fans and cooling devices

8/3/2019 FAns and Cooling Devices

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55FANS AND

COOLING DEVICES

When electrical power is applied to a circuit, the circuit uses that power to perform work.In the case of a PC, work would be the myriad of processing operations that go on through-out the computer every moment. For computers (as with all machines), the conversion of

power into work is not perfecta portion of power is dissipated in the form of heat. Over

time, the buildup of heat will cause a PC to fail prematurely. As a result, it is very impor-

tant that a computer system be properly outfitted to deal with heat.

You might wonder why heat is taken so seriouslyafter all, the vast majority of ICs and

passive components found in a PC dissipate very little heat. Unfortunately, the few com-

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CONTENTS AT A GLANCE

Understanding Cooling MethodologiesNatural convection

Heatsinks

Chassis fans

Fan cards

CPU fans

Liquid cooling (the heat pipe)

Piezoelectric coolers

Cooling ProblemsFan wear

Bad heatsink contact

CPU vibration failure

Sunlight

Thermal cycling

Excessive devices

Dust

AC power problems

Blocked vents

Excessive CPU voltage

Troubleshooting Cooling Problems

Further Study


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NATURAL CONVECTION

Natural (or static) convection is the cooling technique used in most computer monitors.Take a look at the rear enclosure on your monitor, and notice that open slots are along the

top and bottom of the enclosure. The openings beneath the monitor are for air intake and

the upper slots allow air to escape. These slots provide a free flow of air through the mon-

itor. Yet, this process works without the benefit of a fan. You can easily see how this

process works. Let the monitor run for a while, and place your hand over the upper slots

you can feel hot air rising. The warm circuitry inside the running monitor heats the sur-

rounding air, which rises up and out. As warm air is displaced, new cooler air is drawn in

from the bottom slots. Although monitor circuits will heat up (especially the power sup-

ply), none of the components become hot enough to require forced convection.

As you might expect, the success of this method depends on an unobstructed air path. If

Fluffy the cat curls up on top of the monitor, the exhaust vents will be blocked. This in-

terrupts the flow of air, and temperatures inside the monitor will increase. Eventually, you

might notice the display rolling or shifting positionthese are initial warnings that the

monitor circuits are overheating. If the blockage continues for an extended period, the

monitor might fail prematurely. Besides monitors, dot-matrix and ink jet printers typically

rely on convection to cool their circuits.

HEATSINKS

Another rule of heat transfer is that the effectiveness with which heat is transferred depends

on the amount of surface area that is exposed. Check out the radiator in your careach of

those small fins adds a small amount of surface area to the overall radiating surface. This

is the principle behind heatsinks (Fig. 55-1). By adding a heatsink to a heated compo-

nent, you increase the effective surface area which is open to the air. Because more air can

flow over a larger surface (through natural or forced convection), the component stayscooler. For components that become inordinately hot during normal operation (such as

regulator ICs or CPUs), a heatsink is a very simple and inexpensive way to enhance cool-

ing (Fig. 55-2). Of course, heatsinks are not just for ICs. Take a look at the print head of a

dot-matrix printer. Youll find a set of cast aluminum fins set right into the head assembly.

UNDERSTANDING COOLING METHODOLOGIES 1455


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FIGURE 55-1 IERC self-adhesive heatsinks. IERC (International

Electronics Research Corporation), Burbank, CA


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CHASSIS FANS

Although natural convection is fine for cooling monitors and printers, PCs are too crampedand obstructed to establish a consistent air flow. As a result, air must be forced through

the enclosure. This is usually accomplished with one or more fans positioned in the rear

chassis. By positioning the fans blowing out of the enclosure, cooler air can be vacuumed

into the system through strategically located intake slots in the housing. Some systems use

a second fan blowing into the enclosure from the front chassis. This kind of push-pull

cooling develops a very strong air flow.

FAN CARDS

Even with good-quality, chassis-mounted fans, some high-performance systems require an

extra measure of coolingespecially in the CPU area. An ongoing trend in PC cooling is

the use of a fan carda standard-sized expansion board with one or two +12-Vdc fans

mounted to it. This allows you to place the fan card in the immediate vicinity of a CPU ordrive to improve the local air flow. However, fan cards have some limitations. First, many

expansion boards are full-sized, expansion boards, so the chances are very good that at least

one side of the fan card will be somewhat obstructed by another full-slot expansion board.

