Different Types of Heat Transfer and How to Calculate their Values

Heat transfer like the name states is the way that heat transfers from one entity to another.  Heat is defined as a motion of molecules.  So, heat is anything above the absolute temperature of 0 Kelvin (-460 deg. F or -273.15 deg. C).  Thus, heat is relative.  Now, for heat to transfer, we need to have a difference in temperatures.  Energy like heat will always travel from the higher temperatures toward the lower temperatures; and there are three major ways that this can happen; conduction, convection, and radiation.  By the first Law of Thermodynamics, energy is neither created or destroyed, only transferred.  In this blog, I will explain each type of heat transfer.

  • Heat Transfer by Conduction

Conduction is about two stationary objects that are in contact.  The vibration of the molecules of one object will affect the vibration of the molecules adjacent to it.  Examples of conduction would be the cold air outside a window pane in a warm room.  Or a hot iron sitting on your wrinkled pants.  The heat from the hotter object will flow to the cooler object.  Thus, the hot object will become cooler while the cool object will become hotter.  This can be explained in Equation 1:

Equation 1 :

Q = -k * A * (T2 – T1) / x

Q – Heat Transfer (Watts)

k – Thermal Conductivity of material (Watts/K-m)

A – Heat Transfer Area (m2)

T2 – Temperature of object 2 (Kelvin, K)

T1 – Temperature of object 1 (Kelvin, K)

x – Material Thickness (m)

 

  • Heat Transfer by Convection

Convection describes heat transfer between surfaces that are in motion. This happens by moving a fluid which can be a liquid or air across an object.  There are two types, free convection and assisted convection.  Free convection is caused by gravity or buoyancy.  The basement will be cooler than the second floor because hot air will rise.  The density of warm air is less than cold air, so it will rise.  As for assisted or forced convection, the fluid will be moved over a surface with a pump, fan, or some other type of mechanical device.  An example of forced convection would be blowing your breath over your cup of coffee to cool.  Another example is the EXAIR Super Air Amplifier.  This device uses a small amount of compressed air to amplify the volume of ambient air.  When blown across a heated surface, it can cool the object quickly.   The calculation for heat transfer by convection is shown in Equation 2.

Equation 2:

Q = h * A * (T2 – T1)

Q – Heat Transfer (Watts)

h – Convective Coefficient (Watts/K-m2)

A – Heat Transfer Area (m2)

T2 – Temperature of object 2 (Kelvin, K)

T1 – Temperature of object 1 (Kelvin, K)

 

  • Heat Transfer by Radiation

Radiation refers to the transfer of heat through electromagnetic waves. Of course, the largest radiation source is our sun.  You can feel the difference when you wear a black shirt versus a white shirt.  Any object will adsorb, reflect, and transmit the radiation at different values depending on the color, surface finish, and material type.  This is called emissivity.  Emissivity, or e, is a coefficient that determines the ability of that object to adsorb the heat from radiation.  Thus, the value of e is between zero and one, and it is unitless.  By definition, 0 < e < 1.  Thus, a black object can have an emissivity of 1.  .  This is important for the EXAIR Cabinet Cooler Systems.  If the panel is outside and in full sun, we would use the color to determine the additional heat that can be absorbed by your electrical panel.  The equation for radiation heat transfer is shown in Equation 3.

Equation 3:

Q = e * A * s * ((Th)4 – (Tc)4)

Q – Heat Transfer (Watts)

e – Emissivity Coefficient

A – Heat Transfer Area (m2)

s – Stefan-Boltzmann Constant (5.6708 * 10-8 Watts/K4 m2)

Th – Temperature of hot body (Kelvin, K)

Tc – Temperature of cold body (Kelvin, K)

Thank you for reading the blog about the three main methods for heat transfer.  If you need to cool products, or remove the heat, EXAIR has many types of products to accomplish this.  You can contact an Application Engineer to discuss any of your applications dealing with heat and heat transfer.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Image courtesy of Arman Cagle, Creative Commons License

 

Air Amplifier Provides Cooling for HOT Parts

Hot Cylinders

The five C’s of EXAIR products are Cooling, Cleaning, Conserving, Conveying, and Coating.  All EXAIR products are suitable for applications in these areas, with varying degree of possibility.  When it comes to cooling, one of the most suitable EXAIR products is the Super Air Amplifier.

An Air Amplifier can increase the volume of ambient air directed over an specific area, effectively decreasing the cooling time needed in an application.  Air Amplifiers cool effectively due to the fundamental principles of convective heat transfer.  In convective heat transfer, cooling capacity can be increased by increasing the temperature differential between the cooling medium and the object to be cooled, or by increasing the flow of the cooling medium.

An Air Amplifier is the best cooling choice when the material to be cooled is at an extremely high temperature.  For example, in the application above, 903°C (1650°F) cylinders need to be cooled to ambient temperature as quickly as possible. Vortex Tubes are another product our customers consider for cooling applications. Vortex Tubes are the best choice when the area to be cooled is small and the temperature differential is not as large. A Vortex Tube based solution will provide very cold air, but at a lower air flow over a small area and they were not the best choice for the application in the image above.

In the same application, a Super Air Amplifier can provide large volumes of ambient air over a large area, effectively cooling the cylinders much more efficiently.  The cooling can be achieved in less time, and with maximum efficiency of compressed air implementation. Air Amplifiers also offer great benefits over electric fans in this rough environment: they can withstand higher temperatures and there are no moving parts to wear or break.

If you have an application in need of efficient cooling, contact an EXAIR Application Engineer to find out if an Air Amplifier will work for you.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE