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Understanding Heat Transfer

By Bruce W. Maki, Editor

What Is Heat?

Intuitively, we all know what heat is. It's that warm feeling from the sun, from your furnace, from your toaster. When it's cold outside we turn up the heat, on the thermostat, and a machine somewhere creates it.

But some of us want to know more.

Engineers and scientists, for example, need to understand heat. Designing and building a furnace, a stove, a clothes dryer, or an air conditioner requires a full understanding of the science of heat transfer.

Some of us hate paying so much for home heating. Some of us know that inefficient home heating causes avoidable environmental side effects, such as air pollution and greenhouse gases like carbon dioxide.

Heat vs. Temperature:

Heat Is... Temperature Is...

A concept.

A form of energy.

Fully transferable from one object to another.

When an object has heat, (and they all have some heat) it contains a form of internal energy.

 

A property of matter. A block of metal or a pond of water will always have a certain temperature.

A measure of the amount of heat in a certain object, such as the air mass covering your city, or a pot of water on your stove.

 

Consider a few things about atoms and molecules: 

Atoms and molecules don't just sit around. In liquids, they are constantly jostling about. In gases, they are flying around quite fast, slamming into each other and into the container they are held in.

In solids, the atoms don't migrate much, but they vibrate. Constantly. Atoms also vibrate when they are in liquid or gas form.

When atoms (and molecules) receive more heat, they vibrate more. Molecules of liquids jostle faster.  Molecules of gases fly around faster, collide harder, and exert more of a push on their container.

Also, all atoms have electrons which rapidly orbit their nucleus. 

When an atom receives more heat, it's electrons orbit faster, and may even orbit farther away from the nucleus. The electrons could collide with those from a neighboring atom, causing them to orbit faster.

Supposedly, at the coldest possible temperature, around -454 degrees Fahrenheit, which is called absolute zero, all atomic motion stops. Atomic vibration stops, and maybe even electron orbiting. This sounds weird to me. What happens to the electron when it stops orbiting? Does it fall down, no longer held in place by the centripetal force of orbit? Of course, reaching absolute zero has never been done, and later you'll understand why.

 

But this atomic-level knowledge is not critically important to understand heat and heat transfer.

 

Concept: What The Average Person Needs To Know
Energy "That which makes things go."
First Law of Thermodynamics Energy cannot be destroyed or created. Only it's form can be changed. *
Second Law Of Thermodynamics Heat moves from warm objects to colder objects. No exceptions.
  The rate of heat transfer across a given barrier (such as your ceiling) is proportional to the difference in temperature between the two sides.
   

 

* All right, you wise-guy advanced physics types, you caught this one. True, energy can be created, by destroying matter. That is Albert Einstein's principle contribution to science. E=mc² means that tiny amounts of matter can be converted to enormous amounts of heat, as in thermonuclear fission (Nuke plants and atomic bombs) But in all non-nuclear reactions and systems, a trivial amount of matter is destroyed and the total quantity of energy, for all practical purposes, is constant.

 

Moving Heat:

There are three methods of heat transfer: conduction, convection, and radiation.

Method

Description

Conduction Heat energy travels linearly along a material, from high temperature to low temperature. Example: Hold a metal rod in a flame. The hot gases in the flame heat the end of the rod, and eventually your hand will feel a temperature rise.
Convection

When a fluid (liquid or gas) is next to a solid surface of a different temperature, heat will be transferred.

Natural convection: Heated fluids become less dense and rise. This is why flames (unless forced) tend to travel upwards. When people state the common misconception that "heat rises", this is what they mean.

Forced Convection: A circulating pump forces the fluid through a chamber such as a long pipe. Heat is transferred to the pipe, and then to the air at all points around the pipe. Special devices (such as metal fins) amplify the rate of heat transfer in some places, namely, at the radiators.

Radiation

Long-wavelength (infra-red) light emitted by all objects. The amount of heat given off depends on the object's temperature.

The heat our dear planet Earth receives from the Sun comes only in the form of radiation.

Infra-red radiation, being a type of radio wave, travels in a straight line, and can pass through air, glass or a vacuum (such as outer space). It is absorbed when it strikes an object. Radiation heat transfer can be greatly reduced by a shiny, reflective barrier, such as aluminum foil. Some home insulation panels come with a shiny foil surface. Dark surfaces absorb more radiation than light surfaces.

 

About Your House:

Heat transfer in a residence is a combination of all three methods. 

But I will make an assertion based on my technical education. The radiation heat transfer from a typical house in winter is small, so small that is can be considered trivial. However, the radiation heat transfer to a house in summer can be very significant, especially in the South.

