fan optimization

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Warm Air Ambient Air Cooled Process Fluid Hot Process Fluid Optimization of ACHE's By Nick Agius and Jack Horan

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Warm Air

Ambient Air

CooledProcess

Fluid

HotProcess

Fluid

Optimizationof ACHE's

By Nick Agius and Jack Horan

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This technical paper provides a general guide formaintenance and process personnel to troubleshoot and collectdata necessary to help achieve the best possible performance oftheir “Air Cooled Heat Exchangers” (better known as “FinFans®”).

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The term Fin Fan®, is actually aHudson Products Corporation tradename for their own “Air Cooled HeatExchangers” (ACHE’s). These are axialflow fans that move air through fin tubebundles in order to cool the process.

Some plant operators will spray wateron the fin tubes for additional cooling.This practice gives little cooling benefitand adds costly long-term effects.

The effect of this water spray loosensthe L-wrap type of fin that is mostcommon on today’s fin tubes. This L-wrap is only in contact with the tube dueto the tight wrap during manufacturing.When rapidly cooled, due to waterspray, the L-wrap contracts. Whennormal temperature returns, the wrapexpands and is now loose on the tube.

This L-wrap expansion allows the water to get in between the tube and thewrap, minimizing the effect by insulating the wrap from the heat of the tube. Italso allows for build up of calcium carbonate and other deposits, which also actas insulators. This makeshift water spray, often referred to in slang terminologyas “California Coolers,” is not a good idea. The short term gain is more thanoffset by the long term consequences.

If the ACHE's are not meeting the required cooling demands, they needto be examined in the following areas and the necessary data collected. Thiswill allow the end-user to make an informed decision on options to improvethe particular ACHE situation.

• Are the fans leaking air?• What is the actual pitch? Is this the same as originally designed?• What is the actual HP being used? Is there further potential in the

motor?• What kind of drives are installed… V-belt, timing belt or gearbox?• What is the “tip clearance”?• Are there “inlet bells” currently installed?• Are there “seal discs” or “air seals” installed?

If not, is the ACHE design lacking “Seal discs”, or have they beenremoved and not reinstalled?

• Are the fin-tubes clean?• Is the fan running the design speed shown on the original

specification sheet? Is the fan running in the correct direction andare the blades pitched with the leading edge down?

• Are these fans delivering the airflow that the original specificationsheet suggests?

• Is this original design sufficient to meet your requirements now andin the future?

Using “California Coolers” is not a good idea!

Leaking fansIt is normal to see daylight directly through the tube-bundles and this is also

an excellent way to determine if the tubes need to be cleaned. If holes and gapsare seen when viewed from the bottom of the forced draft units (FD), or whenviewed from the top of the induced draft units (ID), then air is leaking. Theseleaks are created from the obvious holes and gaps in the fan box. These holesresult in air and costs going out the sides of the fan instead of through the fintubes. The correct strategy is to push the air through the bundles, not out thesides of the fan housings.

PitchIt is very important to always know and record

the pitch for all fans. Every blade (for eachparticular fan) needs to be at the same pitch (± .5º).It is also very important to know exactly where totake this pitch reading. Most ACHE manufacturershave excellent web sites. This, and additionalinformation including maintenance manuals andfan design software programs, can be found onthese web sites.

The pitch on Hudson fans is taken about 1”inwards from the end of the blade tip. Make surethe pitch-level tool covers the whole width of theblade. If it doesn’t, place a long enough piece ofwood under the pitch-level to get the correct bladeangle reading.

The pitch reading for Moore fans and Chittomfans is taken back on the flat block near the hubinstead of at the end of the blade. The leading edge

of the blade has to be tipped down for the airflow to go up through the fintubes.

The maximum allowable pitch of the fan blade is also important to know, butmany end users feel that if they know this they can set the fan at this maximumpitch and use the variable frequency drives (VFD) to control the fan duringwinter and summer conditions. This is a poor practice because the maximumpitch differs under various running conditions and could place the fan at or neara stalled condition.

The best way to understand this issue is by using the information on theoriginal specification sheet supplied by the cooler manufacturer at the time ofthe original purchase.

If there is any available unused horse power (HP) at the current speed, slowlyramp up the airflow and static pressure using the fan program from the originalmanufacturer to make use of this available HP. It will be evident that the pitchangle will increase if the speed is kept the same. This exercise will reveal thecorrect pitch angle to run the blades. Pitching the blades higher in order toincrease airflow will eventually stall the fan. Some fan program's softwareindicate when the fan is in stall. If the program doesn't, then knowledge of howto read a fan curve is required.

