Standards for fired heater design: Analysis of two dominant heat flux variation factors

[Display omitted] •Evidence is provided that fired heater design standards need correction.•Longitudinal heat flux variability is analysed experimentally and computationally.•Tube circumferential heat flux variability is systematically analysed.•Impact of tube deformations on heat flux variability is evaluated.•Impact of tube alignment in coils with changing diameter is evaluated. Many fired heaters in all parts of the globe are designed according to long-standing standards of the American Petroleum Institute. The relevant API standard 560 is to a large degree based on heat flux non-uniformity factors that have been first graphed early in the 20th century using idealised 2D calculations of radiative heat transfer. The basic set of assumptions and values for these factors has recently been challenged, as they neglect or underestimate several significant aspects of nonuniformity including longitudinal heat flux variation due to up-to-date low-NOx burners, interaction of flames from multiple burners, Coandă effect and non-ideal geometry. This paper brings new data and quantified corrections of two dominant heat flux variation factors (tube circumferential factor and longitudinal factor) that are based on numerical modelling, large-scale laboratory measurements and real-life data from operating fired heater. The new information provides evidence that standardised design calculations of fired heaters for refinery service need urgent update and revision, mainly concerning the longitudinal factor. The results show that real-life heat flux distribution can be significantly less uniform than estimated by traditional design techniques (such as API Design Standards).