Heat transfer fluids will give long and trouble-free service in properly designed and operated systems.

Well-designed and operated heat transfer fluid systems are among the safest, most reliable and most cost-effective heating designs possible. However, allowing the heat transfer fluid to become heavily contaminated or severely degraded can result in:

  • Reduced heat transfer rates.

  • Diminished fuel efficiency.

  • Flow blockages in small diameter or low velocity areas.

  • Extended startup times at low temperatures.

  • Fouling of heat transfer surfaces.

  • Overheating and damage -- or even complete failure -- of heater tubes.

Combinations of the following four mechanisms generally are responsible for causing fouling in transfer fluid systems.

Rust, Dirt and Pipe Scale. Rust and dirt are the most common sources of solids in heat transfer fluid systems. Ordinarily, these infiltrate the system during construction or maintenance.

Oxidation. By far, the most common source of contamination is oxidation of the heat transfer fluid. Small amounts of oxidation are rarely a problem; however, excessive oxidation can create solids and high viscosity compounds that impair system effectiveness.

Thermal Degradation. The thermal degradation rate of any organic thermal liquid is a function of fluid chemistry, system operating temperature and time. Products of degradation can include higher molecular weight compounds and even solids (coke). Carbon formation and buildup can occur when metal surfaces reach excessive temperatures.

Process Contamination. Though less common than oxidation or thermal degradation, process contamination can be a major problem in heat transfer fluid systems. Solids, sludges, decomposition products and reaction products are possible.

Many factors should be considered when developing a cleaning procedure for your heat transfer fluid system, and these general procedures are not intended to cover all situations. Consult with your fluid supplier when developing a cleaning program for your specific situation.


If your system has problems with solids, consider these cleaning techniques. Small diameter particles suspended in heat transfer fluid can be effectively removed via filtration. Solids adhered to surfaces or deposited in low velocity areas cannot be removed until they are presented to the filter.

Glass-fiber-string-wound cartridges commonly are used and are highly effective for this application. Proper filter sizing will consider the solids concentration and required flow rates. Filter housing construction must be adequate for expected temperatures and pressures. For high concentrations of solids, bag filters or other high surface area designs can be employed. Filters with 100 micron or less nominal particle removal ratings should be considered for initial system treatment. Continuous filtration through 10 micron rated filters will maintain system cleanliness.

Sludges, High Viscosity Fluid or Residues

If your system has problems with sludges, high viscosity fluid or residues, consider these cleaning techniques. Some heat transfer fluid manufacturers have developed cleaning fluids that are effective in flushing sludges, fluid residues, degradation products and removing other deposits prior to installation of new heat transfer fluid. The general steps for using these cleaning fluids is outlined below, but your flush fluid supplier will outline the specific steps for your application. Particularly, the maximum recommended temperature for flush fluids may vary depending on fluid chemistry. Consult your flush fluid supplier for specific limitations.

Drain the System. Bring the fluid temperature to 200oF (93oC) and shut down the heater. Continue operating the circulating pumps as long as possible to keep loose solids and sludge in suspension. Drain the system through all low-point drains. Caution must be taken to avoid contact with hot fluid and piping. In areas where gravity draining is not sufficient or possible (e.g., heater coils), compressed nitrogen may be effective in blowing additional fluid from the system. It is important to remove as much of the degraded heat transfer fluid as possible to maximize the effectiveness of the flush fluid in cleaning interior system surfaces.

Fluid removed from the system must be stored, handled and disposed of in accordance with applicable regulations. Consult the MSDS and your environmental, safety and health professionals' guidance. In many cases, used heat transfer fluids can be returned to the heat transfer fluid supplier.

If not already present, install a fine mesh strainer in the system return line to the main circulating pumps.

Flush the System. Fill the system with flush fluid to a proper operating level, including the expansion tank where solids tend to deposit. Filling from low points is suggested. Circulate the entire system at ambient conditions to thoroughly mix the cleaning fluid with any residual heat transfer fluid. Check the return line strainer periodically for plugging from solids that may have been dispersed from fouled areas of the system. For larger systems, less cleaning fluid may be used by cleaning system subsections one at a time.

Gradually heat the circulating flush fluid to about 225oF (107oC) and, using your standard operating procedures, vent any moisture that may have entered the system. When all the moisture has been vented, raise the circulating fluid to the recommended flushing temperature to maximize the solvent characteristics of the flush fluid. Confirm with your fluid supplier the temperature limitations of the flush fluid selected. Maintain full circulation of the entire system for 16 to 24 hr at the flushing temperature.

Remove Used Flush Fluid. Cool the fluid to 200oF or lower, per manufacturer's directions, and repeat the draining procedure. Again, caution must be taken to avoid contact with hot fluid and piping. Remove as much of the cleaning fluid and used fluid mixture as possible. Cleaning fluid removed from the system must be stored, handled and disposed of in accordance with applicable regulations. Consult the MSDS and your environmental, safety and health professionals' guidance.

Inspect After Cleaning. Once the system has been drained completely, visually inspect the system in the areas of low fluid velocity to check for solids that may have fallen out of suspension. Remove any solids discovered.

Start Up Heat Transfer Fluid System After Cleaning. Install new side-stream filter cartridges and place the filter in operation. If the system does not have a side-stream filter to continuously remove solids during normal operation, installation of one should be considered prior to installation of new heat transfer fluid. Refill the system with new heat transfer fluid and start up following proper startup procedures. Care should be taken to vent any moisture that may have entered the system during flushing, draining or refilling. Check with your fluid manufacturer to determine whether small amounts of flush fluid will have an adverse effect on the operation or service life of the new fluid.

Chemical Cleaning of Heat Transfer Fluid Systems

Chemical cleaning can be an alternative to cleaning fluids. Chemical cleaning generally is more costly and requires significantly more time to return a system to operation, and several steps are necessary to ensure effectiveness. System owners typically are responsible for waste disposal. Multiple flushings after cleaning will generate significantly more waste.

If chemical cleaning techniques are to be employed, consult your heat transfer fluid supplier and a chemical cleaning supplier about detailed plans and procedures.

A general outline of the process follows:

  • Drain the heat transfer fluid from the system.

  • Solvent flush circulation.

  • Drain solvent flush.

  • Acidic solution circulation.

  • Flush with water.

  • Caustic and detergent solution circulation.

  • Flush with water.

  • Dry thoroughly.

The chemical cleaning supplier will outline the specific routine that will be used on your heat transfer system.

Process Contamination

If your system has problems with process contamination, consider these cleaning techniques. Removal of process contamination from heat transfer fluid systems must be treated on a case-by-case basis. Degradation products of the process contaminant as well as the possibility for reaction products with the heat transfer fluid must considered. Consult your heat transfer fluid supplier in the event of a process contamination.

Hard Coke

If your system has problems with hard coke, consider these cleaning techniques. When the system has been severely fouled by hard coke deposits, removing the highly inert carbon layer generally requires the use of mechanical cleaning techniques such as sand or bead blasting, wire brushing or high-pressure water jetting.

To maximize the effectiveness of mechanical cleaning techniques, consider using the flush fluid procedure first. This will remove loose solids and sludges. Severely overheating a heat transfer fluid can result in hard coking of heating surfaces. High temperature steam-air decoking of fired heaters has been reported to be effective when properly controlled.