In Part 1 of this series, I explained that choosing the proper heat transfer fluid is the single most important spec decision for optimal process performance when putting together a new thermal fluid system. However, the most visible and challenging component in the thermal fluid system is, of course, the heater. In this installment, I’ll be discussing the challenges involved in properly specifying and selecting a direct-fired thermal fluid heater.
The thermal fluid heater has to perform several jobs efficiently:
- It has to have high combustion efficiency.
- It has to heat the thermal fluid gently enough not to overheat and
damage the fluid.
- It has to have sufficient burner turndown and control capability to
provide proper temperature control to the process.
Heater DesignA large number of heater designs are available, including serpentine coil, helical coil, dual helical coil and cabin heaters. There also are heater designs unique to a single manufacturer. In this survey, the discussion will be limited to serpentine coil, helical coil and dual helical coil heater designs as they represent the bulk of heaters sold into the process and converting industries.
Most thermal fluid heaters -- and all of the heater designs discussed in this article -- are constructed by fabricating a heat transfer coil (or multiple coils) from pipe and installing the coil inside an insulated shell that supports the coil and the burner and also directs the flue gas in the desired path for optimum heat transfer.
Fired thermal fluid heaters transfer heat from the flame to the fluid by two processes: radiant heat transfer and convective heat transfer. In radiant heat transfer, the flame literally shines on the heat transfer surfaces. Radiant heat transfer can account for more than 50 percent of the total heat transfer in the heater, depending on heater coil design and the fuel being burned. In convective heat transfer, the hot flue gas is directed over or past heat transfer coils and transfers heat directly to the pipe wall. It is important to note that the flame should never directly impinge on the heat transfer coil surfaces.
Serpentine Heaters. The oldest design discussed in this article, serpentine heaters have a separate radiant section (the serpentine coil) and a convective section. The cylindrical shell can be oriented either vertically or horizontally, with most of the older heaters being vertical. The serpentine coil consists of pipes that run parallel to the heater shell and are connected to each other at the ends with 180o elbows to form a continuous coil. The coil element spacing exposes the refractory lining of the heater shell to the flame. The radiant heat of the flame shines on the side of the pipe facing the flame and on the exposed refractory behind the pipes. Over time, the exposed refractory becomes hot and re-radiates heat onto the back of the coil. This is the benefit of the serpentine coil design. It spreads the radiant heat flux over more surface area and can heat the fluid more gently. Convective heat is accomplished by directing the fluid gas directly through the radiant section and then through a coil that is made of closely spaced rows of pipe.
Helical Coil Heaters. These heaters are constructed by fabricating a closely wound coil of pipe, which is then stitch welded to form a hollow cylinder. The coil then is mounted inside an insulated heater shell with a small gap between the outside of the coil and the heater shell wall. The radiant section of the coil is the first two-thirds (approximately) of the inside of the coil. When the flue gas gets to the end of the heater shell opposite the burner, it is directed back down the outside of the coil through the gap to the burner end of the heater, where the stack breach is located. Radiant heat flux is controlled by the diameter of the helical coil relative the size of the flame. As with serpentine heaters, these heaters can be mounted either horizontally or vertically. One advantage of the helical coil heater design is the ability to wind multiple parallel pipe circuits. Multiple fluid paths offer considerable flexibility in matching heater capability to process fluid flow requirements. I have seen helical coil heaters with as many as four parallel fluid paths in the heater.
Dual Helical Coil (Three-Pass) Heaters. Also called three-pass heaters, dual coil heaters employ a design where the flue gas makes three passes through the heater. Dual coil heaters employ two helical coils nested one inside the other, which adds additional convective area to the coil system. As with helical coil heaters, the radiant heating section is in the inside of the interior coil. The flue gas makes a pass between the two coils and then a final pass between the outer coil and the heater shell. The advantages of dual coil heaters are that they can be considerably more compact than other designs, and they often have higher combustion efficiency because of the added convective surface area. As with the other designs, these heaters can be mounted either horizontally or vertically. Smaller vertical dual coil heaters often are arranged with the burner mounted on top, firing down into the heater, which results in an extremely compact installation.
Which design is best? The answer to this question depends on literally dozens of considerations, including required fluid flow rate, process fluid temperature setpoint, fluid allowable film temperature, emission restrictions, and what, if any, accessories may be desired to enhance combustion efficiency. Additional considerations include the physical location of the heater and how much room is available to install the heater.
