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There are a number of industrial heating systems to choose from, but you’ve done your research and decided a fired thermal fluid heater is right for your application.
The decisions don’t end there, though. Now, you’ve got to decide what style of thermal fluid heater is best for you.
Thermal fluid heaters come in all sorts of shapes, sizes, and configurations, but the most common heaters in use today are heaters with a helical coil(s). Helical coil heaters can be loose or tight wound. A loose wound coil has space between the tubes that exposes the coils to radiant heat on all sides and also exposes the coil supports to radiant heat. A tight wound coil does not have any space between the tubes, and the tubes are stitch welded together. In tight wound coils the hot gases flow over the inside of the coils where radiant heat transfer happens. The hot gas is then forced to return over the space between the outside of the coils and the shell where convective heat transfer occurs. This is commonly referred to as a two-pass process. Sometimes a second coil surrounding the inside coil can be added. This is commonly called a three-pass configuration.
Serpentine coils are another configuration that is sometimes used. Serpentine coil heaters may be designed in accordance with API 560 and referred to as API heaters. API is a recommended practice for design, materials, fabrication, inspection, testing, etc. for fired heaters that is specific to petroleum refinery applications. API stands for American Petroleum Institute. This recommended practice was created to guide reliable and safe design for fired heaters used to heat crude oil and its components, mostly for separation of those components at high temperatures.
Helical Coil tight wound heaters are normally used for heating stable fluids that are meant to be heat transfer medias. These fluids have stable compositions even at very high temperatures and the main goal is to transfer heat to industrial processes and not separate components as in refineries. These heaters are designed according to ASME Code (American Society of Mechanical Engineers) for the pressure parts, NFPA for safety controls and DIN and AICHE for the thermal design.
Serpentine and helical coil heaters are used for a variety of applications, but there are significant differences that affect your decision to choose one or the other. Since tight wound helical coil heaters and serpentine style heaters are the two most common types of coils for heaters in use, in this article we will limit the comparison to those two types.
The intent of this comparison is not to say that one is better than the other. That’s determined by the application, your needs and the relative cost/benefit of each configuration for the application. But there are some myths, or confusion, when comparing the two that we will address.
Myth #1: API heaters are safer than helical coil heaters
You can’t take risks when it comes to your heating system. That’s why safety standards and recommendations were established; to mitigate the risk of injury from explosion or fire. Standards or recommendations like NFPA 87, API, and ASME codes should be followed strictly by your heater manufacturer to ensure safe practices.
NFPA stands for National Fire Protection Association. NFPA 87 is the standard developed for fluid heaters to minimize fire and explosion hazards. NFPA 87 focuses on combustion components of the heater and its operation. A majority of state and local governing bodies have adopted NFPA 87 as a required practice and can levy fines to companies that are not in compliance.
Every year, significantly more helical coil heaters than serpentine coil heaters are built for a variety of industries in North America, Europe and Asia. Most of these helical coil heaters are built to the broader NFPA 87 standard. A helical coil is a proven design that has been safely and reliably used for over half a century.
Verdict: As long as the manufacturer is following standards and guidelines from a reputable association, such as API, NFPA and ASME, either style heater is a safe option.
Vertical heater with helical coils.
Myth #2: Helical coil heaters are less efficient
This myth really comes down to engineering and design. Helical coil and serpentine coil efficiencies are directly related to the amount of surface area that the designer is applying to the heater. Both styles of heaters can achieve anywhere from 75% to 90% thermal efficiency. Size, engineering, fuel, and outlet temperatures all play major roles in a heater’s efficiency.
When we talk about efficiency, we are really talking about two things; environmental impact and costs to operate. Efficiency is the ratio between the amount of energy (heat) released from combustion and the amount of energy absorbed by the fluid being heated. Energy losses occur due to incomplete combustion, heat escaping to the atmosphere via flue gas discharge, and heat skin radiation. A well-designed heater will minimize these losses of energy. A heater that operates more efficiently will have a more complete combustion, lose less heat to the atmosphere and consequently will release less exhaust into the air. In addition, it will cost less to operate since less fuel is required to make duty.
