Review Index:

General Heat Transfer Guide

Author: Ryan Shrout
Manufacturer: General


This content was originally featured on and has been converted to PC Perspective's website. Some color changes and flaws may appear.

Let’s begin by defining most of the terms that relate to computer cooling.


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


Ambient: style='mso-tab-count:1'>                                   In our
discussion, refers to room conditions, particularly room temperature.


Amperes: style='mso-tab-count:1'>                                  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).


(Centigrade):            A temperature
scale referenced to absolute zero.  Pure
water freezes at 0°C and boils at 100°C (at one atmosphere of pressure). style="mso-spacerun: yes">  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. style="mso-spacerun: yes"> 
Conduction increases with increasing
temperature differential, increasing conduction coefficient, increasing
cross-sectional area, and decreasing material thickness.


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


Convection: style='mso-tab-count:1'>                              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.


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


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. style="mso-spacerun: yes">  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. style="mso-spacerun: yes">  You need only add or subtract 32 when
converting absolute temperatures.


Energy: style='mso-tab-count:1'>                                     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: style='mso-tab-count:1'>                                A temperature
scale referenced to absolute zero.  Pure
water freezes at 32°F and boils at 212°F (at one atmosphere of pressure). style="mso-spacerun: yes">  To convert °F to °C, subtract 32 and divide
by 1.8.


Fan Laws: style='mso-tab-count:1'>                                Equations used
to calculate fan flow, pressure, and power at different fan speeds.


Interface: style='mso-tab-count:1'>                                  The point
where two different solids meet, specifically the die and heat sink. style="mso-spacerun: yes">  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: style='mso-tab-count:1'>                                   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: style='mso-tab-count:1'>                                      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: style='mso-tab-count:1'>                                     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. style="mso-spacerun: yes">  One N-m/s equals one watt. style="mso-spacerun: yes">  A mid-range Duron requires about 42.5 watts
of power.  Electrical power in DC
devices is simply voltage multiplied by current.


Radiation: style='mso-tab-count:1'>                                 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. style="mso-spacerun: yes">  If the object is cooler than its
surroundings, it will receive more energy that it emits. style="mso-spacerun: yes">  The opposite is also true. style="mso-spacerun: yes">  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: style='mso-tab-count:1'>                            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: style='mso-tab-count:1'>                            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. style="mso-spacerun: yes">  Contrast this against “transient”.


Transient: style='mso-tab-count:1'>                                  A condition
marked by change.  Upon a cold-start,
the power consumed in the PC goes from virtually zero to over 200 watts. style="mso-spacerun: yes">  The CPU temperature goes from ambient to
significantly higher until steady-state conditions occur.


is far from an exhaustive list, but covers the basics. style="mso-spacerun: yes"> 
If you wish to look up more terms related to
heat transfer, consider investigating the following terms/words:

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


HSF: style='mso-tab-count:1'>                Heat Sink and Fan. style="mso-spacerun: yes">  The combination of a heat sink and integral
fan used to transfer heat from a component like the CPU, GPU, etc.


PCM: style='mso-tab-count:1'>               Phase Change Material. style="mso-spacerun: yes">  This is a polymer material that changes from
solid to a (very thick) liquid at a specific temperature. style="mso-spacerun: yes">  It serves as a TIM.


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


style='font-size:12.0pt;mso-bidi-font-size:10.0pt'>Abbreviations style='font-size:12.0pt;mso-bidi-font-size:10.0pt'>

A:                     Area. style="mso-spacerun: yes">  Units are commonly m^2


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


cp:                    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:                    Kelvin


kg:                    Kilograms


l:                       Length. style="mso-spacerun: yes">  Units are commonly m


m:                     Meters


r:                     Density. style="mso-spacerun: yes">  Units are commonly kg/m^3


RPM:               Revolutions per


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


T¥:                   Ambient
temperature.  Units are commonly °C


V:                     Volume. style="mso-spacerun: yes">  Units are commonly m^3


style='font-size:12.0pt;mso-bidi-font-size:10.0pt'>Common Conversions style='font-size:12.0pt;mso-bidi-font-size:10.0pt'>

style='font-size:10.0pt;font-family:Arial'>To convert these units:

style='font-size:10.0pt;font-family:Arial'>To these units:

style='font-size:10.0pt;font-family:Arial'>multiply by:





































style='font-size:10.0pt;font-family:Arial'>BTU (British thermal units)









style='font-size:10.0pt;font-family:Arial'>foot-pounds / second







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