Common Heat Transfer Fluid Problems: Thermal Cracking

Problem: Thermal Cracking

Thermal Cracking or degradation occurs when a fluid is heated above the maximum bulk temperature causing the covalent Carbon-Carbon bond to disintegrate into electrovalent bonds. This leads to the formation of molecules with lower molecular weights than the original molecule. Thermal cracking causes a reduction in viscosity, flash point, fire point and auto-ignition temperature. It is not uncommon to discover carbon varnish on the heat transfer surfaces. Typically, this is from a further disintegration of the Carbon-Hydrogen bond. In order to mitigate thermal cracking, it is important to understand ways in which fluid overheating can occur. They include wrong fluid selection, wrong flame impingement, improper startup & shutdown and a low flow regime. 

Cause: 

• Heating a heat transfer fluid above its bulk temperature
• A rapid rise in burner/boiler temperature
• Reduced or low flow of the heat transfer fluid
• Improper flame impingement
  
  

Consequence:

• Decrease in viscosity, flash point, fire point and autoignition temperature
• Carbon varnish forming and fouling heat transfer surfaces

Solution: 

Choosing a fluid with the right thermal properties helps to ensure fluid longevity. Caldera Heat Transfer Fluids are made from high quality base stocks that maintain their thermal stability over an extended temperature range. 

Operators should always start the pump before turning on the heater. This ensures fluid circulation and good mixing prior to heating. It also reduces the residence time of the fluid on the heated surfaces allowing for a steady rise in temperature, helping to prevent fluid cracking.

1. Start the burner on the low fire setting, circulate heat transfer fluid at full flow and slowly raise the temperature
2. Once there is circulation through the heater, the operator should increase the temperature of the bulk fluid by 20 to 25°F (11 to 14°C) increments until the fluid reaches a viscosity of 10 cP
3. Once the fluid reaches 220°F and is pumping smoothly without cavitation, follow the manufacturer’s recommendation for a full fire heat up

Accordingly, during a shutdown of the system, heat must be decreased proportionately, in the reverse of the startup procedures. Keep the fluids circulating until system temperatures drop below the 200°F or 93°C mark, so that the residual heat is removed.

Other recommendations include the proper alignment of burner flame. A faulty flame disperser may cause the coils adjacent to the flames to absorb more heat energy than they can hold. Hence, a heat transfer fluid can be cracked if temperature at the tube surface exceeds the maximum bulk temperature.

It is also sometimes helpful to check for plugged y-strainers and malfunctioning or improperly set bypass valves. This may free up some of the flow that may have been obstructed in the system. It is important that the fluids in heat transfers are circulating while the system operates at a high temperature.