Everything You Need to Know About Combustion Chemistry & Analysis

All substances contain varying amounts of captive energy depending on the substance and how it exists; that is, solid, liquid, or gas.  The uniting of two substances to form other substances is called a “chemical reaction”.  Combustion is a chemical reaction.  This reaction is for the purpose of releasing heat.  As we’ll see, oxygen will always be one of the substances in the reaction, and the other will be a hydrocarbon, mixtures of hydrogen, carbon, sulphur, etc.

Perfect combustion is simply a mixture of fuel and oxygen, with both being completely consumed in the burning process.  The ideal situation would be to provide just enough air in the combustion chamber to insure complete burning of the fuel.  This would be true if it were physically possible to bring each atom of fuel in direct contact with the amount of air required to complete its combustion.  To date, no method has been devised in a combustion chamber so that air and fuel come in complete contact in just the right proportions.

If we reduce the amount of oxygen, in a perfect mixture, we would have a fuel-rich condition.  However, if we increase the amount of oxygen, in a perfect mixture, we now have excess, which does not contribute to the burning process.  Having just the right amount of oxygen (no more, no less) is called the stoichiometric point, or stoichiometric combustion.  The stoichiometric point is also called the 100% air point.

Anything above the 100% point is called excess.  For example, we might use the term 20% excess air to describe a boiler’s air/fuel mixture point.  This means the excess air is running at the 120% level or 20% (higher than stoichiometric) above the perfect mixture point.

Stoichiometric combustion is important since it is the reference point from which we can measure the efficiency of a heating unit.  Air contains 20.9% oxygen and 79.1% nitrogen.  The air/fuel mixture can be described simply as fuel + air.  Keep in mind that air consists of two parts oxygen (02), along with 7.52 parts nitrogen (N2).

If we do a chemical/mathematical explanation of a fuel using natural gas (methane CH4), we can see how various measurable compounds are formed that can be used to calculate how efficiently a heating unit is using its fuel.

A natural gas air mixture can be expressed as CH4 + 202 + 7.52 N2.  Let’s now increase the amount of air by 20% over this perfect mixture point:

20% excess air = 100% + 20% = 120% or 1.2

Therefore, let’s mix methane (CH4) with 1.2 times the normal 100% air 

or CH4 + 1.2 x 2 02 + 1.2 x 7.52 N2 

or CH4 + 2.4 02 + 9 N2

Now, let’s burn this new mixture and show the chemical transformation:

CH4 + 2.4 02 + 9 N2 ® C02 + 2H20 + .4 02 + 9 N2 

Note that .4 parts of 02 exist in the resultant flue gas because it was not used in the burning process; it was excess.

Let’s do a C02 analysis on a dry basis and also an 02 analysis on a dry basis.  In the flue, due to the burning process, we now have:

1 part CO2 + .4 part of 02 + 9 parts of N2 

Therefore, 1 + .4 + 9  = 10.4 total parts.

Note:  We drop the 2 H20 term because our analysis is on a dry basis.

% C02 =   1 part Co2  x  100%  =  1  =  9.61%

                10.4 parts                   10.4

% 02 =  .4 part 02  x  100%  = 3.8%

                   10.4 parts

Finally:     Actual 02  - Theoretical 02  x 100%  = excess air

                          Theoretical 02 

2.4 - 2  x  100%  =  .4  x  100% = 20% excess air 

   2                          2

Therefore, a C02 point of 9.61%, an 02 point of 3.8%, and an excess air point of 20% all mean the same thing in describing the air/fuel mixture point in the combustion process.

For natural gas, we have an ultimate or maximum C02 level of: 

       1              or  11.73% 

1 + 0 + 7.52

This means our excess air is set to 0%, which also means 0% 02 occurs in the flue, allowing the ultimate C02 level to be achieved.

Figure 1.

For a graphical explanation of excess air, refer to Figure 1.  This graph shows a typical theoretical air curve, showing the relationship between the combustion air setting on the boiler and various fuels.  The combustion setting, therefore, can be expressed as any one of the three terms:  excess air, 02, or C02.

Figure 2.

Figure 2 is known as a theoretical air curve.  This curve is for the fuel natural gas and is intended to show % excess air as it relates to being either fuel rich or fuel lean.  The fuel lean side is a safe side. 

Note:     A C02 analysis alone does not provide a safe indication of the combustion air/fuel setting.  Additional measurements of either smoke or CO are recommended.

