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Fans, Heat Sinks, and Cooling Systems

All Pentium processors require the presence of a heat sink and a microprocessor fan for cooling purposes. As Figure 3.15 illustrates, these devices come in many forms, including simple passive heat sinks and fan-cooled, active heat sinks.

Figure 3.15

Figure 3.15 Typical microprocessor cooling systems.

Passive heat sinks are finned metal slabs that can be clipped or glued with a heat-transmitting adhesive (referred to as thermal compound or paste) onto the top of the microprocessor. The fins increase the surface area of the heat sink, enabling it to dissipate heat more rapidly. Active heat sinks add a fan unit to move air across or through the heat sink. The fan moves the heat away from the heat sink and the microprocessor more rapidly.

The original ATX power-supply specification called for these systems to employ power supplies that use a reverse-flow fan that brings in cool air from the back of the unit and blows it directly onto the microprocessor. For this to work properly, the system board must adhere to the ATX form factor guidelines and place the microprocessor in the correct position on the system board. However, this portion of the ATX design specification has almost completely been ignored in favor of exhaust fan designs, which pull air through the system unit, across the system board and processor, and then push it out through the power supply unit.

Slot-based cartridge processors (Pentium II and III processors) also require special heat sink and fan support structures that work with the cartridge package. These units mount vertically on the system board beside the processor cartridge and provide support for the heat sink as well as the fan unit.

The support mechanism is designed so that it plugs into standard predrilled holes in the system board. For repair or upgrading purposes, the fan unit can be removed from the support mechanism and replaced.

In newer Pentium systems, the BIOS interrogates the processor during startup and configures it appropriately. This prevents the user from subjecting the processor to potentially destructive conditions, such as overclocking. In addition, these systems can monitor the health of the processor while it is in operation and take steps to compensate for problems such as overheating. This normally involves speeding up or slowing down the processor fan to maintain a given operating temperature.

The fan module must be one supported by the installed BIOS. If a fan unit is installed that does not have proper stepping in the BIOS routines, the system will not be able to correctly control the fan speed. Therefore, it may not be able to keep the processor cool enough for proper operation. Also, some fans are built better than others. For instance, fans that use ball bearings instead of slip ring bearings tend to run smoother and make less noise. However, they are usually more expensive than the slip ring versions.

BTX Thermal Module

The BTX form factor design is based on creating specific airflow zones within the case. The component responsible for generating the airflow is the BTX Thermal Module. The thermal module combines a heat sink and fan into a special duct that channels the air across the system board's main components. The duct fits tightly against large air vents in the front center portion of the case. The fan draws air in from the front and pushes it directly over the microprocessor mounted under the assembly in a linear flow pattern. The air continues toward the back of the case, passing over the graphics card and major chipset components. A fan in the power-supply unit draws some of the air across the memory devices before exhausting it out through the rear of the unit. Figure 3.16 shows the flow of air through the BTX case.

Figure 3.16

Figure 3.16 Airflow in a BTX system.

Advanced Cooling Systems

As system designers continue to push microprocessors for more speed, they also increase the amount of power that they dissipate. The latest microprocessor design techniques have created processors that generate more than 80 watts of power that must be dissipated as heat. This is more heat than a 60-watt light bulb generates. It is beyond the capabilities of most processor fans and heat sinks to effectively dissipate this much heat.

Simple air-cooling systems cannot create a large enough temperature differential to cool the processor. Therefore, system designers have begun to equip very high-speed systems with refrigerated cooling systems. Originally, the designers adopted water-based cooling systems that cooled and circulated water to carry heat away from the processor. Figure 3.17 shows the components of a sample water-based cooling system typically used to cool processors that have been configured to run in overclocking conditions.

Figure 3.17

Figure 3.17 Water-based microprocessor coolers.

The water cooler system consists of the following:

  • A water reservoir tank
  • A water pump that circulates water throughout the cooling system
  • A condenser coil radiator with fans that cool the water and exhaust heat into the outside atmosphere
  • A CPU cooling block that connects directly to the microprocessor and extracts heat from it

The water pump operates from inside the reservoir tank and forces cooling water through the system. Most of the pumps for these systems are adaptations of home aquarium pumps and are designed for 120Vac operation; therefore, they must have an external power cord.

The CPU cooling block consists of a copper-finned heat sink that mounts to a bracket installed around the microprocessor. Pentium 4 system boards have standard hole patterns already supplied to permit such devices to be attached to them. The heat sink is enclosed in a water jacket that circulates cooling water around the fins. This water jacket removes more heat from the processor faster than an air-cooled heat sink.

Heated water from the CPU cooler is pumped through the radiator. The radiator is composed of several coils of tubing to maximize the surface area that is used to dissipate heat. The additional fans push air across the coils and speed up the radiation process in the same manner as conventional CPU fans do for air-cooled heat sinks. The cooled water returns to the reservoir for recirculation.

More advanced liquid-based cooling systems have migrated to nonwater coolants like those used in residential refrigerators or automobile air conditioners. The components associated with a refrigerated cooling system used with a PC system include the following:

  • An evaporator that mounts on top of the microprocessor.
  • A condenser with cooling fan that mounts to the case so that air can be exhausted to the outside of the case.
  • A compressor that places the cooling liquid under pressure so that it can perform refrigeration.
  • A flow control/expansion device that acts as a restriction in the lines of the system that causes the refrigerant to lose pressure and partially vaporize.
  • Insulated tubing that connects the four major components in a closed-loop cooling circuit.

As Figure 3.18 illustrates, the components of the PC cooling system do not fit inside a typical desktop or tower unit. Instead, they must be used in cases that have been modified for them, or in cases that have been designed specifically for them.

Figure 3.18

Figure 3.18 PC refrigerant coolers.

The four major components of the system are interconnected by a sealed piping system that holds a refrigerant liquid. The compressor is used to compress the refrigerant and pump it through the system. The high-pressure, high-temperature refrigerant first passes through the condenser unit where it exchanges heat with the surrounding air and cools somewhat.

Next, the refrigerant is forced through the flow control/expansion device, which restricts its flow and causes it to lose pressure as it passes through the device. The loss in pressure causes some of the refrigerant to change into a gas. In the process, the gaseous portion of the refrigerant extracts heat from the remaining liquid and thereby cools it.

The refrigerant is then passed through the evaporator on the microprocessor in the form of a warm liquid. As air passes over the evaporator, heat is extracted from the processor body and is passed to the cooler refrigerant. The remainder of the liquid refrigerant becomes a cool gas as it gathers heat from the evaporator and is drawn back to the compressor where the process begins again.

As the air passes over the evaporator and cools, moisture can condense around the processor in the form of condensate. To protect the processor and printed circuit board around it, special insulating foam pads must be mounted around the microprocessor socket. In addition, special heating elements are typically mounted on the backside of the system board under the microprocessor socket position and on top of the processor (as shown in Figure 3.19).

Figure 3.19

Figure 3.19 Condensation prevention.

The BIOS controls the refrigerant cooling system through its Health Management system. This includes monitoring the actual temperature of the microprocessor and manipulating the cooling system to maintain a designated temperature level. It also controls the temperature of the heating element under the printed circuit board.

This technology is not widely used in PCs. Although the military has been using this type of cooling system for more than five years, it is just beginning to be used with commercial PCs. Because the liquid refrigerants used in these systems are considered hazardous to the environment, you must be aware that only individuals licensed to handle refrigerants can legally work on these units.

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