Safe sampling of high temperature heat transfer fluids

Dr Chris I Wright explains how to gain a representative sample of the heat transfer fluid.

Chris Wright is a research scientist, graduating from the University of Leeds in the UK with a BSc and PhD. His research focuses on the use and maintenance of heat transfer fluids in manufacturing and processing, which includes food, pharmaceutical, specialist chemicals and solar sectors.

There are around 4,000 companies operating high temperature heat transfer fluid (HTF) systems in the UK and Ireland (examples of systems are detailed in the Table). A HTF is used as a heat carrier and pumped around a closed system to ensure that the object being heated reaches thermal stability, which is vital in the process of generating homogenous end products, from processed foods to thermoplastic polymer resin used to make fibres for clothing, and containers for liquids and foods. HTFs need to be handled with extreme care, but it is essential that they are sampled on a regular basis as they thermally degrade with prolonged operation at high temperature. Few companies understand that regularly servicing their HTFs and HTF systems can sustain their longevity.

It is a fact that HTFs and O-Rings will degrade over time. The objective of any buying decision, however, is to avoid their accelerated ageing, which can be monitored through a well-designed preventative maintenance plan. When selecting a new O-Ring it is advisable to work with reputable suppliers and manufacturers as they should be able to provide technical data for every O-ring and more specifically their elastomer properties and characteristics. Once a suitable O-ring has been identified, it is recommended that before buying an O-Ring, the buyer should consult a reputable supplier / manufacturer regarding its application and to confirm its technical specifications.

Gaining a representative HTF sample:

1.    Take a closed, hot sample whilst the HTF is circulating in the thermal system (i.e. live) – this is industry best practice and means a representative HTF sample is collected whilst the thermal system is in operation.

2.    A sampling device (with a volume of 500 ml) is used to ensure a live HTF sample is taken as safely as possible without compromising on the accuracy of the HTF being sampled.

3.    The sampling device is closed to air used to gain an accurate assessment of flash temperature, which gives false readings if air is allowed to contaminate the sample.

4.    The sampling device is connected to the system using a Hanson couple.

5.    The sampling line is flushed before HTF collection begins.

6.    The sample is then collected in the sampling device and this is placed in a sampling bucket, thus avoiding contact with the HTF and ensuring any excess HTF is not spilled on the ground.

7.    Once the sampling device is full, it is allowed to cool to ambient temperature.

8.    The HTF is only analysed once it has cooled to avoid air contamination and that all samples are analysed at ambient temperature in a designed laboratory.

9.    Fluid testing enables the assessment of by-products that form during the thermal degradation of the HTF.

10.    It is also useful to take more than one sample to ensure that measurements are reproducible.

In summary, it is important to gain a true representative sample of the HTF whilst it is hot and circulating in a plant. The current article highlights the key steps to sampling a HTF, but it is important to seek the expert advice from a reputable supplier and to consider using a well-designed maintenance plan, such as the Thermocare programme offered by Global Heat Transfer Ltd, to help sustain the life of the HTF.