Hydrocarbon liquids will oxidize when exposed to continuous supplies of fresh air. Many will oxidize at temperatures considerably below the normal operating temperature of thermal fluid systems. Starting at the fluid's smoke point, the rate of oxidation approximately doubles with each 20°F rise in fluid temperature. And the more mixing of fluid and air, the faster and quicker the degradation.

Oxidation Products
Hydrocarbon liquids smoke as they oxidize - like overheated vegetable oil in a skillet on the kitchen stove. The same, normal products of combustion are produced, typically carbon dioxide, carbon monoxide and water vapor.

Organic acids begin to form in the liquid and concentrate as the oxidation continues. The fluid becomes more viscous. This reduces the film coefficient and increases the pressure drop. Higher pressure drop reduces the flow rate which, when combined with lower film coefficients, significantly increases the film temperature inside the heater and accelerates fluid degradation.

As it degrades -- usually at an accelerating rate - the fluid becomes less and less capable of efficiently carrying and transferring heat. Losing its ability to withstand high temperatures, the fluid becomes even more prone to degradation.

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Oxidation Symptoms
Your heat transfer fluid will begin to darken and smell pungent ("acidy" or "vinegary"), as acidic carbonaceous sludge is produced. Eventually, the sludge deposits on all surfaces in the system. Inside the heater these deposits harden and permanently reduce heat transfer. In the heat user these deposits can plug up lines.

Cleaning the System
This procedure will help remove the degraded fluid that contains acids and solid contaminants. The cleaner the system, the longer and more efficiently it will operate. And the fluid will last longer too.

1. Drain the existing fluid as completely as possible.
2. Remove solid matter from system. This may require chemical 
cleaning with oxidizing agents, circulating hydrocarbon solvents or manual scraping of surfaces.
3. Thoroughly flush with heat transfer fluid of the same type you will operate in the system.
4. Shortly after system startup, send a fluid sample to your thermal fluid supplier for analysis.

System Operation
To prevent oxidation, fluid in the expansion tank must be kept cool. If this cannot be done, consider "padding" the system with inert gas. Nitrogen is inexpensive and readily available. Run a line from your nitrogen source to the expansion tank's head space. The gas should flow from the source through an alarmed flow meter, regulator, check valve and into the expansion tank. Install a back-pressure control valve on the tank's vent line along with a relief valve. The back pressure valve will relieve system pressure during start-up. The relief valve should be properly sized to handle a sudden large pressure increase in the tank, such as water flashing to steam. It is not a good idea to use the relief valve to control the back pressure in the tank since the valve may not reseat properly. In addition to protecting the fluid from oxidation, the inert gas will prevent water from condensing in the fluid due to increased ambient temperature and dew point changes.

Technical Support
Contact us by phone, fax or e-mail -- or visit us on the web at www.paratherm.com. In the US and Canada you can call toll-free 800-222-3611. We'll spend the time necessary to insure your system gives you many hours of peak, trouble-free performance.

Questions? We'd like to hear from you. Call toll-free, 800-222-3611 -- or fax or e-mail.

Note: The information and recommendations in this literature are made in good faith and are believed to be correct as of the below date. You, the user or specifier, should independently determine the suitability and fitness of Paratherm heat transfer fluids for use in your specific application. We warrant that the fluids conform to the specifications in Paratherm literature. Because our assistance is furnished without charge, and because we have no control over the fluid's end use or the conditions under which it will be used, we make no other warranties-expressed or implied, including the warranties of merchantability or fitness for a particular use or purpose (recommendations in this bulletin are not intended nor should be construed as approval to infringe on any existing patent). The user's exclusive remedy, and Paratherm's sole liability is limited to refund of the purchase price or replacement of any product proven to be otherwise than as warranted. Paratherm Corporation will not be liable for incidental or consequential damages of any kind. Some product names of companies found in Paratherm literature are registered trademarks. This statement is in lieu of individually noting each.