FIRED HEATERS

KSP Engineering specializes in the engineering and design of a wide range of fired heaters used across multiple industrial sectors, including:

REFINERY

SRU and TGCU (Sulphur Recovery Unit and Tail Gas Clean-Up)
Furnaces of all types
CO Boilers
Waste Processing
Gas treating
Hydrogen Reformers

PETROCHEMICAL

ENERGY CONSERVATION

OUR FIRED HEATERS & REFORMERS

KSP Engineering utilises the state-of-the-art technology in the design and engineering of fired heaters used in refineries, chemical and petrochemical plants. In addition to commercially available software, we also have our own proprietary packages for many specially units. Due to the specific design constraints typical of the different process units, our process design and technologies are divided, accordingly, in the following major applications:

CRUDE ATMOSPHERIC DISTILLATION

The “High Intensity” heaters feature vertical tubes in a box envelope with tubes being fired partly from one and partly from both sides. Very large capacity forced draft burners are installed in the floor of the furnace working at low excess air.
In connection with large capacity, a high efficiency (in the range of 88 to 90%) is always required and reached via different arrangement of the waste heat recovery unit on the flue gases such as:

Direct air preheaters when there are no particular concerns about acid flue gas condensation.
Direct air preheaters with steam-air attemperator on the cold air inlet to prevent low temperature corrosion.
Indirect air preheaters where an intermediate fluid (thermal oil or similar) is heated in the convection section and then pumped around in close circuit to heat up the combustion air. This system is the preferable to avoid any risk of acid flue gas condensation.
Steam raising in waste heat boilers only when steam production is valuable in the overall refinery balance.

Special attention in the design of crude distillation heaters goes to process control, which is using the benefit of DCS optimization which allows to implement all the algorithms to keep a smooth run of the furnace independently of the feed flow variation as well as of the fuel composition fluctuation.

The Crude Unit being the only one processing the entire amount of feed in the Refinery, requires special attention with regards to reliability.
By capital cost reasons there is a definite trend to install very large capacity furnaces, sometimes even only one for the complete Refinery, therefore there is no typical size or typology for such kind of furnaces.
Based on best compromise of operability, reliability and investment cost the selection of the crude heaters for new installation is recommended as follows:

Up to 40 Million kCal/hr thermal capacity the vertical cylindrical type of heaters is preferred. This generally allows good plot arrangement, low cost and reduced number of burners.
From 40 to 150 or more Million kCal/hr thermal capacity the so called “High Intensity” type will allow most compact design and minimal number of burners at the minimum capital cost.

VACUUM DISTILLATION HEATER

Modern refineries are based on maximisation of conversion and require vacuum distillation operation to fit with the flow diagram of the overall process. This means that the vacuum tower operation must be steady without impacts coming from fired heater fluctuation.
The design has to guarantee that the profiles of pressure, temperature and vaporisation from the inlet to the heater to the tower will be achieved during operation independently of feed variation and fuel fluctuation. Fluid-dynamic simulation of the outlet tubes, manifold and transfer line are to be carried out to have the highest possible symmetry of the different flow streams resulting in equal individual flows properties.
A similar DCS control as described for the crude distillation unit is suitable for vacuum distillation as well and is necessary to keep on the long run the above conditions.
Considering the layout of the furnace, capital cost consideration are the same as for crude distillation with smaller heaters being designed as vertical cylindrical and larger one being done to the “high intensity concept”.
Vertical tube heaters will be distributed in the minimum number of streams suitable for the allowable pressure drop even if resulting in large outlet tube diameter. In our experience such tubes, in order to stay safely below the critical velocity, may have diameter as large as 350 mm.

CATALYTIC REFORMING

The main characteristic of fired heaters for catalytic reforming unit is the very high gas feed flows that need to be heated and then reheated several times for the different adiabatic reaction steps.

This implies very large compressors being used to give the system the necessary flowrate. Operating as well as capital cost consideration are, therefore, in favour of low pressure drop multi-pass heater design.

