Electronics Protection - Summer 2016 - (Page 6)

Feature Common Coolant Types and Their Uses in Liquid Cooling Systems Rich Roser, Manager Laird Engineered Thermal Systems Using liquids for heat transfer is an important cooling method in many industries. Selection of the best heat transfer fluid for a cooling system involves consideration of performance, compatibility and maintenance factors. Water has excellent heat transfer properties, making it somewhat of a standard for comparison to other coolant fluids. Among heat transfer fluids, water has superior properties in many regards, with a high specific heat of around 4,200 J/kgK, low viscosity, and no flash point. On the other hand, it has a relatively narrow range of operation, as liquid temperatures leave plain water susceptible to freezing or boiling. Water Purity The quality of street (mains) water depends on its storage, delivery and ultimate source (ground versus surface water). It may contain corrosive impurities, such as chloride, alkaline carbonate salts or suspended solids. For cooling applications involving recirculating water flow, the system may be charged with already filtered or purified water. While some impurities are to be avoided due to potential corrosive effects, completely pure water is hungry for ions and is considered to be an aggressive solvent. Impure water is also an electrolytic bridge to enable galvanic corrosion if the system possesses dissimilar metals. Water as a coolant in a recirculating system is also susceptible to biological fouling. Algae, bacteria or fungi are likely to form depending on the system's exposure to light and heat and the availability of nutrients in the wetted components. The resultant slime or biofilm can inhibit heat transfer between the fluid and wetted surfaces. Sufficient concentration of the additive agent must be considered. For instance, glycol as an additive is commonly used as a control against biological growth, but concentrations less than 20 percent, the effectiveness is limited; in fact, below 1 percent, propylene glycol and ethylene glycol act as a bacterial nutrient. There are several complex and interrelated factors in the selection of different types of water and water/mixtures, as well some design imperatives that drive the need for other heat transfer media. Consider the comparison of propylene glycol (PG) against ethylene glycol (EG). Propylene glycol is much less toxic than ethylene glycol, resulting in easier handling and disposal than ethylene glycol. It also has a higher specific heat than ethylene glycol. However, its thermal conductivity is lower and its viscosity is higher than ethylene glycol, resulting in better overall performance for EG versus PG. In most cases, a mixture of the glycol and water is used with a lower concentration of glycol due to the superior performance of water over either glycol type. EG requires lower concentrations than PG for equivalent freezing point depression, boiling point elevation and burst temperature depression. Operating Temperature Compatibility The suitability of the fluid for operation over the range of operating temperatures is of the upmost importance. This must include consideration of the fluid's phase transitions (boiling and freezing), chemical breakdown of the fluid's chemistry, and the diminishment of the fluid's lubricating and heat transfer properties. Freezing of a fluid will diminish heat transfer at a surface, while boiling is dangerous for systems not designed to withstand overpressure in the fluid containment. Boiling liquid expanding vapor explosion (BLEVE) is a potentially dangerous phenomena that can occur should a sudden rupture of the containment, even if the designed temperature and pressure operational conditions should keep the fluid in the liquid state. Flash points for volatile fluids must also be noted. Most fluids can be assessed for temperature compatibility with readily available printed specifications, with other materials needed to determine situations involving different pressure or unusual operating environments. In cases where a particular combination of fluids is being tailored for use by the user, such as water/glycol combinations, little direct test work by the user is usually required, given the availability of data from manufacturers. 6 Summer 2016 * www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Summer 2016

Electronics Protection - Summer 2016
Contents
Editor's Choice
Common Coolant Types and Their Uses in Liquid Cooling Systems
The Benefits of Vertical Integration for Customized EMI Shielding Products and Services
Cool It Your Way: Versatile Cooling for Electronics Enclosure
Heat Pipes and Vapor Chambers – What’s the Difference?
Securing Electronics in Modern Railway Systems
Enclosures
Thermal
Power
Hardware
Industry News
Calendar of Events

Electronics Protection - Summer 2016