The use of heat pipes has been mainly limited to this field of science until recently, due to cost effectiveness and complex wick construction of heat pipes. There are several applications of heat pipes in this field like
● Spacecraft temperature equalization
● Component cooling, temperature control and radiator design in satellites.
● Other applications include moderator cooling, removal of heat from the reactor at emitter temperature and elimination of troublesome thermal gradients along the emitter and collector in spacecrafts.
Dehumidification and Air Conditioning
In an air conditioning system, the colder the air as it passes over the cooling coil (evaporator), the more the moisture is condensed out. The heat pipe is designed to have one section in the warm incoming stream and the other in the cold outgoing stream. By transferring heat from the warm return air to the cold supply air, the heat pipes create the double effect of pre-cooling the air before it goes to the evaporator and then re-heating it immediately.
Activated by temperature difference and therefore consuming no energy, the heat pipe, due to its pre-cooling effect, allows the evaporator coil to operate at a lower temperature, increasing the moisture removal capability of the air conditioning system by 50-100%. With lower relative humidity, indoor comfort can be achieved at higher thermostat settings, which results in net energy savings. Generally, for each 1Â° F rise in thermostat setting, there is a 7% savings in electricity cost. In addition, the pre-cooling effect of the heat pipe allows the use of a smaller compressor.
Heat pipe technology originally used for space applications has been applied it to laptop computer cooling. It is an ideal, cost effective solution. Its light weight (generally less than 40 grams), small, compact profile, and its passive operation, allow it to meet the demanding requirements of laptops.
For an 8 watt CPU with an environmental temperature no greater than 40Â°C it provides a 6.25Â°C/watt thermal resistance, allowing the processor to run at full speed under any environmental condition by keeping the case temperature at 90Â°C or less.
Figure 2: Heat Sink Inside a Laptop
One end of the heat pipe is attached to the processor with a thin, clip-on mounting plate. The other is attached to the heat sink, in this case, a specially designed keyboard RF shield. This approach uses existing parts to minimize weight and complexity. The heat pipe could also be attached to other physical components suitable as a heat sink to dissipate heat. (See photo of inside of laptop computer).
Because there are no moving parts, there is no maintenance and nothing to break. Some are concerned about the possibility of the fluid leaking from the heat pipe into the electronics. The amount of fluid in a heat pipe of this diameter is less than 1cc. In a properly designed heat pipe, the water is totally contained within the capillary wick structure and is at less than 1 atmosphere of pressure. If the integrity of the heat pipe vessel were ever compromised, air would leak into the heat pipe instead of the water leaking out. Then the fluid would slowly vaporize as it reaches its atmospheric boiling point. A heat pipeâ€™s MTTF is estimated to be over 100,000 hours of use.
Laptop Thermal Control
Heat pipes have proven to be the excepted means of providing thermal control in notebook and Mobil PCs systems. Heat pipes can move and dissipate CPU generate heat selectively throughout the system without affecting temperature sensitive components. Low wattage heat pipes (under 20 watts) have standardized input plates to the heat pipe. The connection to the heat exchanger via the heat pipe can have any number of configurations to accommodate component placement, multiple power ranges and fan options.
The heat pipe solutions for thermal control at this level is a component and overall systems requirement. Not only do the heat pipes take on a different configuration with multiple heat pipes and cooling fins, but also airflow becomes the critical design factor. Heat pipes designed to move 75 watts are usually flat with fin stacks from three to six inches, in many cases with fins mounted on each side of the CPU input pad. Input pads are standard using stand-offs, transition sockets, and bolster plates on the bottom of the PC board. The spring clips used on the fan/heat sink combination wonâ€™t work here. Airflow management is important in the overall efficiency of the heat pipe and should be calculated along with the intended heat pipe design.
Figure 3: Heat Pipe on a 500 MHz System
Thermal solutions are normally designed with multiple heat pipes, dedicated airflow and maximum input area. Fins stacks typically extend over both sides of the CPU. Input attachment to the CPU is with stand-offs, transition sockets or bolster plates.
The 500MHz operating systemÂ shown in Figure 3 uses two thermal products, heat pipes to transfer the CPU heat (100 to 300 watts) and a second internal or external cooling source. Input power is generated from multiple CPUs and components with single or multiple heat pipes. Cooling temperatures on the output range from -0Â° C to – 40Â° C. This system requires thermal isolation because of dewpoint considerations.
Heat pipes are manufactured in a multitude of sizes and shapes. Unusual application geometry can be easily accommodated by the heat pipeâ€™s versatility to be shaped as a heat transport device. If some range of motion is required, heat pipes can even be made of flexible material.
Two of the most common are:
Constant Temperature: The heat pipe maintains a constant temperature or temperature range.
Diode: The heat pipe will allow heat transfer in only one direction.
Figure 4: Flexible Style Heat Pipes
Flat heat pipes are typically used for cooling printed circuit boards or for heat leveling to produce an isothermal plane. Mega flats are several flat heat pipes sandwiched together.
Figure 5: Flat Style Heat Pipe Materials
Some of the flat heat pipes manufactured are:
XY Mega Flats: Surface maintained within .01Â° F isothermal with concentrated load centers. 6″ X 6″ Mega Flat: Dissipated 850 watts from a printed circuit board.
Weight Reduction Mega Flats:
Standard – aluminum construction.
Lightweight – Â½ the weight of aluminum.
Very light weight – 1/3 the weight of aluminum.
SEM C and SEM E Mega Flats in stock. Low and light weight coefficient of thermal expansion (CTE) Mega Flats – any CTE from 2 to 10. Alloy H: 70% more conductive than, or 40% less weight than copper clad invar.
Cost Effectiveness of Heat Pipes
The cost of heat pipes designed for laptop use is very competitive compared to other alternatives. Cost is partially offset and justified by improved system reliability and the increased life of cooler running electronics. Heat pipes, in quantity, cost a few dollars each while an entire cooling system will cost between $5 – $10 in production quantities, depending on the final design. Standard design products are available to reduce cost even further. Heat pipe manufacture has been a difficult area to compete in. Simple in concept, but difficult to apply commercially, the heat pipe is a very elusive technology & holds the key to the future of heat transferÂ and its allied applications.