Intro to Computer Systems

Chapter 9: Power Considerations

Thermal Design Power

The use of energy from computing equipment affects not just its power consumption, but also its form factor - all of that energy ends up becoming heat, and it needs to be dissipated in order to prevent the system from overheating.

Heatsink Design and Operation

A heatsink/fan unit on an AMD processor.
A heatsink/fan unit on an AMD processor.

The common method to cooling hot-running processors is a heatsink. This is a block of metal made out of a heat conducting metal such as aluminium or copper, broken up into fins such that air blowing around the fins carries the heat away. They are typically referred to as a heatsink/fan unit, or HSF. To minimise noise, the speed of the fan blowing air over the heatsink is monitored and controlled by the system. In times of low load (i.e. low CPU power dissipation), the fan slows down to ensure quieter operation.

FrostyTech, a site reviewing computer heatsink technology, dissected and investigated a stock Intel heatsink/fan unit.
A 10000rpm desktop hard disk, encased in a heatsink for heat dissipation reasons.
A 10000rpm desktop hard disk, encased in a heatsink for
heat dissipation reasons.

Heatsinks are used to spread the output heat of many internal components; not just the CPU, but also GPUs, chipsets, voltage regulators, high-end memory modules, and hard disks. In compact cases and laptops, heat pipes are used to transmit the heat from one part of the case to another, where it can be more conveniently dissipated.

A heatsink employing a copper base and heatpipe, to aluminium fins.
A heatsink (upside down in the picture) employing a copper base and heatpipe, to aluminium fins.

Airflow

Once heat is transferred from heat-sensitive components, the hot air must be evacuated. Failure to do this will mean that the ambient temperature inside the case increases, reducing the heatsinks' ability to effectively dissipate heat and ultimately cause the system to overheat.

The flow of air into, within, and out of a case can have significant effect on the efficiency of heatsinks, and ultimately, component cooling.

Positive vs Negative Pressure

Positive pressure is when a case has more airflow going into it, than out of it - that is, the air pressure in the case is higher than the outside air. This has the effect of air trying to escape out of the case wherever it can.

Negative pressure is when the pressure inside the case is lower than the air outside, this drawing air from any seam or crease in the case. This is to be avoided, mainly for reasons of dust accumulation: most indoor air is rather dusty, and a negative pressure case will draw the dust into all parts of the case.

Technibble.com has a short article on air pressure within cases.

Airflow Management

Fans can either blow air into the case (intake), or out of the case (exhaust). Most cases have a large number of fan vents, but the choice of how to vent air is up to the system builder. In order to keep positive case pressure, the balance of airflow should tilt towards intake fans. Some forced air exhaust is still important, to help hot air out of the case.

Some higher quality cases, and many cases from major manufacturers like Apple, Dell and HP, have cases that are designed into thermal zones. This seeks to isolate each major heat producer in the case and cool them independently, such that each gets a clean and uninterrupted airflow, and heat does not pool within the case.

Apple's Power Macintosh G5 case is one example of thermal zoning.
Apple's Power Macintosh G5 case is one example of thermal zoning.

For example, in the image below, the case is separated into four thermal zones: mass storage cooling (top), expansion card cooling (middle), CPU and memory cooling (bottom), and a hidden fourth zone at the very bottom of the case for the power supply.