During my time working in a refinery, an option came up to use cheaper water as a cooling medium for heat exchangers cooling petrochemicals. These were the following interesting training notes I had made at the time.
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| Typical bank of shell and tube heat exchangers |
Why a special Heat Transfer Fluid (HTF) (i.e. Thermal Oil) is better for cooling than water:
1.
Better temperature control using thermal oil
system v cooling water which was outside the battery limits of the plant (i.e. cooling
tower system, ect)
2.
Thermal oil is better to be around heated
processes and nearby electricals such as level leg instrument, where elvated
temperatures are expected (above 100°C). Here cooling water may evaporate (e.g.
film boiling) and be ineffective especially through the small tubes on the
level leg instruments.
3.
The capacity of cooling water was limited at the
time (~100k GPM), given cooling medium is used inside the shells of the HEXs, we
may need large amounts of cooling water. Therefore, if there is a small flow of
cooling water there will be potential for water evaporating (phase change
possible & micro boiling) in the shell
4.
Thermal oil can be considered as a secondary
safety system, as it gives immediate emergency cooling, if needed
Using Diesel as a Coolant
v HTF like Thermal Oil within the Coolant pump around system:
1.
Lubrication properties may not be the same as
Thermal oil, especially when lubricating the Coolant pumps
2.
Diesel is a Class 2 hazard, and will face
further scrutiny, i.e. requiring another HAZOP study
3.
Heat capacity / transfer comparison – are they
comparable?
4.
Can diesel be preheated – and is there a risk of
flash off say at around 100°C? (we were not expecting to preheat but just in
case, if we were to over cool a distillation reflux circulation loop. At the time
we did not want anything below 100°C returning to the distillation column, e.g.
as steam will form leading to bogging in packing section.
5.
Can you still pump the through the coolant
circulation pump, close of vapour pressure of diesel? The centrifugal pump needs
to be analysed – as previously it was rated based on thermal oil which can a
flow rate. But maybe the maximum flowrate through the coolant circulation pump
will be less for diesel hence less coolant duty for the heat exchangers? Vendor/manufacturer
would need to confirm with updated flow curves
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| Typical draining off coolant around a level leg instrument |
Some general points about a special Heat Transfer Fluid (HTF)
1.
There are 2 ways to control process temperatures
in the distillation column circulation loop:
a.
Control temperature of the coolant via the secondary
coolant heat exchanger
b.
Control flow rate of coolant through the direct Heat
exchanger
2.
We will control temperature of the coolant (HTF)
as it is better than flow rate control for the following reasons:
a.
To maintain a constant film temperature – say
within HEX tubes and shell material (temperature gradient)
b.
A lot of velocity is lost as the coolant enters
the large volume of the shell side of the HEX, hence leading to coarse control
if we were to control the flowrate
c.
Easier to control temperature of coolant within
the coolant system with opening of valve
3.
Other considerations
a.
Would an inline filter needed to ensure coolant
(Thermal Oil or diesel) contains no solid deposits? – this would require
further inspection if any
b.
Safety concerns - regarding welding pipe into
the coolant supply/return header:
i. There
was a concern/risk of flashing off/fire and danger
ii. Confirm
with safety datasheet for the HFT
iii. Recommendations
to add a catch/isolate & then add steam purge into line to ensure no
residue HTF coolant in line
iv. Also
add fire blankets/shield when welder cut into coolant HTF line
Hope this has been useful, and please comment, like and subscribe below
Regards
Mr C
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