The use of parallel inlet/outlet manifold and low pressure drop tubes hairpins having very large radius return bends located in the firebox is the most common solution. In order to further minimise the coil tube length, the double fired arrangement is preferred when high duties are involved even though other consideration may time to time suggest different solutions.

Manifolds may be top (fig.5) or bottom installed, with the last option being preferred in our experience since the load of the radiant coil may be taken by the concrete foundation instead of needing unnecessary additional structure in the radiant steelwork envelope.

Due to the very tight layout of the multistage reactors special experience is needed in the design of the manifold expansion which has to be fully taken inside the radiant box by the movements of the radiant coil. A complex system of spring supports and guides has to be studied to achieve this result.

Modern heaters are based on low pressure drop design therefore can not accept the high pressure drop of conventional tube bundle convection coil. As a consequence, the process service in the heaters is segregated in the radiant section and has very low thermal efficiency with flue gas leaving radiant section typically around 800°C. Since the catalytic reformers size is in general very large, it is imperative to recover the waste heat up to an overall efficiency of around 90% with or without air preheating. This means that the raising steam is an unavoidable solution but it is still possible to use part or all of the waste heat to preheat the charge to the different re-boiling columns connected to the unit (splitter, debutanizer etc.) bearing in mind the heat transfer optimization concept of pinch technology as well as the needs of flexibility in start up and operation.

STEAM REFORMING FOR HYDROGEN MANUFACTURING

Among the various kind of fired heaters in refinery service, the steam reforming furnaces are the ones that have been subject to more improvements in the past years. These improvements were achieved in the areas of major concern:

Tube metallurgy
Catalyst
Coil configuration and firing arrangement
Inlet and outlet headers system

Tube metallurgy improved to the latest generation HP 35 Modified Microalloys grade (25 Ni 35 Cr + Nb + micro quantities of special elements). The implementation of such new materials has led to much thinner tube thickness at same design conditions (typically 9 mm).

At same time the new generation of catalysts developed by the leading catalyst manufacturers has allowed to design at higher heat flux still avoiding carbon formation and or catalyst damages (typically 70.000 – 80.000 kCal/m2 hr).

The coil arrangement has evolved from the old side fired layout to the most modern top fired one where vertical lanes of tubes fed from the top are fired on both sides by burners installed in the arch of the furnace. This layout allows:

○ Modularity of size: more lanes of tubes may be added in one single radiant box up to Hydrogen production of 150.000+ Nm3/hr.

Optimized heat flux: the major part of the heat is given where required, i.e. at the top cold section of the tubes
Optimized use of tube material: due to the fairly flat tube skin temperature profile the minimum required thickness is almost the same in each section along the tube length.

Very important improvements have been achieved also in the field of mechanical design of the outlet headers where, in order to avoid any mechanical stress to the catalyst tube, long flexible tubing is used to connect the inlet and the outlet headers to each of the individual catalyst tubes. The load of the tubes, therefore, is easily taken by the floor of the furnaces and by the top spring supports with no movement other than vertical expansion and no external load other than the dead weight of the catalyst tube themselves.

The high temperatures involved in the process of steam reforming result in very high temperature of the flue gases leaving the radiant section. Several solutions are available to recover the heat from such gases:

Hydrocarbon feed preheating
Hydrocarbon/steam mixed flow preheating
Boiler feed water preheating
Steam generation
Steam superheating
Combustion air preheating

Depending on the economic value given to steam production in the overall refinery balance, such production may be maximized or limited to the steam flow required for the steam reforming reaction only. The unit will then be either a net steam exporter or a self sufficient steam producer. In any case the approach of the pinch technology has to be followed in order to have the best utilization of the heat source at the various temperatures. which in turn will allow to optimize the selection of metallurgy for the tubes of the different streams.

