Hydrocarbon Processing - January 2021 - 50

Heat Transfer
an operating temperature above
575°F-600°F (302°C-316°C).
2.	 Compare different HTFs
and recommend an optimal
hot oil for operating temperatures
above 680°F (360°C).
It is important to note that several operating refineries and chemical plants are still
using the same hot oil that was selected 40
yr-50 yr ago. These old plants could be
running their hot oil systems inefficiently
and experiencing degradation problems,
as mentioned here. Any plant or refinery
can use this approach to solve a degradation problem and select an optimal HTF
suitable and appropriate for their system.

METHOD PARAMETERS
USED FOR COMPARISON
ULSD and proprietary heat transfer
fluidsb,c,d were selected for comparison
in this study because they exhibit the
desired properties for the temperature
range of 550°F-680°F (288°C-360°C).
Note that ULSD has also been observed
to break down fairly rapidly above 600°F
(316°C) similar to the base oila. The following parameters are estimated for each
hot oil fluid, and the description and significance of each parameter are detailed.
Vapor pressure. The vapor pressure is

the pressure exerted by a pure component at equilibrium, at any temperature,
when both liquid and vapor phases exist.2
Vapor pressure increases with an increase
in temperature. Low vapor pressures are
preferred in hot services to avoid any vaporization within the system. A two-phase
hot oil system has higher frictional losses
and requires more pumping energy, which
equates to a higher operating cost. Also,
a two-phase system requires additional
piping supports, adding to project capital
costs. Vapor pressure values for the proprietary heat transfer fluidsb,c,d are available.3,4,5
Typically, refinery diesel stream data and a
proprietary softwaree simulation are used
for estimating ULSD properties.
Fluid properties. Basic fluid properties-
such as heat capacity (Cp, Btu/lb-°F), thermal conductivity (k, Btu/hr-ft-°F), viscosity (μ, cP) and density (ρ, lb/ft3)-are used
to estimate the thermal and hydraulic process parameters. Fluid properties for the
proprietary heat transfer fluidsb,c,d are available.3,4,5 Typically, refinery diesel stream
data and a proprietary software simulationa
are used for estimating ULSD properties.
These fluid properties play an important
role in determining the thermal and hydraulic performance of a hot oil.
Absorbed heat duty and mass flowrate. The heat duty (Q ) is the heat ab-

FIG. 2. Examples of fouling caused by rapid
degradation of hot oil in a heat exchanger.

50 JANUARY 2021 | HydrocarbonProcessing.com

sorbed by the hot oil heater, which is 48.1
MMBtu/hr. Based on the process requirements, the supply temperature from
the hot oil heater is 680°F (360°C) and
the return temperature is 551°F (288°C).
The differential temperature is the difference between the supply and return
temperatures (i.e., ∆T = 680°F-550°F =
129°F). The duty and differential tem-

peratures are used to estimate the mass
flowrate of a hot oil through the hot oil
system. The mass flowrate can then be
used to estimate volumetric flowrate of a
hot oil. The heat equation (Eq. 1) is:
Q = m × Cp × ∆T(1)
where m is the mass flowrate of the hot oil
in lb/hr, Q is the heat absorbed, Cp is the
heat capacity of hot oil in Btu/lb-°F, and
∆T is the differential temperature in °F.
Volumetric flowrates. The hydraulic

equation (Eq. 2) is used to convert the mass
flowrate into the volumetric flowrate (VF),
which can then be used to estimate the velocity and the Reynolds (Re)number:
lb
hr
(2)
VF = ( gpm ) =
500.5 SG
m

where (Eq. 3) SG is specific gravity =

(

)

density of hot oil lb/ft 3
(3)
density of water 62.4 lb/ft3

(

)

Velocity, Reynolds number and
Prandtl number. The velocity through

an 8-in. nominal diameter pipe (7.981in. inner diameter) is estimated for each
hot oil. This diameter is selected based
on an approximate 10 ft/sec velocity
for liquids. The corresponding Re and
Prandtl (Pr) numbers are also calculated.6 Re and Pr numbers are then used to
estimate the heat transfer coefficients, as
shown in Eq. 4:
VF ( gpm )
ft ⎞
(4)
V ⎛⎜
=
⎟ =
⎝ sec ⎠

Area of tube

VF ( gpm ) × 1 USG × 1 minute
3.142
× ( d /12 )2
7.48 ft3 × 60 sec ×
4

where d is the diameter of the hot oil tube
in inches (Eqs. 5 and 6):
inch × 1 ft ⎞
lb
ft
ρ ⎛⎜ 3 ⎞⎟ × V ⎛ ⎞ × d ⎛
⎝ sec ⎠
⎝ 12 inch ⎠
⎝ ft ⎠
⎛ cP × 0.000672 lb ⎞
ft-sec ⎟
µ ⎜⎜
⎟
(5)
1 cP
⎟⎠
⎜⎝
Re =

⎛ cP × 0.000672 lb ⎞
Btu ⎞
⎜
ft-sec ⎟
Cp ⎛
×µ⎜
⎟
⎝ lb - F ⎠
1 cP
⎟⎠
⎜⎝
Pr =
Btu ⎞

k⎛
⎝ hr - ft - F ⎠
(6)
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