Thermal energy transfer associated with a change of phase can be a valuable phenomenon for engineers designing systems for electronics cooling. Solid to liquid phase change materials were described in the May 2005 Technical Data column and the topic for this issue is liquid to vapor phase change. The energy required to change a substance from a liquid state to a vapor state is commonly termed the heat of vaporization. The energy required to change the substance from a liquid (or solid for sublimation) to vapor is directly related to overcoming the intermolecular bonding force in the liquid state. The energy released when a vapor is condensed to a liquid is numerically equal to the heat of vaporization but has an opposite sign and is commonly termed the heat of condensation. The most common applications of this thermodynamic property in the electronics cooling community are associated with boiling heat transfer. However, it is almost summertime in Texas as I am writing this article and I appreciate my body’s evaporative cooling system.
Utilizing phase change for temporary thermal energy storage has several benefits such as allowing heat rejection to occur over a longer time period, which in turn allows a smaller heat sink. However, compared to solid to liquid phase change, the volume change associated with liquid to vapor phase change is significantly larger, often greater than two orders of magnitude. Even though heats of vaporization are typically larger than heats of fusion for a particular material, the large volume change limits the ability to take advantage of this property for simple thermal energy storage. When one is not required to contain and reuse the vapor, such as in a single use application, expendable coolants that undergo a liquid-vapor phase change as they absorb heat are a consideration.
More commonly in electronics cooling, the use of heat of vaporization occurs in closed systems where heat removal is augmented by the vaporization property and then released elsewhere in the system when the vapor is condensed. Many examples exist for cooling hardware using this property including heat pipes, pumped refrigerant, and spray cooling. Frequently, the operating pressure of these systems is adjusted to provide an attractive boiling point for the coolant. Desirable qualities for a coolant that will operate in a two-phase mode include a high heat of vaporization, acceptable boiling temperature and pressure, low corrosion potential, low toxicity, environmentally friendly, and low cost. The heat of vaporization and melting temperature for several common substances at standard temperature and pressure are listed in Table 1. Note that these values are typically measured at the normal boiling point of the substance and corrected to 298 K, but the correction is typically small, often on the order of the uncertainty in the measurement.
Table 1. Heat of Vaporization at Standard Temperature and Pressure [1, 2]
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References
- Bolz, R., and Tuve, G., (Eds.), “CRC Handbook of Tables for Applied Engineering Science, Second Edition,” Boca Raton: CRC Press, 1973.
- Tuma, P., “Fluoroketone C2F5C(O)CF(CF3)2 as a Heat Transfer Fluid for Passive and Pumped 2-Phase Applications,” Proceedings, SEMI-THERM 24 Conference, March 2008, pp. 173-179.