VISBREAKING

Fired heaters in Visbreaking service (as well as in any cracking application) are to be considered as reactor furnaces and their design needs all necessary knowledge of cracking kinetics in general and Visbreaking process specifically.

Based on the feed composition and the coil geometry, Manmont takes the advantage of simulation programs which enables designers to predict the conversion rate and product composition at different temperature / pressure / heat flux conditions. The model will also predict the tube skin and film temperature profiles allowing to avoid film overheating and keeping control on the formation of undesired by-products (like the asphaltenes) that may have negative impact on the stabilization of the fuel oil produced in the Visbreaking unit.

The program takes also into account both liquid phase and vapour phase reactions and contains a model for the prediction of coke build up at a given forecast of running conditions.

As far as heater coil geometry is concerned, the simulation program shows no particular need for any specific lay out, therefore the most economical arrangement may be preferred, like the vertical cylindrical one.

CRACKING FURNACE

Cracked gas at high temperature comes out of the radiant coil and passes through the quench system so that temperature of the hydrocarbons goes down and further cracking is frozen. In inlet section of the quench exchanger is most critical part in whole assembly since it experiences huge temperature difference between cold side i.e. boiler feed water and hot side i.e. cracked gas. The thermal stress analysis is performed to determine the suitability of the material for the given process conditions. The routing of riser and downcomer lines have to be designed to minimize pressure drop for natural circulation. After quenching in the quench exchanger, the cracked gas is routed to quench fitting in furnace Type II for further cooling.

The quench system receives BFW from the steam drum which is located at top of the furnaces. The BFW to the steam drum comes from the convection section after preheating in economizer bundles. The heat from the cracked gas is utilized to generate saturated high-pressure steam in quench exchangers. For the liquid furnaces, the cracked gas after first stage of quenching is routed to the quench fitting. The quench fitting reduces cracked gas temperature by direct injection of quench oil. The mixing and rapid cooling of cracked gas is achieved in specially designed injection device.

The boiler feed water (BFW) is preheated in the convection section prior to entering the steam drum. The steam drum provides saturated BFW to the quench exchangers to reduce cracked gas temperature below required cracking temperature while generating saturated high-pressure steam. The quench system operates on the natural circulation. The saturated steam is fed back to steam drum which provides saturated steam to superheater bundles in the convection section for the steam superheating.

After mixing steam with feed to lower the hydrocarbon partial pressure, the mixture is superheated in HTC-1 and HTC-2 bundles before it enters crossover header for the distribution of feed and steam mixture to the split coils. Crossover is a manifold which is connected to each of coil inlets via venturi nozzle to feed them with hydrocarbon and steam mixture at constant temperature and pressure. The pressure drop in the cross over header is optimized in such a manner that each pass will have equal flow through the venturi nozzles. Installation and design of the crossover shall be very precise as it holds the pigtails on it which connects to the radiant coils. Due to thermal expansion, the designs of crossovers as well as associated support need to be investigated in very much in details. The faulty design of crossover might lead to improper distribution of feed through the header and in the worst cases cracks on pigtail joint which may lead to explosive environment.

For the furnace designed for liquid cracking, the dilution steam is superheated in dilution steam super heater prior to the mixing with hydrocarbons so that it helps in complete vaporization of the naphtha feedstock.

In the radiant coils (placed in the radiant boxes), the cracking occurs to produces olefins and byproducts. Due to optimized heat flux via floor and wall burners, the whole coil length experiences a uniform heat from outside so that temperature gradient will be similar for all the coils. The combination of floor and wall firing provides excellent radiant section efficiency and good heat distribution. The coils have no fixed support as whole system hangs on the hangers without any bend fixed on the furnace floor. The hanging system helps the coil to relax in the high thermal conditions.

Each of the radiant boxes consists of floor and side wall burners, 70 – 75 percent of the total required fired duty is given by the floor burners and 30 -25 percent of total required fired duty is given by wall burners. A combination of wall with floor firing makes the fuel gas piping arrangement complex. The performance of the burners is very much dependent on the available pressure at inlet. The fuel gas piping arrangement shall take care of maximum allowable pressure drop so that furnace shall not experience any bottleneck due to lack of fuel gas pressure at burner.