Since both styles are able to achieve similar efficiency levels, it’s important to note that the capital invested in a helical coil heater is usually lower than a serpentine coil heater. Helical coil construction is less labor intensive than serpentine heaters since there are no return bends to be welded. Therefore, the initial cost of ownership of a helical coil heater is going to be less.
There are several ways to build helical coils. Some coils are tight wound. Others are loose wound. Some manufacturers use two-pass and others use three-pass designs. With all the different options, it’s important to work with a manufacturer that offers multiple styles and can give you the most efficient heater for your application.
Verdict: To claim one style is more efficient over the other is misleading.
Myth #3: Serpentine coil heaters offer more options for customization
Again, this myth really comes down to the manufacturer’s design capabilities. While some only offer standard size helical coil heaters and options, others offer fully customized solutions. For example, Heatec offers a variety of helical coil heaters in sizes up to 80 million BTU/Hr that are fully customizable for the application and available space.
There really isn’t a limit on the size of a helical coil heater. The only limiting factor is shipping constraints. Helical coil heaters are shipped in one piece, fully assembled and tested at the factory. This minimizes field work and consequently saves installation time and cost. Serpentine coils are normally field erected for duties over 30 to 40 million BTU/Hr, which increases the installation costs and can generate unforeseen assembly issues.
Like serpentine coil heaters, helical coil heaters can be made in a vertical or horizontal configuration. They can even be portable if that’s what is needed. They can be fitted with low NOx burners and offer an array of controls. An economizer (heat exchanger) can be added to help boost efficiency.
Vertical heater with serpentine coils.
Standard size heaters or stock heaters are a great option for those that might be looking for a quick turn around and they are typically more budget friendly since most of the engineering has already been done. And they can usually be modified to give you exactly what you need.
Verdict: Both heater styles are versatile and fully customizable.
Myth #4: Serpentine coil heaters last longer
As long as your heater is properly maintained and operated within the parameters the heater was designed for, either style is going to last their designed life. Most heaters are designed for a lifespan of about 25 years but it is not uncommon to see helical coil heaters operating for 30 plus years.
Make sure your heater has a good warranty from the manufacturer. If a coil (helical or serpentine) is going to fail from manufacturer defects, it’s going to happen within the first few years of operation. If there are not any manufacturer defects, the coil will last the life of the heater with proper operation and maintenance of the heating system. Upgrades to controls and other components can easily be retrofitted throughout the life of the heater without replacing the entire unit.
Verdict: Both style heaters can last for decades, as long as they are maintained and operated properly.
Myth #5: Helical coil heaters are harder to maintain
Most of the components, like burners, valves, actuators, controls, sensors, etc., found on a helical coil heater are similar to those found on serpentine heaters. These components are the typical the wear items found on all heaters. Accessibility to replace and repair those components depends on how they are laid-out and designed by the manufacturer. Since the components are mostly similar, this myth basically boils down to the differences in coil design.
A serpentine coil heater designed per API standards requires a space between each tube in the tube bundles. The rationale behind the design is for each tube to be replaceable individually and for easy access to each section. However, the serpentine coil tube supports are constantly exposed to the radiation of the flame and require frequent maintenance. The supports eventual failure also influences the serpentine tube failure.
Helical coils rarely have to be repaired or replaced as long as the system is properly operated and maintained. The coil supports are not exposed to any radiant heat and will last the life of the heater. In the rare event the helical coil does fail, normally due to faulty operation or maintenance of the controls, a section of the coil can be repaired or replaced in the field. If the entire coil needs to be replaced, good design makes the process relatively simple. For example, Heatec designs their heaters so that all that is required is to unbolt the heater head and slide the damaged coil out and slide a new one in. Typically, no cutting is required.
Verdict: Aside from the shared wear components, a heater with a helical coil should be less maintenance than one with serpentine coils.
The bottom line is that either style heater can be safe, efficient, and reliable. If you are unsure which one you should go with, talk to a specialist in process heating. A manufacturer like Astec, who sells all types of heaters, will ask the right questions to provide the best solution for your application and needs without bias.
For more information on choosing the right heater visit www.astecindustries.com, or give Astec a call at 423-821-5200.
