There are many heater manufacturers and what can seem like an endless array of heater design options. Each of these myriad designs was conceived with a particular application, environment or budget in mind. Here is a review of a few important factors to consider when comparing heater types and heater options.
Cost is always a factor for any purchase; however, the true cost of a heater is more complex than just the initial purchase price. The tradeoff between capital costs, operating costs and long-term maintenance costs must be considered in a proper evaluation. A well-designed and engineered heating system may cost more initially than an off-the-shelf heater, or one with a lower efficiency. However, the less expensive options can become far more expensive than you think. Once operating costs such as fuel costs, thermal fluid replacement, parts and maintenance begin to accumulate over the first years of use, the initial purchase price savings may not seem as significant. Fuel efficiency requirements vary depending on fuel type, fuel availability, application and run times.
A simple process requiring low supply temperatures and an inexpensive thermal fluid may not demand a customized, highly efficient system. In these cases, a less expensive heater option may be ideal. In direct contrast, a more complex or demanding application requiring high temperatures and a more expensive fluid will certainly see the benefits of a properly designed, engineered heating system. The difference in cost for a system designed specifically for an application with higher efficiencies, higher heating temperatures, or heavy-duty operation typically can be made up in short payback periods when looking at the total equipment operating costs.
It is important to pay close attention to the method of fuel efficiency calculation used by different vendors. These numbers can be skewed based on the chosen method of calculation, and seemingly small differences in efficiency can actually represent large differences in operating cost. Compare carefully, and do not be afraid to ask potential vendors to explain their efficiency claims, support their design calculations and guarantee the stated efficiency at your operating conditions.
Design Parameters to Consider
Surface area of the heater coil (heat transfer surface) is crucial. A generous surface area is the starting point for a sound heater design. Inadequate surface area in your heater coil will limit your efficiency and lead to a higher film temperature. This ultimately means thermally degrading your expensive heat transfer fluid. Running poor quality oil in your system will shorten coil life and decrease heater efficiency over time.
Of course, the primary benefit to a reduced coil surface is less cost. Be sure to carefully consider coil surface area and relative heat flux rates when comparing heater options. This is an important area where initial savings can result in massive costs down the road.
Fluid velocity and pressure drop are inherently related and vital in engineering an appropriate heater for any application. In general, a higher fluid velocity means better heat transfer and lower film temperatures. This comes with a tradeoff though, as pressure drop increases with fluid velocity. Higher pressure drop means more pump horsepower, which indirectly increases your operating costs due to the increase in power consumption. You can have one heater design that has very low pressure drop - resulting in low fluid velocities that yield higher film temperature. Another heater design masks a low coil surface area with extremely high velocities - resulting in reasonable film temperatures but very high pressure drop. This design parameter is just one piece of the puzzle. In order to compare more appropriately, be sure to obtain fluid velocities, pressure drops, film temperatures and heat flux rates. Discuss and understand what pressure drops your heat user system may allow in addition to what minimum and maximum flows are safe for your heater.
Combustion chamber size and flame impingement also are critical to heater design. The biggest potential for damaging the coil and fluid lies within the radiant zone of the heater. A well-sized combustion chamber will ensure proper heat release within the radiant zone, but it also ensures that there is no potential for flame impingement on the coil. Flame impingement causes local hot spots on a heater coil, which result in localized fluid degradation and potentially even coil damage. Ask potential suppliers to discuss the combustion chamber size as it relates to the burner flame size and its proximity to the heater coil.
Helical Coil Heater Design Types
Armed with a basic understanding of what is important to your application and facility, you now need to select the right heater design. Unfortunately, it is not always obvious which type is best for each application. Many manufacturers will tout the merits of their specific designs, and you need to be able to understand how those differences relate to your needs. Here are summaries of several types as well as the pros and cons of each relative to performance, cost and footprint.
Single Helical Coil Thermal Fluid Heater. The single coil is a simple, cost-effective design. Using a single helical coil, this design allows for two passes of flue gas along the heat transfer surface. The single helical coil is simple to design and fabricate, and it can be built in horizontal or vertical configurations.
Generally speaking, this heater style will have lower efficiencies than other heaters with more available surface area. In some cases, a manufacturer will require an additional waste heat recovery unit to achieve the efficiencies available via other base heater designs. High exhaust temperatures at the first turn require internal insulation to protect the heater shell from extreme temperatures. This internal insulation is subject to wear and could need to be replaced over time.
Double Helical Coil Thermal Fluid Heater. This design incorporates two concentric helical coils, one inside of the other. This allows for three passes of flue gases, and this additional pass of flue gas (compared with a single coil heater) allows for additional heat transfer surface for a comparably sized unit. The additional surface area typically makes these heaters more efficient than a comparable single coil design.
The double helical coil usually is more expensive than a single helical coil design. It can be configured in either a horizontal or vertical arrangement. The design allows for a large heat duty in a small overall footprint. If size is an issue for your facility, this may well be a deciding factor. Larger duty requirements may dictate that a dual helical design be used as it is difficult to build a shippable single coil unit above 25 million BTU/hr.
In a double helical coil design, the flue gases are relatively cool when first contacting the heater shell, eliminating the need for most internal shell insulation. This minimizes long-term insulation maintenance and replacement.
Radiant-Convective Thermal Fluid Heater. Worldwide, the traditional radiant-convective style heater is the most well-known and commonly used style of direct-fired heater. These heaters are physically larger than their helical coil counterparts, utilizing both a bare tube radiant zone in combination with a separate bare or finned convection zone. Radiant-convective style heaters are more complicated to design and build, and they are therefore typically more expensive than other heater types.
With this design, a range of design capabilities allows for flexible performance and configuration options. Efficiencies of a new heater can vary from 70 to 90 percent, and footprints can be small and tall with vertical-cylindrical heaters, or shorter and wider with cabin-style heaters.
Several manufactures have their own proprietary design for the radiant convective style of heater. However, the most widely accepted standard design is API 560, which has been used for many years in the oil and gas industry to heat petroleum streams in various stages of production. This specification is often applied to thermal fluid systems, which in almost all cases is unnecessary. This specification is extremely conservative in nature and, in the authors’ opinion, is really only necessary when heating a non-homogeneous petrochemical stream with varying thermophysical properties. When applied to a thermal fluid - an engineered fluid with well-known and homogeneous properties - it adds unnecessary size, cost and complexity.
Now, armed with thermal fluid heater knowledge, specify your next heater with more confidence.