Coker Heater PDF
Coker Heater PDF
Coker Heater PDF
heaters
Film temperature control is critical to the sucessful design of fired heaters,
especially for heaters employed in upgrading heavy feedstocks
D
irect-fired heaters have to a costly result. Secondly, if Fouling requires more heat
been widely used in the the film temperature exceeds input and a hotter tube metal
oil refining and chemical the limitation, the stationary temperature to maintain the
process industries to heat the fluid film on the inside tube same heater outlet temperature.
crude oil contained in tubular surfaces is subject to thermal These factors cause heaters to
coils by the combustion of fuel decomposition, which results shut down much more
within an internally insulated in coke deposition at that loca- frequently and eventually
enclosure. A successful fired tion. Coke deposits increase reduce the whole plant’s
heater design relies on many resistance to heat transfer and profitability.
factors. Film temperature raise the tube metal’s tempera- Due to the importance of the
control is one of the key factors ture. Once the tube wall film temperature, its control has
that play a crucial role in fired temperature reaches the design become a hot topic for fired
heater design, particularly for temperature, the heater must heater designs, especially for
units processing heavy feed- be shut down for decoking crude heaters, vacuum heaters
stocks that are thermally to avoid coil damage. Thirdly, and coker heaters. In this article,
unstable, such as Canadian oil overheating of the fluid film some feasible methods of
sands-based feedstocks. accelerates the fouling rate. controlling the film temperature
Film temperature determines
the susceptibility of a process
fluid towards coking. Bulk oil 380
temperature plus a temperature
360
rise across the oil film sets the
Film temperature, ºC
Film temperature, ºC
340
film temperature, it is advisable
to reduce tube sizes for tubes 320
with peak film temperature 300
only, to minimise the increase
in pressure drop caused by 280
reducing tube sizes. 260
240
Double fired vs single fired
A fired heater can be single or 220 Single fired
double fired. The heat flux on Double fired
200
the tube’s circumferential
surface is not uniform because From inlet to outlet
of the shading of radiant heat.
The single-fired heater receives Figure 3 Comparison of the film temperature between single and double firing
radiant heat on one side of the
process tubes (directly from the can be calculated by the follow- of a vacuum heater with a
burner flame), while the other ing equations:4 design duty of 50 MW. It can be
side of the tubes, facing the seen that using double fired can
heater wall, gains radiant heat greatly reduce the film tempera-
from the refractory. The portion Tf = Tb + ∆Tf (2) ture of the radiant coil for the
of the tube facing the burners heater.
has a higher local heat flux,
qm Do (3)
while the side facing the refrac- ∆Tf = ( ) Lowering the average heat flux
tory is much lower. For a given Kf Di The first step in designing a
fired heater with nominal two fired heater is to set up the
diameter tube spacing and a where Tf and Tb are film allowable average radiant heat
very uniform longitudinal heat temperature and oil bulk flux. For a given heater, either
flux distribution, the local peak temperature, respectively. ∆Tf single or double fired, it is
heat flux (qm) is approximately is the film temperature rise and helpful to control the film
1.8 times the average heat flux Kf is a film heat transfer temperature by lowering the
(qa) for single-fired heating. In coefficient. average heat flux. From
contrast, the double-fired heater From the equations, it can be Equation 1, the localised heat
has radiant heat on both sides seen that it is the localised heat flux reduces with lower aver-
of the tubes, which greatly flux, not the average heat flux, age heat flux, no matter
reduces the peak flux to about that directly governs the film whether it is single or double
1.2 times the average heat flux.3 temperature. For a heater with a fired. Oil film temperature
The correlations mentioned given average heat flux, a depends on the heat flux and
above for single and double double-fired heater has a lower oil mass velocity. Decreasing
fired can be simply represented localised heat flux distribution the heat flux reduces the oil
in the following equation: than a single-fired heater. A film temperature at a fixed
lower localised heat flux reduces mass velocity.5
qm = Xqa (1) the film temperature at that The average radiant section
location. Figure 3 shows a heat flux is defined as the total
where X represents the time comparison of the film tempera- radiant section absorbed duty
factor, which is approximately ture between single and double divided by the total radiant
equal to 1.8 and 1.2 for single firing for the same heater with section tube surface area. For a
and double fired, respectively. the same average heat flux. The given radiant duty of a fired
The local film temperature results are from the simulation heater, the only way to lower