## Heat Transfer – How Energy Can Move

Heat. One word can bring to mind so many different things from cooking to sun tanning. But what is heat and how does it move. Heat is essentially a form of energy that flows in the form of changing temperatures; this form of energy will flow from high to low. When you describe something as being hot, you are actually describing that the item in question has a higher temperature than your hand thus the thermal (heat) energy is flowing from that object to your hand. This phenomenon is what is referred to as heat transfer. Heat transfer can be observed all the way down to the atomic scale with the property known as specific heat. Every molecule and atom can carry a set amount of energy which is denoted by specific heat; this value is the ration of energy (usually in Joules) divided by the mass multiplied by the temperature (J/g°C).

But how does this heat move from object to object? On the atomic scale, the atoms are storing the energy which will cause electrons to enter into an excited state and rapidly switch between shells. When the electron returns back to a lower shell (closer to the nucleus) energy is released; the energy released is then absorbed by atoms at a lower energy state and will continue until the thermal energy is equal between the two objects. Heat has four fundamental modes of transferring energy from surface to surface and they are as follows:

Advection is the physical transport of a fluid from point A to point B, which includes all internal thermal energy stored inside. Advection can be seen as one of the simpler ways of heat transfer.

Conduction
Conduction can also be referred to as diffusion and is the transfer of energy between two objects that have made physical contact. When the two objects come into contact with each other thermal energy will flow from the object with the higher temp to the object with the lower temp. A good example of this is placing ice in a glass of water. The temperature is much lower than the room temperature therefore the thermal energy will flow from the water to the ice.

Convection
Convection is the transfer of thermal energy between an object and a fluid in motion. The faster the fluid moves the faster heat is transferred. This relies on the specific heat property of a molecule in order to determine the rate at which heat will be transferred. The low the specific heat of a molecule the faster and more volume of the fluid will need to move in order to get full affect of convection. Convection is used in modern ovens in order to get a more even heat through out the food while cooking.

Radiation is the transfer of thermal energy through empty space and does require a material between the two objects. Going back to the how thermal energy is released from atoms; when the electron returns to a lower energy shell the energy is released in the form of light ranging from infrared light to UV light. Energy in the form of light can then be absorbed by an object in the form of heat. Everyone experiences radiation transfer every day when you walk outside; the light from the sun’s radiation is what keeps this planet habitable.

EXAIR’s engineered compressed air products are used every day to force air over hot surfaces to cool, as well as dry and/or blow off hot materials. Let us help you to understand and solve your heat transfer situations.

If you have any questions about compressed air systems or want more information on any of EXAIR’s products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
Visit us on the Web

## Solar Radiation and Electrical Enclosures

Before I get started, I should mention that I’m quite fond of the sun. Now, I don’t spend a lot of my time working on a “bronze god” kind of tan, nor do I own the proper equipment that would have allowed me to observe, in first-person, the recent transit of the planet Venus (I did see quite a few amazing photos and videos on the internet, though). I did make it a point to be outside during the last solar eclipse that was visible in the Midwest. It was a perfect day for it too…a bright clear day in May 1994, my girlfriend & I had a lunch date, and we were lucky enough to get a table on the patio. Her father had a pair of welding goggles that she was able to borrow a lens from, and our server got us free dessert for letting her look at the eclipse through them. That’s one of a gazillion fond date memories I have with the girl that agreed to be my wife, two years later. Solar eclipse AND free dessert…it’s actually near the top of that list…

If I’m spending any time at all in the sun, though, I don’t take chances, because, as fond as I am of the sun, I am equally un-fond of sunburn. Which brings me to my topic today: solar radiation.

We are right in the middle of a very busy Cabinet Cooler season at EXAIR. Most of the applications we’re getting calls on are for indoor installations, but more than a few callers are in need of cooling for an outdoor electrical enclosure. A prime consideration in determining the total heat load for these is the heat absorption due to solar radiation, or “sun load.”

First and foremost, if you have an electrical cabinet outdoors, you can eliminate sun load by simply preventing the sun’s rays from shining on it. If you can mount it to a north wall, or get it under an awning, that’s ideal. And it’s all you have to do.

If you can’t control the mounting location, and your enclosure has to be exposed to direct sunlight, then you’re most likely going to need a significantly higher amount of cooling. Just this morning, I helped a caller determine the heat load on a small outdoor enclosure in the deep South…before accounting for sun load, they were looking at a total heat load of 450 BTU/hr, which our smallest Cabinet Cooler System is more than capable of handling. Adding in sun load, though, their mid-day/midsummer heat load was over 2,500 BTU/hr, which would require a system that consumed more compressed air. We encourage using as little air as possible; they’re looking at what it would take to put an awning on it.

If you have any say in it, the color/finish of the cabinet’s exterior makes a big difference, too. A polished aluminum, or white painted, surface will absorb less than 20% of the solar radiation as a black cabinet. And if it’s grey, the lighter the better.

Like I said, it’s a busy Cabinet Cooler season here, but we always have time for, and welcome, the next caller. Don’t forget, you’ll receive a free AC Sensor with any Cabinet Cooler purchase through then end of July…for anyone who loses track of the calendar (like I do), that’s next week. Anyway, we look forward to seeing how we can help. Give us a call!

Russ Bowman
Application Engineer
EXAIR Corporation
(513)671-3322 local
(800)923-9247 toll free
(513)671-3363 fax
Web: http://www.exair.com
Blog: http://blog.exair.com/