Balanced Draft Air Preheater Troubleshooting

Excess O2 and/or draft limitations experienced on balanced draft furnaces are most likely caused by any of the following (in order of decreasing likelihood):

·????????Fouled air preheater

·????????Air preheater corrosion/damage and corresponding interstream leaks (overwhelming ID fan)

·????????Damper (damaged, slipped linkages, improper lineups, etc.) or fan problems

·????????Inadequate design for desired capacity

·????????Fouled convection fins or fouled SCR (if SCR present)

When analyzing air preheaters to rule out the above causes, the air and flue gas flowrates need to be established, which will most likely require a mass balance and/or a simple combustion calculation.

The excess oxygen content of the firebox flue gas can be used along with the fuel flow rate and fuel heating value in a simple combustion calculation to back calculate the required airflow.?For tight fireboxes the air leakage could be insignificant, however know sources of unmetered air ingress can further complicate the accuracy.?The calculated air and flue gas flow values are highly dependent on the accuracy of the instrumentation.?Being off by ~20% due to instrumentation inaccuracies it not uncommon. ??If combustion air is metered, this value should be compared with the combustion calculation noted above to gain confidence in the air flow rate.

Once air and flue gas flow rates are determined, the air preheater airside and flue gas side “clean pressure drop” can be predicted basis normalizing with:

·????????the air preheater OEM’s design thermal rating sheet, or better yet:

·????????a known clean air preheater condition pressure drop at flow rate

These conditions can be compared to measured pressure differentials across the flue gas side and combustion airside of the air preheater.?In some cases of grossly fouled air preheaters, integer multipliers on the clean pressure drop have been observed.

In addition to evaluating the hydraulics of the air preheater, thermal performance degradation can be evaluated via several different methods:

·????????Trending hot flue gas air preheater inlet temperature minus hot preheated combustion air outlet temperature over a long period time trend (years).?An increasing trend suggests air preheater fouling.?Figure 1 below describes this method.

·????????Setting up a fouling trend calculator over a long period time trend, using the following:

Simplifications:

·????????The LMTD correction factor can be backed out from the original design data, or from a known clean condition, then set fixed.??Although this is not accurate, your fouling resistance calculation will be underpredicted because as the temperatures deviate due to fouling, the correction factor will increase towards unity.

·????????Because the airside flow and flue gas side flows are comparable to each other, and neglecting wall resistance the fouled overall coefficient can be approximated as:

Where Uclean is taken from the original manufacturer’s design information (correlated to multiple flow conditions), or predicted from known clean conditions and used to calculated N below.

The overall fouling trend (over long periods of time) can then be plotted as shown in Figure 2 using the equation below.?

Similar type of fouling trends have also been carried out with feed/effluent exchangers as well as lean/rich amine plate exchangers.

Figure 1 – Example of a simplified trend suggesting fouling over time of an air preheater

Figure 2 – Example of calculated fouling trend of an air preheater.

Air preheater fouling typically manifests on the cold end of the flue gas side.?However, refractory debris sometimes falls within the hot flue gas end and causes obstructions.?It is important to check these areas during visual internal inspections.?Less likely (but possible) fouling / corrosion can occur on the air side in moist or coastal environments.?

If an air preheater has experienced significant corrosion (likely flue gas due point corrosion), the higher pressure air-side will leak into the flue gas stream.?In gross cases, this can overload the ID fan and/or starve the burners for air.?The amount of air leakage into the flue gas stream can be quantified by a handheld combustion gas analyzer taking a sample of both upstream and downstream of the air preheater’s flue gas side.?A noticeable increase in oxygen content in the flue gas downstream side will indicate leakage and thus a compromised air preheater.

ID and FD Fans:

With the estimated air flow and flue gas flow, along with the pressure differential measured across each fan, the fan curves can be used to evaluated fan performance and identify deficiencies.?Shaft speed verification and amperage draw can provide additional confirmation of the fan performance.?Watching a needle spike on a handheld manual Magnehelic pressure gage can provide additional insight that a filtered pressure transmitter will not show. ?

Louvers / Dampers:

Damper shafts, linkages, and actuators should be observed during actuation and positions confirmed in the field with their respective values at the control console.?It is possible for linkages to become loose, therefore it may be necessary to scrutinize linkage positions and shaft markings.?Most shaft ends on louvered dampers have a scribe mark indicating the blade position – all shafts should have scribe marks aligned together.?Visualizing the linkage movement compared with connecting linkages could give insight to slipped linkages as is the case in Figure 3.?

In some cases, airfoil blades internally bolted to the shaft may become loose and wobbly, potentially giving slight erratic draft fluctuations – this can only be visualized/corrected from the inside of the ductwork.

If in doubt, a pressure survey upstream and downstream of the dampers can provide insight as to whether all blades are fully open or not.

Figure 3 – Visualization of a slipped linkage on the 3rd shaft from bottom.?Notice inconsistency of shaft end marks (two louvers are open, two are closed).

#heattransfer #thermalengineering #furnace #heatexchanger