The next step is to speed-up the fan in order to eliminate this stalledcondition. Understanding the relationship between static pressure and airflow is4

Hudson Fan pitch is measured 1" in from theend of the blade tip.

necessary to be able to input any air-flow increases in the fan program. SeeFormulas and Rules on page 9. This is also available on an Excel spreadsheet.Contact Nick Agius ([email protected]) for a copy.

Speeding up the fan is not as easy if it is driven by a gearbox, but it is easyenough to purchase a new set of gears to change the ratio of the box.

When increasing the speed on a belt drive it is easiest to increase the size ofthe motor pulley. This often has a hidden benefit because in many cases thedriver pulley is already well worn and in need of replacement. It's generallyeasier to change, compared to the larger driven pulley and costs less. The largerdiameter on this driver pulley not only increases the fan speed, but also lowersthe bearing load on the motor and increases the overall belt drive HP rating.

NoiseIncreasing fan speed has very few disadvantages other than increased noise

levels. Higher tip speed and any increase in airflow generally means morenoise. Be aware of local noise codes when making ACHE upgrades. Speciallow noise blades are available. Contact the author for more information on thissubject.

Never do this!One major problem often encountered, is when the fan pitch is set by using

the amp reading of the motor. This method rarely results in the fan running inits' ‘sweet spot’. The fan will draw more amperage when it is in stall orapproaching stall. Therefore, using amperage as the only reference is not goingto give the desired result. This could end up with less airflow and higher powercosts. If, however, the fan speed is high enough and the pitch is flat enough,then the fan is running very efficiently. If this is the case then amp readings canbe used to fine tune and get the fan running at its optimum pitch and efficiency.

Is the HP being used efficiently?It is very important to know if the HP is being used efficiently in order to

maximize the ACHE systems. Take the amperage and voltage readings on allthree phases, average them and insert the values into this formula. The motorefficiency and power factor is also required. This will give a more accuratereading on the HP being used by the motor than simply reading amps.

HP= voltage x actual AMP reading x 1.732 x motor efficiency x power factor.746

Upgrading to new high efficiency motors is recommended. These new motorsare almost 10% higher in efficiency compared to motors from 20 years ago.This 10% adds up to increased cooling for the same power costs.

In addition to higher efficiency, consider the next size HP motor. Check tosee if the motor leads and related equipment can handle the newer larger motor.Higher HP means more airflow and more cooling. If the capital commitment ismade to upgrade to a high efficiency motor, then upgrade to the next size motorif it means no other modifications.

It is critical that the motor is running in the correct direction for the fanblade. All ACHE's turn counter-clockwise when viewed from under the fan.

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DrivesDrives are a major area of opportunity for big improvements on ACHE’s. If

using V-belt drives, one should consider converting to one of the many timingbelt options available today. TheACHE market has long recognizedthe advantages of belt drives thatdon’t slip. One of the top performingand very recognized belts available onthe market is the Gates Poly Chain®

belt. Gates has an excellent rangemade especially for ACHE withlighter driven pulleys. Gates also hasthe narrowest belt available, whichhelps reduce motor bearing loads andoverall drive costs.

The 20mm wide (14mm pitch) PolyChain® belt will handle most ACHEapplications ranging from 1 to 50HP,depending on the size of the driverpulley. The belt drives must be

designed correctly. When a belt drive is designed with a motor pulley that is toosmall, it will create a problem for the motor bearings.

Gates has a downloadable program called “Design Flex II”. This has aspecial function for ACHE’s.

All timing belts are considered 98% efficient. Once they are installed, alignedand tensioned correctly, the belt drive may never have to be touched again.Experience dictates that these belts stretch slightly at first. After running for afew hours, and re-tensioned, they will not require any further attention for thelife of the belt.

A V-belt is considered 97% efficient only if it is running on new pulleys andtensioned and aligned correctly, but this rarely happens on ACHE’s. The driveris so small, and the driven is so large, that this belt slips most of the time.Slipping V-belts on an ACHE reduce efficiency and waste money. V-belts tendto slip more in the hot summer when cooling requirements are at their highestlevel. V-belts stretch by design so they need to be re-tensioned several timesduring their life span. The problem with many ACHE’s is that they often do notget the necessary attention required to maintain the V-belt drives correctly.

Tension information for all drive belts is on the drive sheets provided by yourbelt supplier. Make sure the maintenance staff have tension tester tools and thecorrect tension information for each drive. Writing the tension (in pounds) andthe deflection force (in inches) on the side of the framework near the fanensures that the information is readily available to the maintenance staff anddecreases the opportunity for improper tension, which is the leading cause of alltiming belt failures.

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Tip ClearanceTip clearance is a major issue and is

relatively easy to fix. The blade tip has thehighest surface speed of the entire blade,therefore having a good tight tip clearance willpay back with increased airflow. Fans create anarea of low pressure on the suction side of thefan. If there is any amount of tip clearance thelow pressure area creates an opportunity for theair that is under high pressure on the oppositeside of the fan to flow back to the low pressureside, thereby causing recirculation of air. Themore tip clearance, the higher the air flow willbe from the high pressure side back to the lowpressure side. The objective is to move that aironce through the fin tubes, not to have it constantly recirculating at the tip ofthe blade. The industry suggests that good tip clearance can improve fanperformance by 2 to 3% (depending on existing clearances). Here is a guide asto what is considered acceptable for tip clearance for the different sizes of fans.

3 ft fan through and including 9 ft fan 1/4" to 1/2"10 ft fan through and including 11ft fan 1/4" to 5/8"12 ft fan through and including 16 ft fan 1/4" to 3/4"18 ft and up (cooling towers) 1/4" to 1"

The simplest way to remedy poor tip clearance is with a product called tipseal. This is a metal (honeycomb aluminum) seal that attaches to the inside ofthe existing fan ring with metal screws. An installer can easily adjust the tip sealto the above requirements, using a tool that attaches to the fan blade.

Inlet BellsAn inlet bell can be part

of the fan ring or can beadded later. It is a simplecurved section of fiberglassthat is screwed to the inletof the fan ring. It is easy tocut by hand and to attach inmost cases. This curvedfiberglass housing allowsair to enter the fan in asmooth manner and hasbeen measured to increasefan efficiency by 2 to 3%.In some cases it can besomewhat difficult toretrofit the expanded metalguard after installation of theinlet bell, but it is worth the effort to install these inlet bells in order to achievethe extra efficiency.

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ACHE with tipseal installed

ACHE with inlet bell installed.

Seal discs or air sealsThese are the large plastic or metal covers that are normally attached to the

fan hub, providing a barrier for air that tries to re-circulate back down, close to the center of the fan.The center of the fan is in an area of low bladesurface speed, and therefore is a low-pressure zone.The pressurized air that has just passed through thefan naturally tries to flow to this low pressure area. Ifthe seal disc is not installed, the fan can be losingefficiency by allowing this air to be re-circulated.This seal disc should be about 25% of the fan'sdiameter. If there is no seal, or if the seal disc isbroken, this can affect the fan efficiency by 2 to 3%.

CleaningIn many cases it is easy to determine if the tube

bundles are dirty simply by looking up through the finbundle with louvers open towards a light source. If daylight cannot be seenwhen looking up at a position of 11 o'clock or 1 o'clock, the bundles need to becleaned. There are many different types of cleaning methods available. It is bestto find a contractor in your area who can offer different cleaning options. Someof the fan manufacturers have information on their web sites pertaining tocleaning. A general practice is to get the bundles cleaned once a year.

Fan speedA physical survey of the fan drives helps determine the exact speed. Compare

this to the original specification sheet to see if it has sped up or slowed downover the years. Increasing speed is generally advantageous, provided that themaximum recommended tip speed of the fan blade is not exceeded, and thatincreased noise is not a concern. On the other hand, slowing the fan speed canresult in a stalled condition. When using V-belts, an actual shaft-speed shouldbe recorded. As stated earlier the V-belts slip so the accurate shaft speed cannotbe determined by pulley size alone.

Are the fans delivering the designed airflow, and isthis enough?

This is difficult to determine without outsidehelp. Some end users have the tools andtraining to be able to perform airflow tests onACHE’s, but in most cases it might be better tohire a local contractor who is experienced,knows how to measure airflow and can offeroverall ACHE's solutions.

If it is determined that the ACHE isdelivering it's best possible airflow based on allthe discussions so far in this paper, but theprocess still requires more cooling, then havethe process engineers review the application.The cooling demands might be more than theexisting ACHEs can handle with the currentblade design.

Nick Agius and Bob Moore perform airflow testingon forced draft (FD) ACHE's.

Seal disc

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Keep in mind that before a decision is made to purchase any new exchangersthat the type of blade can also greatly increase cooling. Experience has shownthat most aluminum fan blades are unable to achieve the airflow that was on theoriginal specification sheet due to their inefficient blade design. This paper isnot meant to endorse any particular fan manufacturer. Technical papers, airflowtests and other information that deal specifically with the issue of fan blades areavailable from the author.

The blade design can greatlyaffect the airflow of any fan andthere are high efficiency bladesavailable to fit any ACHE unit. Lowefficiency straight cord aluminumfan blades, can have efficiencies aslow as 35 to 55%. Today’s modernfans are more aerodynamic, withefficiencies as high as 75 to 85%.

