Heat transfer fluid: Properties, limitations, and more

What makes an optimally functioning temperature calibration bath incapable of reaching temperatures within its operating range? The answer lies in the chosen heat transfer fluid’s temperature-dependent properties.

High temperature limitations of heat transfer fluids

The following heat transfer fluid properties may limit the upper temperature range of a temperature calibration bath:

• Evaporation rate
• Boiling point
• Flash point

Flash point

At a certain upper temperature, a fluid produces enough vapor to be ignited; this is known as the flash point. Accumulated vapor is ignited, creating a vapor flash, and ends once the vapor is gone. The flash point introduces safety concerns that limit the upper end of the temperature range.

Evaporation rate

The vapor pressure of a fluid is an indication of a liquid’s evaporation rate. Heat transfer fluids with high vapor pressure evaporate quickly. For example, ethyl alcohol evaporates more quickly than water at the same temperature. At 25 °C, ethyl alcohol has a vapor pressure of 0.08 atm while water has a vapor pressure of 0.03 atm. Rapid evaporation is costly because fluid must be replaced. Rapid evaporation also causes heat loss that impacts temperature calibration bath performance. Due to this limitation silicone oils are commonly considered as they can cover a wide range of temperatures. 
Boiling point

Vapor pressure increases as temperatures rise. When it’s equal to the surrounding environmental pressure (usually atmospheric), fluid boils. Higher altitudes have lower atmospheric pressures, which means a lower boiling point. When a fluid is boiling, its temperature is unable to rise any higher until it completely becomes vapor.

Low temperature limitations of heat transfer fluids

Fluid properties that could limit the lower temperature range are:

• Fluid viscosity
• Freezing point

Fluid viscosity

Fluid viscosity is a measure of a fluid’s resistance to flow. We usually think of it as the thickness of the fluid. When the kinematic viscosity of a heat transfer fluid is higher than 10 centistokes, it strains the calibration bath’s stirring and pumping mechanisms. In addition, the fluid won’t transfer heat uniformly which can express as instability in the bath’s performance. Viscosity changes with fluid types and temperature.

Freezing point

Usually, the viscosity of a heat transfer fluid will become a problem long before the fluid freezes (for example, ethylene glycol). This is more commonly noted as pour point for oils, the lowest temperature a fluid will flow or pour when cooled.. But there are exceptions. For example, water will freeze at 0 °C, but its viscosity won’t become a problem until slush starts to form.

One heat transfer fluid may not cover your bath’s entire range

It seems reasonable to assume that just because a calibration bath can be used over its entire operating range the fluid can, too.

However, a clearer picture forms when we consider the usable temperature range of the heat transfer fluid in the bath. Flashpoints, viscosity, and evaporation may hinder our plans to cover the entire operating range of the bath with a single fluid. Although a fluid with the desired properties may not yet exist, at least we have an explanation for the behavior of calibration baths.