Commonly referred to as an inline heater, circulation heaters provide a clean and efficient means to heat flowing gases or liquids. They consist of an assembly of heating elements, pressure vessel (tank), terminal housing, mounting hardware, insulation and inlet/outlet connections. Depending on the process conditions, circulation heaters can be manufactured utilizing different element sheath materials, vessel materials, terminal housings, and power and pressure ratings. Additionally, circulation heaters are designed so that the flow is forced directly across the heating elements to deliver rapid response and even heat transfer.
In addition to circulation heaters being simple to install and maintain, their largest benefits are efficiency, response time, even heat transfer, turndown ratio and clean operation. Many companies are seeking such benefits via electric heating solutions as a clean alternative to large, gas-fired heating systems. To realize these benefits, however, the heater must be designed correctly. This only can be accomplished when the application is truly understood.
1. Application Conditions
Circulation heaters are used in many process applications and are popular in the chemical processing, hydrocarbon refining, power generation and natural gas processing industries. They can be used to heat air, nitrogen, hydrogen, water (including deionized, potable, clean and process) and oil (including hydraulic, lubricating, fuel and crude). They also can be used for superheating, a process where a vapor is heated to very high temperatures to increase its entropy value.
To ensure that the heater will provide a long-lasting solution that meets the needs of the process and application, the heater must be engineered considering the following specifications:
- Chemical composition, viscosity, thermal properties and corrosive nature of the medium being heated.
- Minimum and maximum flow rate.
- Operating and design pressures.
- Minimum and maximum operating and design temperatures.
- Installation location (indoor or outdoor).
- Mounting orientation (horizontal or vertical).
- Ambient environmental conditions.
- Power available (volts, phase, amps).
- Temperature sensor requirements.
- Temperature control requirements.
- Pressure drop requirements.
- Electrical area classification requirements (Class 1, Div. 1 and 2, Groups B, C, D).
- UL, CSA and other third-party certification requirements.
By defining the greatest number of variables related to your application in advance, you optimize your selection.
A circulation heater is designed in such a way that the flow is forced directly across the heating elements to deliver rapid response and even heat transfer. Shown here is a Watlow circulation heater. Images courtesy of Watlow
Materials of construction are a critical decision. Standard heating element materials available include steel, 316 stainless steel and alloy 800/840. There also are standard vessel materials to choose from, including carbon steel and 316 stainless steel. In addition to knowing specific application details, one of the biggest factors to consider in selecting the proper materials for the heater is the chemical composition and corrosiveness of the media being heated.
Typically, one will find standard designs available for quick delivery for common applications. Many specialty applications should consider a custom design, however. There are even designs available where the media can be forced through an isolated flow path (usually 316 stainless steel or Inconel tubing) so that it never comes in direct contact with the heating element. This is particularly useful when heating sensitive, corrosive or flammable media.
A more even heat transfer can be achieved through proper design. Shown here is the Watlow Optimax heat exchanger. Images courtesy of Watlow
3. Watt Density
Another factor that cannot be overlooked is the watt density. How hot will the element operate? This is a significant factor that can help determine the life of heater. Also, it is analyzed to determine the physical size and number of elements used. Typically, the higher the heater’s operating temperature, the shorter its life, but several other factors can contribute to shorten operating life. Applications with corrosion potential and high operating or process temperature will require special attention.
Calculations must be performed to determine the wattage requirement for the application to ensure the heater can meet the proper heatup rate and other requirements of the application. Wattage and watt density generally are determined by reviewing the type of media being heated, the flow rate and the maximum delta (temperature difference, or ΔT) across the heater. Keep in mind that baffles can be used to optimize or change the flow of the media to improve heat transfer.
A watt density must be chosen that will result in both safe operating element-sheath temperatures and fluid integrity. Traditional thought and training for years has encouraged lower watt densities to extend heater life and safety. With new technologies emerging to enhance flow and improve the heat transfer of air and gases through the vessel and across the elements, higher watt densities and smaller footprints can be utilized. This results in a smaller, lighter and more efficient solution. In larger applications with multiple heaters, sometimes an entire heater bundle and vessel can be eliminated.
FIGURE 1. There are general suggestions and guidelines for orientation to ensure that the heating elements stay completely immersed and to help reduce the terminal enclosure temperature. This figure illustrates standoff and terminal enclosure considerations. Images courtesy of Watlow
4. Heater Orientation
Available space and orientation also should be top of mind when designing a heater. The following general suggestions and guidelines for heater orientation can be followed to help ensure that the heating elements stay completely immersed:
For liquids, the heater should be oriented horizontally with the inlet and outlet connections facing up. Typically, the inlet is nearer to the terminal housing, but in a low temperature liquid application, this is not critical.
Alternatively, for liquids, the heater can be oriented vertically with the terminal enclosure at the bottom and the inlet closest to the bottom. This will help to ensure that the elements always will be immersed during operation.
For air or gases, the heater should be oriented horizontally with the inlet connection closest to the terminal enclosure.
When oriented vertically, the heater should display the terminal enclosure and inlet connections at the bottom. Both orientations are done to help reduce the temperatures in the terminal enclosure (figure 1).
Following these general recommendations also can help reduce the terminal enclosure temperature.
5. Standoffs and Terminal Enclosures
A standoff can be included in the design to reduce the temperature inside of the terminal enclosure. This is a common area for failure due to high operating temperatures, heat generation from electrical connections or even other factors overlooked in the design.
The design of the standoff should be analyzed carefully as another way to extend the life of the heater and provide a safe solution. This is especially true when a temperature class requirement is specified by the user, or the equipment is being installed in an explosive environment. In cases such as these, the designer must consider the customer-provided T-Code so that the maximum surface temperature of the enclosure will never exceed that rating.
The proper terminal enclosure must be selected based on the provided application details. General-purpose, moisture-resistant, explosionproof and moisture-resistant/explosionproof enclosures all are options. In cases where CSA and other third-party certifications may be required, the media being heated, the process temperature, the mounting orientation and the maximum working pressure will need to be specified.
Finally, one needs to select a proper control system to ensure reliable, safe and accurate control for the heater. Utilizing a silicon controlled rectifier (SCR) power control system with a PID temperature controller, the heater can respond quickly and accurately to varying process conditions, providing exceedingly tight control. SCRs are imperative where precise control — or reducing scrap — is critical. Typically, a temperature sensor is located in the outlet of the heater to provide feedback to the process temperature control system for accurate control. Another temperature sensor should be attached directly to the sheath of one or more of the elements to provide immediate feedback to a high limit, safety controller. This helps prevent an overtemperature situation due to improper flow, a low liquid level or sludge buildup.
In conclusion, there are a number of factors to consider and decisions that need to be made in order to effectively select a properly engineered circulation heater for a given application. If shortcuts are taken, the results can be both costly and dangerous. It is critical that the process is approached with an engineering professional to ensure that everything is done correctly and in the best interest of the heater’s life and productivity.