What Are External Electric Tank Heaters?

External Electric Tank Heaters: Eliminate Concerns about Material Compatibility and High Watt Densities when Dealing with Viscous, Aggressive, or Erosive Contents

Many industries use large fiberglass or steel tanks that need to be heated. A common way to heat the tank contents is with electric heating elements designed to penetrate the tank through an existing flanged or threaded opening. This type of heating element, known as an immersion heater, is put directly in to the process media and heats the tank contents. While this heating method is efficient, it's not alway practical. Some tanks contain very viscous, chemically aggressive, or physically erosive media. Using an immersion heater for these applications gets tricky. Very careful consideration needs to be taken when determining proper watt density, maximum sheath temperatures, and material compatibility.

Another way to electrically heat steel or fiberglass tanks is with externally mounted "pad" heaters, also known as heater panels. This type of heating element is mounted directly to the exterior tank surface and provides evenly distributed heating over a large area of the tank. These heaters are designed to deliver the required amount of energy (watts) distributed over a larger surface area, and provide "gentle" heating that will not burn or react with the process media. Furthermore, since heating panels are mounted externally, there's no concern with corrosion or erosion from aggressive or abrasive contents.

External electric panel heaters are constructed of flat foil based heating elements or nichrome resistance wire patterns sandwiched between flexible and tough silicone or fiberglass layers. Because of the heating element pattern, even distribution of heat is assured with no hot spots. Panel heaters come in many standard sizes, volts and wattages. Custom shape and sizes are available too for tanks with complex geometries. Installation is easy and requires no special tools or skills.  Heaters are usually installed by banding them to the tank with metal bands or polyester straps, which takes only a matter of minutes.

For optimal performance and greatest energy savings, external panel heaters should be insulated to minimize backside loses.  The job of the insulation is to direct the majority of the heating panels energy into the tank. Failure to insulate allows for heat energy to escape to the surrounding area.

For tank heating applications where immersion heater sheath temperatures, or sheath material compatibility is a concern, or where penetrating the tank is impractical, externally mounted electric heating panels are an excellent solutions.

Use External Electric Heating Panels for Ease and Simplicity

ELP tank heater construction

When it comes to heating process tanks electrically, you have two options - immersion or externally mounted panels. Immersion heaters must penetrate the tank wall, or be placed of over-the-side in an open top tank. Both approaches add cost and/or safety considerations. Sometimes entry to the tank may not be possible at all.  A good alternative are electric heating panels.

One type of exterior heater design, the epoxy laminated panel (or ELP), is a very versatile tank heater. Possessing unique flexibility, ELP heating panels can be used in a variety of heating applications ranging from freeze protection to process heating up to 212 deg. F . These panels can be used on either vertical or horizontal tanks of metallic or non-metallic construction. They can be used in dry, wet, or corrosive environments in ordinary or approved hazardous locations.

ELP heating panels are quick and easy to install, require no special tools or skills. Heaters are usually installed by banding them to the tank with metal bands or polyester straps. Installation of several ELP heating panels takes only a few minutes.

Check out the following document for more information on the epoxy laminated panel heaters.

Epoxy Laminated Tank Heater Panels from Hotfoil EHS, Inc.

FM Approved Silicone Rubber Heaters for Tank and Vessel Heating

Silicone rubber electric heating element for tank and vessel heating.

Silicone rubber electric heating elements are well proven, tried and tested flexible heaters for heating plastic or metallic tanks and vessels. Their low mass offers virtually no thermal inertia and within minutes can achieve elevated temperatures. They are generally easy to apply to the surface of tanks, chutes, or hoppers and can be custom designed in virtually any shape, all that is required is a sketch of the heated area and information about the electrical connections.

Silicone rubber electric heating elements are constructed by vulcanizing a nichrome heating element to the silicone rubber layer to produce a homogenous laminate. Two layers of 0.028” thick silicone rubber compound, sheath cured on one side, are calendared with high strength fiberglass mesh and vulcanized together under heat and pressure. This bonding sandwiches the resistance heating element between the two layers. An electric grounding mesh is located on the assembly and a further layer of silicone rubber is applied and vulcanized, making a homogenous assembly.