In other words, a given C02 value can occur on either side of the stoichiometric.

Excess air is the preferred term to describe the combustion setting on the safe side of the stoichiometric.  In order to arrive at the excess air value, an 02 measurement is performed.

Figure 3.

Figure 3 is a theoretical air curve chart for fuel oil.

Again, C02 can appear on both the fuel rich and excess airsides of stoichiometric.  This is a very important point worth repeating. 

Warning:         When doing combustion testing, if you only rely on C02 percentage alone, you can get high C02 readings and be on the fuel rich side of the stoichiometric.   Avoid the fuel rich side since partially burned fuel will result in carbon monoxide (C0), a gas that in large enough concentration can kill!

See Figure 1 again.  Note that testing for oxygen, 02, insures being on the excess airside of combustion and correlates directly toC02 readings, regardless of the fuel being burned. 

Note:  Measuring 02 in the flue gases is the preferred method for combustion analysis.

All modern electronic portable combustion analyzers use an 02 cell.  They may be able to display C02, but it will be calculated or computed from the 02 reading. 

Now that we have a handle on what happens when we “burn” something, how can this knowledge be put to use?  It should be clear that we could test a heating unit with some instruments that can measure the products of combustion, and find out how efficiently the unit is using its fuel.  We could even “tune up” a burner to burn more efficiently.  A combustion analysis can also diagnose problems with the burner.

There is nothing mysterious about combustion efficiency.  It is simply 100% minus the percentage of heating value lost through the stack.  If stack losses are 25% of the heating value for a given fuel, then the combustion efficiency is 75%.

Combustion efficiency calculations start with assuming complete combustion and then applying three basic factors:

1. The heating valve for a given fuel.
2. Net stack temperatures.
3. Percentage of oxygen by volume.

 The Heating Value of a fuel is the stoichiometric air/fuel mixture, or put another way, the potential energy in the fuel.  The Net Stack Temperature is the temperature that the gases are raised above the temperature of the primary and secondary air, usually ambient air temperature.  In some commercial-industrial applications, the primary air will be pre-heated.

If we only want to know the efficiency of a particular burner, we need only an instrument to find percent 02, a thermometer, and a combustion efficiency calculator or chart for the fuel being used.  Combustion efficiency calculators, such as the Bacharach 10-5064, or combustion efficiency charts (see Figure 4), have taken into consideration the heating value of a fuel, so it is only necessary to find the net stack temperature, percent 02, and insert these figures into the chart or calculator. 

Figure 4.

As an example, a natural gas furnace with a net stack temperature of 350oF and an 02 reading of 7 1/2% is 80% efficient (see Figure 4).  What this means, is that 80% of the heating value of the gas is being used to heat with and 20% is wasted.  Another way to look at it is to say that for about every dollar of fuel spent, 80 cents is returned as usable heat and 20 cents is going out the stack.  At today’s and future fuel prices, this is expensive.  At today’s cost of natural gas, by increasing a unit’s efficiency from 80% to 85% will save about $7.00 per $100 of fuel cost.

A contractor can use simple overall combustion efficiency to compare a customer’s operating unit to a proposed modern high-efficiency unit and estimate fuel cost savings to show payback.  It’s a useful selling tool, but is not true combustion analysis.  For combustion analysis, we want to know more than just net stack temperatures and percent 02.  We also want to know C0 parts per million (PPM), draft, smoke (if oil), and for large industrial burners, sulfur dioxide (PPM) and oxides of nitrogen (PPM).  With all that information, we can then properly analyze the burner’s performance, diagnose problems, and tune the burner for optimum efficiency.

Combustion analysis used to be done by taking tests the old-fashioned way that is with oristats, sometimes called “cocktail shakers”.  Tuning a burner using those devices was very time consuming.  Modern electronic combustion analyzers let you see the results of changes made during a burner tune-up immediately.  They are accurate and much easier to use than oristats.  A prime example of a modern analyzer is the Bacharach Model 300. 

A fully equipped 300 can display on large LED readouts:

1. Stack temperature,
2. PPM C0
3. Percent 02
4. Percent excess air
5. Percent efficiency
6. Percent stack loss
7. Percent C02 
8. PPM NOX 
9. PPM S02  

It can print out the data and can transfer the data to a computer. 

