Using Thermosiphons in Electronics Cooling
Thermosiphons (aka thermosyphons) are devices that However, there are two major distinctions between these transport heat from point A to point B through liquid/vapor heat transport vehicles. Thermosiphons are gravity driven flow. They are sealed systems consisting of an evaporator, units and heat pipes are capillary driven. The connection condenser, connective piping and a working fluid that can between the condenser and the evaporator in a heat pipe transition to vapor phase. Figure 1 is a schematic of a typical is a wick system in an adiabatic area. In a thermosiphon thermosiphon used in three different positions. a simple pipe connects the condenser with the evaporator. Second, for a thermosiphon to start working, nucleate boil- The operation of a thermosiphon is rather straightforward. ing is required. Though a heat pipe may require a kick-start, The evaporator is attached to the heat source. Heat causes it does not have to be in the boiling phase to function. the liquid to boil and rise in the thermosiphon column. This vapor condenses and accumulates back in the evaporator. Thermosiphons are used in many markets, from coffee mak- Therefore, without the use of any external mechanical de- ers to solar heaters to electronics cooling. There are two vice or internal capillarary system, e.g., pump or wick, the primary designs for thermosiphons: sealed tube and loop liquid is steadily circulated between the evaporator and the types. In a sealed tube thermosiphon, the evaporator and condenser. the condenser are usually at opposite ends of a bundle of straight, individual thermosiphon tubes, and the exhaust and Thermosiphons are sometimes referred to as heat pipes. supply ducts are adjacent to each other (this arrangement is
Figure 1. A Typical Thermosiphon in Three Different Orientations.
Condenser Rising Tube
Evaporator Chamber
Falling Tube Heat Source
Figure 2. A Loop Thermosiphon with Condenser and Evaporator at Different Locations [1].
similar to a heat pipe system). Sealed tubes are illustrated in Disadvantages Figure 1. With loop type thermosiphons, the evaporator and • Adds to the first cost, and to the fan power, to overcome condenser coils are installed independently in the ducts and its resistance are interconnected by short lengths of working fluid piping • Requires placement so that the liquid condensate can (this configuration is somewhat similar to that of a coil loop return to the evaporator by gravity system). A loop thermosiphon is shown in Figure 2. • Uses two adjacent air streams in the sealed tube type • May need a significant temperature difference to initiate Of the two types, the loop thermosiphon is the most com- boiling monly used for electronics enclosures because of the diffi- • Requires that relatively clean air streams and may re- culty in vertically placing a sealed tube configuration in most quire filtration electronic chassis. In a loop type thermosiphon, the working fluid type, volume and filling process play important roles in overall perfor- In general, the advantages and disadvantages of thermosi- mance. A properly designed and fabricated thermosiphon phons can be summarized as follows: contains almost no non-condensables, such as air, and the Advantages fluid is equivalent to about 20% of the volume of the evapo- • Passive heat exchange with no moving parts rator. This is a rule-of-thumb number and must be specifi- • Relatively space efficient cally looked at for a given application. Figure 3 shows an • Cooling or heating equipment size can sometimes example of a loop thermosiphon system in a PC application be reduced [1]. The evaporator is placed on the microprocessor and the • The moisture removal capacity of existing cooling condenser is placed further away near a fan area of the PC equipment can be improved enclosure. • No cross-contamination between air streams
April 2008 |Qpedia FUTURE COOLING
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0.0 0 10 20 30 40 Angle, (o)
Figure 5. Thermosiphon Thermal Resistance as a Function of Inclination Angle [1].
Figure 3. Thermosiphon Application on a PC Microprocessor with Once the temperature overshoot takes place (potentially Condenser Next to the Chassis Fan [1]. from dry-out) the response is obviously different, as shown in Figures 4 and 5. However, this does not mean that all ther- The Effect of Orientation mosiphons will respond the same way. Those shown in Fig- Orientation impacts the performance of thermosiphons much ure 1 can be used in a horizontal configuration provided that more than it affects heat pipes. they are designed for such placement. But they will operate at a lesser effectiveness had they been used in a vertical As shown in Figure 4, this evaporator’s temperature goes very high and potentially dries out as its angle reaches 40o. Figure 5 shows the thermal resistance of the thermosiphon
140 Evaporator 120
100 C
80
60 Temperature, 40 Figure 6. A Stacked Boiling Structure Formed by Joining Multiple 20 Layers [1].
0 -80 -60 -40 -20 0 20 40 60 mode. Beitelmal et. al., provide several examples of such Angle (o) improvements[1]. Figure 6 shows one of these concepts.
Figure 4. Evaporator Temperature as a Function of a Thermosiphon’s Angle [1]. Heat Transfer Enhancement Since nucleate boiling plays a key role in the operation of a as a function of its orientation. thermosiphon, much effort has been put in place to improve boiling heat transfer. The latter has been a field of research Interestingly, the thermal resistance and the evaporator tem- for many years and there are numerous examples where dif- perature remain fairly steady for a large inclination angle. ferent enhancements have been deployed.
6 Transient Response applications, the power dissipation is high and the thermal A thermosiphon is a heat transport vehicle that requires nu- resistance between die and the evaporator is made small by cleate boiling to function. At the same time, its evaporator design. Storey performed excellent work in capturing, both is attached to the heat dissipating components. The boiling analytically and experimentally, the transient effects that oc- dynamics is a transient process and subject to many fluctua- cur in the evaporator [2]. Figures 7, 8 and 9 depict such tran- tions. Therefore, the transience that occurs in the evaporator sience for energy flux, pool height and vapor temperature as will have a direct impact on the die. In most thermosiphon a function of height for a thermosiphon.
0.248 25 0.246 0.244 20 ] 0.242 [m 0.240 ght i s 15 tt
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10 Po 0.234 5 0.232 0.230 0 0 1000 2000 3000 4000 5000 6000 7000 8000 0 1000 2000 3000 4000 5000 6000 7000 8000 Time [s] Time [s] Figure 8. Pool Height in the Condenser as a Function of Time [2]. Qin Qout
Figure 7. Input and Output Power to the Thermosiphon as a Function of Time [2].
April 2008 |Qpedia FUTURE COOLING
In summary, thermosiphons are attractive thermal transport 330 325 vehicles that can be very beneficial for transferring heat to ]
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por 295
Va 290 metals are being used in high temperature environments, 285 there is the potential for a high degree of fouling and of pos- 0 1000 2000 3000 4000 5000 6000 7000 8000 Time [s] sible galvanic effects. Furthermore, because thermosiphons are typically evacuated before filling, and degassed before Figure 9. An Evaporator’s Vapor Temperature as a Function of sealing, there is always a chance for leakage and loss of Time [2]. liquid from the system. If that occurs, this highly effective As shown in Figures 6 through 8, the dynamics that occur heat transport system will be reduced to a hollow, thin walled in a thermosiphon is time sensitive, and these fluctuations tube, incapable of transferring high heat fluxes. ■ may have an adverse effect on the reliability of the pack- age. Temperature cycles may impact electrical performance References: and physical reliability. As a result of higher temperatures, 1. Beitelmal, H. and Patel, D., Two-Phase Loop: Com- the device may operate at a lower frequency. Also, if the pact Thermosyphon, HPL-2002-6, Jan 2002. package is highly conductive, these fluctuations may cause 2. Storey, J., Modeling the Transient Response of a unplanned temperature cycling for the device and may ad- Thermosyphon, Ph.D. Dissertation, Brigham Young Uni- versely impact its expected life. versity, 2003.
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