a substance that has a poor heat conductivity. Example: wood, plastic, paper, glass, water, etc.

In everyday life, can you find home appliances utilizing the principle works in conduction heat transfer concepts, among others: electric irons, solder. Why is household equipment such as irons, solder, pots, pans there is a handle of an insulating material? It aims to inhibit heat conduction so as not to come down to us.

2.      Convection  

      Convection represents the transfer of heat by circulation or movement of the hot particles to cooler areas. This is another intuitive concept as we know that hot air or hot water rises. Air or water surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current. 

   Convection is the transfer of heat in a substance that is accompanied by movement of the particles of these substances. Convection occurs because of differences in density of the substance. You understand convection, among others:

• In the liquid due to differences in the density of the substance, eg water heating systems, hot water flow system 

• In the matter of gas due to differences in air pressure, eg the occurrence of onshore winds and sea breezes, air ventilation systems, to get the cooler air in the room mounted air conditioner or fan, and factory chimneys.

Of the activities that you do can be concluded that, the flow in the glass caused by differences in density of the substance. The water that touches the bottom of the beaker is heated by conduction. Due to heat receiving water, the water will expand and become less dense. Water is more dense at the top of a hot water push it down toward the top. This movement raises the current convection. In the heated liquid will have decreased so that the density of the flow rises to the top. 

3.      Radiation

      How can the sun's heat energy to the earth? We already know that between the sun with the earth in the form of the vacuum, so the heat from the sun reaches the earth without going through intermediary substances. Heat transfer without going through an intermediary substance or medium is called radiation / delivery. Examples of heat transfer by radiation, for example, we held activities at camp, at night the cold is often light a campfire. 

   Thermal radiation represents the transfer of heat by electromagnetic radiation. The sun, or an electric radiator, are perfect examples of radiating objects producing heat. Bright, shiny materials, such as radiant barriers, reflect radiation while dull, black materials absorb it. Good insulators are not necessarily good radiant barriers, and vice versa. An example of radiant barrier is the bright material you can put on the windshield of a car to protect it from overheating in summer. 

    

    Thermal radiation of the sun is of particular importance when it comes to thermal insulation. During the cold months, that energy is most welcome and houses should be designed to let it in. In summer, on the contrary, thermal radiation of the sun should be kept away from the house indoor in order to maintain a cool interior.

     Roof and windows are the most concerned about thermal radiation. For the roof, it is possible to place radiant barriers and/or to build a ventilated roof which allow the radiant heat to dissipate via convection. 

      Regarding windows, it is possible to find reflective glazing, which maximizes light transmission while reflecting solar radiation. Another type of glazing called low E glass (low emissive), has a metallic or oxide coating, which reduces transfer of heat or cold through the glass while still allowing light through. In any case, the most efficient way to avoid radiant heat from windows in summer is to protect them from direct sun exposition. That can be done using overhangs (see figure below), shutters or vegetation.

When we are in our bodies by the fire was warm because of the heat transfer from the fire into our bodies by radiation. Although there is air around us that can move heat by convection, but the air is a poor heat conductor (insulator). If the fire pit with us put a baffle or a veil, was the warmth of the campfire we can not feel anymore. This means there is no heat to our body, because it was blocked by sealing it. From the campfire events can be concluded that:

a. the incident radiation, heat moves in the form of light, because light can propagate in a vacuum, so heat can propagate in a vacuum;

b. heat radiation can be prevented by giving the veil / cover that may hinder the light emitted from the light source.

For make it more understanding, lets watch the video below.