The heaters are used primarily for metallic and non-metallic tank and vessel heating. Their flexibility, both physically and design-wise, allow the heaters to be used for a wide variety of applications. Silicone rubber heaters are water-resistant and may be used for outdoor or “wet” applications, as long as the thermal insulation or exterior covering over the heaters is weatherproofed.

Silicone rubber heating systems must be correctly temperature controlled. The controls and all electrical components must be approved for the area as classified. All wiring must follow Federal, State and Local codes and be fully compliant with the NEC as it relates to the project. If in doubt, please contact an industrial heating applications expert for assistance.

For more information about silicone rubber tank and vessel heaters visit this link.

Heat Transfer - The Basics

Industrial hopper heaters are
example of conductive heat transfer.

Heat transfer is the movement of heat from one body or substance to another by radiation, conduction, convection, or a combination of these processes.

When heating a pan of water over a gas flame for example, all three forms a heat transfer are taking place. Heat from the flame radiates in all directions. Conduction takes place with the transfer of heat from the burner to the metal pan. This heat transfer is also responsible for making the handle hot after a period of time. Water is heated by the process up convection which is a circular movement caused by heated water rising and cold water falling.

The process of heat transfer also occurs when an object cools. If a mug of hot coffee is left standing on a cold kitchen countertop, its temperature will gradually decrease as heat is lost. The heat energy dissipates by conduction through the mug to the table top by convection as the liquid rises, cools and sinks, and by the radiation of heat into the surrounding air.

One way to conserve the heat a liquid and prevent heat transfer is to place it in a thermos. The use a vacuum chamber with silvered surfaces, along with low conductive materials, can greatly improve the amount of heat or cold that is lost to the surrounding environment. In between the silvered glass walls up a thermos lies a vacuum. In the case of a hot liquid heat transfer by convection through the vacuum is greatly restricted due to the absence have air molecules necessary to facilitate the transfer of heat. The lack of physical contact between the inside and outside walls of the thermos due to this airless space also greatly inhibits the movement of heat by conduction. Heat loss by radiation is prevented by the silvered walls reflecting radiant energy back into the thermos. Some conduction of heat through the stopper in glass can be expected but this too is limited because they are made of materials with very low conductivity. Thus the temperatures are both hot and cold liquids can be maintained by a properly designed thermos that limits the transfer energy through radiation convection and conduction.

Heat capacity is the amount of heat required to change the temperature of an object or substance by one degree Celsius. The heat capacity of water varies depending on its phase. As solid ice, the heat capacity of water is .5 calories per gram for every one degree Celsius, which means it takes half a calorie to raise the temperature of one gram of ice one degree Celsius.

As a liquid, water heat capacity is one calorie per gram for every one degree Celsius, so it takes one calorie of heat energy to raise one gram of water one degree Celsius.

The processes of phase change between solid, liquid, and gas also require a specific amount of heat energy. The amount of energy required to change a liquid into a solid, or a solid into a liquid, is known as heat of fusion. The amount of heat required to change one gram of ice to water is 80 calories. Similarly the heat vaporization is the energy required to transform a liquid into a gas. It requires 540 calories to change one gram of liquid water into a gas. With these values its easy to calculate exactly how many calories of heat energy are required to transform one gram a ice at absolute zero to steam.

To warm 1 gram of ice from -273 degrees Celsius to 0 degrees celsius would be 273 times .5 gram per calorie or about 140 calories. The phase-change of one gram a ice to liquid water requires 80 calories. Then to heat the water from zero degrees Celsius to 100 degrees Celsius with the heat capacity at one calorie per gram, would require 100 calories. The final phase change it one gram a boiling water to steam would require an additional 540 calories. Adding all of these values together yields 860 calories, the amount of heat energy it takes to transform one gram gram of ice at absolute zero to steam.

Watch this video for an illustration of the above: