Aircoolers optiimization

8/10/2019 Aircoolers optiimization

1/3

54 HYDROCARBON ASIA, OCT-DEC 2011 Visit our website at: http://www.safan.com

Heat

change

T

E

C

H

N

O

L

O

G

Y

Air Fin CoolerOptimisation for Offshore& Onshore Application

Unlike shell and tube exchangers, air coolerdesign is totally different. In shell and tubeexchangers, utility flowrate is fixed, basedon cooling duty. In the case of the air cooler,

air flow rate is an open variable for designing. Forgiven cooling duty, at a higher air flowrate, airoutlet temperature drops, providing higher tem-perature gradient (EMTD) for heat transfer. Athigher air flowrate, air side heat transfer coeffi-

cient (HTC) increases, because of higher velocityand air side turbulence. Thus both will tend toreduce heat transfer area (HTA), at the cost ofhigher fan power.

Requirements of offshore and onshore air coolerdesign are quite different. For offshore structures, aircooler footprint and weight should be minimum, toreduce overall structure cost. For major offshoreapplications, tube material is exotic (expensive). Thusminimum HTAis economic.

For onshore or plant air coolers, generally spaceis not a constraint. For onshore applications, air

cooler can be placed on technological structure,pipe rack or ground.

This paper will discuss various design options tosuite offshore and onshore (plant) requirements.Basically this involves optimisation ofair cooler footprint, HTA and fan power.

If the air cooler designer or the client is notaware of possible design variations; then aircooler design may not be the best for the givenproject requirement.These will be illustrated bythe Lean Amine Air Cooler example. Examplesdemonstrate five alternate designs, to suit vari-

Air coolers might be similar to heat exchangers on surface, but delving a little

deeper will result in the one seeing the differences. This paper will discuss variousair cooler design options to suite offshore and onshore (plant) requirements.

ous layout requirement with huge variation inHTA and air flow rate (power) requirement.

Based on site location, the electric power supplycan be cheap or costly. Tube material can be CS toexpensive alloys. Based on fluid service, tube mate-rial is fixed. Thus air cooler design involves carefuloptimisation of HTA and power cost, using avail-able space for the air cooler. For expensive tubes,HTA can be minimised by increasing power con-

sumption. All these criteria can change, air coolerdesign significantly.

To suit above variation in requirements air coolersallow a wide variation in hardware design; e.g.number of bays, number of bundles per bay, numberof tube rows, number of tubes per rows, tube passes,tube length, number of fans, fan diameter and fanpower, etc. Tube size and fin design can also changeoverall air cooler design. Generally based on applica-tion and project specification the tube size, materialand fin design can be fixed.

For offshore applications, a compact air cooler is

designed using higher air flow rates (higher EMTD)and with maximum possible tube rows. Generallythis option requires higher power demand.

For onshore (plant) applications, available space isutilised, to optimize fan power and HTAof air cooler.

Above optimisation should be done with consid-eration of following constraints.

Maximum possible bundle width & tube length,to suite transport & manufacturing requirements

Minimum Number of fans per bay, maxi-mum fan power to avoid gear drive, processcontrol etc.

Heat

change


2/3

HYDROCARBON ASIA, OCT-DEC 2011 55

Project contraints (available space etc.)Budget {Total cost = hardware including structure

cost and operating cost (fan power)}Example of Lean Amine Air Cooler from Amine

Recovery Unit (ARU)After regeneration, lean amine is cooled in lean /

rich exchanger, followed by lean amine air cooler.125 tones / hr of lean amine is cooled from 72 C to65C, with the cooling duty of 1 MW.

Table 1 illustrates five alternate air cooler designs.For each design refer tube length, air cooler width,

number of tube rows and total fan power. For samecooling duty, all four parameters widely vary to suitedifferent project requirement. Figure 1 represents thevisual effect of foot print area and tube layouts forfive alternate designs.

Air side thermal resistance is highest among all

resistances. Thus increase in air flowrate has doubleeffect of increasing EMTDwith increase in overallHTC. Both will reduce required HTA. But fan powerincreases with increase in air flowrate or with in-crease in number of tube rows.

Design A has second lowest powerrequirementwith reasonable HTA. While design C has highestair flow / powerrequirement, with smallest HTA.As Design C has highest power with three fans, &similar length as design A; thus design C is notattractive for any application. Thus Design A is suit-

able for onshore / plant application.Design D hassmallest tube lengthwith reasonable

power requirement, thus suitable for offshore struc-ture or pipe rack. Smaller length of Design D isachieved with six tube rows and highest number ofshorter tubes per row.

Notes :1) Blue values are minimum and red values are maximum, for indicated parameter.2) Bract values indicates ratio of highest value to smallest value, for indicated parameter.3) $ - Bay width for design E (for two bundle)4) ** Approximate foot print area of air cooler.


3/3

56 HYDROCARBON ASIA, OCT-DEC 2011 Visit our website at: http://www.safan.com

Heat

change

Design B has 40% more power requirement as

compared to Design A. If power is costly or spaceis not a constraint, then design A is best choice. ButDesign B offers 50% smaller foot print (width xlength)as compare to design A. Thus design B can

be selected, when space is a constraint, for techno-logical or offshore structure. Design B has odd

pass, thus inlet and outlet are atopposite sides. This should bediscussed with Piping depart-ment; as this is not a commonpractice. Smaller Foot print ofDesign B is achieved using sixtube rows, with fewer tubes perrow; as compared to design A.

Design E has smallest fanpower with smallest HTA re-quirement.But this design occu-

pies biggest foot print areaamong all options (Two tube bun-dle per bay). Thus Design E is onlysuitable for plant, with plenty ofspace for air cooler. Forexpensivetube material &/or costly powersupply; Design E is most attrac-tive option among all options.

Conclusion

Thus it is clear that, by vary-ing air flow rate, number of tube rows, passes,

tube length and number of tubes per row, manyalternative designs are possible for singleair cooler.

Thus air cooler design involves; carefuloptimisation of power, space and hardware tosuite project needs. Enquiry Number 10/12-07HA

This publication thanks Mr. ManishShah for providing this paper Mr.Manish Shah has received Degree inPetrochemical Engineering from MITIndia. Mr. Shah is UK Charter engi-

neer (CEng, MIChemE) with 16 years of processengineering experience in leading various feasi-

bility study, concept, feed, basic and detail engi-neering projects for Oil & Gas, Refinery andPetrochemical units. In the field of heat transfer,Mr. Shah has presented several papers in inter-national conferences and magazine. Currently,He is working with Ranhill Worleyparsons SdnBhd as a Lead Process Engineer. His main spe-cialization is in exchanger (shell and tube andair cooler) and Tray/packed column designing.

He is proficient in Steady state simulation usingHYSYS, UniSim, T-SWEET/PROMAX, AspenPlus and PRO II. He has done extensive work inflare and blowdown system designing for oiland gas facility. He has done Process design ofoffshore & onshore oil and gas facilities withgas compression, gas liquid water separation,oil stabilisation, produced water system, acidgas removal and gas Dehydration. Process de-sign of downstream industry includes LPG re-covery, SRU (Sulphur recovery unit), TGTU,ARU, SWS and water phase sulphur oxidation.Process design of midstream industry includesLSG (Low Sulphur Gasoline), DHDS (DieselHydro De-Sulphurisation), Butene-1 and MildHydro cracking unit.

Documents

Aircoolers optiimization