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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 P4 with 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 Ê/W.
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 table gives the main design characteristics of MLHPs:
Fig.3 and Fig.4 present the external view of copper-water miniature loop heat pipes with a flat and cylindrical evaporator. 
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.  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.
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 system total thermal resistance “thermocontact surface of the CPU simulator - ambient air” decreases from 0.3 up to 0.18ºÑ/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).
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 5,7ºÑ and the temperature of the motherboard – by 3,9ºÑ 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 .
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 IÍ 4400À 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 22ºÑ 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.
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