As long as the fan card is blowing away from the adjacent expansion board, this should not

present a problem. If the fan card is blowing toward the adjacent expansion board, a region

of turbulent air will be produced that reduces the fan cards overall effectiveness.

Another concern is EMI produced by the fan motors. As inductive devices, fans are no-

torious for producing unwanted electromagnetic interference. If the fan card is placed in

close proximity of a sensitive device, such as a drive controller or video capture board, the

electrical noise produced by the fans can degrade the other devices performance, or cause

operating errors. There is also the possibility that electrical noise from the fans might

travel back along the +12-Vdc voltage line and interfere with other devices in the system

using +12 Vdc. It is always wise to approach fan cards with a certain amount of suspi-cionespecially if you have problems with the device once the fan card is installed.

CPU FANS

CPUs have always run hot and the reason is readily understandablea single microscopic

transistor dissipates virtually no heat, but the combined heat from over 5 million transis-

1456 FANS AND COOLING DEVICES

FIGURE 55-2 An IERC

CPU fan

and

heatsink.

IERC

(International

Electronics

Research

Corporation),

Burbank, CA


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tors crammed into a wafer the size of a fingernail becomes extreme. The surface temper-

ature of a Pentium processor can easily exceed 100C. With such a strong concentrationof heat, even forced air through a heatsink can leave a CPU running hot. To manage heat

in the latest and Pentium, Pentium MMX, and Pentium II processors, a CPU fan can be

used. Basically, a CPU fan mounts a small, high-speed fan, which blows down into a

heatsink assembly. The inrushing air cools the heatsink (and the CPU) very effectively.

Unfortunately, the CPU fan has some disadvantages. First, the added height of a

heatsink/fan combination can obstruct full-length expansion boards. This is typical of

poorly designed AT-style motherboards, although ATX and NLX motherboards position

the CPU well away from expansion slots. Similarly, if your particular motherboard places

the CPU under a low-hanging drive or other chassis obstruction, a CPU fan might not fit.

Before using a CPU fan, be sure that you have several cubic inches of available space over

the CPU. The CPU fan also requires power. Check that you have a power connector avail-

able from the power supply and be sure that the CPU fan assembly comes with a built-in

Y connector. Another possible problem involves vibration. Because the fan is now phys-ically coupled to the CPU, there is a bit of debate over what (if any) damage is done to the

CPU by fan vibrations over the long term.

LIQUID COOLING (THE HEAT PIPE)

CPU cooling can also be accomplished through conductive devices generally known as

heat pipes. A small number of liquid cooling devices are available for CPUsmost of

which are used in Pentium notebook systems that do not have the space for heatsinks or

CPU fans. The drawback to a liquid-cooled system is clear enougha breach in the cool-

ing loop can deposit liquid onto the motherboard and result in real damage. Extra expense

is another consideration.

The basic principles behind the heat pipe are well-known among refrigeration profes-

sionals, but are not all that intuitive to computer technicians. Basically, a vacuum-tight

tube is filled with a low boiling point fluid. The tube is run through a small heatsink fitted

over the CPU. The advantage here is that because heat does not have to dissipate to the air,

the heatsink can be quite thin (Figure 55-3).

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If you must use a Y splitter cable to power a CPU heatsink/fan, never split power from a

hard drive or another crucial drive. Split power from a floppy drive instead.

Keyboard PC board

Insulator

Evaporator

Condenser

Heat pipe

CPU

FIGURE 55-3 Basic diagram of a CPU heat pipe.


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This CPU heatsink is known as the evaporator because the fluid running through it is

evaporated (flashed to vapor) by the CPUs heat. Because the process of evaporation isa cooling process, heat is transferred from the CPU to the evaporated fluid, which then

travels through the tube, where it runs through a somewhat larger large metal plate, known

as the condenser. The condenser is usually located under the notebooks keyboard as-

sembly, where enough empty space is available for the heat to dissipate and allow the fluid

to return to its liquid state. Once the liquid returns to its liquid state, it is free to re-circu-

late back to the heatsink. Two of the most interesting elements of the heat pump are that

it requires absolutely no electricity and it has no moving parts. So long as the evaporator

remains in good contact with the CPU, the heat pump should continue operating indefi-

nitely. Circulation is driven by the natural phase changes of the liquid.

