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 Amplifiers Speed Up Cooling Of Cast Parts

Do you like soup? I like soup. Especially on cold days in the winter. Living down south apparently ruined me for cold weather, because, even though I’ve been here in Ohio for 25 years, I still get a chronic chill in early November that won’t let go until about April. March, if I’m lucky. A nice, hot bowl of soup gives me a temporary respite from that dreaded chill, though, so yeah…I like soup.

Sometimes (OK; most of the time) I like it so much I don’t want to wait for it to cool (just slightly) to a temperature that won’t scald my tongue, so I resort to the age-old practice of blowing on those first few spoonfuls. Even though my breath is a fairly consistent 98.6F (give or take,) it’s still quite effective at transferring enough heat out for pain-free consumption. There are two reasons I’m thinking about this right now:

First reason: I’ve been working with an engineer at a large automotive plant…they were cooling a production run of metal cast parts with a series of fans. It ran pretty slowly, and they had a line of those pedestal mounted fans “waving at the parts as they went by.” The thought was, they could direct a stream of cooling air by using the focused flow of an Air Amplifier, and this might just allow them to speed up the line. And they were right. They tried a few Model 6041 1-1/4″ Aluminum Adjustable Air Amplifiers, with very favorable results. So favorable, in fact, that they ordered (40) more to outfit other casting lines in the plant, in arrangements similar to this:

With (16) models to choose from, EXAIR Air Amplifiers are a quick and easy way to provide a tremendous amount of cooling air flow from a compact, lightweight product.
With (16) models to choose from, EXAIR Air Amplifiers are a quick and easy way to provide a tremendous amount of cooling air flow from a compact, lightweight product.

Just like it might take more than one “blow” to cool off a spoonful of soup, they have installed multiple Air Amplifiers, in succession, on the lines, depending on the size, shape, and mass of the part. And the precise adjustability of the Adjustable Air Amplifiers allows them to dial in the optimum air flow, while minimizing their compressed air consumption. So the Production and Facilities folks are all very happy.

And (because I know you’re wondering) the second reason I’m thinking about conductive/convective heat transfer via air movement:

I trust you had already guessed what was for lunch today.
I trust you had already guessed what was for lunch today.

Russ Bowman
Application Engineer
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