The more air that is pushedthrough the fin tubes, the better. Thisincreased air-flow will have apositive effect on cooling.

Note: This is not the case on awet “cooling tower” where moreair can pull the water curtain too far into the tower and create other problems.

Other issues for best performance practices onACHE’s

It should be noted that a 10% increase in airflow relates to an improvedthermal duty increase for ACHE’s as follows:

ACHE Service Condensing Liquid Cooling Vapor cooling Viscous coolingMin Duty 5% 3% 2% 1/2%Max Duty 7% 6% 5% 1%

Formulas and rulesCFM (airflow) = Fan RPM, (Airflow varies in direct proportion to RPM)SP (static pressure) = Fan (RPM x RPM), (Pressure capacity varies

with the square of a change in RPM’s), or a 10% increase in fan speed equals a21% increase in static. 1.1 x 1.1 = 1.21. Or the square root of 1.21= 1.1.

HP = Fan (RPM x RPM x RPM), (Power required varies with the cube ofthe change in RPM). So a 10% increase in fan speed means 1.1 x 1.1 x 1.1 =1.33 or 33% increase in HP.

10% more flow = 21% more static pressure = 33% more HP. This isconservative as the actual numbers are normally… 10% / 18% / 29% increase inHP.

Will a fan, in stall, produce higher static pressure? High system resistance attoo low of a fan speed will produce fan stall. However, when the fan is in stall,or more correctly, as it moves more into stall, the static pressure will increasebut the airflow will decrease and this will typically produce increased vibration(flutter) and increased noise. From above, it sounds like the fan is in stall.

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Nick Agius testing induced draft (ID) with special testing tool.

Winter and summer pitchWinter and summer pitch is more of an issue in the colder northern regions

and with VFD’s this issue can be controlled very easily. The air is heavier in thewinter, so if the pitch is set for summer conditions the motor will overload.With VFD’s, the speed can be turned down to control this condition. It is verycommon for plants to have a winter and summer pitch.

With the necessary information and knowledge to be able to read the fancurves or use a fan program, the end-user can design summer and winterspeeds. This way the pitch can remain the same all year.

The following are the benefits of leaving the pitch the same throughout theyear:

It is faster and easier to change a motor pulley than change the pitch of all theblades. Changing just the motor pulley is not considered a confined space issueso less manpower is needed with quicker results.

The small pulleys wear fast and should be replaced from time to timeregardless. These are very inexpensive parts. The money saved on the quickerturnaround, with fewer people, (no man-watch required) will more than pay fornew pulleys every year.

If the driver pulley is kept in good condition, by replacing it every sixmonths, the result is a better belt life and less downtime on the fans.

VibrationIf experiencing vibration, a few things can be examined before the experts

are called in.There are three main types of frequencies to be concerned about related to an

ACHE.Motor frequencyIf the vibration frequency is at motor speed, then investigation needs to focus

on the motor, belt or gear drive. Is the belt tensioned correctly? Are the bearingsokay? Is the drive aligned?

Fan frequencyIf the vibration frequency is at the shaft speed, then this kind of vibration is

associated with the fan only. Look for blade tracking issues; blade sweep issues(which is the distance between each blade); or look for an unbalanced fan blade.Are all the blade pitches for a particular fan within 1/2 of a degree total?

Blade pass frequencyIf the frequency is fan speed times the number of blades, look for loose parts

in the structure. Are any welds broken? Is this structure flimsy? Are the bearingand motor bolts all tight?

Keep in mind that if everything is fine your blade pass frequency shouldalways be the dominant frequency.

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Windmilling ACHEIf the system is built where more than one fan

shares the same housing, the fans can windmillbackwards when one or more fans are turned off. Thiscan be very destructive to the drive system once thisfan gets a signal to go forward. Again VFD’s cancontrol this. Another way of solving this problem iswith an “Anti-Rotation Device”. It was developed andpatented in the mid 90’s to solve this problem. It’s asimple, mechanical device that bolts onto the largefan pulley with no additional modifications required.For further information please e-mail Nick Agius [email protected].

ReferencesHudson products Corp: “The Basics of Air Cooled Heat Exchangers”, September 2000.Monroe, RC: “Minimizing Fan Energy Cost”, Chemical engineering, May 1985.Robert Giammaruti: “Performance Improvements to Existing Air-Cooled Heat Exchangers”CTI institute paper TP04-13 February 2004.Charles Taylor: “Maintenance of ACHE” May 2005.

Anti-Rotation Device.

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