August 2017

Heat Transfer

Revamp fired heaters with a common air-preheating system to increase capacity

In any process plant, fired heaters are generally considered to be one of the most optimally designed pieces of equipment.

Ahamad, S., Vallavanatt, R., Bechtel Corp.

In any process plant, fired heaters are generally considered to be one of the most optimally designed pieces of equipment. Any margin provided during the design stage is consumed very quickly during debottlenecking. Plant operators work hard to extract every possible absorbed heat duty from a fired heater.

In many refineries, fired heaters are often over-fired beyond their design limits, mainly to meet increased capacity requirements. Once its operation has been maximized and the fired heater capacity cannot be increased any further, the only available option is to continually revamp the fired heater to improve capacity and efficiency.

Revamping a fired heater is not an easy task, and it becomes even more challenging when multiple fired heaters share a common air-preheating (APH) system. Any modification to one fired heater will affect other fired heaters. This commonality does provide opportunities, however, as fired heaters can be used in combination. It is possible for one fired heater to compensate for another fired heater if they are revamped carefully.

This article will detail a case study that illustrates the concept of a revamp of fired heaters that share a common APH.

APH system

A typical fired heater APH system consists mainly of an APH, an induced draft (ID) fan, a forced draft (FD) fan, ducting and dampers. The flue gas from the fired heater stack is diverted to an APH, where it exchanges heat with combustion air. The cold flue gas from the APH is sent back to the stack through an ID fan. An FD fan supplies combustion air to the APH. The air is heated in the APH and hot air is supplied to fired heater burners. FIG. 1 shows a simple schematic of two fired heaters with a common APH system.

FIG. 1. Two fired heaters sharing a common APH system.
FIG. 1. Two fired heaters sharing a common APH system.


The primary purpose for using an APH in a process fired heater is to increase the overall thermal efficiency, thereby saving on fuel consumption. The potential for energy savings grows with an increase in fired heater size. A typical APH will increase the fired heater efficiency by approximately 10%. In most cases, the payback period on the installation of an APH is less than 3 yr.

Fuel generally contains hydrogen sulfide (H2S) or sulfur, which converts into sulfur dioxide (SO2)/sulfur trioxide (SO3). The APH’s heat transfer surface is subject to cold end corrosion attack caused by the condensation of SO3, which results in APH tube leakage. The amount of sulfur in fuel limits the temperature up to which flue gas can be cooled in an APH.

Revamp objectives

The revamp of a fired heater is carried out for one or more of the following reasons:

  • Increasing capacity
  • Improving thermal efficiency and operating severity
  • Extending run length
  • Reducing nitrogen oxide (NOx) emissions.

A fired heater revamp can achieve increased capacity and efficiency at much lower costs without an extended operational shutdown. Generally, fired heater revamp costs are significantly lower compared to installing a new fired heater for additional capacity requirements.

Revamp options

Options to achieve the revamp objectives for any fired heater include:

  • Changing the tube size and number of passes
  • Augmenting the heat transfer area in the convection section
  • Increasing the radiant coil tube size, mainly for pressure drop consideration.

Changing radiant tube size and number of passes

Often, changing the radiant tube size and number of passes can increase throughput. In many services, additional flow results in critical velocity in the fired heater tube, particularly in outlet tubes. This can be mitigated by changing a few tubes to a larger size in the outlet section of the fired heater. In such cases, critical velocity should be checked for each tube to ensure that it does not exceed prescribed limits.

The pressure drop through the coils increases at an approximate ratio of the square of the flowrate increase. The number of passes can be increased where a substantial increase in flowrate exists, and the increase in the number of flow passes substantially reduces the fluid pressure drop. However, other operating parameters, such as fluid film temperature and fluid mass velocity, should be carefully evaluated when increasing the number of passes.

In most fired heaters, the radiant section fire box dimensions cannot be changed. This is due to limiting considerations that include foundations, heater structure, burner layout, heater piping limitations, etc. These limitations do not allow an increase in the heat transfer surface area in the radiant section.