The 300 can do all this for seven selectable fuels: 

1. Natural gas
2. Number 2 fuel oil
3. Number 6 fuel oil
4. Propane
5. Coal
6. Wood
7. Bagasse. 

With the addition of a smoke tester, draft gauge, manometer, and oil pressure gauge, the service technician would have every instrument needed to do a complete combustion analysis, tune-up, or diagnose problems of any burner.

Combustion efficiency gives us an overall view of a burner’s performance.  Using our instruments, these problems can be isolated, and through interpreting the readings, the problems can be corrected.

Only the manufacturer of a piece of equipment knows what the recommended 02, net temperature, draft, etc. should be.  The design of the equipment will dictate what the “correct” settings, the optimum operating parameters, should be.  If the manufacturer’s specifications are not available, some general guidelines will have to be used, along with federal, state, and local codes.  Government or utility regulators often set excess air settings and allowable C0 concentrations.

In general, most equipment will operate between 16% and 30% excess air, that’s 3% to 5% 02 (see Figure 1).

Over fire draft of .02 inches water column (W.C.) is acceptable with flue draft .02 to .04 inches W.C. greater than the over fire draft.

Net stack temperatures vary according to the fuel used.  Non-condensing units are not designed to withstand condensation, so the stack temperatures must be maintained above the dew point. 

Rough guidelines for minimum net stack temperatures are: 

1.  Natural gas 250°F
2.  Number 2 fuel oil 275°F
3.  Number 5 fuel oil 300°F
4.  Coal 325°F
5.  Wood 400°F.

A 0 PPM C0 reading is ideal, but not practical.  No code or manufacturer specification allows 400 PPM or more C0.  One can reasonably expect to achieve C0 readings below 100 PPM.

When “tuning up” a burner, the exact adjustments depend entirely on the equipment’s design, size, and make.  Every attempt should be made to follow the manufacturer’s specifications.  A general procedure would be to complete all maintenance.  Clean the heat exchange surfaces, oil equipment, replace defective parts, etc.

For burners using heavy oils, check the preheat temperature.  If unknown start at 250oF and reduce the temperature until best combustion is achieved.  Check and adjust fuel input.  Operate the burner for at least 15 minutes.  If a modulating burner, operate at high fire until the unit is at normal operating temperature.  Check and set excess air settings.  Check for C0 (and smoke, if an oil burner).  Reset air settings until C0 and smoke are acceptable levels, check net stack temperatures.  On modulating burners, check all settings at low fire and at several points over the firing range.  Adjust as necessary.  If a dual fuel burner, repeat the procedures for both fuels.

The modulating boiler is a combustion system that adjusts its firing level and steam production to meet a changing output demand.  A process steam boiler is a good example of a system that has this modulating or variable firing rate capability.

The Model 300 is capable of measuring combustion efficiency at each firing rate or combustion load point.  To put it simply, the Model 300 measures all the necessary combustion process parameters and in turn automatically calculates and displays the actual combustion efficiency for each boiler operating point.

To optimize efficiency at any boiler point is quite simple.  Connect the Model 300’s probe to the boiler’s sampling location and adjust the air in steps of 10% excess air and measure the efficiency.  Next, plot a curve (efficiency vs. excess air) and determine the mechanical position necessary for the ideal process set point.  Repeat the procedure at various boiler load points over the normal operating range.  The procedure merely consists of adjusting the boiler in order to obtain the maximum safe combustion efficiency.  To assure continued maximum safe efficiency, regular tests are repeated to ensure proper handling of the key parameter variations. Changes in relative humidity, variations in the fuel’s heating value, and changing combustion air temperatures must always be considered as possible random parameter variables, which affect combustion efficiency.  A random variation in the humidity, for example, can cause the concentration of oxygen in the air to vary from 20.9% at 0% RH to 20.40% at 100% RH (at an air temperature of 70oF).  This humidity factory will cause a variation in the oxygen measurement of .5% 02, at a 20% 02 set point.  This difference will result in a corresponding change in combustion efficiency of .2% or more.

Using the Model 300 to spot check these combustion parameters ensures that the process does not change and become unsafe.  Knowing the amount of combustibles present, determining the operating conditions over a range of fuel compositions, noting ambient temperature variations, and knowing the current humidity will allow the boiler set points to be maintained.

