storm technologies - useful air heater formulae

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September 2009 Volume No.8

STORM TECHNOLOGIES, INC.411 North Depot Street PO Box 429

Albemarle, NC 28002Phone: 704/983-2040, Fax: 704/982-9657

www.stormeng.com

The Cost Impact of Poor Airheater PerformanceStephen Hall, Senior Project Engineer and Storm Technologies, Inc. Team

Many plants are complacent with their air heater leakage. Itis common knowledge that air heater leakage is detrimentalto performance, but the extent and severity as well as theimpact on the bottom line are often overlooked. When youbegin to look at the complete picture, the more importantmaintaining minimal air heater leakage becomes.

Leakage through a regenerative Ljungstrom style air heateroccurs in several ways. With all of the moving parts, there

are plenty of ways for air to go where it is not supposed to go. Air can escape through the radial seals on both the cold andhot side of the rotor. It can also bypass the rotor completelyand go into the furnace unheated. Similarly the gas canbypass the rotor and go into the backend equipment un-cooled. While less common, Rothemuhle air heaters havesimilar leakage paths. In addition, air is carried into the gasstream by the baskets directly. Basket cleanliness plays arole in air heater leakage. As the baskets become plugged,the basket differential increases. The gas will then take thepath of least resistance and will increasingly pass aroundthe basket instead of through it. In addition, since the fanshave to pull harder on the gas to get it to pass through the

air heater, the air on the other side of the seals is pulled

harder as well. Therefore, it is important to make sure theair heater soot blowers are working properly and as often asrequired. All seals should be set to the proper clearance tominimize the amount of air in-leakage and gas bypass.

Tubular air heaters are often assumed to have zero leakagebecause of the design. However, as small holes begin todevelop, they rapidly increase in size. Design leakageon a tubular air heater is 0%, but 7-8% is not uncommon.

Therefore regardless of type, air heater leakage is a problemshared by all three. Tubes need to be plugged as theydevelop holes. However, if too many tubes are plugged, thedifferential across the air heater begins to increase. Thiswill exacerbate the leakage through the remaining tubes.If this is the case, there are few options to correcting theleakage beyond re-tubing the affected circuits. This highmaintenance cost is one of the reasons regenerative airheaters are more common than recuperative tubular styleair heaters.

Air leakage negatively impacts ow rate and temperature.The air leaking into the gas ow adds to the ow going through

the air heater and subsequent downstream equipment. Inaddition, the air temperature is lower than that of the gasstream and thus reduces the gas temperature. Sometimespoor air heater performance goes unnoticed because thegas outlet temperatures are so low; the thinking is “Becausethe gas outlet temperatures are low, my air heater mustbe working well.” However, if you were to evaluate the airheater performance on the true gas outlet temperature, thetemperature of the gas without any diluting air, then theperformance would not look quite as impressive. Since thegas is heating air that never reaches the furnace, this heatis more or less thrown away. In this respect, air heaterleakage is a net heat rate penalty.

Perhaps the biggest impact poor air heater leakage has isnot on the air heater itself, but on the FD and ID fans. As theair in-leakage increases, the FD and ID fans have to workharder to maintain the desired drafts and ows into andout of the furnace. Not only do the fans have to push/pullmore ow, but even more costly, they have to operate at apressure higher than design. In the summer months whenthe ambient air has a greater volume, it is possible that thefans can no longer keep up with the increased demandsplaced on the system. If the unit cannot make full loadduring the hours when the price of energy is at its greatest,the nancial impact of air heater leakage is quite large.



As important as air heater leakage is, it is rarely measuredproperly. If oxygen probes are present at the air heatergas inlet and outlet then the leakage can be calculated.However, just a few probes do not get a good qualityrepresentation of the entire duct. A proper test grid withtest points distributed evenly across the duct is requiredto capture the stratications in the ue gas. A proper testgrid can sometimes even help determine the source ofthe leakage. Typically higher oxygen concentrations arefound closer to the sector plates on a Ljungstrom-style

air heater. Depending on where the test grid is, it couldalso help determine which tube circuits are leaking on atubular air heater. For highly stratied ows, a weightedaverage should be used based on a gas velocity. Leakagetesting does have its limitations. It cannot measure air orgas bypass around the baskets. Therefore, true leakage ishigher than “air in-leakage.”

When evaluating air heater leakage it is important to lookat the other performance factors as well. These include airand gas side efciency, x-ratio, and total thermal efciency.

A thermal efciency of an air heater is required to fullyunderstand the true performance of an air heater. However,the testing required for that is even more demanding than aPTC test. Multipoint probes can dramatically decrease thetesting time and personnel required for a PTC or thermalstyle test. Weekly monitoring is a good diagnostic tool andwith the right equipment, not a huge time investment.

