PTQ Q2 2022 Issue

Utilising future rows in combination with air preheaters retrofit projects Continuing with the same narrative of efficiency improvement, retrofit - ting air preheaters (APH) on existing fired heaters is becoming increas - ingly popular. Over many years, the increase in fuel costs has driven eco - nomics to favour additional invest - ment in APH systems. However, the addition of air preheat is never a ‘plug-and-play’ option. The fired heater must also be evaluated in detail. Invariably, the radiant heat flux increases when any natural draft heater is retrofitted with an APH, which can force major critical param - eters beyond the safe operating win - dow. The basic fact that the average radiant flux increases while retrofit - ting an APH will lead to a multitude of repercussions, already discussed in this article. This increase in radi - ant flux can be mitigated by utilising the future rows while executing such revamps. A case study presented below elaborates on the same. As shown in Table 3 , the unwel - come increase in radiant flux, seen in the base case retrofitted with an APH, is arrested by the use of future rows, which also ensures that the BWT remains within the design operating window. An inter - esting thing to note here is that the efficiency increment on future row addition, as shown in the case of the natural draft heater, is not per - ceived in the balanced draft heater. However, investment in future rows does bring down APH area requirements. This is because the air temperature entering the burner for the case retrofitted with future rows is lower than the case without future rows, owing to the lower flue gas entry temperature to the APH. This has an important corollary. It is always advisable to evaluate the expected performance and actual benefit if future rows are to be uti - lised with an efficiency improve - ment objective! Takeaways Future rows in the convection sec - tion of fired heaters are included as per the provisions of established design standards, and they can be an immense help in the low-cost

Increased efficiency by adding future rows of convection tubes

Heater after years

Retrofitting with two new rows of convection tubes

of operation

Process absorbed duty, MMKcal/hr



Fired duty, MMKcal/hr


25.6 405

Flue gas temperature at the exit of convection section, °C 470

Fuel efficiency, %



Flue gas side pressure drop, mmWC

0.75 247


Fluid side pressure drop, kPa


Fuel fired, kg/hr



Table 2

ple solution for efficiency increase at minimal investment. A funda - mental question that may arise is why, in the first place, was the fired heater not designed for the highest level of efficiency for a particular process condition? Why is the need to utilise future rows appearing? The answer to this query lies in process terminal condition changes and the age of the equipment. The fired heater, when newly designed, is obviously designed for the high - est level of efficiency achievable. However, with time, operating conditions of the unit change, for example, owing to fouling of the convection tubes. External foul - ing of convection tubes may be so severe in certain instances that the flue gas temperature exiting the convection section may increase by more than 50°C. The fins of the convection section tubes are also prone to mechanical wear and tear, which further reduces the heat transfer rate, increasing fuel firing and elevating the flue gas temper - ature at the exit of the convection section going to the stack. Table 2

shows a typical case study depict - ing how additional rows can ease out the added burden encoun - tered with ageing or surface area deterioration. Thus, retrofitting this existing fired heater with two new rows will enable savings of around 900 tonnes of fuel annually. Considering heav - ily fluctuating natural gas prices and other geopolitical factors, this may lead to an annual saving of more than $150 000, which more than justifies the use of these addi - tional rows. However, a few critical parameters must be evaluated well in advance. The flue gas-side pres - sure drop across the furnace con - vection section increases with the addition of future rows. To add to it, the flue gas temperature enter - ing the stack reduces sharply. These factors are critical in maintaining the draft inside the fired heater, especially considering it is a natural draft furnace. Hence, the draft and peripherals must be well evaluated, as the simplicity of a revamp can be eaten away if stack modification complexities arise.

Benefits of adding two future rows for air preheater retrofit case

Natural draft

Base case

Base case


retrofitted with retrofitted with

(base case)

APH (without future rows)

APH and with

future rows of convection tubes

Absorbed process duty, MMKcal/hr


20 22

20 22

Fired duty, MMkcal/hr


Average radiant flux, Btu/hr.ft 2 Bridgewall temperature, °C

12 000

12 600

12 100




Fuel fired, kg/hr


2227 449


Flue gas temperature at exit of convection 470 at exit of


section/entering the APH, °C

convection section. APH not applicable

Flue gas temperature entering stack, °C




APH duty, MMKcal/hr

Not applicable



Table 3

66 PTQQ 2 2022

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