The modulating boiler’s correct air/fuel ratio depends upon the particular demand (load) placed on the boiler.  The correct operating parameters must be determined at each load condition.  This “profile”, when completed, is mechanically locked in place to ensure repeatability.  Graphing efficiency vs. excess air, at each load point, enables the operator to locate and set the process for the maximum efficiency over the entire boiler operating range.

In small (automatic) control systems, a jackshaft is used for modulating control.  A modulating motor controls each load position in order to automatically adjust to air and fuel linkage.  A cam is used as an adjustment to alter the air/fuel ratio and is considered part of the control mechanical linkage.  By placing the burner on “bypass”, manually setting a physical load position and constructing the necessary graphs, the optimum set point can be established.  Here again, use the Model 300 to determine the correct operating conditions.  This procedure is executed for each 10% and 20% load position.  This procedure, in other words, is repeated for each desired load point.

On larger control processes, the operator switches to “manual” and adjusts the air/fuel ratio at each load position.  A graph is still produced and a final set point is established.  The set point information, obtained by utilizing the Model 300, is then programmed in to a controller.  The “trim” controller has (by design) a limited operating range, since it corrects or trims the air and fuel linkage to compensate for the various parameters previously noted.  One final note on automatic control systems; the trim control must not be used to correct malfunctions in the boiler and must always have a slower response time than the main or overall control loop.  This limited range adjustment capability, on working boilers, is to prevent large-scale changes, which can cause major disruptions in the combustion process.  The Model 300, therefore, is a necessary tool for all modulating boilers regardless of automatic control type or basic mechanical adjustment configuration.  The 300 is needed to ensure correct operating conditions for combustion systems that must be readjusted for each and every output demand.

For many years, the combustion efficiency rating for new furnaces and boilers was in the range of 75% to 82%.  This average figure of 80% combustion efficiency was considered as the optimum performance level.  Times have changed.  Now combustion efficiencies are typically in the 90% plus range.  There are many reasons for this overall major improvement in heating unit performance.

•     The “condensing” design enables the recovery of the latent heat lost in the high stack temperatures previously required to maintain water in a vapor form.

•     The addition of “draft inducer” fans to provide a constant draft and to eliminate natural draw after burner shutdown.

•     Utilization of outside air for combustion air thus reducing the need for using inside (living area) oxygen for combustion.

•     Improved heat exchanger designs and better utilization of circulating air for more efficient scrubbing of the heat transfer surfaces in hot air applications.

•     The elimination of the constant pilot flame by the incorporation of electronic ignition systems.

•     The elimination of the dangerous hot stack by replacing with small diameter “room temperature” tubing.

•     Venting has been greatly simplified, thus reducing the possibilities of incorrectly sized venting and improper chimney size and height.  Installation, therefore, allows the heating unit to be free from previous installation and design restrictions.

Water vapor is present in the flue gases produced by the combustion of all fossil fuels.  The combustion process needs an air/fuel mixture, and this mixture already contains water vapor, just as the air we breathe contains a certain amount of water vapor, depending on the relative humidity.  In addition, water is a product of combustion for any fuel containing hydrogen or hydrocarbons.  Fuels such as methane and propane contain large amounts of hydrogen, but even coal contains some hydrogen in the form of entrapped hydrocarbons.  It takes energy to heat and thus raise the temperature of all this water vapor in flue gas.  This energy is latent heat.  If hot flue gases are allowed to cool, energy is released.  If water vapor is allowed to cool to the point where it condenses into a liquid (the dew point temperature), a great deal of energy is released.  This energy is the latent heat of evaporation.  A “condensing” furnace or boiler recaptures this latent heat (associated with both the raising of water temperature until it is fully vaporized and cooling the water vapor until it is fully condensed), and uses it to heat the boiler water or furnace air.

The amount of energy required to vaporize water (or conversely, the energy released when water vapor condenses) is staggering.  At atmospheric pressure, it takes only 142 BTU to raise the temperature of one pound of water from 70°F to its boiling point, 212°F.  However, once this pound of water reaches 212°F, it takes almost 1000 BTU to convert it from a liquid to a vapor, the latent heat.  Condensing furnaces recapture this heat.  A conventional furnace lets this heat escape up the stack.

The theoretical maximum heating value is the total heat, which can be obtained from the combustion of a specific amount of a given fuel, mixed with the correct amount of combustion air (at the stoichiometric point).  With the combustion starting temperature at 60°F, the combustion process is allowed to completely take place, and finally the flue gases (products of complete combustion) are allowed to cool back to 60°F.  The heat released due to combustion is measured over this entire range.