Cracking is an endothermic reaction which requires heat for chemical reactions. The heat is given to the coils via floor and wall burners. The typical efficiency of firebox i.e. heat transferred to the cracking reaction, is in the vicinity of 42-45% which means that rest of the heat flows to the convection section for heat recovery.

Superheater bundles are split so that an intermediate boiler feed water injection to control the temperature of superheater steam can be installed. The split coil provides an excellent run length for liquid/gas feedstock.

COKING

All considerations about the need for a rigorous kinetics modelling program, Visbreaking unit, are in general valid also for the Coking unit heaters as well. However, the higher criticality of Coking versus Visbreaking reaction result in some specific differences:

Faster coke build-up means shorter run time and requires more than one heater (typically two) to be installed in the unit. This allows to run at reduced capacity during the decoking operation.
Much lower residence time / higher mass velocity is required to have the smallest possible difference between bulk and film temperature, thus allowing for the required conversion rate at minimal coke build-up on the inner tube surface.

Simulation software and field feedback data show that the preferable configuration is the horizontal tube lay out with two heaters running in parallel.

HYDROCRACKING

Selection of heaters for the Hydrocracking units is strongly impacted by two main factors:

○ Relatively high process outlet temperature
○ Very high process pressure (typically 200 bars)

These two constraints result in the need for very thick high alloy tubes making the coil lay out the major factor in the competitive design of the heaters. To cope with the above, state of the art technology is to use the double fired tubes / vertical box heater featuring stainless steel coil material (typically ASTM grade 321H or 347H).

The double fired arrangement, where tubes are fired from both sides at minimal heat flux misdistribution and the optimized tube diameter selection allow keeping the tube thickness of such high alloy material within the range of commercially available coil material. Since no other specific process need is limiting the design, the most economical vertical tubes arrangement is selected.

OUR ENGINEERING SERVICE

Our comprehensive fired heater engineering services encompass the entire lifecycle of your heating equipment, from initial design and fabrication oversight to ongoing maintenance and optimization.

Our team of dedicated engineers possesses the expertise to tackle any challenge, whether it’s designing a new heater to meet your specific process needs, troubleshooting operational issues, or maximizing efficiency through performance assessments and upgrades.

We are committed to providing a single point of contact and a collaborative approach, ensuring your fired heater operates safely, reliably, and cost-effectively throughout its lifespan.

Energy Conservation

The bulk of the energy requirements of the world is presently derived from fossil fuel sources. These sources are getting depleted at a faster rate.

Since alternative sources have not been developed to full extent, conservation of energy has assumed great importance today, in particular when the energy cost is also increasing day by day.

It may be very impressive to note that a refinery with moderate to low furnaces efficiency is firing the equivalent of 4% of the crude if processes and that improvement of efficiency up to 90% may save 0.5% to 1% of total processed crude. This means, in a 12 MM t/y refinery a saving of more than 100.000 t/y.

For complete combustion with minimum excess air one should exercise control on the following:

Excess air monitoring
Air Infiltration
Combustion air pressure

1) COMPLETE COMBUSTION WITH MINIMUM EXCESS AIR

Besides abnormal increase in stack losses, with the increase in excess air, the ingress of too much excess air lowers the flame temperature and consequently reduces furnaces heating rate. If too little excess air were used, the combustion would be incomplete and lot of fuel will be wasted and will be carried away by flue gases in form of unburnt combustible gases such as carbon monoxide, hydrogen and unburnt hydrocarbons. Thus, it is significant to use appropriate control system to monitor proper Air/Fuel ratio to enable the heater to operate with adequate excess air level. Besides this, proper handling of fuel oil can also contribute to the Energy Conservation. While handling liquid fuel oils, some of the points which everybody should always remember are:

Split oil is not retrievable. Plug all leakages
Impurities in furnace oil affect combustion. Filter oil in stages
Oil has to be preheated to obtain the right viscosity to the burn. Provide adequate preheater capacity.

