2. INTRODUCTION
Three of the condensers at the PX Plant were limiting
(1) THFR Overhead Condenser
(2) STRIPPER 2 Overhead Condenser
(3) NHT Reactor Outlet Condenser
The above condensers were limiting either in summer conditions or creating constraint for
increase in throughput, leading to problems in pressure control and finally to the loss of
important fractions in the flare.
So, the aim of the present work is to explore different ways for improving the duty of the
present condensers.
3. APPROACH TO SOLVE THE PROBLEM
Study of the associated columns.
Study of air-cooled heat exchangers/field visit to THFR fin fan cooler.
Researching for different ways of improving heat transfer.
Collection of data of the condensers from GA Drawings.
Introduction to HTRI
Matching the base case/plant case for the condensers.
Moving ahead with the different types of changes.
Finally formulating a table with different changes.
4. BASE CASE VALIDATION / PLANT CASE
DATA FOR
DUTY
(MW)
THFR
CONDENSER
STRIPPER 2
CONDENSER
NHT OUTLET
CONDENSER
DESIGN DATA 2.6 6.23 2.13
HTRI DATA 2.503 6.25 2.17
IN SUMMER 1.9 4.6 2
5. DEBOTTLENECKING OPTIONS
Change in Fin Type/Shape/Material/Density.
Use of Tube Inserts
Change in Air Flow Rate.
Looking for changes in material of tubes.
Changing the no. of tube passes
Going for additional Tube row/Bundle.
6. CHANGES IN FINS
MATERIAL
FIN MATERIAL OVERDESIGN (%)
CARBON STEEL - 14.86 %
STAINLESS STEEL - 36.41 %
COPPER 5.08 %
ALUMINUM 0.0 %
The above table was generated by simulation in HTRI considering
Aluminum as the base for performance comparison.
7. FIN TYPE / DENSITY
HTRI offers 88 different types of fin types, all the types were simulated one by
one. Among the different types of fin types, the best one was the fin type with
fin code 625 Extruded 1000.
The performance of the fins have been measured for a fixed air flow rate.
The specifications of these fins are also in complete agreement with the
specifications of the present tubes.
FIN TYPE BASE
THICKNESS(mm)
TIP
THICKNESS(mm
CURRENT FINS 0.432 0.216
625EXTRUDED
1000
0.457 0.305
This Fin type with fin density of 11 fins/inch gives a duty increment of 5-6%
8. FIN SHAPE
HTRI provides simulation for three different shapes:
Circular, Serrated, Rectangular
The three shapes were simulated, one by one keeping the same air flow rate,
same fin height.
FIN GEOMETRY OVERDESIGN (%)
CIRCULAR FINS 0.0
SERRATED FINS
SPLIT SEGMENT WIDTH(0.5
MM)
7.27
RECTANGULAR FINS - 40
The above table was generated by considering circular fins as the basis for
performance Comparison.
9. CIRCULAR vs SERRATED
By simulation, it was found that a duty increment of 4-8% can be achieved with
the serrated fins(split width 0.5 mm), instead of circular fins.
10. TWISTED TAPES
Twisted tapes induce turbulence and superimposed vortex motion(swirl flow)
causing a thinner boundary layer and consequently resulting in a high heat
transfer coefficient, at the same time they increase the pressure drop.
The Heat transfer enhancement efficiency and the Nusselt number increases by
decreasing the twist ratio.
11. SPECIFICATIONS OF TWISTED TAPES
In the market, tube inserts with L/D ratio between 6 and 18 are only available. The
thickness of these tapes, usually range between 1mm and 0.5mm.
To find the best tape in terms of pressure drop as well as heat transfer, a simulation
was carried out on HTRI condenser with different twisted tape specifications.
12. AIR FLOW RATE
Air flow rate is the single most important variable in the operation of
ACHE(Air-cooled heat exchangers).
Factors such as tube fouling, reverse air flow reduce the air flow rate to the
bundle.
The above factors combined with the increased ambient air temperature in
summer reduce the duty by a significant amount.
The air temperature was chosen as 40˚C for all the cases, considering the
summer conditions.
An air flow increase of 20% is taken.
An increase of 10-18% is observed in the duty just by increasing the air flow
rate by 20%.
13. TUBE MATERIAL / ROW, BUNDLE INCREMENT
Among the 12 different kinds of materials that HTRI offers, carbon steel
and MO Plus S32950 gave the best and comparable results.
MO Plus gives a nominal duty increment of 0.5% as compared to carbon
steel.
ROW INCREMENT/ BUNDLE INCREMENT WERE TRIED IN THE CASE OF THFR
CONDENSER ONLY.
14. CHANGING THE NO. OF PASSES/ NO. OF TUBES
IN PASSES
Increasing the number of passes was not fruitful in any of the cases.
THFR CASE
The above case came out to be the best, and was possible only in the case of additional
bundle, which gives a duty increment of 16%.
In STRIPPER 2 CONDENSER, by decreasing the present no. of tubes from 93 to 83 in
second pass, a duty increment of 5% as well as the pressure drop decreased by 13.51%.
15. RESULTS
All the combinations of the different changes were tried.
CONDENSER FIN CHANGE
(CIRCULAR)
FIN
(SERRATED
AIR FLOW TUBE INSERTS
THFR CONDENSER 5.55% 12.73% 10.35% 4.14 %
STRIPPER 2
CONDENSER
3.31% 5.4% 17.12% 3.59%
NHT REACTOR
CONDENSER
4.93% 13.04% 9.22% -------
17. BEST RESULTS
After simulating for different combinations, the combination with best
duty increment was, the fin change + air flow rate change
Comparing the best results on the scale of circular and serrated fins.
FIN CHANGE +
AIR
THFR
CONDENSER
STRIPPER 2
CONDENSER
NHT REACTOR
CONDENSER
CIRCULAR FINS 15.84% 20.71% 15.21%
SERRATED 26% 23% 26%
PRESENT FINS
(SERRATING)
17% 18% 17%
18. OTHER WORKS
THFR CASE FIN
CHANGE
TUBE
INSERTS
AIR FLOW
INCREASE
FIN
CHANGE+AIR
ADDITIONAL
ROW(14.19%)
27.9% 18.52% 27.48% 43.32%
ADDITIONAL
BUNDLE(99.75%)
110.9% 108.34% 121.02% 151.85%
19. CONCLUSION
Different changes and their combinations were tried to increase the heat
transfer in ACHEs.
Among the different changes tried, the maximum increment in duty was
obtained in the case of fin change + air flow increase
The above case offers more increment in the case of serrated fins
Detailed tables with different combinations have been generated
