Glossary and Conversions
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Let’s begin by defining some of the terms that relate fans with computer cooling.Glossary
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.
ACFM: Actual Cubic Feet per Minute. This is a measure of airflow referenced to the current density of the gas. The mass flow rate of the air equals the ACFM multiplied by the air density.
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).
Axial Fan: A fan that propels air in a direction parallel to its axis of rotation. Virtually all fans used in computers today are of this type. The alternative is a centrifugal (or radial) fan.
Celsius (Centigrade): A temperature scale. 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.
CFM: Cubic feet per minute. This is a general measure of volumetric flow rate. Fans are normally rated in terms of CFM. In order for fan ratings to have meaning, they must be tested under identical, rigidly controlled conditions.
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.
Decibels: A logarithmic scale used in measuring sound.
Denver: A city located in the state of Colorado, United States of America. Approximate elevation of the city above sea level is one mile, 5280 feet, 1609 meters.
Differential Temperature: The difference between two temperatures. Convection between a solid and liquid depends on temperature differential. 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 footpounds of work. In metric units, the common units are called “joules”. One joule equals one Newtonmeter. In metric units, if you weigh 850 Newton’s and climb straight up a 3 meter ladder, you do 2550 Nm of work. Energy divided by time is called “power”.
Fahrenheit: A temperature scale. 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. Fahrenheit and Rankine have the same scale, but are offset by 459.67 relative to one another. ie.: 0°R equals –459.67°F and 0°F = 459.67°R.
Fan Laws: Equations used to calculate fan flow, pressure, and power at different fan speeds, different air temperatures, and different air pressures.
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 Nm / 60 seconds = 42.5 Nm/s. One Nm/s equals one watt. A midrange Duron requires about 42.5 watts of power. Electrical power in DC devices is simply voltage multiplied by current.
SCFM: Standard Cubic Feet per Minute. An absolute measure of air flow rate. “Standard” defines the air density by specifying a temperature of 70°F (~21.1°C) and pressure of one atmosphere (~14.7 psi, 101.3 kPa). SCFM equals ACFM only when the temperature is 70°F (21.1°C) and pressure equals one atmosphere (101.3 kPa). A fan producing 38CFM at standard conditions produces 38ACFM in Denver, but only about 31SCFM as we shall see later.
Sound Power Level: The amount of power dissipated in the form of sound. Contrast to “Sound Pressure Level”.
Sound Pressure Level: The localized air pressure fluctuation due to a specific sound source. For a given sound power level, the sound pressure level drops as distance to the sound source increases.
Static Pressure: Static pressure (abbreviated SP) is the uniform force exerted equally in all directions by a liquid or gas. It does not include any force from motion or acceleration of the liquid or gas. It is akin to the potential energy of a system.
Steady state: A condition of equilibrium where all things are constant. Power consumption and temperatures no longer change once steadystate occurs. In actuality, computers never truly reach steadystate; although, they tend to get pretty close. Contrast this against “transient”.
Total Pressure: Total pressure is the sum of static and velocity pressure. Not including temperature changes, it is the sum energy potential of liquid or gas.
Transient: A condition marked by change. Upon a coldstart, the power consumed in the PC goes from virtually zero to over 200 watts. The CPU temperature goes from ambient to significantly higher until steadystate conditions occur.
Velocity Pressure: Velocity pressure is the energy associated with a liquid or gas based upon its velocity and density. Velocity pressure is proportional to the square of velocity. It is akin to the kinetic energy of a system.
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 
Pressure 


atmospheres 
lbf/in^2 (psi) 
14.696 
atmospheres 
kPa 
101.325 
lbf/in^2 
kPa 
6.895 
kPa 
newtons / m^2 
1000 
inches of water 
psi 
0.0361 
inches of water 
kPa 
0.249 
Flow Rate 


CFM 
m^3/min 
.028317 
Energy 


footpounds 
newtonmeters 
1.3558 
footpounds 
joules 
1.3558 
BTU (British thermal
units) 
joules 
1055.06 
Calories 
joules 
4.1868 
Power 


footpounds / second 
watts 
1.3558 
horsepower 
watts 
745.7 