Combustion Control SystemSpecifying the burner and combustion control system of a thermal fluid heater is as important a decision as the design of the heater. The burner has to match the requirements of the heater, both in heat input and flame shape. The burner also has to have enough turndown capability to match the variable heat demands of the process. This is one item that is often overlooked by the inexperienced. The combustion control system has to conform to the requirements of national codes and standards plus the unique demands of the owner’s loss risk insurer.
Fuel selection also is important. While natural gas is the most convenient fuel choice, availability may make it necessary to use other fuels, either as a backup or as the primary fuel. Backup and secondary fuels include propane and fuel oil (No. 2 and No. 6). Other fuels include wood chips and vegetable oil. Many burners are equipped to burn two fuels, the most common combination being natural gas and No. 2 fuel oil.
Burners and Burner Turndown. While a few heater manufacturers install a proprietary burner of their own design and manufacture, most heaters are equipped with a burner and combustion control system supplied by a third party. Standard industrial burners are mounted on the heater and have a duct running to a separate combustion air blower mounted nearby. Package burners have the combustion air blower mounted on the burner as a single unit. Package burners often are less expensive and will work well in many applications; however, burners with remote blowers can offer flexibility in mounting and are easier to adapt to special requirements such as heated combustion air or emission control hardware.
The ability of the burner to turn down (or “throttle”) rests in the design of the burner; it does not reside in the control system. If the required process heat load is less than the lowest output that the burner can attain, the thermal fluid temperature will increase until the temperature exceeds a preset limit, at which time the burner will be turned off by the control system. The thermal fluid temperature in the system will then immediately begin to drop. At a temperature below the fluid setpoint temperature, the control system will trigger the “call for heat” relay, which will enable the flame safety programmer to restart the burner. If the low fire output of the burner is higher than the lowest heat demand of the process, the burner will continually cycle on and off. As a result, the thermal fluid temperature can cycle as much as 40oF (22oC). For this reason, it is as important to understand the lowest demand for heat as it is to understand the maximum in order to properly specify the burner’s capability.
Emissions. It is important to have the thermal fluid heater and burner manufacturer provide estimates of the emissions that can be expected from the equipment. This information can be used to submit to local authorities to meet permitting requirements.
Combustion Efficiency and Efficiency Enhancement. It is possible to spend more money in one year for fuel for a thermal fluid heater than was paid for the heater in the first place. That said, it should be obvious that selecting a heater with optimum combustion efficiency should be one of the owner’s primary concerns when specifying a new heater. Combustion efficiency is determined largely by the heater’s efficient extraction of heat in the convective section. In addition to the heater’s basic design, there are numerous ancillary devices that can be installed to enhance efficiency. They include:
- Economizer. This device is a
gas-to-liquid heat exchanger that allows the flue gas leaving the heater to
heat the returning thermal fluid before it enters the heater. Simply stated, an
economizer is an extension of the heater’s convective section.
- Combustion Air Preheater. In an air preheater,
combustion air from a remotely mounted combustion air blower is directed
through a heat exchanger, where the combustion air is heated by the exiting
flue gas. These devices are very effective but can contribute to an increase in
NOX production. The manufacturer’s emission estimates
should be reviewed by plant environmental personnel as well as local
authorities before specifying a combustion air preheater.
Codes and StandardsWhen specifying a fired thermal fluid heater, the specification should include the requirement that the coil be manufactured in strict accordance with ASME Section VIII, Division 1, including the U-1 stamp on the heater coil and the inclusion of properly sized pressure relief devices. Pressure relief calculations should be included as part of the manufacturer’s submittal data.
Additional codes and standards that need to be referenced in the specification of the heater include NFPA standards for burner controls; the ANSI piping codes; the National Electrical Code; insurance standards (such as Factory Mutual) for fuel train design; federal and state environmental regulations; and OSHA.
Development of a detailed purchase specification will help ensure that the heater will conform to the many codes and standards that help govern the manufacture of this equipment and will help ensure that manufacturers’ proposals will offer equivalent pieces of equipment.
SidebarUse the links at the bottom of the page to continue reading this
six-part series on specifying a thermal fluid heating system. You've
Specifying a Thermal Fluid System: A 6-Part Series
Part 2: Fired Thermal Fluid Heaters
Other parts in this series include:
Part 1: Choosing the Thermal Fluid
Part 3: Electric Thermal Fluids Heaters
Part 4: Thermal Fluid Pumps
Part 5: The Expansion Tank
Part 6: Piping Materials, Valves and Insulation