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A noiseless system for computer cooling


The HTD Laboratory has developed and successfully tested noiseless (fanless) cooling systems for the central processors of desktop personal computers. The systems were created on the basis of Loop Heat Pipes. Processors Intel P4with a clock rate of 2.8 GHz and Athlon XP 2500+with a clock rate of 1.83 GHz were cooled. The capacity dissipated by the processors mentioned above at the maximum load was about 70 W. The total mass of the cooling systems, including the LHP and the radiator, depending on the modification, was from 0.85 to 3.2 kg. The total thermal resistance of the system was in the range from 0.5-0.8 /W.
When the systems were tested as parts of computers by a test program providing the maximum loading of the processors, the temperature at their contact surface was in the range from 64-77 º at an ambient temperature of 26 º.
During simulation tests of the cooling systems temperatures of 82-95 º at the contact surface of a thermal simulator of the processor were achieved at capacities of 110-120 W.

External view of system blocks with a noiseless system of cooling of the central processor.


Copper-water miniature loop heat pipes

About 20 different variants of copper-water miniature loop heat pipes (MLHPs) with a cylindrical and a flat evaporator intended for cooling promising CPUs of portable computers were developed and tested since 2001.
The maximum capacity achieved by the best specimens by the devices was 160 W. The indicated value is not limiting. The limitation on the heat load was determined by the temperature of the evaporator wall , which should not exceed 100º. The minimum value of the total thermal resistance TeTamb/Q, equal to 0.46 K/W was demonstrated by an MLHP with a flat evaporator at a heat load of 140 W and an evaporator temperature of 88º. The maximum heat transfer coefficient in the evaporator equal to 61000 W/m2K and its minimum thermal resistance of 0.048 K/W have been achieved at a heat load of 160 W and an evaporator temperature of 98º.
The thermal characteristics of the devices are given in Fig.1 and Fig.2.

Fig.1. Heat-load dependence of the evaporator temperature


Fig.1. Heat-load dependence of the evaporator temperature

Fig.2. Heat-load dependence of the total thermal resistance


Fig.2. Heat-load dependence of the total thermal resistance

The table gives the main design characteristics of MLHPs:

Characteristic Cylindrical evaporator Flat evaporator
Evaporator diameter, mm 6 --
Evaporator thickness, mm -- 3,3
Evaporator active-zone length, mm 20 20
Evaporator interface area, cm2 4,0 3,6
Condenser length, mm 61 61
Vapor-line diameter, mm 2,5 3,0
Liquid-line diameter, mm 2,5 2,0
MLHP effective length, mm 290 290

Fig.3 and Fig.4 present the external view of copper-water miniature loop heat pipes with a flat and cylindrical evaporator.

Fig.3. Mini LHP with a flat evaporator


Fig.3. Mini LHP with a flat evaporator

Fig.4. Mini LHP with a cylindrical evaporator


Fig.4. Mini LHP with a cylindrical evaporator


Passive-and-active CPU cooling system for a desktop PC.

A cooling system for CPU of a desktop PC was developed and tested successfully in the laboratory conditions. The system was created on the base of a copper-water loop heat pipe (LHP) equipped with a flat-oval evaporator 7 mm in thickness and with lines for vapor and liquid 4 mm in diameter.
An aluminum radiator, the sizes of which correspond to the sidewall of the system block body midi-tower, was used as a heat sink (Fig. 5).



Fig. 5. CPU cooling system on the base of a copper-water loop heat pipe.

In the passive state at an ambient temperature of 22ºC the cooling system held a temperature of 70ºC on the thermocontact surface of the CPU thermal simulator at a heat load of about 90 W.
In the active state, when the radiator was blown by a low-noise fan, a temperature of 70ºC was reached at a heat load of 160 W.


Compact cooler for desktop PCs.

At present in the Laboratory of Heat-Transfer Devices a new compact cooler AC-1 for desktop PCs created on the base of a copper-water LHP is being tested. An axial-flow fan (92x92x25 mm), which has an flow rate of 40 CFM and creates a noise at the level of 25 dBA at 2200 RPM, is used in the cooler. The cooler sizes are 100x100x75 mm, its weight is 585 gram.
The cooler was tested with a CPU thermal simulator at an ambient temperature of 22º.
The results of tests are presented in the table.

Heat load, W Temperature on the LHP
thermal interface, º
Temperature on the simulator
thermocontact surface, º
50
100
150
200
250
36,7
40,4
47,1
54,5
62,8
37,4
41,8
49,1
57,1
66,2

The system total thermal resistance thermocontact surface of the CPU simulator - ambient air decreases from 0.3 up to 0.18º/W at a heat load increase from 50 up to 250 W. No signs of a heat-exchange crisis were observed at a maximum value of 250 W reached during the testing.

