The Benefits of Switching Indirect Thermal Heat Transfer Mediums from Gas to Electric

The industrial landscape has always been defined by continuous evolution, with sustainability and energy efficiency at the forefront of our current technological advancements. One such development is the rise of electric heating in indirect thermal processing. Up until recently, most manufacturers have relied on the combustion of fuel for heat transfer processes, but the shift towards electric heating is gaining momentum as many other industries continue to do the same. Not unlike those other industries, this movement is driven largely by a combination of environmental concerns and potential economic benefits for manufacturers. 

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However, are these concerns and potential benefits significant enough to warrant refactoring your thermal processes to electric? Let’s dive into the available options on the market.

Considering Heat Transfer Mediums

To better evaluate what a shift to electric means in terms of indirect thermal heating, a clear understanding of the process involved is necessary. Specifically, indirect thermal processes rely on a heat transfer medium, which is isolated from the process materials by a metal wall—in other words, the material being processed never comes into direct contact with the heat transfer medium. Thus, the heat transfer medium conveys heat via conduction through the vessel walls. The selection of the heat transfer medium plays a direct role in the efficiency, economics, and sustainability of the overall drying or thermal process. 

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The most common types of heat transfer media in drying or heating systems are steam, hot oil, and more recently, electric elements. Each medium carries a unique consideration for selection, advantages, and disadvantages. We’ll provide a brief overview of three common mediums and share how we’ve recently employed electric heating on our own technologies, followed by an analysis of whether or not electric elements are right for your process. 

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When evaluating or selecting your heat transfer source, there are a few primary considerations:

1. Process temperature needed for the application
2. Existing utilities/Infrastructure
3. Safety and Reliability
4. Mechanical and operational requirements

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Traditional Mediums: Steam & Hot Oil

To fully understand the benefits of switching your thermal processing systems to electric, it is helpful to begin by outlining the advantages and limitations of existing industry standards. As we mentioned previously, the two prevailing mediums are steam and hot oil. We will first review these two mediums, then compare how electric heating stacks up against each of them. 

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Medium #1: Steam

Steam is one of the most common heat transfer fluids, and for good reason, as it is very good at conducting heat through metal. Steam is also one of the fastest mediums in heating up and cooling down, giving it the advantage in installations with frequent start-stop situations. Process steam is primarily generated by heating water in a boiler, with the most common heat source for boilers being the combustion of a fuel, typically natural gas. In some cases, an electric steam boiler may be used using immersion-type heating elements.

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Steam can be practically applied up to 185°C at 150 psig. Anything over and above this temperature may require special consideration, equipment, and process design. Typically, when we design applications that require higher than 185°C, we will recommend hot oil as a heat medium. 

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Additionally, steam is one of the best conductors of heat through metal. It comes up to temperature quickly and cools down quickly. It also allows for a bit more responsive design when on-the-fly changes are necessary to ensure a quality product. 

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One potential drawback is that having and operating a steam boiler at your facility does require specialized/licensed personnel. This is less of a concern for existing facilities already operating boilers, but for new facilities planning out their utility requirements, it becomes a necessary consideration. Also, unless using low-pressure steam (<15 psig), the thermal equipment requires a code vessel. This requires certified welders and a code stamp for approval, both on the initial build and any subsequent repair or rework after installation. 

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Summary of steam as a heat transfer medium:

• Process steam is typically generated in a natural gas or electric boiler. 
• It is a fluid medium that is carried in pipes or hoses. 
• Boiler requires special permitting, engineering, and onsite personnel.
• Steam is quicker than other mediums to heat and cool. 
• The practical upper temperature/pressure limit of steam in the process is 185°C/360°F. 

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Medium #2: Hot Oil

Hot oil is generally considered less efficient than steam in terms of conducting heat through metal. However, hot oil is capable of stable operating temperatures up to 300°C, providing far more driving force when heating or drying materials than steam. 

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Similar to a steam boiler, a hot oil heater can employ a variety of methods to heat the oil, including flame-heating or electric heating. Beyond opting to use a hot oil system, you may be presented with options for the type of oil to be used. In this case, it's advised to work with your vendor, or the supplier of the hot oil system to determine the best fit. 

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If a given application is capable of operating in the elevated temperature ranges afforded by hot oil, the equipment itself will likely be smaller than a system running steam at 150 psig. This is due to the additional driving force of the increased temperature, thus, improving the overall efficiency and performance. 

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Hot oil is applied in a recirculation system, which also improves overall efficiency. Depending on the actual oil being used, the cycle count or overall lifespan of the oil will vary. Also, as the hot oil exits the process technology, it will lose some, but not all of its heat. This in turn minimizes the energy required in reheating the oil back to the input temperature. However, unlike steam, hot oil processes are slow to heat and slow to cool, making them less practical for installations that aren’t running 24/7. 

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Like steam, hot oil is commonly operated over 15 psig, requiring code vessel design, manufactured by a certified shop. This limits where you can manufacture and service your equipment.

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Lastly, there is additional safety consideration in operating a hot oil system given the elevated temperatures, the existing potential for any leaks (however rare), and fire hazards. Plant staff must take strong precautions in operating and maintaining a hot oil system and related components.

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Summary of hot oil as a heat transfer medium:

• Hot oil is a heat transfer fluid that is typically heated in a burner, fueled by natural gas combustion or electric immersion.
• It requires specialized piping for safe operation. 
• Able to be recirculated, but will degrade over time and require replacement.
• The practical upper temperature of hot oil systems is 350°C/660°F.