2) PERFORMANCE AT TURNDOWN CONDITIONS

Proper selection and sizing of burners are crucial to ensure they operate efficiently across a wide range of firing conditions. This includes the ability to perform well at turndown, which refers to the burner’s ability to operate at lower firing rates without compromising performance or safety. Turndown performance is particularly important in processes that require flexibility and varying heat demands, ensuring that the burner can adjust to these conditions while maintaining energy efficiency.

When sizing burners for turndown, factors such as the combustion system’s design, fuel type, and the required heat output need to be carefully considered. Burners that are oversized may struggle with stable operation at lower firing rates, while those that are undersized may not be able to provide adequate heat when needed. Selecting the right burner size and configuration allows for smooth transitions between full-load and low-load operations, improving the overall efficiency and lifespan of the system.

Ensuring excellent performance at turndown conditions is not only about meeting operational needs but also about maintaining safety and minimizing environmental impact. Well-sized and well-designed burners help reduce the risk of flame instability, excessive emissions, and inefficient fuel use, contributing to more sustainable operations in the long run.

3) PROPER HEAT DISTRIBUTION

The selection of the appropriate type of burners is a fundamental consideration in ensuring proper heat distribution within a furnace or combustion system. Different burner types, such as premixed, diffusion flame, or staged burners, offer distinct advantages depending on the specific application and desired heat distribution characteristics. Choosing the right burner not only affects combustion efficiency but also plays a crucial role in how uniformly heat is distributed across the furnace, which is essential for optimizing energy utilization.

In addition to burner selection, the layout of the burners within the furnace significantly impacts the overall heat distribution. A well-designed burner arrangement allows for balanced heat delivery across the entire combustion chamber, minimizing hot spots and temperature gradients that can lead to inefficiencies or equipment damage. By strategically positioning burners, the system can ensure more even heat coverage, promoting better performance and reducing the need for excessive energy consumption.

The geometry of the furnace itself also plays a critical role in heat distribution. Factors such as furnace shape, volume, and the flow path of combustion gases must be carefully designed to support efficient heat transfer throughout the system. For instance, the integration of baffles, heat exchangers, and optimal gas flow patterns can enhance the uniformity of heat dispersion, ensuring that all areas of the furnace are effectively heated while preventing localized overheating or underheating, which can impact process outcomes.

4) REDUCING HEAT LOSSES FROM FURNACE OPENINGS

Furnace openings represent a significant source of heat loss, which can drastically reduce the efficiency of the system and increase operational costs. Heat tends to escape through areas such as furnace doors, inspection ports, and other access points, leading to wasted energy and poor temperature control. Therefore, it is essential to ensure that all openings in the furnace are tightly sealed to minimize these losses. Properly designed seals and insulation materials can help prevent heat from escaping, improving both energy efficiency and furnace performance.

In addition to the doors, other furnace openings must also be properly insulated. The refractory lining used around access points should be sufficiently thick and made from materials with excellent insulation properties. This prevents heat from escaping around areas such as windows or inspection ports, ensuring that the furnace retains as much heat as possible. Regular inspections and maintenance of these seals and linings are also essential to ensure their ongoing effectiveness. By addressing heat losses from all furnace openings, operators can significantly improve energy utilization, reduce fuel consumption, and extend the operational life of the furnace.

5) PRESSURE CONTROL

Maintaining proper pressure conditions inside the heater is essential for ensuring efficient and uniform heat distribution. Correct pressure regulation prevents fluctuations that could disrupt the combustion process, leading to uneven heating and inefficient fuel usage. By controlling the internal pressure, the system ensures that the heat is distributed evenly throughout the furnace, optimizing the combustion process and improving overall energy efficiency. This, in turn, helps to achieve more consistent and reliable performance, reducing energy consumption and operational costs.