Because condensing furnaces allow the flue gases to cool to the condensing point, the flue or exit temperature is typically around 100°F.  In the non-condensing units, it is important to prevent condensation by keeping stack temperature above minimum values.  For instance, stack temperatures must be at least 75 to 100°F higher than steam temperature in steam boilers or the water temperature for hot water boilers and heaters.  If the water temperature is 180°F, for example, the stack temperature must be at least 250°F. The main point is non-condensing units are not designed to withstand condensation.  Therefore, the stack temperature must be maintained above the dew point.

Depending on the fuel and other conditions. the increase in efficiency due to condensing is in the range of 5% to almost 20%.  This is the gain due to recapturing the latent heat.

In condensing units, the key measurement is stack temperature.  If the stack temperature is around 100oF, we have a condensing unit, which should yield an improvement in combustion efficiency as compared to non-condensing furnaces and boilers.  Net stack temperatures (above ambient) of 40, 30, 20, or even 10°F are possible.

The second point of discussion is on the use of electrical fans to induce a draft.  The purpose of a draft inducer is really twofold.  Its first purpose is to pull flue gases evenly through the heat exchanger.  Its second advantage is to eliminate the need for a chimney.  The flue gases can now be blown through a sidewall vent system.

Efficiency is actually helped by a few percentage points because of two draft inducer effects.  First a stable and constant flow of flue gases is achieved over the heat exchanger and secondly, the stand-by heat loss is reduced, since there will be no suction through the heating unit, caused by a chimney “draw” during burner-off periods.

The key measurement, in this case, is the flue gas sample (either C02 or 02) taken at the same location as stack temperature.  This will be at a positive pressure of around 0.3" of water, depending on the particular furnace or boiler.  The draft-inducer’s fan typically develops around 1.5" of water negative pressure for its function of drawing the flue gases through the heat transfer mechanism.  Sometimes flue sampling may involve dealing with a safety device used to shut off the fuel supply in the event of draft-inducer failure.  This safety device is usually in the form of a pressure switch.

The third and final point deals with furnaces and boilers, which utilize outside air for combustion air.  The major advantage of bringing in outside air is the elimination of the risk of depleting breathing air by using heated inside air for combustion purposes.

The net stack temperature (the difference between the inlet air temperature and the exhaust gas temperature) is very important to the efficiency of non-condensing furnaces, but has little effect on the efficiency of condensing furnaces.  In condensing furnaces, the relationship of the actual stack (exhaust gas) temperature to the dew point of the exhaust is the more important factor, because of the very large amount of heat liberated when the stack gas is cooled below the dew point.

Dealing with the measurement aspects of new high-efficiency furnaces and boilers is quite simple.  First choose an instrument that can measure, calculate, and display combustion efficiency to 99.9% and also one that can automatically take primary air temperature into consideration.  Remember, the outlet temperature must be around 100oF and will be under a slight positive pressure when dealing with draft inducers.  A safety switch must be dealt with on certain units.  Always check the particular manufacturer for measurement locations and procedures.  Finally, combustion efficiency figures seem to run 1% or 2% higher than the manufacturers’ rating numbers.  If a certain furnace or boiler has a rating or AFUE (Annual Fuel Utilization Efficiency) number 92%, the combustion efficiency will be around 93.5%.

The concept of having stack temperatures below the dew point, eliminating the need for a chimney, and bringing in outside air creates a challenging instrumentation need.

The Bacharach hand held Fyrite II is the ideal instrument for testing condensing furnaces and boilers.  The Model 300 can also be used. Figure 5 and Figure 6 show the difference in the location of the sampling point for typical condensing (Figure 6) and non-condensing (Figure 5) units.

Figures 5 and 6.

Location of the sampling holes to take the various measurements is very important.  For residential and light commercial or industrial equipment, the following recommendations are applicable.

Oil Gun Burners:  Locate the sampling point as close the furnace breaching as possible, and at least six inches upstream from the furnace side of the draft regulator.

Gas Burners:  Locate the sampling hole at least six inches upstream from the furnace side of the draft diverter or hood, and as close to the furnace breaching as possible.  A probe can also be inserted down in the flue through a draft diverter or hood.