Comparative testing of a new cooler and a production run aluminum body cooler Intel/Sunyo Denki supplied with a fan with a number of rotations of 2700 RPM was conducted at the Ural computer engineering plant (Ekaterinburg).
The testing was carried out outside the body of the system unit in the combination of a motherboard ASUS P5LD2-VM/LGA 775 Socets with a two-core CPU processor Intel Pentium&reg4 640 (3,2GHz/2M) at an ambient temperature of 27,2º. A testing program S&M was used for testing.

Testing results are presented in the table.

Cooler Tcore1, º Tcore2, º Tmb, º Tev, º
Intel/Sunyo Denki
AC-1
56,7
51,0
47,3
47,5
38,3
34,4
-
50,0


Tcore1 is temperature of the 1st processor sensor
Tcore1 is temperature of the 2nd processor sensor
Tmb is temperature of the sensor on the motherboard
Tev is temperature on the thermal interface of the LHP evaporator.

It follows from the comparison of the indicated temperature values with the results, which were obtained earlier during the testing of a new cooler with a CPU thermal simulator, that the maximum power dissipated by a real processor makes up a value of about 130 W. Testing with a real processor demonstrated that the new cooler reduced the temperature of the first CPU core by 5,7º and the temperature of the motherboard by 3,9º in comparison with a run production cooler. The temperature of the second processor core remained approximately at the same level at this.

The comparative testing of the cooler AC 1a and a cooler Cooler Master (Taiwan) supplied with aluminum finning with the area of 3000 cm2, three heat pipes 6 mm in diameter each and 92 mm fan PL92S12M-5 was conducted. The sizes of Cooler Master with a casing are 145x125x100 mm, its weight is 804 g. The coolers testing was being conducted with a CPU heat simulator and with a fan Cooler Master at 1800 and 3200 RPM at an ambient temperature of 22ºC .
The results of the tests are represented in the table which shows the values of the temperature measured on the thermocontact surface of the CPU heat simulator with varied heat load changing within the range from 50 to 250 W.

Heat load, W 50 100 150 200 250
1800 RPM
Cooler Master
AC 1a
36,8
35,3
50,7
46,2
64,7
57,2
78,2
67,5
91,7
77,8
3200 RPM
Cooler Master
AC 1a
33,1
32,6
42,5
39,7
51,9
46,5
60,5
52,9
70,0
59,9

Within the range of heat load from 100 to 250 W at 3200 RPM the total thermal resistance of Cooler Master was changing from 0, 20 K/W to 0,19 K/W. For AC 1a these values were making up 0,19 K/W and 0,15 K/W accordingly.

External view of the cooler AC-1a

A new cooler AC-3/2 has been developed equipped with an aluminum heat sink with finning area of 0.24 m2. The cooler sizes are 150x120x85 mm, its weight is 766 gram. As coolers AC-1 and AC-2 it is created on the base of a copper-water LHP with flat-oval evaporator 7mm in thick.

The comparative tests have been conducted of the AC-3/2 with the cooler Ice Hammer 4400A that has four 8mm copper-water heat pipes. The sizes of I 4400 are 150x126x75 mm, its weight is 835 gram, aluminum finning area is about 0.9 m2. The coolers were tested with a CPU thermal simulator at an ambient temperature of 22º with the sizes of 35x35 mm and 120 mm fan Ice Hammer at 1900 RPM. The ambient temperature was 20ºC.

Tests results are presented below on graph of the heat load dependence on the thermocontact surface temperature the CPU thermal simulator.

Ammonia loop heat pipes with a nominal capacity of 130 W were developed to remove heat from CPUs installed in a computer server. The devices were equipped with cylindrical evaporators 8 mm in diameter, an active zone 40 mm in length, vapor and liquid lines 2.5 mm in diameter. The LHP effective length is about 500 mm.

A flexible loop heat pipe with an effective length of 2 m, supplied with a cylindrical evaporator with an active zone 60 mm in length, vapor and liquid lines 2 mm in diameter was developed. This LHP can change its length and bend at an angle of 180. The maximum capacity of the device is 100 W at a thermal resistance of 0.19 C/W.

Miniature loop heat pipes with a complicated configuration of a heat-transfer zone were developed to remove heat from low-sized high-heat objects. The devices can operate at different orientation in the range of heat loads from 5 to 120 W. The minimum thermal resistance equal to 0.18 C/W is achieved at heat loads from 60 to 120 W.

Miniature loop heat pipes


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