Increasing convection surface

The convection section accounts for approximately 30% (or more) of the total absorbed heat duty in a typical fired heater. The convection section heat transfer surface can be augmented with additional surface to further increase the absorbed duty. This addition is much simpler and more cost effective compared to adding heat transfer area in the radiant section. The flue gas temperature approach—defined as flue gas temperature leaving convection minus process inlet temperature—can be reduced to as low as 100°F of the feed inlet temperature.

The convection section heat transfer area can be increased in several ways. Some of the most frequently used revamp options are:

  • The installation of additional tube rows (e.g., the use of future rows space and/or installation of additional rows by modifying the convection breaching section).
  • Replacing bare tubes with extended-surface tubes and/or increasing the extension ratio of the extended surface (i.e., increase the extended surface area per unit length of tube by changing the configuration).
  • Increasing the number of tubes per rows. This is an expensive but very effective revamp option, and can also be used to reduce flue gas pressure drop through the convection section. In many cases, flue gas pressure drop increases with an increase in capacity, which can result in stack limitation. An existing stack can be reused with a wider convection section.

Increasing APH duty and its effect on the radiant section

An APH is used to recover the residual heat from cold flue gas. This heat from the flue gas is absorbed in combustion air, which is then used for combustion in the firebox. The heat from the APH gets recycled in the radiant section, resulting in an increase of radiant heat flux. APHs are generally designed for an approximately 20% addition capacity (design margin). The designers tend to use this additional capacity during a revamp, but this option should be used very carefully.

FIG. 2 shows the increase in radiant flux with an increase in APH absorbed duty. The radiant heat flux increases proportionally with the increase in APH size. Therefore, the preferred option is always to increase the heat transfer area in convection to cool down the flue gas to the extent possible in the convection section. This reduces the APH duty, resulting in lower radiant heat flux and longer fired heater run length.

FIG. 2. Effect of APH size on radiant heat flux.
FIG. 2. Effect of APH size on radiant heat flux.

Revamp case study system description

Two fired heaters (F01 and F02) shared a common APH system. Both fired heaters were vertical cylindrical heaters with convection sections.

Two services were being heated in these two fired heaters: F01 had process service (P1) in the radiant section only, while F02 had process service (P2) heated in the radiant and convection sections. A side stream from service P2 was taken to the convection section of F01.

P1’s absorbed heat duty requirement increased by 29%, and the process fluid flowrate increased by 41%. This increase in absorbed duty resulted in very high radiant heat flux, much higher than the allowable radiant heat flux limits for this heater. At the same time, the increase in fluid flowrate resulted in substantial fluid pressure drop. Using the existing heater with revised process conditions, the original pressure drop was doubled and the radiant heat flux for F01 was increased by approximately 30%.

A substantial increase in bridge-wall temperature led to higher-than-design temperature for tube supports, refractory and other components. It was infeasible to use the existing fired heater for revised process conditions. TABLE 1 summarizes the existing vs. required flow and absorbed duty of F01 and F02.


Conventional scheme to achieve revised capacity. As the absorbed heat duty and process fluid flowrate increase substantially, the conventional way to achieve the revised capacity is to install a small fired heater for additional capacity for P1. This new heater must also be connected with existing fired heaters and the APH system. This option costs many times more than the revamp scheme detailed here.

REVAMP SCHEME

A revamp scheme has been developed to meet the revised process condition. An overall system approach was used, and individual equipment was evaluated in isolation and as part of the overall system effect, particularly considering the potential effects on other equipment. The two main targets for this revamp—increased fluid flowrate and increased absorbed heat duty—required that all components be analyzed to select the best revamp option and ensure the performance of the revamp fired heater and air-preheater system. The existing and revamped heater coil configuration schemes are shown in FIG. 3. The existing and revamped convection sections for fired heater F01 are shown in FIG. 4.

