How do I choose a heat transfer fluid?

When choosing a heat transfer fluid, there are many things to consider to ensure optimum performance. There are four main categories of heat transfer fluid:

  • Water (including deionised water)
  • Glycol/water combinations (ethylene glycol/water or propylene glycol combination)
  • Oils
  • PFPEs

Water is one of the most efficient and highest quality products available for heat transfer by theory, but freezes and boils, providing a limited temperature range within which water alone can be used as a heat transfer medium. Glycol and water combinations are able to offer very high stability at higher temperatures and often require a lower start up temperature than oils. Oils are often non-toxic, efficient, and cost-effective, performing well at moderate to high temperatures. The benefits and drawbacks of each of these heat transfer fluids will be discussed in a later section of this article.

PFPE, or Perfluoropolyether, is a type of fluorinated fluid that can operate at both temperature extremes, with typical operating temperatures ranging from -70ºC to 290ºC. PFPE is able to offer high thermal stability, has good dielectric properties and is chemically inert, compatible with metals, plastics and elastomers. However, PFPEs are expensive and have seeking properties.  

To select an efficient heat transfer fluid, you must first know the minimum and maximum operating temperatures of your equipment. Fluid with a lower operating temperature will offer protection from premature degradation of your equipment when turning on. It is important to consider the thermal stability, heat transfer efficiency, and expansion rate of heat transfer fluids, and ensure that the fluid will meet the requirements of both the process and chiller.

Several questions must be asked regarding the operating conditions of the heat transfer fluid to ensure maximum compatibility.

  • Is the fluid able to operate efficiently throughout the entire process cycle, transitioning between temperatures quickly?
  • What is the local climate?
  • Will the chiller be located outside and so open to the atmosphere?
  • Is this a continuous process?
  • Does this process require heat transfer fluids with a food-grade rating?

To ensure that phase separation does not occur, it is essential to consider fluid compatibility when replacing existing heat transfer fluids. As it is incredibly challenging to completely drain fluid from a system, any remnants of expired fluid that do not effectively mix with replacement fluid could cause pump cavitation and unnecessary wear and tear on other parts throughout the system. It is also important to ensure that there is no contamination of heat transfer fluids during maintenance, as contaminants are incompatible with most heat transfer fluids and will immediately degrade their efficiency. When considering compatibility, it is also essential to ensure that the heat transfer fluid is compatible with the construction materials of your equipment to prevent leeching of particles into the fluid.

There are four crucial factors that should be compared when choosing a heat transfer fluid:

  • Heat transfer efficiency
  • Temperature range
  • Material compatibility 
  • Thermal stability 

The heat transfer efficiency of a heat transfer fluid is determined by several characteristics. An ideal heat transfer fluid will have:

  • Low viscosity 
  • High density 
  • High thermal conductivity 
  • High specific heat capacity 

With the right balance of these characteristics, a heat transfer fluid will provide better heat transfer efficiency at a range of operating temperatures and flow conditions.

Any impurities should be removed from the heat transfer fluid to prevent fluid degradation and fouling on surfaces within the system. This extends the life of heat transfer fluids and reduces the amount of maintenance required.

When comparing the thermal stability of heat transfer fluids, it is important to remember that a product with higher thermal stability will provide higher heat transfer efficiency for a longer period and poses a lower risk of system damage or safety hazards when used in accordance with instructions.

Consider how frequently heat transfer fluids need to be replaced and whether a specialist waste removal service will be required to safely dispose of expired fluids. Ensure that the cost of this has been factored into the budget. It is also worth noting that although selecting a heat transfer fluid with an extended temperature range will provide a safety cushion, it can come at an enhanced cost, so discuss specific system requirements with the supplier.

Take time to discuss the requirements of your chiller with the manufacturer and take note of their heat transfer fluid recommendations. Working with a heat transfer fluid supplier who can provide technical support through the lifecycle of the product is beneficial, as fluid degradation will naturally occur.

In summary, choosing the right heat transfer fluid is essential in protecting against accelerated degradation, machine failure, increased maintenance, and decreased efficiency. Ensuring that fluids work in synchronisation with the specifications of the chiller and that regular preventative maintenance is undertaken will protect both the chiller and the fluid from damage. Expert advice should be sought from the heat transfer fluid supplier or chiller manufacturer to ensure that the right fluid is selected, and a proper maintenance plan formed. This will ensure that the operation runs with increased safety, efficiency, and reliability, with decreased downtime.

Applied Thermal Control Ltd.
39 Hayhill Industrial Estate, Barrow upon Soar, Leicestershire, LE12 8LD.
Telephone: +44 (0) 1530 83 99 98
E-Mail: sales@app-therm.com
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