The thermal efficiency of the Fired Heater is dependent upon the heat losses from the system. The primary heat loss occurs from the stack flue gases. This loss is dependent on the mass of the flue gas and the temperature of the flue gas. The combustion air quantity is dependent on the excess Oxygen in the flue gas. The second parameter is the temperature of the flue gases. There are two limitations to flue gas temperature reduction. The first one is that the flue gas temperature needs to be higher than the inlet fluid temperature for the heat transfer. The minimum approach temperature recommended is 30 F or 15 C. The second limitation is the minimum tube metal temperature, and it must be about 11 C ( 20 F) higher than the acid dew point of the flue gases. The Sulfur is present in trace quantities in the fuel gas. The sulfur trioxide mixes with water vapor to form sulfuric acid. The sulfuric acid raises the dew point of the flue gases substantially. These two limitations and the casing heat losses in the range of 1-2% limit the thermal efficiency to around 94-95%. Most heaters end up with a thermal efficiency of around 90-94%.

Fired heater design involves a number of steps in a particular order. The major design steps involved in fired heaters are as follows:

1. Finalize design basis of fired heaters

2. Perform combustion calculations

3. Carryout overall heat balance of fired heater- calculate firing rate, stack temperature, fuel firing

4. Air preheater sizing if applicable

5. Fluid side coil size and number of passes to be estimated

6. Fluid side heat transfer calculations

7. Develop preliminary radiant section sizing

8. Develop preliminary convection section layout

9. Perform fluid side pressure drop calculations

10. Perform radiant section sizing calculations

11. Perform shield section heat transfer rating

12. Carryout convection section calculations

13. Calculate flue gas pressure drop in the convection section

14. Carryout stack sizing

15. Estimate Maximum Tube Metal Temperature

16. Carryout tube thickness calculations

17. Calculate casing heat loss and refractory thickness calculations

The maximum achievable efficiency is different for each heater based on process feed inlet temperature, the fuel-fired, the fuel cost. The flue gas temperature approaches these days can be as low as 15C. The approach is also based on the material of the construction of tubes. Heaters with high process feed inlet temperature require an air preheater or steam generation system to improve thermal efficiency. Minimum flue gas stack temperature is decided based on the acid dew point of the fuel-fired, as the minimum metal temperature shall be 15°C above the acid dew point. The acid dew point is calculated based on the sulfur content present in the fuel. The casing heat loss is typically around 2.5% for the forced draft system and 1.5% for the natural draft system. Fuel gas costs play a significant role in payback calculations. Fuel gas costs are $3/MMBtu in the USA, while they range between $9-12 per MMBtu in Asia. Air preheat systems cost as much as 50-70% of the heater. The efficiency improvement must be weighed along with the increased investment cost.

Acid dewpoint condensation is a very common problem in air preheaters. The main reason for this acid dewpoint corrosion is the low metal temperature. There can be several reasons for low metal temperature. The main reason is the low combustion air temperature during winters. Another reason could be low flue gas temperature. Cold end corrosion could also be due to localized cold spots in the air preheater due to non-uniform airflow to the air preheater.

Here are two common ways to eliminate acid dew point corrosion: 
- Preheat combustion air
- Bypass air around air preheater

Acid dewpoint corrosion is directly linked with the sulfur content in the fuel gas. It will be ideal to monitor the sulfur content and link it with the combustion air temperature to maximize efficiency. Another measure will be to install thermocouples in the air preheater to monitor the metal temperature in real-time.

Steam air preheater is used to avoid dew point corrosion in air preheaters. Most fuel gases will have a trace quantity of sulfur compounds present, leading to sulfur trioxide formation. If the fuel gas is free from Sulfur, then a steam air preheater may not be required. Steam air preheaters are most effective when the ambient air temperature drops below design temperature during winters or nights, causing dew point condensation. Preheating air is one of the safest ways to keep the corrosion away and ensure long air preheater life.

This is an exciting question. We face this question every time we are designing new furnaces or revamping old furnaces. Natural draft furnaces are the most common and first choice of designers. The stack height provides the required draft in the furnace. The efficiency of these furnaces is typically balanced by the draft available in the stack. In the last 30 years, all furnaces are required to provide a minimum 60 m ( 200 ft) tall stack for lowering the pollution at the grade level. These tall stacks provide plenty of draft, and that can be used to provide a tubular air preheater on top of the convection section. It is possible to provide 90% efficient natural draft furnaces. Forced draft furnaces are very rare in the refining industry. They are used whenever we need short flames. Forced draft firing is used in down-fired steam methane reformers very often. Induced draft fans are used with furnaces with extensive convection sections or convection sections with high flue gas pressure drops. In these furnaces, the stack can not overcome the draft losses. The induced draft fan is installed to overcome the draft losses and provide the required draft in the radiant section. Balanced draft furnaces have forced draft and induced draft fans and are used with air preheating and waste heat recovery systems. FD fan supplies combustion air to the air preheater, and the preheated air is provided to the burners. The hot flue gases from the stack are diverted and sent to the air preheater, where the heat is recovered from the flue gases, and cold flue gases are sent back to the stack. The efficiency of these furnaces can go up to 94-95%.

• Generally fired heaters are designed to burn refinery fuel gases. The hydrogen content in refinery fuel gases is around 10 – 30 vol%.

• The heat content of hydrogen is more than double as compared to any other hydrocarbon(C1-C6), which results in lower fuel mass flowrate for the same firing rate.

• The density of hydrogen fuel is 9.5 times lower than the density of natural gas, which results in high volumetric fuel flowrate for the same fired duty.