New Medium: Electric Elements

Like hot oil, electric jackets aren’t as efficient, or effective in conducting heat as steam. However, electric heating makes up for this lower efficiency in providing higher temperatures up to 800°C. Electric heating also requires much less consideration upon installation. There are no boiler, burner, or high-pressure utility lines to be installed, with the only special requirement being basic wiring of the heating elements to the controller. Additionally, electric heating reduces onsite combustion, lowering overall plant emissions. Technologies heated fully by electric heat also don’t require code stamps and certified shops. 

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However, it is important to note that electrically heated jackets have a finite life compared to the more reliable steam and hot oil jackets. The life of these elements ultimately depends on the operating temperatures, the environment, and the quality of the installation of the components. When an application calls for the extreme end of the temperature range (480°C/900°F and above), the jacket material of construction needs to change (from stainless/carbon steel, for example) to a more exotic alloy. 

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Summary of electricity as a heat transfer medium:

• Electric heating eliminates the fluid commonly employed 
• It doesn’t require specialized piping, plumbing, or onsite personnel. 
• Electric elements have a finite life and will require replacement in time.
• The practical upper temperature of electrically heated systems is 900°C/1600°F.

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Let’s Compare

Now that we have a clear understanding of all three heat transfer mediums, let’s see how they stack up against one another: 

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• Process Temperature Limits
  • Steam: up to 185°C/360°F
  • Hot oil: up to 350°C/660°F
  • Electric: up to 800°C/1600°F
    
    
• Vessel Requirements
  • Steam and hot oil typically require code stamps and certified code shops.
  • Electrically heated vessels require an exotic alloy if operating above 900°F.
    
    
• Efficiency
  • Steam is the most efficient heat transfer medium in terms of conducting heat through metal.
  • Hot oil and electricity are less efficient, but make up for this in higher achievable temperatures.
    
    
• Time to Heat/Cool
  • For operations requiring frequent starting/stopping, steam allows for the fastest heat-up and cool-down.
  • Hot oil and electric heating take longer to ensure even heating and to minimize thermal shock on the equipment.

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As you can see, each medium has its clear advantages depending on the processing system being used. Before concluding which one might be best for you, let’s take a look at how we’ve made the move to electric at Bepex.

How Bepex Is Utilizing Electric Elements

We pride ourselves in innovation at Bepex and employing electric heating in new and unique ways is just one of our latest projects. It is not yet possible to move 100% to electric with every process without any significant drawbacks, however, below are some examples of how we have adopted electric elements on some of our core technologies:

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• Fully Electrically Heated Technologies:
  • The Solidaire thin-layer paddle dryer is our first technology fully heated by electric elements.some text
    • Electric heating is accomplished via multiple, independently controlled heating elements mounted on the vessel/body, for precise control over temperature input.  
  • The Turbulizer is a high-shear paddle mixer, featuring a similar physical design to our Solidaire. The vessel is a cylindrical housing, allowing it to use a similar design incorporating electric elements. This has been implemented initially on mixing/agglomeration applications when using a binder with a high melting temperature.
    
    
• Partial Electric Indirect Technologies:
  • In some units, a combination of heat transfer mediums can be applied using electric heating on the vessel and a heat transfer fluid on the rotor. For example, on both our TorusDisc and Thermascrew technologies, the vessels themselves can be heated electrically, either by installed elements or a blanketed design. some text
    • The rotors on these technologies are still heated using more traditional steam or hot oil if heating is necessary. 

• Electric heating for process gas in direct drying technologies:
  • In direct drying systems, the heat transfer fluid is typically hot gas/heated process air that comes in direct contact with the process material. In recent years, more interest has been expressed in heating this process gas electrically, eliminating the carbon emissions that would have been involved in a more common gas burner.
  • One example is our PCX Dispersion Flash Dryer, which utilizes a heated gas stream to dry and convey process materials. In new installations, we are increasingly being requested to include electric air heaters in lieu of flued combustion burners. 

Should You Make the Switch to Electric Elements?

Switching your thermal processing system from steam or hot oil to electric heating elements offers several notable advantages. Electric heating elements can achieve significantly higher temperatures, up to 800°C, compared to steam's 185°C and hot oil's 350°C. This higher temperature capability can enhance the efficiency and performance of processes requiring extreme heat. Additionally, electric heating elements simplify installation and maintenance since they do not require boilers, burners, or high-pressure utility lines. The primary installation requirement of basic wiring to the controller also reduces infrastructure costs and complexity.

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Furthermore, electric heating eliminates onsite combustion, leading to lower overall plant emissions and a safer working environment. This transition contributes to a cleaner environment and helps industries meet stringent emission regulations, promoting sustainable practices. Although electric elements have a finite lifespan and may require exotic alloys for very high temperatures, their benefits in temperature range, installation ease, and environmental impact make them an attractive alternative for many thermal processing applications.

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Unlike steam and hot oil systems that necessitate code vessels and certified personnel, electrically heated systems typically avoid these regulatory hurdles, streamlining compliance and reducing operational risks. Thus, in terms of cost savings, electric elements reduce the need for extensive infrastructure and specialized personnel required for steam and hot oil systems, potentially lowering installation and maintenance costs. Over the long term, the higher efficiency and lower energy consumption of electric elements, combined with reduced regulatory compliance expenses, can lead to substantial financial benefits.

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In a vacuum, the best heat transfer medium for any given project typically comes down to the 4 primary considerations outlined in the first section of this article, with electric elements likely not being the best solution 100% of the time based on those criteria. It is imperative that as you consider a heat source for your system, you work with a vendor or multiple vendors that have practical experience with each and can help you make the right decision for your specific application requirements. 

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If you still have questions about electric elements or heat transfer mediums for your upcoming project or existing processes, please fill out our simple contact form and one of our team members will be happy to assist you.