Temperature control units (TCUs) — whether a water bath, refrigerated/heated bath circulator, laboratory circulator or recirculating chiller — are vital pieces of equipment for countless processes and tools. When it comes to selecting a new or replacement model, it is acceptable to focus on new displays, advanced control systems and reduced footprints because they all have their importance. But in order to optimize the system for its application (those processes and tools), it makes sense to better understand the life blood of any TCU installation — the fluid.

Physical and chemical properties of the fluid, including temperature range, specific heat, thermal capacity, viscosity, density, material compatibility, conductivity and pH, play a vital role in TCU and application performance and longevity. Using the correct fluid — maintained properly — will help prevent performance loss, corrosion, biological fouling and premature failure of the TCU and associated application.

When operating within the  41 to 203°F (5 to 95°C) range, water often is the fluid of choice. But, simply relying on the nearest supply of tap water is unlikely to be the best choice for the productivity and longevity of both your application and the TCU.

Distilled water, deionized water or filtered water that meets the TCU manufacturer’s specifications and is compatible with the application should be used. Alternatives such as water treated with additives (freeze-point suppressants, algaecides and inhibitors) and specialized fluids such as silicone oils and synthetic fluids also can be used. These all have their own pros and cons affecting how well suited they are to certain applications (table 1).

So, before you choose a TCU fluid, it is worth first understanding and defining the various properties common to the different fluids available (figure 1).

Pay Attention to Fluid Temperature Limits

The usable temperature range of a fluid is affected by its viscosity and, if it is nonflammable, the fire point or boiling point. Some fluid manufacturers list the temperature range of their fluids all the way down to the actual pour point. (At that point, it is likely too viscous to be used in a TCU.) Many also list temperatures close to the flashpoint or fire point even though directive EN 61010 (IEC 1010) states that the highest working temperature for a fluid in a TCU must be limited to 45°F (25°C) below the fire point of the fluid. Do not compromise on safety: Always check the fluid data sheet for flashpoint, fire point and autoignition temperatures.

Because some fluids absorb and reject heat easier than others, the energy required to change the temperature of a fluid, and its subsequent ability to transfer heat to or from the application, is primarily related to the specific heat values and thermal conductivity of the fluid. Fluids with higher specific heat and thermal conductivities will be better at moving heat away from or into an application.

When planning to use a temperature range beyond the capabilities of water, thermal expansion must be considered. For instance, oils will expand to a much greater extent than water over a similar temperature change. Bear this in mind to avoid TCU overflow.

Fluid Viscosity and Fluid Intermixing

A fluid’s viscosity and density influence its ability to be pumped, whether it is within a bath or out to the application and back. Fluids with a higher viscosity and/or a higher density create more pressure loss when pumped through tubing, resulting in lower flow. Fluids that have a higher viscosity at lower temperatures can also form an insulating boundary layer on the refrigeration coils in a bath that may reduce its cooling capacity.

Some fluids just do not mix with the materials they come into contact with. Reactions between fluids and the TCU or application can result in swelling of the elastomers, corrosion or even disintegration of the seals. The scale of wetted-material compatibility for a fluid often is given in a range such as from A (Excellent) to D (Severe Effect). While it is easy enough if the desired fluid has an A rating for all wetted materials in use, it is much more difficult to know what the long-term implications of a B or C rating would be. Short-term testing may give some indication, but it does not alleviate all concerns.

In conclusion, there is more to getting a new TCU than evaluating the available features. If chosen wisely, such a decision can help to guarantee your TCU has a long and thermal-stable life.