In most cases, conductive heat transfer happens more rapidly than convective heat transfer. That is, heat transfer through solid materials is more profound than that in liquids or gases. You’ve probably experienced this in your everyday lives, knowing that we can “sense” heat transfer when we feel warm or cold ourselves. The heat or cold that we feel is known as heat flux, which is heat transfer per unit area. Therefore, in most cases if we have a metallic object and a roomful of air at the same temperature, touching the metallic object will feel warmer (be careful of burns!) than simply standing in the room and absorbing the ambient temperature.

Wear gloves when touching hot metal surfaces!

Why does this happen? Intuitively, solids are denser than liquids and gases, meaning the molecules in solids are more closely-packed. This means that it is easier for heat to be transferred from molecule to molecule in solids, which would explain why heat transfers faster in solids.

When designing an apparatus for heat transfer purposes, one must consider two things: cost and effectiveness. Natural convection (such as with air) is relatively inexpensive because air is everywhere, but it isn’t as effective as using a metallic solid for heat transfer purposes. However, metals can be expensive. Therefore, some form of middle-ground is often desireable. This can be seen in computers, where fins conduct heat away from, say, a processor, and a fan blows the heat away in a process called forced convection.

(Image from WellBake)

Fins are efficient for heat transfer purposes, like in this computer.

What makes fins so ubiquitous in heat transfer applications? First, it is helpful to understand that the amount of heat an object can transfer is directly related to the surface area of the object that is in contact with ambient surroundings, such as air here on Earth. Two other factors that affect heat transfer are temperature difference and type of material (some materials conduct heat better than others: think metal versus cloth, like an oven mitt). So, imagine that we have a flat sheet of metal and a small cube of the same metal. Both are at the same temperature, have the same volume (consist of the same amount of metal), and are in the same room (so the ambient air temperature is the same for both). If the sheet has twice as much exposed surface area to ambient air as compared to the small cube, then the sheet has the capability of transferring twice as much heat as the cube, even though they have the same physical volume in our example.

Second, businesses like to get as much “bang for the buck,” just like consumers. This means a company that needs to design a cooling mechanism for a computer processor would want to maximize cooling ability while minimizing cost for raw materials in their mechanism. Trying to increase the amount of heat transfer by increasing the temperature difference (such as actively cooling the surrounding air) or using a better heat-conducting metal can drive up costs significantly. This leaves the option of increasing exposed surface area for increasing heat transferring capability, and this is where fins come into play.

Fins essentially increase the surface area of an object in need of cooling, which increases the rate at which heat is transferred away from it. By making fins long and slender, like what we see on chipsets inside computers, businesses attain their desired heat transfer capability while keeping the amount of raw material required at a minimum. In the end, we have a win-win situation by using fins: businesses cut down on costs while consumers have devices that don’t overheat and fail.

(Image from Wikipedia.)

Fallen Snow in Ithaca, NY

Last week, the eastern half of the United States was hit by a extreme cold front which brought temperatures down to low temperatures as low as 30 degrees below zero Fahrenheit with the wind chill in some places.

Isn’t wind a peculiar thing? It can make a decently chilly day to freezing cold. And in the summertime, a breeze in a terribly hot day can make the day seem bearable. There isn’t anything different in the air. And there isn’t anything magical about it. It is purely heat transfer via convection and conduction. In both instances, the wind stirs and mixes the air and removes heat from the body.

Let’s take a look at conduction only. You’ve probably noticed that a typical metal pot with boiling water can get hot very quickly on the outside because metals have relatively high termal conductivity. (In fact, this is also what makes metals so amazing with electric circuits, with copper and gold having the highest electric conductivities around. It has something to do with their highly unique metallic bonds.) Sometimes, though, we’d like to insulate things as best we can. This might be your morning coffee in a Styrofoam cup, or your home during the winter.

Units for Thermal Conductivity

Thermal conductivity for copper is about 380 W/m-K, where glass is about 1.1 W/m-K. Water is about 0.6 and air is 0.025. Arguably, air is the best all-around insulator that isn’t also synthetic or a rare material.

Isn’t that just a little weird? Air is the best at retaining heat, but thinking back to 20 mph wind conditions and we might find something wrong with this picture. Because air is a gas, once it’s moving, it can get messy and turblent; wind makes it feel colder than it really is. Standing air prevents the mixing of hot and cold air. That’s why a calm 25 degree day is more bearable than a 35 degree windy day.

Let’s take a quick look at our glass windows. Have you noticed that good, insulated windows are double glazed or triple glazed. Essentially, there is a layer of standing air in between two or three glass panes. We can picture little packets of heat traveling from inside the house to the outside, where it’s cold. While it can walk through glass pretty easily, but once it hits the layer of standing air, the “rate of heat escape” becomes over 40 times slower through the window surface. (Keep in mind we are not considering gaps and drafty windows.)

That is not to say that the window glass will not feel cold. The cold that you might feel is the heat escaping your hand and into the glass and has little to do with the standing air.

So, whenever and wherever you are trying to insulate from the cold this winter, make sure you put as much standing air in between you and the cold. Just make sure you don’t have any leaks though.

Stay warm!

(Fallen snow image from my Flickr.)