Editing Computer cooling (section)

==Air cooling==
{{Further|Computer fan}}

===Fans===
Fans are used when natural convection is insufficient to remove heat. Fans may be fitted to the computer case or attached to CPUs, GPUs, chipsets, [[power supply]] units (PSUs), [[Hard Disk|hard drive]]s, or as cards plugged into an expansion slot. Common fan sizes include 40, 60, 80, 92, 120, and 140&nbsp;mm. 200, 230, 250 and 300&nbsp;mm fans are sometimes used in high-performance personal computers.

====Performance of fans in chassis====
[[File:Chassis-plans-fan-curve.jpg|thumb|Typical fan and [[chassis impedance]] curves]]A computer has a certain resistance to air flowing through the chassis and components. This is the sum of all the smaller impediments to air flow, such as the inlet and outlet openings, air filters, internal chassis, and electronic components. Fans are simple air pumps that provide pressure to the air of the inlet side relative to the output side. That pressure difference moves air through the chassis, with air flowing to areas of lower pressure.

Fans generally have two published specifications: free air flow and maximum differential pressure. Free air flow is the amount of air a fan will move with zero back-pressure. Maximum differential pressure is the amount of pressure a fan can generate when completely blocked. In between these two extremes are a series of corresponding measurements of flow versus pressure which is usually presented as a graph. Each fan model will have a unique curve, like the dashed curves in the adjacent illustration.<ref>{{Cite web|title = Cooling and Noise in Rugged Industrial Computers|url = http://www.chassis-plans.com/white_paper_cooling_and_noise.html|website = Chassis Plans Rugged Computers and LCD Displays|access-date = 2016-02-11|language = en-US|archive-url = https://web.archive.org/web/20140107184852/http://www.chassis-plans.com/white_paper_cooling_and_noise.html|archive-date = 7 January 2014|url-status = live}}</ref>

====Parallel vis-à-vis series installation====
Fans can be installed parallel to each other, in series, or a combination of both. Parallel installation would be fans mounted side by side. Series installation would be a second fan in line with another fan such as an inlet fan and an exhaust fan. To simplify the discussion, it is assumed the fans are the same model.

Parallel fans will provide double the free air flow but no additional driving pressure. Series installation, on the other hand, will double the available static pressure but not increase the free air flow rate. The adjacent illustration shows a single fan versus two fans in parallel with a maximum pressure of {{convert|0.15|in}} of water and a doubled flow rate of about {{convert|72|cuft/min}}.

Note that air flow changes as the square root of the pressure. Thus, doubling the pressure will only increase the flow 1.41 ([[square root of 2|{{radic|2}}]]) times, not twice as might be assumed. Another way of looking at this is that the pressure must go up by a factor of four to double the flow rate.

To determine flow rate through a chassis, the chassis impedance curve can be measured by imposing an arbitrary pressure at the inlet to the chassis and measuring the flow through the chassis. This requires fairly sophisticated equipment. With the chassis impedance curve (represented by the solid red and black lines on the adjacent curve) determined, the actual flow through the chassis as generated by a particular fan configuration is graphically shown where the chassis impedance curve crosses the fan curve. The slope of the chassis impedance curve is a square root function, where doubling the flow rate required four times the differential pressure.

In this particular example, adding a second fan provided marginal improvement with the flow for both configurations being approximately {{convert|27|-|28|cuft/min}}. While not shown on the plot, a second fan in series would provide slightly better performance than the parallel installation. {{Citation needed|date=February 2013}}

====Temperature vis-à-vis flow rate====
The equation for required airflow through a chassis is

: <math>\text{CFM} = \frac{P}{Cp \times r \times dT}</math>

where
* <math>\text{CFM}</math> = [[cubic feet per minute|Cubic Feet per Minute]] ({{convert|1|cuft/min|disp=out}})
* <math>P</math> = Heat Transferred ([[kW]])
* <math>Cp</math> = Specific Heat of Air
* <math>r</math> = Density
* <math>dT</math> = Change in Temperature (in °F)

A simple conservative rule of thumb for cooling flow requirements, discounting such effects as heat loss through the chassis walls and laminar versus turbulent flow, and accounting for the constants for specific heat and density at sea level is:

: <math>\text{CFM} = \frac{3.16 \times P}{\text{allowed temperature rise in} ^\circ F}</math>

: <math>\text{CFM} = \frac{1.76 \times P}{\text{allowed temperature rise in} ^\circ C}</math>

For example, a typical chassis with 500 watts of load, {{convert|130|°F|abbr=on}} maximum internal temperature in a {{convert|100|°F|abbr=on}} environment, i.e. a difference of {{convert|30|F-change|abbr=on}}:

: <math>\text{CFM} = \frac{3.16 \times 500\ \text{W}}{(130 - 100)} = 53</math>

This would be actual flow through the chassis and not the free air rating of the fan. It should also be noted that "Q", the heat transferred, is a function of the heat transfer efficiency of a CPU or GPU cooler to the airflow.

===Piezoelectric pump===
A "dual piezo cooling jet", patented by [[General Electric|GE]], uses vibrations to pump air through the device. The initial device is three millimetres thick and consists of two [[nickel]] discs that are connected on either side to a sliver of piezoelectric ceramics. An alternating current passed through the ceramic component causes it to expand and contract at up to 150 times per second so that the nickel discs act like a bellows. Contracted, the edges of the discs are pushed apart and suck in hot air. Expanding brings the nickel discs together, expelling the air at high velocity.

The device has no bearings and does not require a motor. It is thinner and consumes less energy than typical fans. The jet can move the same amount of air as a cooling fan twice its size while consuming half as much electricity and at lower cost.<ref>{{cite web|url=http://www.gizmag.com/ge-dual-piezo-cooling-jet/25447/|title=GE's "dual piezo cooling jet" could enable even cooler gadgets|website=gizmag.com|date=14 December 2012|access-date=20 April 2013|archive-url=https://web.archive.org/web/20130721040030/http://www.gizmag.com/ge-dual-piezo-cooling-jet/25447/|archive-date=21 July 2013|url-status=live}}</ref>