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

 

3D Printing with Chocolate

Everyone seems to be talking about 3D printing lately. Last week, I received an email from a customer who had a new idea for 3D printing.

Chocolate!

Well I was intrigued. The customer wanted to modify current 3D Printing technology to work chocolate. There was obviously several hurdles. For instance, using a vat of molten chocolate as opposed to typical material, cleaning, and replacement parts to make a food safe low-cost printer. Her biggest problem was how to cool the chocolate after the application of each successive layer upon dispensing, so the chocolate didn’t pool into an amorphous blob.

She came to me asking about the Adjustable Spot Cooler. This product caught her attention because of the ease of installation with the magnetic base, the adjustable temperature control and instant cold air response. The magnetic base could be incorporated into her design fairly easily. The adjustable temperature control would allow her to decrease the temperature and decrease the cold flow at the same time. If she found that the force of the compressed air was damaging the printing process, reducing the cold flow would allow her to use a colder temperature to harden the layer that had just been used.  Finely the compressed air could be rapidly controlled with a solenoid to only run when the cold air is needed, which would limit compressed air cost.

Because of the high freezing point of chocolate and overall size constraints, I recommended that she first try a model 3204 Vortex Tube. A small Vortex Tube, which could use as little as 4 SCFM of compressed air and provide up to 3.2 SFCM of cold air at fifty degrees below the compressed air temperature, would be more than capable of forming a shell on the surface area of each extrusion. It is reasonable to assume that this air temperature would be around 20 degrees Fahrenheit, which could create a delicious chocolate shell for the next layer of chocolate be deposited.  She was able to buy the magnetic base, model 9029, separately to aid in her installation.Chocolate tools

Hopefully, you read this after lunch, because I made myself hungry looking for chocolate pictures, but I found what I would print for Christmas.

Dave Woerner
Application Engineer
davewoerner@exair.com
@EXAIR_DW

 

Don’t Get Fired, Install A Cabinet Cooler System

If the weatherman is correct, it is going to be a sizzling hot summer this year with the southern tier states experiencing triple digit temperatures and the rest of the nation in the 90’s.

When it gets hot, controls fail. When controls fail, production shuts down. When production shuts down, the boss yells at you. When the boss yells at you, you tell him where to go. When you tell the boss where to go, you get fired. Don’t get fired. Install an EXAIR Cabinet Cooler system.

EXAIR Cabinet Cooler systems incorporate a Vortex Tube to produce cold air from compressed air – with no moving parts. The compact Cabinet Cooler system can be installed in minutes through a standard electrical knockout. NEMA 12, 4, and 4X Cabinet Cooler systems that match the NEMA rating of the enclosure are available in many cooling capacities for large and small control panels.

EXAIR has a staff of engineers that will assist you in selecting the proper cooler.

Give us a call at 1-800-903-9247.

Joe Panfalone
Application Engineer

Phone (513) 671-3322
Fax   (513) 671-3363
Web: www.exair.com
Twitter: www.twitter.com/exair_jp
Facebook: http://www.facebook.com/exair