Moreover, proper pressure control also helps in preventing the infiltration of outside air into the furnace. If external air enters the system, it can interfere with the combustion process, causing instability and reducing the heater’s efficiency.

The aspect of furnace pressure control plays a crucial role in energy conservation. Consistent pressure regulation helps to maintain the correct air-fuel mixture for optimal combustion, thereby minimizing energy losses. Additionally, modern pressure control systems can automatically adjust to changing conditions, ensuring that the furnace operates at peak efficiency at all times. This proactive approach to pressure management is an essential part of any energy-saving strategy, helping to reduce both fuel consumption and the environmental impact of the heating process.

6) MINIMISING WALL LOSSES

The choice of refractory and insulation materials is crucial in minimizing wall losses and enhancing overall furnace efficiency. High-quality refractory materials with excellent thermal resistance help prevent heat from escaping through the furnace walls, ensuring that the energy generated inside the furnace is retained for effective use. Refractory linings that are properly selected and installed can significantly reduce the amount of heat lost, which not only improves fuel efficiency but also promotes a more stable and controlled temperature environment within the furnace. This reduces the need for excessive fuel input and ensures that the system operates at its optimal performance.

In addition to refractory materials, the use of advanced insulation is equally important in minimizing wall losses. Insulation materials with superior thermal conductivity help maintain high temperatures inside the furnace while preventing heat from dissipating into the surroundings. The combination of effective refractory linings and insulating materials provides a barrier that retains heat, reduces energy wastage, and ultimately lowers operating costs. Proper maintenance and regular inspection of these materials are necessary to ensure their continued performance. By investing in the right materials for furnace walls, companies can achieve significant fuel savings, lower emissions, and improve the overall energy efficiency of their heating systems.

7) WASTE HEAT RECOVERY FROM FURNACE FLUE GASES

A major part of the heat being wasted can be recovered by utilisation of sensible heat of the furnace flue gases. In addition to heating of the process stream in the convection section, heat recovery can be affected by steam generation and/or combustion air preheating. The most common and effective way to utilise the waste heat in fired heater is preheating of combustion air. The preheating of combustion air may be:

Direct type with flue gas – combustion air heat exchanger in finned cast iron tubes.
Indirect type with flue gas – heating medium convection exchanger and heating medium – combustion air exchanger at ground. In this case a drum and pumps system are installed to assure heating medium circulation.

The introduction of preheated air into existing furnace requires a redesign of the furnace due to change in heat load distribution. The investment required to incorporate air preheat system in the existing furnaces are very attractive having a pay back period between one to two years.

Increase of furnace efficiency by air preheating is one of the powerful contributions of energy conservation. The fundamental concept is to reduce the temperature of flue gases and transferring heat from the flue gas to incoming combustion air. An air preheater system saves furnace fuel by transferring heat from flue gases to combustion air. Furnace flue gas temperature is reduced and operating efficiency is correspondingly increased.

The main advantages of heating of combustion air can be listed as below:

Saving in fuel consumption
Increase in flame temperature
Improvement in combustion
Reduction in initial heating time
Reduction in scale losses

Thus, we can say, that higher the air preheat, higher would be the saving in fuel consumption. But to what extent, the air can be preheater is influenced by various factors like:

Volume and temperature of exit flue gases
Suitability of burners employed, to withstand higher temperature
Economic sizing of Air Preheater
Economic evaluation of the choice of stack temperature Considering sulphur dew point or sulphur corrosion phenomenon

The above is again depending on the sulphur content in the fuel being used.

Heaters' Services

Process & Hydraulics Design
PFD & P&ID Development
Control Philosophy Development
Piping Stress Analysis Piping 3D Modelling
Structural Engineering and 3D Modelling
Mechanical Engineering
Refractory Lining Engineering
Electrical & Instrument Engineering

KSP