Hydrocarbon Processing - August 2021 - 71

Heat Transfer
K. BATRA and N. AGARWAL,
Engineers India Ltd., New Delhi, India
Embrace tube metal temperature factor (FT
in fired heater design
Fired heaters are one of the most critical pieces of equipment
in a refinery. They are capital intensive and occupy significant
plot space, so due consideration must be given to all factors influencing
a fired heater design. Estimation of peak heat fluxes and
tube metal temperatures (TMTs) are some of the critical parameters
that have significant impact on heater design. While providing
process specifications, licensors may restrict the furnace
designer on maximum allowable average radiant heat flux based
on their own design considerations. Most furnace designers will
strive to design the furnace at this maximum allowable average
radiant heat flux value provided by the licensor, as it would minimize
the heater cost, making its design competitive.
TMT plays a significant role in the final selected average radiant
heat flux. A method to estimate the maximum TMT based on
average radiant heat flux is specified in API 530, which underlines
the various parameters and correlations to be considered for calculation.1
The
maximum TMT is estimated by calculating temperature
drop using the peak heat flux and resistance offered by
fluid film, fouling/coke thickness and tube thickness. Therefore,
the estimation of peak heat flux is vital in the calculation for maximum
TMT and final consideration of average radiant heat flux.
The adopted average radiant heat flux is multiplied by factors-such
as circumferential heat flux (FC
flux (FL
) and tube metal temperature factor (FT
and FL
), longitudinal heat
)-to arrive at
the peak radiant heat flux, which will be used to determine the
maximum TMT. Literature and guidelines2,3,4
taining to the impact of FC
factors on heater design. FC
in similar ways, irrespective of service. However, the FT
are available perand
FL
factors depend on tube arrangement, firing pattern and heater
geometry. Moreover, these two factors (FC
dependent on the service, more precisely the process fluid temperature
difference at the inlet and outlet of the radiant section.
In most cases, the FT
and FL) are applicable
factor is
factor has an insignificant impact and it
is conservatively considered as 1. Therefore, designers may turn
a blind eye to its impact in peak flux contribution. This strategy
works well for heaters where the temperature difference in the
radiant section is small. In some cases, however, this disregard
can lead to over-sizing of fired heaters by as much as 30%, leading
to substantially higher investment and limiting the heater
turndown capabilities. Being conversant with the FT
factor can
prevent confusion and multiple iterations by furnace designers,
and can help in making the heater design more competitive.
The applicability of the FT
equations governing its use are discussed here. An example of a
factor in TMT calculations and the
FIG. 1. Heat flux distribution in a firebox.
Hydrocarbon Processing | AUGUST 2021 71
)
hydrogen heater in a hydroprocessing unit is provided, where assuming
an FT
factor of 1 may lead to providing an additional area
more significant than required and almost no turndown. Also,
an example of a product fractionator reboiler is discussed, where
no significant impact exists when considering the FT
factor as 1.
Variation of peak/maximum local heat flux in a firebox.
For process fired heaters, the average radiant heat flux is calculated
by dividing the absorbed duty in the radiant section by the
total outside surface area of the coil in the radiant section. The
average radiant heat flux is expressed in Btu/hr-ft2
or MMKcal/
hr-m2
. However, the average radiant heat flux does not represent
the harshest condition in a fired heater. Within a firebox, the
local heat fluxes vary significantly, as the flue gas temperature
profile is not uniform. For a radiant firebox calculation, a wellstirred
firebox is assumed. In reality, due to flue gas flow patterns
and recirculation, the flue gas temperature may be only 537°C
(1,000°F) at the heater floor, while at arch the flue temperature
may reach 788°C (1,450°F). FIG. 1 represents the heat flux distribution
in a firebox.
Eq. 1 is used for estimating peak heat flux in a fired heater:
Pass #1 inlet
Pass #2 inlet
Pass #1 outlet
Pass #2 outlet
Peak flux
Heater elevation
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