Although the heat pump is a relatively simple and reliable mechanism, it poses some

unique problems for small computer assemblies. If you upgrade or repair mobile comput-

ers at all, you will need to know the important issues for heat-pipe assembly and installation:

s Be careful of liquids Remember that you are basically dealing with a delicate liquid

vessel mounted in the bowels of your mobile computer. As a result, you must be ex-

tremely careful during disassembly and reassembly procedures. Crimping the tube at

any point will reduce the pipes effectiveness. Breaking the heat pipe can result in

chemicals being spilled into the main board.

s Be careful of dust and debris As you saw, the evaporated liquid sheds its heat in the

condenser, which is located under the keyboard. But just like convection heating in the

home, heat will have trouble leaving the condenser if it is insulatedcovered with

dust and debris accumulated from long periods of use. If you find yourself working on

a mobile PC using a heat pipe, be sure that the condenser is clean and free of dust. You

might choose to blow away any dust and debris with compressed air.

s Be careful of contact As far as the CPU is concerned, the heat pipes evaporator is just

another heatsink. Like any heatsink, there must be good physical contact between theCPU and the evaporator for proper cooling to occur. If the evaporator is left loose (or

otherwise mounted incorrectly), the heat pipe will be ineffective.

PIEZOELECTRIC COOLERS

Piezoelectric devices (calledPeltier coolers) mount a layer of piezoelectric material over

the CPU. As the crystal layer vibrates, a temperature differential develops, through which

it actually results in a cooler surface (applied against the CPU). The problem with piezo-

electric chillers is their expense, as well as the yet-unclear potential for CPU damage

from long-term exposure to vibration.

Cooling ProblemsCooling is often the most overlooked and neglected features of a PC. In many off-the-

shelf systems, cooling is sufficient. But over time, constant use, environmental factors,

and upgrades, the cooling plan might need to be reviewed or revised. As with so many



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other elements of PC service, successful troubleshooting means knowing where to look.

This part of the chapter shows you the factors to consider when evaluating PC cooling andcooling problems.

FAN WEAR

Fans do not last forever. They are electromechanical devices; eventually, the motor or ro-

tating shaft will wear out and fail. Normally, PC cooling fans are very quiet devicesthey

have to be because loud operation in a home or office environment would quickly become

maddening. The first sign of fan failure is excessive noise. A persistent buzz or grinding

sound immediately points to a fan problem. The fan motor might also be unusually hot.

In extreme cases, the fan will hang up and stop altogether (it might start again if you nudge

it gently). The best way to deal with a cranky fan is to replace it outright. A new fan must

have the same three major characteristics of the original fan: (1) physical mounting di-

mensions, (2) operating voltageusually +12 Vdc, and (3) air flow rate.

BAD HEATSINK CONTACT

For a heatsink to be effective, it must have a strong physical connection to the host IC.

This ensures that the maximum amount of heat is transferred from the IC to the

heatsink/fan. The better that contact is, the more heat is transferred into the heatsink, and

the cooler the IC runs. The heatsink/fan can be installed in a variety of ways, but clip-

ping the IC and heatsink together has been easiest and most popular. However, not all

heatsink clips are tight, and even a tight clip does not guarantee good contact. Check to

see that the heatsink is attached securely and be sure to add a layer of thermally conduc-

tive compound (typically a thick white cream) between the IC and heatsink. Thermal

grease fills in any air space between the IC and heatsink, so heat transfer is enhanced. Sus-

pect heatsink problems if the system locks up randomly for no apparent reason or if theCPU suffers chronic failures.

One of the more recent trends in heatsink marketing is the use of stick-on heatsinks

just peel off an adhesive backing and stick the heatsink in place. It sounds terrific in prin-

ciple, but adhesive is often more of a thermal insulator than a thermal conductor. In

general, go with the clip-on heatsinks wherever possible.

