Air In-LeakageCorrected Gas Outlet TemperatureX-RatioGas Side EfficiencyPressure Drop

The air heater is the last heat trap in most utility boilersystems. Once flue gas passes through the air heater it isrejected out of the stack, in most systems. Dry gaslosses, which is any remaining heat in the flue gas, islost energy, which reduces the boiler efficiency/heatrate. Dry gas losses account for the largest losses in PTC4.0 boiler efficiency testing. Air heater performance isdirectly impacted by the conditions in which it operatesincluding flow rates, entering temperatures, andpressures.

Air heater performance is typically evaluated bythese factors:

While each of these factors are important in reviewingair heater efficiency all are required to understand thetrue performance.

In balanced draft systems, there are many locations inwhich air in-leakage can occur and each has differentimpacts on the system and efficiency. The air in-leakage negatively impacts flow rates and temperaturesthroughout the system. Air leaking into the gas flowadds to the flow going through the air heater andsubsequent downstream equipment. In addition, thediluting 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. However, if youwere to evaluate the air heater performance on theactual Corrected Gas Outlet Temperature, thetemperature of the gas without any diluting air, thenthe performance would not look quite as impressive.

One of the biggest impacts air heater leakage has isnot on the air heater itself, but on the ID fans andback end equipment. As the air in-leakage increases,the fans work harder to maintain the desired draftsand flows into and out of the furnace. Not only dothe fans have to move more volume, but even morecostly, they must operate at a pressure higher thandesign. In the summer months when the ambient airhas a greater volume, it’s possible that the fans can nolonger keep up with the increased demands placed onthe system. If the unit cannot make full load duringthe hours when the price of energy is at its greatest,the financial impact of air heater leakage is quitelarge.

UNDERSTANDING AIR PREHEATERPERFORMANCE

STORM TECHNOLOGIES, INC. www.stormeng.com

Some typical rules of thumb

Parameter Change Percent Changein Heat Rate

0.25%

0.15%

0.29%

0.35%

0.05%

1%

1%

10℉

10℉

1%Air Heater Leakage

Air Heater Effectiveness

Outlet Has O2

Corrected Outlet Gas TempChange (12,000 Btu/Lb Coal)

Corrected Outlet Gas TempChange (8,000 Btu/Lb Coal)

Air in-leakage after the furnace and prior to the airheater is not typically measured during a typical airheater performance test (ASME PTC 4.3), however airin-leakage here impacts not only combustion but theair heater performance. Infiltrating air at the boilerroof/penthouse penetrations, back pass, economizerhoppers or expansion joints before the O2 probes areseen by the system O2 probes as combustion air whichin turn robs air from entering the furnace. Air enteringafter the temperature quench point does little tonothing for combustion of carbon. This type of air in-leakage often results in higher carbon in ash, increasedfurnace exit gas temperatures, and secondarycombustion. Infiltrating air impacts air heaterperformance because it is air entering the system whichdid not pass through the air heater. Reducing theairflow and increasing the flue gas flow results inreduced air heater performance. This air heaterperformance parameter is evaluated with the X-ratio,which is the ratio of the heat capacity of the air passingthrough the air heater to the heat capacity of flue gaspassing through the air heater. Reduced airflow and/orincreased flue gas flow will thus reduce the X-ratio.

Boiler or backpass air in-leakageIncrease in tempering airIncreased primary air ratios requiring lower millinlet temperature and thus more tempering air Changes in fuel moistureAny changes in cold air bypassHot air recirculationWhile less of an impact, excess air

Factors that can Impact X-RatioAny air that enters the system and does not passthrough the air heater will impact the air heater X-ratio.

Air heater air in-leakage is measured and calculatedbased on flue gas constituents at the air heater inletand outlet. In the ASME PTC 4.3 test code thepercent air in-leakage is calculated on a mass flow ratebasis in terms of wet products of combustion perpound of as fired fuel entering and leaving the airheater. This test requires a full analysis of the flue gas,fuel analysis and carbon in ash, however an empiricalcalculation can be utilized which only requires O2measurements at the air heater inlet and outlet whichis based on a volume basis rather than mass basis.Storm has found this empirical or abbreviated leakagecalculation to produce results very close to the fullPTC 4.3 test and can be performed at a fraction of thecost and time.

Entering gas temperature change will cause the exitgas temperature to change in the same direction. Forquick calculation:

Exit Gas Temp Change=Entering Gas TempChange*(1-Gas Side Efficiency)

For example, with a gas side efficiency of 65%, if theentering gas temperature increases 10°F then the gasoutlet temperature will likely increase approximately3.5°F.

10℉ x (1 - 0.65) = 3.5℉

Other Factors Impacting Air Heater PerformanceEntering air temperature change will cause air heaterexit gas temperatures to trend in the same direction.This is due to changes in temperature head difference.For quick calculation:

Exit Gas Temp Change = Entering Air TempChange*Gas Side Efficiency

For example, with a gas side efficiency of 65%, if theentering air temperature increases 10°F then the airheater gas outlet temperature will likely increaseapproximately 6.5°F.

10℉ x 0.65 = 6.5℉

Storm Technologies, Inc. has always strived toprovide each plant we are involved at with a pathtowards improved efficiency and heat rate. Often, asshown in the following example, we find throughstandard air heater testing that the performance ofthe air heater has degraded to a point where therecould be 2% or more of efficiency and heat rateimprovements available on the table.

The largest limitation to increasing air heaterperformance is the requirement to stay above the gasoutlet acid dew point temperature. If the average coldend temperature or gas exit temperatures drop toolow, they can impart a great deal of damage over ashort period of time to the airheater and anydownstream equipment.

Design Leakage8%

MeasuredLeakage

32% - 35%

36℉ Abovedesign

corrected gasoutlet

temperature

In the past Storm would have conducted the performance testing and possible a condition inspection of theradial and circumferential seals and left the other details to the facility. However, in the fourth quarter of 2020,Storm expanded our air heater service capabilities with the addition of industry expert Kent Ritter. Kent Ritterhas over 40 years of experience in the air heater industry with regenerative-style air heaters. With Kent onboardat Storm Technologies, Inc. we can now provide a fully comprehensive list of services.

APH Components SupplierVendor Review and PricingShop Review and Quality Audit ofAPH Components at ManufacturingFacilityGuarantee Review/VerificationTechnical Support, Progress Review,and Condition Assessment Proven Synergetic TeamManagement for Multiple Vendorsand Sub-Contracts

Expanded Air Heater Services

Airheater Performance TestingASME PTC 4.3 Boiler EfficiencyFlue Gas VelocityComprehensive Boiler Testing

Project management

Performance Testing

With our expanded list of air heater services, we can now offer a complete package to measure the performanceof your air heaters, inspect, repair, and ensure your air heaters are not elevating the heat rate of your facility.Please give us a call today to find out how we can better support you and your air heater needs in 2021.

Performance testing (Abbreviated andComplete ASME PTC 4.3) Detailed air heater inspection servicesAir heater project managementOnsite trainingAir heater inventory and spare partassessmentsEngineering review of air heaterTechnical support and 3rd party review ofAPH work/performance

Life extension and planning of air heatercomponentsReview of APH proposals, developspecifications and work scope forconstructionReview cost benefits and ROIOutage planning and management services“Turn-Key” construction management ofAPH components

- Storm Technologies TeamWritten by: Adam McClellan, P.E.

411 N. Depot St. | Albemarle, NC 28001 www.stormeng.com +(1) 704-983-2040