Conduction occurs through the walls, floors, and ceiling of your house, especially through the wallboard and wooden framing. Concrete is a fairly good conductor of heat, so concrete floors pose a challenge in colder climates.

Convection may be the biggest factor in heat loss. Inside an un-insulated wall, the wallboard is constantly warmed by the room. Just inside the wall cavity, the air is heated by the wallboard, and rises up. At the top of the cavity the air transfers heat to the cold exterior wall surface, which conducts the heat to the cold outside air. Kiss your money goodbye. 

So What Does Insulation Do:

Some insulation products, such as foam plastics, have low coefficients of conduction (they don't transfer heat too well). This is good. 

But glass is a very good conductor of heat. How does fiberglass, the most popular insulation, do any good? Because the glass fibers are tiny, and they create tiny pockets of dead air space. Convection barely occurs in these. Tiny pockets of air are actually a decent insulator (I'll explain later) When there are billions of tiny pockets of dead air space surrounding your house, heat transfer is reduced. 

Fabrics (i.e. your clothes) work in the same way. Cotton would be a great home insulator, but it is flammable (a big problem) and will rot if it gets wet (an even bigger problem). Fiberglass does not have these problems.

Cellulose insulation, which is just finely shredded newspapers, is another excellent product. But it has to be treated with fire retardant and rot-resistant chemicals. Cellulose is usually blown into place. It has a habit of settling over time, which makes it a poor choice for wall insulation. But for ceilings, it's economy can't be topped.

The Great Freebie:

There is something called a thermal air film that surrounds all objects. This means that the molecules of air that are very close to your house tend to want to stay put, and not move much even when they are warmer than their neighboring air molecules. This is caused by plain old friction. The net result is that you get a small, extra amount of insulation value just from having a simple barrier, such as your wall or windows. And you get not one but TWO air films, inside and outside.

Now about that fiberglass insulation: it's this air film that makes it so effective. Replicated many times over, it adds up to a good insulation value, made from a good conductor.

Some people say that single pane windows get more R-value from their air films than from the glass. I believe it. I'll try to dig up some books on this.

For years I have been building "Interior Storm Windows". I use that shrink-film plastic that 3M makes, and wants you to discard every year. But I install the film on a custom-made wooden frame that fits snugly inside the window opening, so the whole unit can be removed in the spring and replaced in the fall. And these things work! My intention was to reduce air infiltration, which they do, but I know there is an added benefit: two more air films. I can tell by the reduction in condensation on the windows. What condensation forms is always frost and always on the outer panes of glass, never on the plastic "interior storm window". The inner layer stays warm, which is what you want.

 

That "Delta-Tee" Thing:

Near the beginning I stated that the rate of heat transfer across a barrier is proportional to the difference in temperatures between the two sides. This is very important, because the rate of heat transfer is also proportional to the rate at which your money flies out the window.

delta T = T(in) - T(out)

where:

  • delta T is the difference in temperatures (I can't make the Greek symbol, so I'll spell it out)
  • T(in) is the indoor temperature
  • T(out) in the outdoor temperature

Right now, the outside temperature is 12 degrees. My office is 72 degrees. The difference in temperature is 60 degrees. (Hey, this is Northern Michigan in February... it could be worse)

I used to live in Northern Ontario. It gets much colder there.  Minus 18 would not be uncommon. That would be a delta T of 90 degrees. The same building on such a night would consume 90/60 or 1.5 times as much energy.

Or consider this: When I finally call it quits for the night, I'll turn the thermostat down to 62 degrees. That will lower the delta T to 50 degrees. The rate of heat loss (and fuel consumption) will be 50/60 or 83.3% of the current rate, after the system reaches equilibrium. 

This means...

Your home's energy consumption is directly affected by your behavior, namely, how high you set the thermostat.

Without making any changes to your home's insulation, you can reduce your heating bills by:

  • Setting your thermostat to a lower temperature at night or when you are away. The lower the better. But don't let your house get below freezing (or you'll say goodbye to your pipes).
  • An automatic, programmable thermostat can do this for you. These cost a few bucks, but they pay for themselves quickly, possibly within a month or two.
  • Shutting off the heat registers in unused rooms. Keep the doors closed too. Be careful with bathrooms: frozen and burst pipes will cost more to repair than any heat savings.

 

Sources:

Engineering Thermodynamics by William C. Reynolds and Henry C. Perkins,
1977, Publisher: McGraw-Hill

 

Fundamentals Of Heat Transfer by Lindon C. Thomas,
1980, Publisher: Prentice-Hall

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Written February 17, 2000