CPU VIBRATION FAILURE

CPU heatsink/fans are generally regarded as one of the most effective CPU cooling devices

available, but there is a certain amount of debate over the effect of long-term fan vibration

on the CPU. Some manufacturers argue that because the fan is physically attached to the

heatsink (and CPU), the fans vibrations will be carried directly into the CPU, which will

shorten the CPUs working life. However, no studies are available to prove or disprove

COOLING PROBLEMS 1459


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Thermal grease is toxic and can stain clothing. When using thermal grease, be sure to

work carefully and avoid getting it on hands and clothing.


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that possibility. As a result, you should rely on your own experience when checking or

recommending CPU heatsinks. Normally, it is reasonable to expect that the cooler CPUshould run longer and more reliably. So, if you find that the CPU fails frequently when fit-

ted with a CPU fan, try a large heatsink instead.

SUNLIGHT

Anyone who has ever been in the sunlight understands how warming it can be. This nat-

ural warming can be magnified through glass, so sunlight indoors can feel even warmer.

When a PC sits exposed to sunlight for prolonged periods, the metal enclosures tend to

pick up much of that heat (and heat the air inside). Although sunlight alone rarely provides

enough heating to endanger the system, it can intensify the cooling demands while the sys-

tem is operating. As a general rule, keep the PC and its peripherals out of direct sunlight

for extended periods of time.

THERMAL CYCLING

Turn it off or leave it on? This is the perennial PC question and one that continues to

be a hotbed of debate among technicians. The best way to answer this question is to ap-

proach it both theoretically and practically. Theoretically, each time material heats up,

it expands. When the material cools down again, it contracts. Thus, every time a PC is

turned on, the ICs, solder joints, and wiring expand until the system reaches a stable op-

erating temperature. When the PC is turned off, it gradually cools and its components

contract until the PC returns to room temperature. Over time, this accordion effect

of expansion and contraction (or thermal cycling) is known to cause material to fatigue

and fracturean IC breaks down or a solder joint becomes intermittent. As a conse-

quence of this effect, long-time PC veterans argue that the PC should be left on con-

stantly. This allows the system to achieve a stable temperature, so thermal cycling iseliminated.

From a practical standpoint, however, the view is a bit different. First, the damaging ef-

fect of thermal cycling depends on the amount of temperature difference. Frankly, todays

PCs just dont get that hot (although the CPUs can blaze if not properly cooled). Cooler

PCs are effected less by thermal damage. Drive wear is also another non-issue, with cur-

rent, cool-running designs exhibiting MTBFs of over 300,000 hours. The other consider-

ation is the rising cost of powerwhich is wasted by simply leaving the PC on overnight.

On the other hand, there is no reason to power down the PC each time you get up for a cof-

fee. Ultimately, the current thinking is to go ahead and turn the PC off overnight or when-

ever you must leave the system for more than a few hours.

EXCESSIVE DEVICESMany PCs are eventually upgraded with more RAM, more drives, new CPUs, etc.

Each new device added to the system contributes to its overall heat production. For

heavily expanded systems, it might be necessary to augment cooling with a supple-

mental exhaust fan or inlet fan. The general yardstick for judging the need for extra

cooling is to feel the air exhausting from the system. If the air feels comfortable or



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somewhat warm, chances are that cooling is adequate. If the air feels hot, its time to

add a new fan. True, this is a rather subjective means of measurement, but it is accu-rate enough for most situations.

DUST

Perhaps the most significant problem of reliable, long-term PC cooling is dust, which

is always present in everyday air. For the purposes of this book, dust includes other

contaminants, such as pet hair and cigarette smoke. Dust has two effects on the PC.

First, dust collects on the fan blades and intake vents. This interrupts and limits air

flow into and out of the system. Second, dust can collect on a printed circuit board,

where air flow is limited. The dust acts as a thermal blanket, which prevents normal

convective cooling. When upgrading or servicing a PC, vacuum any accumulations of

dust in or around the system.

AC POWER PROBLEMS

Power supplies can be serious sources of heatespecially in the regulator portion of the

supply, which is designed to maintain a stable voltage output as ac input levels and load

demands change. If ac climbs over its nominal value, the regulator must work harder to

maintain a constant outputthis results in excessive power-supply heating. Lagging ac

levels result in larger amounts of current being drawn to keep the power output steady

this also causes extra heating. Persistent power-supply failures or unusually hot operation

might suggest problems with ac (or an overloaded supply).

