Problems with Closed-Loop Temperature Control Systems

Thermal-liquid temperature control systems offer key benefits over other techniques of cooling and heating. Some benefits are low maintenance, efficiency, lack of cool or hot spots and the total control of temperature. One difficulty is system fouling, which can decrease product output and equipment performance, and lead to enhanced downtime and undesirable product quality. Fouling can happen when the heat transfer fluid is degraded. The “bruised” fluid cannot transfer the same quantity of heat or flow at the same rate.

Fluid Damage

Overheating, oxidation, and contamination are the three ways that can bruise the fluid.

Overheating

Overheating happens when the film temperature of the fluid rises above its optional maximum. The fluid begins to distil with the smaller molecules starting to boil and vaporize. Viscosity increases flow reduction. This allows the fluid to remain in contact with the heated surface and absorbs more amount of heat. As temperatures keeps on increasing, the fluid’s molecules begin cracking and sooty carbon is realised by the fluid upon breaking of its chemical bonds. If this problem is not checked, the process occurs repeatedly until the reduction of all the fluid into carbon.

Oxidation

Oxidation happens when hot fluid comes in contact with atmospheric oxygen. The fluid will start to smoke and will become corrosive and acidic. As the fluid oxidizes, it tends to become less resistant and less efficient to heat.

Contamination

Contamination occurs due to foreign matter in the system. In new systems, unfinished cleaning can leave spatter, weld slag, and flux, quench oils, dust and dirt, and protective lacquers to mix with the fluid. In current systems, dirty bruised fluid could stay, or a non-compatible fluid could have been added.

System Problems

Overheating is due to loss of input heat control or a reduction in fluid flow.

Input Heat Control

Issues with the equipment include improper flame propagation in fossil-fired heaters, and controller glitches or human failure. “Bumps” and power failures can fall the pump motor out. The fluid, which is stagnant in the tubing of fossil-fired heaters, absorbs heat radiating and convecting from the refractory and begins to boil.

Flow

Reduction in flow is the most common reason for overheating. As flows reduce, turbulence and fluid velocity also reduce, and the heat transfer fluid stays in extended contact with the heated surfaces. Even though the bulk fluid temperature of the system may not alter much, the film temperature can increase greatly and rapidly. This additional heat is not transferred as fast to the bulk of the fluid, and the fluid degrades at the heated surface.

As molecules at the film layer decompose, carbon can be created. This carbon sticks to the heated surfaces and bakes on– thickening as the procedure repeats itself and consecutive layers are added. Difficult to eliminate, the carbon coating behaves as an insulator both on electrical heater elements and in heater tubing, and can adversely affect flows. The carbon that escapes or splits away from the heated surface is transported across the system, and can lodge in clog small channels, restrictions, and hang up control valves, further adding to the issue.

Also, the fluid is now additionally viscous. Since it does not flow as well, it stays in contact with the heated surface longer, picking up even more heat and continuing to overheat, decompose and foul.

Flows may reduce for a number of reasons such as contaminants lodged in valves, lines, or strainers, bypass valves hanging up, out-of-spec components, pump problems, or the wrong valves accidentally closed.

Solutions

The following are some solution recommendations:

  • The maximum film or bulk temperature of any heat transfer fluid should never be exceeded. This is possible, but in practice not very simple or cheap.
  • Heat transfer fluid, which does not foul when extremely overheated, must be specified.
  • Paratherm NF® and OR® Fluids – These will not carbon-coat the internal surfaces of the system when relentlessly overheated. They are designed for service to 600°F, the odorless, crystal clear fluids offer exceptional heat transfer and are low in viscosity. They are exceptionally reliable, 100% non-toxic and cost-effective. They have cleared Bioassay and can be integrated with used lube oils and locally recycled, or burned for BTU value.
  • If extremely overheated, the fluids evolve carbon granules, which stay in suspension, cannot stick to the internal surfaces of the system, and can be effortlessly filtered out.

This information has been sourced, reviewed and adapted from materials provided by Paratherm.

For more information on this source, please visit Paratherm.

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