Heat transfer is the study about how heat moves from one place to the other. There are three basic ways in which heat transfers. They are conduction, convection and radiation. Conduction happens within and through solid objects that are in contact of each other. Convection always involves fluid. Radiation happens all the time on any surface but not much heat is passed unless the surface temperature is very high.
Conduction, convection, and radiation heat transfer exists all round us. We use heat transfer to cook foods, to make products, and to manage our living environment. This image showing the three ways of heat transfer. You can see conduction is in solids, convection is associated with liquid or gas, and radiation will pass through space until it is absorbed. Yes, there are convection and radiation under the pot (or any fire) – that is how the pot gets hot.
Most of us are involved in using, producing, and purchasing heat transfer products, such as the cooking pot, an air conditioner, or a heat exchanger. Learning heat transfer principles will help us to choose and to make a proper product and to use a product correctly.
Heat conducted through a solid wall Q at a given time t can be calculated with the formula in this image. We can see the heat conducted is directly proportional to surface area A and the temperature difference (Thot -Tcold) but inversely proportional to the wall thickness d. The proportional constant k is called the conductivity.
|Water at 20° C||0.0014||0.6|
|Air at 0° C||0.000057||0.024|
Conductivity, or more specifically thermal conductivity, is different with different materials as you can see from the table on the right. Conductivities of metals are about a thousand times more than that of plastics. That is why most of heat exchangers are built with metals, not plastics.
Another interesting fact to notice is that air has about half the conductivity as wool. This make air a very good thermal insulator. When making heat transfer products, especially insulation products, we use foam or fiber to create a lot of air voids. These small air pockets provide the best of insulation.
The main function of the foam or the fiber are actually to hold the air and prevent it from moving. When air moves, it is not a good insulator anymore as we will see in the next section.
Heat convection happens between a solid surface and a fluid flowing over the surface when there is a temperature difference. The amount of heat transferred Q at a given time t can be calculated with the formula in this image. It is directly proportional to surface area A and temperature difference (Thot -Tcold). The proportional constant hc is called the convection heat transfer coefficient.
Many factors contribute to the convection coefficient. In general the higher flow velocity, higher heat capacity, and higher density, but lower viscosity of the fluid will lead to higher convection heat transfer coefficient. Now referring the table on the right. Free convection caused only by temperature difference has much lower convection coefficient because the fluid is not moving as fast as forced flow. Gas has smaller convection coefficient than liquid because it has lower density than liquid. Water has higher convection coefficient than oil because it has lower viscosity.
Other facts we can see from this table are that water boiling or condensing has huge convection coefficient than other forms of convection. This is because the large amount of latent heat involved. Water has large latent heat than most of other liquids. Water boiling or condensing can take or give a large among of heat energy in the form of latent heat without changing water temperature.
Heat radiation and absorption happens among objects. The simplest calculation is to consider just one object radiating into and absorbing heat from its surroundings as seen in this image. The object is at absolute temperature To while the surroundings is at Ts. The object has a surface area of A and emissivity of e. One important fact to notice here is that the radiation power is proportional to the forth power of the absolute temperatures.
The emissivity of a object surface is a property of the material and the surface finishing. The emissivity is 1 if the surface can absorb all the radiation hitting it. The emissivity is 0 if the surface can absorb no radiation. To stress this point, there is nothing called “absorpty”. Thermal absorption or light absorption is always measured by emissivity. Emissivity is always equals to the portion of radiation (light or thermal) absorbed by the surface. The other portions are reflected or passed through.
As we see in the table to the right, emissivity is between 0 and 1 for all materials regardless the surface condition is. The more polished the surface is the smaller emissivity will be. This can be easily understood that polished surfaces reflect radiation better. More radiation is reflected than absorbed.
Understanding how heat transfers can help us in many aspects of work and living. The Lowe’s website has a very good page that is adopted here:
Types of Heat Transfer
There are three methods for heat transfer: conduction, convection and radiation. Knowing each type and will give you a better understanding of how insulation and weatherstripping systems protect your conditioned space.
Conduction is the transfer of heat through solid objects.
- Example: When it’s cold outside, uninsulated walls and windows become cooler on the inside.
- Result: More energy is required to replace the heat lost warming the inside of the walls and windows.
- Preventive Measures: Insulation , low-e insulated windows, storm windows and storm doors slow the migration of heat energy and help maintain the temperature in the conditioned space. Slowing the energy transfer saves on the amount of energy needed to maintain the conditioned space.
Convection is the transfer of heat through liquids or gases.
- Example: When cold air enters your home, it mixes with warm air. Heat energy is transferred to the cooler air, and the overall temperature of the room is lowered.
- Result: More energy is required to replace the heat transferred to the cooler air.
- Preventive Measures: Weatherstripping, house wrap, caulk and expanding foam greatly reduce the uncontrolled flow of air into or out of your home. Reducing the uncontrolled flow of air into or out of the home reduces the amount of energy needed to heat unconditioned air.
Radiation is the transfer of heat through space in the form of electromagnetic energy.
- Example: When sunlight enters an air-conditioned room through a single pane window, heat energy is generated in the room.
- Result: The air conditioning system must run longer and work harder to overcome the heat gained through the window.
- Preventive Measures: Low-e insulated windows, blinds and awnings all lessen the heat gained by sunlight entering through windows.
Insulate and Save Energy
Check out these thermal images of houses and think how much energy cost would that be. An infrared thermometer is under $20 for digit only or a couple hundreds with image display. Buy on and scan your house to see where the heat leaks are.
In cold or hot climates, a well-insulated and weatherstripped home is more energy-efficient and costs less to heat or cool. Most of the preventive measures listed here are simple fixes that you can do yourself. For a small investment of time and money, you can see a large reduction in the amount of energy required to heat and cool your home. Along with the reduction in energy use, your monthly energy bill should be lower, saving your hard-earned cash for other things.