BLOCKED VENTS

Air needs a clear path into and out of the system, which is usually accommodated throughvent slots located around the enclosure. If the vent slots are obstructed or blocked, air flow

might be interrupted (this is especially detrimental to devices relying on natural convec-

tion for cooling). You can see the importance of proper ventilation by reviewing the in-

stallation guidelines for almost any piece of consumer electronics. Most guidelines

recommend that you leave several inches of free space on each side of the enclosure. Be

sure that vent slots are unobstructed, and clear of dust or debris.

EXCESSIVE CPU VOLTAGE

Not all CPUs use the same voltage. Intel, AMD, and Cyrix CPUs use slightly different

voltage levels for proper operation (usually all around +3.3 Vdc). If the CPU voltage

is tweaked a bit too high for the particular CPU, it will generate excessive heat. An-

other typical oversight comes with the use of Pentium MMX (or compatible) proces-

sors that use dual voltages (usually +2.8 Vdc and +3.3 Vdc) for reduced power and

lower heating. Again, if the CPU voltage is set too high, excessive heating will result,

which can shorten the CPUs working life. Such devices as Autotimes Processor

Protector (Fig. 55-4) are tools that you can use to quickly and efficiently verify the

CPU voltage(s).

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Troubleshooting Cooling ProblemsCooling problems manifest themselves in a variety of waysusually through intermittent

system operation and frequent failures. This part of the chapter is intended to illustrate

some of the more perplexing cooling problems that you should be aware of.

Symptom 55-1. The fan is producing an unusual amount of noise, but it

seems to be working properly This can often happen after replacing a fan and it is

typically the result of fan vibrations being introduced to the PC chassis. Although this is

rarely harmful to the system, it can become quite annoying. Check the way in which the

new fan is mounted and be sure that any damping material is in place. Otherwise, you

might try adding small standoffs of foam around each mounting screw to dampen the vi-

brations. Of course, if the fan is original, it might be wearing out and need to be replaced.

Try a new fan.

Symptom 55-2. The fan has stopped turning First, check to see if the fan is an

intermittent-type controller by a small internal thermostat. If so, it simply might havestopped normally. You should see it start and stop as required. However, most fans turn

continuously, so if the fan has stopped, it might have become disconnected or it might

have failed. Check the fans power connection. If the problem persists, try a new fan.

Symptom 55-3. The CPU freezes intermittently This is a classic sign of CPU

overheating. Although overheating will not necessarily destroy a CPU immediately, pro-


FIGURE 55-4 The Processor Protector CPU voltage

measurement tool from Autotime. Autotime


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longed or repeated overheating can precipitate a permanent failure. Check the heatsink or

CPU fan attached to the CPU. Warning: be sure to let the system cool before touching theheatsink. If the cooling device is loose, re-attach it securely (be sure to use thermal com-

pound). If the heatsink is secure, but overheating continues, try a more aggressive device,

such as a CPU fan.

Symptom 55-4. Frequent CPU failures occur Chronic CPU failures are rare oc-

currences and can often be traced to insufficient cooling. If the CPU does not have a

heatsink, try adding one. If a heatsink is already attached, try a larger heatsink or CPU fan.

However, if a CPU fan is already in use, there might be a vibration problem, which will

shorten the CPUs working life. Try downgrading to a regular heatsink or use an alter-

native cooling device, such as a Peltier cooler.

Symptom 55-5. Frequent drive failures occur This is typical of the hard drive

in an overloaded system. When replacing the hard drive, take careful note of the exhaustheat and the overall number of devices in the system. If the exhaust is unusually warm or

if many adjacent drives are in the system, try mounting the replacement drive by itself

away from other drivesmaybe in a rear drive bay. If possible, try mounting the drive

vertically. If it is impossible to relocate the offending drive, try adding a supplemental fan

or a fan card to improve air flow over the drive.

Further StudyThats all for Chapter 55. Be sure to review the glossary and chapter questions on the ac-

companying CD. If you have access to the Internet, take a look at some of these thermal-

control resources:

Autotime: http://www.autotime.com

PC Power and Cooling: http://www.pcpowercooling.com

FURTHER STUDY 1463


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