How to use immersion heaters in chemical process applications

Immersion heaters have a variety of applications in the chemical process industries. Knowing which heaters to specify and how to install them can make a manufacturing process more cost-efficient. It’s time to learn about immersion heater types; typical applications; and selecting, sizing, specifying, installing and using the heater.

Immersion heaters, as the name implies, are immersed in water, oils, solvents, process solutions, molten materials and gases, where they release all their heat within the fluid, which makes them nearly 100% energy-efficient. Immersion heaters are offered in a wide variety of sizes, kilowatt ratings, voltages, terminations, sheath materials and accessories. They are often custom engineered for a specific application.

The basic immersion heater configurations are the screw plug, flange, pipe insert or bayonet, circulation or in-line, booster, and of an over-the-side style. They’re usually available in a round tubular design or a flat tubular design. The flat variety can operate at a higher watt density without overheating the sheath. Heaters also are grouped into two categories – pressurized (closed) systems and non-pressurized (open tank) systems.

Pressurized systems

The square flange immersion heater is used in industrial water boilers and storage tanks holding degreasing solvents, fuel oils, heat-transfer fluids and caustic solutions. The assembly consists of a round or flat tubular heater brazed or welded to a four- or six-bolt flange with screw lug or threaded stud terminals for wiring connections. These heaters bolt directly to a mating companion flange that is welded to a tank wall or nozzle. Assembly change is as easy as unbolting the flange and replacing the heater, thus minimizing extensive equipment downtime.

The screw plug heater fits a threaded opening or full or half pipe coupling in a tank wall. Applications include high-purity water, oils, caustic cleaners, chemical baths, glycol solutions, liquid paraffin, process water and clean-water rinse tanks.

ANSI flange heaters are through-the-side heaters for liquid immersion applications that require high wattage in large tanks. The applications are similar to those of screw plug heaters, but ANSI flange heaters are used in high-pressure applications such as superheated steam, compressed gases or liquids. Pipe insert, or bayonet, heaters are used for heating liquids in extremely large storage tanks. The heater is mounted inside a pressure-tight bayonet pipe that mates to a flange connection on the side of the tank, thus supplying the pressure boundary that allows removal of the heater without draining the tank.

Circulation, or in-line, heaters are all-in-one units with the heater mounted inside its own insulated tank. The heater has inlet and outlet piping and the liquid or gas flows through the tank. By the time the material exits, it’s heated to the proper temperature. This design has fast response and even heat distribution. Heaters can be as small as a 1-1/4-in. NPT screw plug size to as large as 14 in. diameter. Custom units have been made up to 48 in. nominal pipe size. Booster heaters are a type of circulation heater for applications using lower wattage, including in-line operations or engine preheating. Booster heaters with copper and steel sheaths are used for heating water and oils, respectively.

An innovative circulation heater is available for applications that demand precise temperature control for gases and other fluids. Rapid response heat exchangers provide faster thermal response and higher power in a smaller footprint when compared to most other conventional circulation heaters. These typically have low wattage requirements and are single phase.

Non-pressurized systems

Over-the-side heaters are formed into L and O shapes and are installed in the top of a tank, with the heated portion immersed along the side or at the bottom. Over-the-side heaters provide even heat distribution. They’re portable, easily removed for cleaning of heaters and tanks, and provide ample working area inside the tank. A variety of optional sheath materials, kilowatt ratings, terminal enclosures and mounting methods are available. Over-the-side heaters are designed for heating of water, oils, solvents, salts and acids. Often, they’re used for freeze protection.

Two other over-the-side style heaters are the thin-profile vertical loop heater is a tubular heater design that hangs over the side of an open tank and a drum heater that easily fits into the bung hole of a 55-gallon drum and is used for melting heat-sensitive materials, such as paraffin (wax), lard, grease, various oils, and other viscous fluids. A pre-wired thermostat protects the material from overheating.

Cost comparisons

Specific application characteristics or requirements limit most heater choices. Square flanges and screw plugs are generally the most economical solution while ANSI flange heaters and circulation heaters are usually more costly as their size and power requirements are much greater.

Selecting a heater

Most electrical heating problems can be solved by converting the heat requirement to its equivalent electrical power. Whatever the application, the method for determining the power requirement considers the following.

Properties of the material to be heated: It’s important to know the type and quality of the fluid being heated. For example, if the fluid is water, is it clean or contaminated or potable?

Acids cause corrosion and insulating buildup on the heater sheath, which can cause overheating and heater failure. If the fluid is thick and viscous oil, it requires a low watt density, whereas light oil could tolerate sheath watt densities to 30 W/sq. in. to 40 W/sq. in., avoiding coking by considering the oil’s viscosity, specific heat and thermal conductivity.