• Due to the increased fuel volumetric flowrate, fuel lines, flow control valves in fuel lines, and burners may limit, if not designed with sufficient margins.

• The other impact of changing the fuel from RFG to hydrogen is on the burner flames. Hydrogen has a flame speed of 10 times more than methane.

• The higher flame speed increases the flame temperature locally which can generate high levels of NOx.

• Chances of flashback of hydrogen at lower loads is high, leading to extinguishing of flame and release of unburnt hydrocarbon into the firebox.

• The advantage of hydrogen rich fuel is that it helps in reducing the carbon dioxide emissions significantly

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Serrated vs Solid Fins: Finned tubes with segmented fins show higher turbulence as compared to smooth fins. Solid fins are more popular in case of fired heaters due to its high mechanical strength. The comparison of a few parameters is as follows:

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Fins vs Studs: Studs are expansive as compared to fins and are generally used for heavy oil firing. Basic comparison of studs and fins is as follows:

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• For the same heat duty, fired heater design may vary with a number of constraints, like; plot space availability,acoil pressure drop, type of process feed etc.

• For the VC heaters, required plot space is lower as compared to the horizontal tube heaters.

• VC heaters are preferred generally for non-critical refinery services such as reboilers, hot oil, hydrocrackers etc. For critical services like crude, vacuum coker, visbreaker preferred choice is horizontal tube heaters.

• The radiant section absorbs around 60-70% of the total heat duty of the fired heater. It is generally designed for radiant section heat flux of 8,000-12,000 Btu/hr.ft2 depending on the service.

• API 560 is the guide for designing the refinery fired heaters. Mechanical design constraints are available in API 560 for each part of a fired heater like radiant section, convection section, stack, ducts, burners etc.

• Process design parameters are generally kept as per the design industry typical practices. A fired heater for the same duty can be designed in multiple ways with varying the process parameters like flux, flue gas mass velocity in convection, etc. Selecting the right fired heater design can define the heater performance for its entire service life.

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• Presence of Sulphur in the fuel is one of the factor which controls the maximum possible efficiency of a fired heater.

• During the combustion, the fuel bound-sulphur oxidizes rapidly to sulphur dioxide (SO2). 3-5% SO2 further oxidizes to sulphur trioxide (SO3). The SO3 reacts with the water vapor (that is one of the product of combustion of hydrocarbons) and forms sulfuric acid(H2SO4).

• The acid present in flue gases starts condensing at low temperatures. The temperature at which the acid starts condensing is called the acid dew point for the given fuel gas/ fuel oil and combustion air.

• The flue gas temperature leaving the fired heater stack shall be 8°C - 14°C (15°F - 25°F) above the calculated acid dew point as per API standard 560 (fifth edition 2016).

• At flue gas temperature below the acid dew point cooling of flue gases results in condensation of sulfuric acid which leads to severe corrosion of carbon steel, low alloy steel, stainless steel and other alloys.

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• Generally , high emissivity coating is applied on the radiant tubes where the expected tube metal temperatures are very high. In heaters like naphtha reformers, ethylene cracking heaters, high temperature is required to promote the chemical reactions in the tubes and improve the yield of desired products.

• In the reformers, usually the tube material is 9Cr-1Mo, Which oxidizes at temperature above 950°F and results in corrosion of radiant tubes and shorter tube life. Scale formation on outer surface of tubes increases with increase in temperature.

• It not only affects the mechanical integrity of radiant tubes but also results in various operational issues.

• High emissivity ceramic coating helps in protecting the radiant tubes against the high temperature flames and maintaining the metallurgical reliability and stability.

• The design heat flux depends on the service of the heater. Heaters designed with higher heat flux maybe cheaper but will lead to lesser service life as compared to the heater designed with lower heat flux.

• The typical average radiant heat flux of various heater services are as follows:-

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Major factors affecting the radiant coil material are:

• Operating Temperature

• Tube Service Life

• Oxidation, Carburization, Sulfidation

• Cost

• Ductility and Weldability

• Creep – Rupture Strength

Generally, the fired heater tube material is selected for a service life of 100,000 hrs at design conditions. The actual service life of heater tubes varies depending on the severity of conditions they are subjected to during heater operation.

Enhancement of heater capacity is dependent on various process and fire side parameters:

Process Side

• Requirement of additional heat transfer area, if limited

• Allowable pressure drop

• Effect on heat flux, max. TMT

• Flow regime

Fire Side

• Burner capacity to achieve required heat release

• Ability to handle additional loads for ducts, fans and stack

• Adequacy of existing tube supports, fins to withstand more severe temperatures.

Another aspect is current heater operation. The heater and its auxiliary equipment may require further modifications on case-to-case basis.

• The optimum location of oxygen analysers in fired heaters is at radiant section exit (arch) and convection section exit or stack.

• The analyser at radiant exit indicates the excess air used for combustion.

• If the convection exit analyser shows the same O2 reading, it assures no tramp air leakage through opening i.e. peep doors, header boxes, tube penetrations etc. If it is lower than that at radiant arch, it means that afterburning is taking place in convection section.

• For the optimum operation the draft inside the heater shall be negative everywhere. Generally arch is the minimum draft location in the fired heater. Designers ensures atleast 0.1 in WC negative draft at arch level wil 120% of design flue gas flowrate as per API 560.

• The draft at arch is calculated from stack effect – flue gas pressure drop (considered from radiant arch to top of stack).

• Stack effect is movement of ambient air into the heater resulting from buoyancy due to density difference in hot flue gas and ambient air resulting from the temperature difference.