Air in-leakage also effects downstream equipment likescrubbers, precipitators, and baghouses. ESPs areparticularly sensitive to temperatures and velocities. Air in-leakage negatively affects both. The higher velocity willalso increase the erosion rates due to yash.

Leakage testing only identies problem areas. However,knowing where to concentrate your work during the next

outage will make your time and money more productive.Expansion joints, casing, and air heater seals are commonplaces for air in-leakage. Storm Technologies hasextensive experience in measuring air heater leakage andhelping customers understand the impact it has on bothperformance and heat rate.

A more detailed illustration ofthe Air Heater leakage paths ona Ljungstrom style air heater isillustrated as follows:

Consequences of each owpath (leakage):

No penalty with intendedow pathThis air never getsheated and thus lowersthe air heater outlettemperature loweringheat rateThis air is waste volume.It makes the FD fan andthe ID fan work harderincreasing auxiliaryhorsepower.

1.

2.

3.

The most damaging leakage because it steals

heat from the gas and increases the workload onthe FD and ID fans: a heat penalty and an auxiliaryhorsepower penaltyNo penalty with intended ow pathThis heat is wasted and directly increases the dry gasloss which is a heat rate / boiler efciency penalty

In our experience, utilization of high performance seals and/or incorporating a performance testing program as part ofan airheater overhaul and/or refurbishment is necessary toevaluate the design parameters and/or challenges upfrontthat can be evaluated prior to installation of new designbaskets and/or high performance seals. Performancefactors typically measured in a complete air heater test by

Storm Technologies include:

Air In-LeakageCorrected Gas Outlet TemperatureX-RatioGas Side Efciency

4.

5.6.

1.2.3.4.

Gas Inlet Test Grid

(Temperature, % Oxygen, CO ppm,

NOX ppm & Static Pressure)

Gas Outlet Test Grid

(Temperature, %

Oxygen, CO ppm, NOX

ppm & Static Pressure)

Inlet and outlet air temperature

and pressure averages required

for complete air heater

performance analysis



In addition to test factors, we also recommend comprehensivediagnostic testing programs be implemented as well toevaluate the ring system, total system efciency, air in-leakage and/or plant heat rate opportunities. Typical,average cost impacts, based on a median average of utilityplant performance opportunities with air heater and theinter-related stealth losses related to non-optimum airowmanagement and/or air in-leakage upstream of the APHare illustrated in the chart to the right.

Storm Technologies primary focus is to provide costeffective and value added solutions to its customers in thepower generating industry. Storm’s engineering servicesinclude combustion and boiler performance improvements,heat rate programs, pulverizer optimization programs,specialized testing programs and airow measurement,control and improvement. Storm fabricates its products inhouse and is accredited by American Society of MechanicalEngineers (ASME) and National Board in accordance withthe applicable rules of the National Board and ASME Boilerand Pressure Vessel Code.

Paragon Airheater Technologies specializes in themanufacturing of high-performance seals, basketedelements, and replacementparts for rotary, regenerativeairheaters. Paragonprovides trained technicalpersonnel to perform routineinspection services, conditionassessments, outage planning,diagnostic services oremergency maintenance andrepair. Paragon’s exclusiveand patented design for bothradial/axial and circumferential/bypass seals are specically

designed to provide maximumefciency while withstandingthe harsh operating conditionsnormally encountered in the airheater.

www.paragonairheater.com

With that said, we askthe question, “Have youhad your Pulmonary ttest?” Contact StormTechnologies today if youare interested in learningmore about optimizingcombustion and/orairheater performance.

www.stormeng.com

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Loss profit due toincreased auxiliarypower: $625,000

Thermal losses dueto radial seal leakage:$55,000

Cost of fuel to produceextra motor horsepower:$155,000

Load limitationsdue to lack of fancapacity:

Thermal loss of bypass(circumferential) sealleakage: $27,500

High carbon in ash:$1,450,000

De-superheating sprayflows: $1,250,000

Air in-leakage from thepenthouse and/or convectionpass (upstream of theairheater): $1,500,000

Excess or non-optimumcombustion airflow:$1,350,000

Measurable losses associated with air heaters (Total): $1,737,500 (Red)

Measurable losses related to boiler air in-leakage and/or poor combustionairflow management (Total): $6,175,000 (Blue)

Note: basis was a 500 MW

unit @ ~2.50/MMBTU (or

$50/ton) fuel costs.

Together Storm Technologies,Inc and Paragon Air HeaterTechnologies represent theheart and breathing power ofsynergistic solutions workingtogether by offering highperformance equipment andsolutions for combustion systemsand air heaters.

“The Heart” “The Lungs”

storm technologies - useful air heater formulae

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