Features

“Cooling System Technology that leads clients' business to success”

High Cooling Performance/Low Noise Measures:

High Performance Liquid Cooling System

Toshiki Ogawara Tomoaki Ikeda

1. Introduction

The heating value of devices has tended to rise in re-

cent years, reflecting the increasingly high-speed and

high-performance nature of information equipment such

as personal computers. The heat density in devices has

also been rising with miniaturization and high density

mounting. Technical advances in microprocessor units

(MPU), which serve as the brain of computers, have been

especially remarkable, with continual gains in speed,

performance and capacity. Accompanying these gains,

the heating value of microprocessors has been rising rap-

idly. That in turn increases the need for a cooling device

with high cooling performance, to cool the microprocessor.

Another recent trend has been growing demand for in-

formation equipment to generate less noise, as an envi-

ronmental measure.

SANYO DENKI has been developing cooling devices

that respond to these demands for high cooling perform-

ance and reduced noise.

But we believe that the needs of our customers will

continue to rise in the future. And conventional cooling

devices using the air cooling system may ultimately be

unable keep pace.

Anticipating that situation, we have been developing a

cooling device that employs an alternative technology in

the form of a liquid cooling system.

This paper discusses the technical problems of the

conventional air cooling MPU cooler, and introduces the

new liquid cooling system developed by SANYO DENKI.

For more details, please refer to the article introducing

the new product: “High Performance Liquid Cooling

System SAN ACE MC Liquid” in this series.

2. Issues with the Conventional Approach

To respond to growing demand from customers for

better cooling performance, SANYO DENKI has intro-

duced cooling devices for microprocessors using the air

cooling system. This system consists of a cooling fan and

heat sink.

The structure of a typical air cooling MPU air condi-

tioner is explained as follows.

An air cooling MPU cooler consists of three parts: a fan

motor, a heat sink, and thermal interface material (TIM).

Heat from the MPU is conducted to the heat sink

through the TIM. Heat conducted from the MPU is dif-

fused throughout the heat sink because the heat sink

material has excellent heat conduction. Aluminum, coo-

per or a combination of the two are used for the main

currents of the heat sink, balancing performance and

manufacturing cost. In addition, a fin is used to generate

a heat radiation effect by enlarging the surface area of

the heat sink as much as possible. The fan motor is

added to force wind towards this fin.

Fig. 1 shows an example of an air cooling MPU cooler.

Fan Motor

Heat Sink

Fig.1 Example of an Air Cooling MPU Cooler

The air cooling MPU cooler generates greater heat

conduction from the heat sink into the air by using the

fan motor to force air into the heat sink.

Heat conduction from the heat sink into the air is

based on the following concept.

The concept is called convection heat transfer, in which

heat moves from a solid wall to fluid when the fluid flows

along the surface of the solid, as shown in Fig. 2. When

the fluid is forced along by a fan or other means, it is

called forced convection heat transfer.

The heat transfer value using forced convection heat

transfer can be expressed as shown below. This is called

Newton’s cooling law.

Ｑ＝ｈ・Ｓ・(Ｔ 1-Ｔ 2)＝ｈ・Ｓ・⊿Ｔ・・・・・(1)

Ｑ： Heat transfer value(Ｗ)

Ｓ： Surface area of the solid(Heat Sink)(ｍ

)

ｈ： Heat Transfer Coefficient(W/ｍ

・K)

T １： Surface temperature of the solid (℃)

T ２： Temperature of the fluid (℃)

⊿T ： Temperature difference (Ｔ 1-Ｔ 2) (K)

This shows the degree of ease of heat transfer. The

larger the transfer, the greater the heat transfer coeffi-

cient (W/ｍ

・K).

It is generally difficult to show the heat transfer coeffi-

cient because the value changes significantly, according

to the nature of the fluid, the state of fluid flow, and the

shape of the solid wall, but is proportional to the 0.8th

power of the velocity of the flow in the case of a turbulent

flow. When the flow of fluid is a laminar flow, the heat

transfer coefficient is in proportion to the 0.5th power of

the velocity of flow

Fig. 3 shows the range of a rough heat transfer coefficient.

