General Heat Transfer Guide

Absolute Temperature:            A temperature referenced to absolute zero.  Absolute zero is the temperature at which molecular activity ceases.  Absolute zero equals 0 Kelvin, -273.15°C, and –459.67°F.  All material properties change according to temperature.

 

Ambient:                                   In our discussion, refers to room conditions, particularly room temperature.

 

Amperes:                                  Units for measuring the amount of electrical current.  Electrical current is analogous to a rate of flow such as gallons per minute (liquid flow) or cubic feet per minute (airflow).

 

Celsius (Centigrade):            A temperature scale referenced to absolute zero.  Pure water freezes at 0°C and boils at 100°C (at one atmosphere of pressure).  To convert °C to °F, multiply by 1.8 and add 32.  Celsius and Kelvin have the same scale, but are offset from one another by 273.15.  i.e.:  0 K equals –273.15°C and 273.15 K equals 0°C.

 

Conduction:                              Heat transfer through a solid material.  A heat sink conducts heat from the die to its fins/pins.  Conduction increases with increasing temperature differential, increasing conduction coefficient, increasing cross-sectional area, and decreasing material thickness.

 

Conduction Coefficient:            A measure of how efficiently a solid conducts heat.  Among all materials, diamonds have the best conduction coefficient.  Among materials commonly used in electronics, the top conductors are silver, copper, and gold.  Aluminum is also a respectable conductor.

 

Convection:                              Heat transfer from a solid into a liquid or gas.  The energy transferred through the heat sink leaves via convection to air or water.  Convection increases with increasing temperature differential, increasing surface area, and increasing convection coefficient.

 

Convection Coefficient:            A measure of how efficiently a fluid (liquid or gas) transfers heat to or from a solid.  This value depends on many factors including fluid density, fluid speed, fluid viscosity, solid geometry, and a few others not mentioned here.

 

Differential Temperature:            The difference between two temperatures.  Conduction through a solid and convection between a solid and liquid depends on temperature differential.  If a given solid has a temperature on one side of 25°C and 35°C on the other, the heat transferred will be identical if the temperatures become 45°C and 55°C.  To convert a differential temperature in °C to °F, multiply by 1.8.  To convert a differential temperature in °F to °C, divide by 1.8.  Do not add or subtract 32 when converting differentials.  You need only add or subtract 32 when converting absolute temperatures.

 

Energy:                                     Energy has units of force multiplied by distance.  It is commonly referred to as “work”.  If you weigh 200 pounds and climb straight up a ten foot ladder, you do 200*10 foot-pounds of work.  In metric units, the common units are called “joules”.  One joule equals one Newton-meter.  In metric units, if you weigh 850 Newton’s and climb straight up a 3 meter ladder, you do 2550 N-m of work.  Energy divided by time is called “power”.

 

Fahrenheit:                                A temperature scale referenced to absolute zero.  Pure water freezes at 32°F and boils at 212°F (at one atmosphere of pressure).  To convert °F to °C, subtract 32 and divide by 1.8.

 

Fan Laws:                                Equations used to calculate fan flow, pressure, and power at different fan speeds.

 

Interface:                                  The point where two different solids meet, specifically the die and heat sink.  Due to microscopic irregularities, contact between the two is not perfect.  This imperfection reduces the conduction between the two solids and requires corrective action to improve conduction.

 

Lapping:                                   A process used to improve the surface finish of a heat sink.  This consists of sanding the surface to reduce its roughness and improve contact with the die.

 

Peltier:                                      An electrical device that uses two materials with different electrical potentials to cause heat transfer across the device.  A Peltier cooler still requires a means for getting the heat that is conducts away from itself and into the surrounding air.

 

Power:                                     A measure of how quickly work is performed.  Work divided by time equals power.  From our “Energy” example above, if the 850 Newton person climbed the 3 meter ladder in one minute, the power would be 2550 N-m / 60 seconds = 42.5 N-m/s.  One N-m/s equals one watt.  A mid-range Duron requires about 42.5 watts of power.  Electrical power in DC devices is simply voltage multiplied by current.

 

Radiation:                                 As it pertains to heat transfer, radiation is the exchange of thermal energy via the emission and reception of photons.  Depending on the temperature of a given object with respect to its environment, the net energy transfer may be either into the object or away from the object.  If the object is cooler than its surroundings, it will receive more energy that it emits.  The opposite is also true.  The level of energy compared to conduction and convection is generally low in the PC world due to small differentials between objects.  Radiation depends upon an object’s size, shape, temperature, and emissivity.

 

Specific heat:                            A measure of thermal energy storage capacity for a material.  A high specific heat means that more thermal energy may be stored in a given mass of material.

 

Steady-state:                            A condition of equilibrium where all things are constant.  Power consumption and temperatures no longer change once steady-state occurs.  In actuality, computers never truly reach steady-state; although, they tend to get pretty close.  Contrast this against “transient”.

 

Transient:                                  A condition marked by change.  Upon a cold-start, the power consumed in the PC goes from virtually zero to over 200 watts.  The CPU temperature goes from ambient to significantly higher until steady-state conditions occur.

 

This is far from an exhaustive list, but covers the basics.  If you wish to look up more terms related to heat transfer, consider investigating the following terms/words:

Reynolds number, Nusselt number, Prandtl number, Biot number, Fourier number, Log-mean temperature difference.

Acronyms

HSF:                Heat Sink and Fan.  The combination of a heat sink and integral fan used to transfer heat from a component like the CPU, GPU, etc.

 

PCM:               Phase Change Material.  This is a polymer material that changes from solid to a (very thick) liquid at a specific temperature.  It serves as a TIM.

 

TIM:                Thermal Interface Material.  A material designed to improve conductivity between two mating solid surfaces.

 

Abbreviations

A:                     Area.  Units are commonly m^2

 

CFM:               Cubic feet per minute.  A measure of airflow. A holdover of the Imperial Units system.

 

c_p:                    Specific heat.  Units are commonly J / kg – K.

 

h:                      Convection coefficient.  Units are commonly W / m^2 – K

 

k:                     Conduction coefficient.  Units are commonly W / m – K

 

K:                    Kelvin temperature

 

kg:                    Kilograms

 

l:                       Length.  Units are commonly m

 

m:                     Meters

 

r:                     Density.  Units are commonly kg/m^3

 

RPM:               Revolutions per minute

 

T₁, T₂:              Various temperatures.  Units are commonly °C

 

T_¥:                   Ambient temperature.  Units are commonly °C

 

V:                     Volume.  Units are commonly m^3

 

Common Conversions

To convert these units:	To these units:	multiply by:
Length
inch	centimeter	2.54
inch	meter	.0254
feet	meter	.3048
Area
inch^2	mm^2	645.16
inch^2	m^2	.00064516
Volume
feet^3	m^3	.028317
Energy
foot-pounds	Newton-meters	1.3558
foot-pounds	joules	1.3558
BTU (British thermal units)	joules	1055.06
Calories	joules	4.1868
Power
foot-pounds / second	watts	1.3558
horsepower	watts	745.7