Designing the block: requirements

As with every design, you have to gather a set of requirements before you can start designing. The KiloWatt waterjacket is designed according to this set of requirements:

• Plain water as the coolant.
  Water is cheap, has a very high intrinsic energy (4.18kJ kg^-1 K^-1), a low viscosity, it is nontoxic, nonflammable and it is not agressive.
  
  
• Optimized for a coolant flowrate of approximately 400 liters per hour.
  Magnetic drive water pumps with a pumping rate of about 600-1200 liters per hour are cheap, easy to obtain, small, silent and robust. When these pumps are used in a closed circuit liquid cooling system, the total flow rate of the coolant drops to 300-500 liters per hour due to flow resistance in the radiator, hoses and waterjacket.
  
  
• Optimized for a power dissipation of at least 400 Watts into the waterjacket.
  A TEC (Thermo-electric cooler, also known as a Peltier element) with enough heat pumping capacity to be useful on today's CPU's will spit out 250-500 Watts of heat.
  
  
• Optimized for a power density of about 30 Watts per square centimeter.
  The type of heat load that will be put onto the waterjacket makes quite a difference for the design. The two types of heat load that are applicable are:
  (1) A small heat producing surface with a high power density. The core of a processor like the Intel Pentium III FC-PGA or AMD Athlon are examples of this type of heat load. Much heat is produced over a small area. For the design, this means that a heat spreader is required to distribute the heat over the waterjacket.
  (2) A larger heat producting surface with a lower power density. The hot side of a TEC or a capped processor like the Intel Pentium IV are examples of this type of load. Since the heat is already distributed over a larger area, less heat spreading is required. This means that the thinkness of the interface between the heat producing area and the coolant can be reduced. This results in a lower thermal resistance between the heat producing area and the coolant.
  Since the KiloWatt waterjacket will be used together with a TEC, the waterjacket needs to be optimized for a type 2 heat load.
  
  
• Large coolant <-> waterjacket contact area.
  A large coolant <-> waterjacket contact area will help decrease the thermal resistance of the waterjacket at the cost of a higher coolant flow resistance.
  
  
• High coolant speed in the waterjacket.
  Water tends to stick onto a metal surface. This is not desirable since the thermal conductivity of water is very low. A high coolant speed will decrease the thickness of the film sticking to the metal walls. Finally this decreases thermal resistance. The cost of a high coolant speed is an increase of the coolant flow resistance.
  
  
• Turbulent flow of the coolant.
  A turbulent flow of the coolant is better in removing heat from metal surfaces than a non-turbulent flow. Although almost every design provides enough turbulence, adding some extra doesn't harm. The cost of a turbulent coolant flow is, again, an increase of the coolant flow resistance.
  
  
• Small size.
  The waterjacket must fit on today's Socket-370 and Socket-A mainboards. This limits the size to approximately 50x60mm. The maximal height is less critical, and depends on the case used.
  
  
• Easy to build with a minimum of required tools
  The design of the waterjacket should not require specialist's tools or materials, so more people are able to build the waterjacket.
  
  

Designing the block: construction tradeoffs

Choice of the construction material

Since at least the base plate (the part of the waterjacket that removes the heat from the heat source) of the waterjacket needs to be thermally conductive, it makes sense to construct the entire waterjacket from a thermally well conducting metal. In the tables below the properties of some metals and alliages are shown.

Properties of some metals

Metal name	Thermal conductivity (W m-1 K-1)	Density (kg m-3)	Solderable
Pure aluminium	237	2.70	No
Pure copper	390	8.96	Yes
Pure silver	429	10.5	Yes
Pure gold	318	19.3	Yes
Pure iron	80.4	7.87	Yes

Properties of some alliages

Alliage name	Thermal conductivity (W m-1 K-1)	Density (kg m-3)	Solderable
'Construction' aluminium	~150	~2.70	No
E-Cu57 copper (99.9% Cu)	350	8.96	Yes
CuSil	~500	-	-
Yellow copper	120	8.5	Yes
Steel (99% Fe, 0.8% C)	50	7.8	Yes

The best candidate metals for the waterjacket are aluminium, copper, silver and CuSil. While silver and CuSil both provide a high thermal conductivity, they are also very expensive. Copper and aluminium are also quite suitable, and a lot cheaper. Copper has the advantage of a higher thermal conductivity, while aluminium is lighter and a bit cheaper than copper. But, it is almost impossible to get pieces of the pure metals; usually what you get is an alliage which improves certain properties of the metal. And that is seldom thermal conductivity. Look at the thermal conductivity values for common alliages like E-Cu57 and 'construction' aluminium. A widely available copper alliage outperforms a widely available aluminium alliage three times! (please note that this might not be true for all aluminium and copper alliages)

Copper wins the battle.


Construction method

When constructing a waterjacket, you can build it up from pieces of sheet metal, or start with a thick block of metal, and cut out the desired shape. The sheet metal method is easier to work with and allows for more complex shapes and experimentation. This is how the KiloWatt waterjacket will be constructed.


The design

The following design is able to fullfill all the requirements:

Design of the KiloWatt waterjacket.

The baseplate of the waterjacket is a piece of 60mm long, 50mm wide, 5mm thick sheet copper. A 5mm thick baseplate is a good compromise between strength, the weight, the extra thermal resistance of the baseplate and the heat spreading. When 400W of heat is dumped into the waterjacket, a temperature difference of about 4 Kelvin between the two sides of the baseplate is generated. A thinner baseplate would generate less temperature difference, but it would also be weaker and unable to efficiently distribute the heat. A thicker baseplate would generate a too large temperature difference between the two sides of the baseplate.
In the baseplate many 20mm long, 3mm thick copper pins are mounted. These provide turbulence and extra contact area between the coolant and the metal. The thickness of the pins is an arbitrary choice. A square spiral of 1mm thick copper sheet narrows the channel to increase coolant speed. The sides of the waterjacket are made of 3mm thick sheet copper. This prevents bending of the sides when the waterjacket is clamped firmly onto the CPU. The top of the waterjacket is made out of a piece of sheet copper and a piece of 12mm thick copper pipe.

The coolant enters the waterjacket through the upper pipe, so it directly cools the hotspot in the middle of the jacket. From there the coolant travels down the spiral until it exits the waterjacket through the copper pipe displayed left.
All the separate parts of the waterjacket, except the top, are soldered together with copper hardsolder or silversolder. Both types of solder have a melting point of around 710 degrees Centigrade, and both are very good in conducting heat. They also provide a very strong bond.
When everything is mounted, hardsoldered and cleaned the top can be soldered onto the waterjacket with standard 40/60 SnPb solder. This allows for easy removal of the top so the waterjacket can be inspected and cleaned after a few years of duty.