3|SANYO DENKI Technical Report No18 Nov. 2004

Fig. 2 Movement of Heat by Convective Heat Transfer

Fig. 3 Outline Value of Heat Transfer Coefficient

(1) As the expression shows, the heat transfer

coefficient improves by increasing the surface area. The

heat transfer coefficient also grows, by speeding up the

flow velocity of air. This improves the heat transfer

value.

SANYO DENKI has been using a larger heat sink with

larger heat radiation area and a high air volume fan to

achieve high cooling performance as the calorific value

increases with improvements in the MPU.

This approach has raised new problems, such as

increasing size and mass. The heat sink manufacturing

technology has become more complex with improvements

in the surface area. And noise has increased as the

rotation speed of the fan motor has accelerated to gener-

ate high air volume.

Nonetheless, we continue to push back the limits of

cooling efficiency and noise reduction using air cooling

technology.

3. New Demands and Our Response

The Thermally Speed Controlled Fan, the rotation

speed of which is changeable depending on the tempera-

ture of the inner housing, is now being used for MPU

coolers, in response to customer demands for high cooling

performance and low noise. Fan rotation speed is sup-

pressed and noise is reduced at low temperatures when

there is still a margin in cooling performance. Fan rota-

tion speed is increased to provide cooling as the tem-

perature of the inner housing rises with device operation.

The MPU cooler, which combines high cooling perform-

ance and low noise, has been commercialized, employing

the Thermally Speed Controlled Fan.

We have recently introduced an air cooling MPU cooler

that has not only the thermally controlled function, but

also a function that carefully controls the rotation speed

of the fan based on the state of MPU operation. This

helps to reduce the noise generated by information

equipment such as personal computers. This function

lowers noise by suppressing the rotation speed of the fan

when the MPU load is small, which is namely when less

heat is generated by the MPU, even if the temperature

inside the housing is high.

Consequently, we now have a product that considers

noise when it is built into a customer’s device, achieving

a coexistence of high cooling performance and low noise.

As customers increasingly value silent information

equipment, there will be growing demand for cooling de-

vices to generate even less noise.

Responding to these demand, the high performance

liquid cooling system, SAN ACE MC Liquid uses the es-

tablished cooling technology of the liquid cooling system

to simultaneously achieve high cooling performance and

low noise.

4. New Solution: Liquid Cooling System

The liquid cooling system consists of a cold plate, a ra-

diator, a pump, a tube, and a fan motor.（See Fig. 4）

In the liquid cooling system, a cold plate absorbs the

heat from the MPU, and liquid flows on the cold plate the

deprive the cold plate of its heat. After removing the heat

from the cold plate, the liquid itself is warm, so it is sent

to the radiator, and then radiated from the radiator by

the air sent by the fan motor. The liquid is thus cooled,

and then again circulates around the cold plate once

again depriving the cold plate of its heat. An electric

pump is used to circulate the liquid.

The heat exchange using the cold plate and radiator is

based on Newton’s cooling law, explained in clause 2 of

this text.

The liquid cooling system is using liquid as a heat

transmission medium. Because the heat transfer

coefficient of liquid is very high (refer to Fig. 3) compared

with air, the liquid can form a cooling system with a

large heat transfer value and high cooling performance.

Although liquid cooling is an established technology, it

has now begun to be used for information equipment

such as personal computers.

Surface Area S[m

]

Q [

Solid（Heat Generation）

Fluid

Heat Transfer Coefficient：h

T2 < T1

(Temp:T1)

(Temp:T2)

Heat Transfer Coefficient [W/m

・K]

※ Flow Velocity of Air : 3～15m/s

Flow Velocity of Liquid : 0.3～1.5m/s

Evaporativ

Cooling

Forced

Convectio

Conditions

3 4

Natural

Convectio

10 10

10 10 10

Liquid (Water)

Liquid

(Water)

SANYO DENKI Technical Report No18 Nov. 2004|4