FIG. 3. Existing vs. revamped coil configuration.
FIG. 3. Existing vs. revamped coil configuration.

FIG. 4. Existing vs. revamped convection section for fired heater F01.
FIG. 4. Existing vs. revamped convection section for fired heater F01.

F01: Increased flowrate

The increase in feed flowrate resulted in higher mass velocity and higher fluid pressure drop in the existing coil configuration. For most revamps, the number of heater passes can be doubled, resulting in a revamp case pressure drop of almost one-eighth of the original pressure drop.

Because the flowrate increased substantially, the pressure drop increased. Therefore, the revamp option was to double the number of passes to reduce the pressure drop without the need to change the existing pumps.

Increased absorbed heat duty

In most heaters, it is easier to augment additional heat transfer surface in the convection section than in the radiant section. In many cases, depending on the type of firebox, it is not possible to increase the radiant section heat transfer area. Additional heat duty can be extracted in the convection section by increasing the heat transfer area in that section. Using the flue gas temperature approach, the temperature difference between the flue gas exit and the feed inlet can be reduced to approximately 100°F.

The absorbed heat duty requirement increased substantially for P1. This cannot be achieved by any modification in the radiant section: because it is a vertical cylindrical heater, additional space is unavailable to install any incremental heat transfer surface area. It was recommended to use part of the convection section to recover additional heat duty. To achieve the additional absorbed duty, the bottom six rows of the convection section were re-tubed and used for P1. As a consequence of this modification, the heat transfer area for P2 was reduced.

Use of future rows

One way to increase the absorbed heat duty for P2 is to use the future rows available in this heater convection section. So, two available future rows in the convection section were used for P2, reducing the flue gas temperature leaving the convection section and, in turn, reducing the combustion air temperature leaving the air-preheater. A lower combustion air temperature helped to reduce the radiant heat flux.

This revamp option for F01 reduced the absorbed duty for P2 in the F01 convection section. However, this was compensated for by modification and increasing the firing rate in F02. The overall absorbed duty for P2 was achieved by modifying F02.

A summary of F01 performance parameters is listed in TABLE 2. Despite a substantial increase in capacity and absorbed duty for P1, F01 performance parameters were still within allowable design limits after revamp.

F02: Moderate increased duty

This heater required few changes, but additional heat duty had to be recovered to compensate for duty loss in F01. As mentioned, the installation of two available future rows in this heater’s convection section accomplished this. Additional heat duty was increased by slightly increasing the firing rate, which must be carefully evaluated to avoid adverse effects on heater performance. By increasing the absorbed duty in this heater, the overall absorbed duty for P2 was achieved.

A summary of F02 performance parameters is also listed in TABLE 2. F02 required only a moderate increase in duty, and part of the absorbed duty of P2 was shifted from F01 to F02. Without major modification, except for the additional two rows of tubes in the convection section, the duty requirement of P2 was met within allowable design limits after revamp.

Burners

The suitability of burners for revised firing were checked for both fired heaters. The revised firing rates for both heaters were within allowable limits. No additional burner modifications were required.

APH

The suitability of the APH was checked for revamp design conditions, and a summary of its performance parameters are listed in TABLE 3. Although the fired heaters’ absorbed duty increased overall, the air temperature leaving the APH was still below the existing heater design conditions. This helped to improve the performance of the radiant section and the overall fired heater run length.

FD and ID fans

The suitability of the FD and ID fans were checked for revamp design conditions, and a summary of their performance parameters is listed in TABLE 4. There was an increase in flue gas and air flowrates. The new operating flowrates were slightly above the existing heater design flowrates, but much below the fan design flowrates. Flue gas and air side pressure drops were also below fans design case pressure drops. Both FD and ID fans were suitable for the new design conditions without any modifications.

Takeaways

A fired heater revamp can be used for increasing capacity and/or efficiency at a much lower cost compared to installing an additional heater. All possible revamp options should be carefully evaluated, as any errors or omissions can adversely affect overall performance and increase the revamp cost. HP

 

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