electric thermal fluid heating system

Specifying Electric Thermal Fluid Heating Systems

There are ten examination that can really aid in choosing what type of electric thermal fluid heating system will fit for your industry here are a few:

Many processes today require an indirect method of heating, which implies the use of a heat transfer medium instead of circulating the process directly into a heater. Thermal oil, water, glycol mixtures and steam are the most commonly used heat mediums for indirect heating. Thermal oil often is preferred over steam for high temperature applications because there is less maintenance such as water conditioning and blow down systems. Steam processes require higher pressures when compared to oil systems at comparable temperatures as well as more ongoing regulatory oversight.

Know Your Heat Transfer Fluid

Several types of heat transfer fluids are available, and each has its own particular set of advantages and disadvantages. If you consider only the heat transfer properties, water is an ideal heat transfer medium, but it will obviously build pressure as the temperature increases beyond the boiling point. Thermal oils offer high temperature capability at low pressures. They include synthetics or aromatics, petroleum-based fluids and silicone. Molten salt has become popular as an ultra-high temperature heat transfer fluid, offering operation up to 1,000°F (538°C). Not all thermal fluids will be suited to your process. Different fluids will have different boiling points, flashpoints and operating temperatures. It is important to know the vapor pressure of the fluid because this will determine whether there is a need for a pressurized expansion tank. Maximum film temperatures must be considered because the fluid flows over the heating elements. The fluid you select will be a significant investment. The time spent selecting that fluid will be good insurance to protect that investment.

Choosing the Heater

Heater selection is one of the most important steps in the thermal fluid heating system. Things to consider are heater watt density, velocity through the heater, heater orientation and control method. The watt density of the electric heater is critical to protect the oil from degradation. Typical watt densities of heaters are 30 W/in2 for water/glycol; 20 W/in2 for lower temperature hot oil systems; and 12 to 15 W/in2 for higher temperature hot oil units and heat transfer salts. Smaller diameter heaters are preferred due to the increased velocity over the heating elements. Typically, a 1:1 ratio between flow measured in gallons per minute and kilowatts will result in approximately 15°F (8.3°C) temperature rise through the heater.

Specifying Heater Controls

Heater controls are just as important as heater selection in a thermal fluid heating system. Thyristor control is preferred over on/off control to allow the heater to run at lower temperatures and maintain tight control. Always keep the over-temperature safety system separate from the control system. Use dedicated safety contactors to provide positive shutoff of the power to the heater. This ensures a fail-safe method of protection.

Know the Types of Insulation

Many types of insulation are available on the market but not all of them are suitable for thermal fluid system service. Fiberglass insulation is easy and inexpensive to install for water and water-glycol applications. For thermal oil applications, fiberglass should be avoided as it creates a fire hazard. Instead, closed-cell insulation such as foam glass should always be used. Ensure there is enough insulation on the heater and piping to prevent injuries to personnel. OSHA typically recommends a surface temperature limit of 140°F (60°C) although you should verify with your local authority or facility to be certain. Insulation also reduces heat losses to the atmosphere and preserves system efficiency. Even if personnel safety is not in question, your equipment and piping should still be insulated.

Liquid Expansion

As the thermal fluid is heated, it will expand or possibly even contract. Water and water-glycol have minimal expansion — typically 10 percent or less. Thermal oil can expand as much as 30 to 40 percent; this will vary widely based on the oil type and manufacturer. Molten salt actually contracts as it melts and experiences little expansion as it continues to heat.

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