For Larger Equipment:  Locate the sampling point downstream from the last heat exchange device (such as an economizer, recuperate, or similar device).  Locating the point after the last heat exchanger ensures that the net temperature will provide an accurate indication of the effectiveness of the exchangers.  However, the further the point is from the last exchanger, the more heat will be lost through the duct or stack to the atmosphere and the greater the chance of dilution from air leakage, reducing the accuracy of the test.

Turbulence of the flue gases can sometimes cause samples taken from a certain portion of the duct to be misrepresentative of the flue gases.  Usually, going 8.5 duct diameters downstream of an elbow or other cause of turbulence will eliminate this effect.  To make certain that the sample taken from larger ducts or stacks is representative; it is generally a good idea to take several measurements with the probe inserted at various depths into the duct or stack.  If the indications at these various points differ, take their average for calculations.

Be very careful of air leakage into the duct or stack that can adversely affect the accuracy of the percent oxygen by volume indications.  This will increase the oxygen percentage beyond that caused by excess air.

Another sampling hole to measure over fire draft should be made so that a draft gauge sampling tube with a few feet of 1/4" OD copper tube will be centered approximately a foot above a combustion chamber.  This hole should be sealed after use.

Previously, the Bacharach Model 300 combustion analyzer’s general specifications were given.  Now that we are more familiar with combustion analysis and what can be achieved with good analysis, the Model 300’s features will be discussed in depth.

The Model 300’s 23-foot long cable allows connection to large systems, so the user is close to the 300, not close to the probe insertion location.  The large readouts let the user make adjustments and watch the displays to see the results of those adjustments.

The Model 300 directly measures and displays flue gas oxygen content in the range 0.0 to 25.0% 02, carbon monoxide content in the range of 0 - 3000 PPM C0, and the actual primary or flue gas temperature in a range 0° to 2100°F.  It also computes and displays combustion efficiency (0 to 99.9), C02 content (0 to 20%), excess air (0 to 250%), and stack loss (0 to 99.9%).

Remember our dry gas analysis did not take into account the water vapor (H20)?  Well, the Model 300 measures C0, relates this C0 value to C0, H2, and H20, and includes this measured parameter in the combustion efficiency calculation.  The Model 300 not only eliminates the need to graph combustibles along with combustion efficiency, but also performs the necessary subtraction operation based on a family of combustibles curves.  The Model 300 determines and displays combustion efficiency with combustibles present--automatically!

Process work and heating season applications are of no real consequence to the Model 300.  Process boilers and furnaces like hot water heaters in homes are used year-round.  Therefore, applications exist year-round, regardless of climate.  The Model 300 is probably less seasonal for the reason of heavy usage in industrial process work.

Consider the length of time the combustion process will be analyzed.  In other words, how long with the instrument be sampling and measuring in the stack or flue?  If a boiler is being studied over time (5, 10, 20, 45, 60 min., etc) the Model 300 is the choice.  The Model 300 is a short-term monitor, which can be attached to an industrial size boiler for a given period of time.

Nearly half of all the natural gas consumed in the United States can be attributed to the industrial sector, which includes both boilers and industrial furnaces.  The industrial furnace is really an example of a high temperature flue gas application.  The sample must be cooled below the upper temperature rating of the analyzer and, of course, the efficiency readings will not be correct.  Combustion efficiency must be accomplished using actual net stack temperatures and, therefore, cooling to a lower value is only for determining an excess air indication.

In the industrial sector, that of the industrial furnace, the temperature range is 400°F to 4,000°F.  The same relative improvement we find in the boiler sector is possible.  However, instead of dealing with improving a boiler’s efficiency from, say 72% to 77%, or decreasing the boiler’s 02 level from 7% to 2%, the improvement is achieved by reducing the furnace’s excess air from 50% to 10%.  Since the total fuel usage potential is similar to the boiler market, the fuel savings possibilities are also similar.  The 300 can handle up to 2100°, but if stack temperature exceeds 2100°F, it’s easy to make up your own custom high-temperature sampling and cooling assembly.  Merely choose tubing that closely matches a particular analyzer’s probe dimensions.  Measure the stack diameter, at the desired sampling location, and cut a new high-temperature probe to equal this measurement.  Experience has shown that when a probe has 50% of its overall length left exposed to ambient temperature, the cooling caused by the heat-sink effect is quite large.  Next attach a convenient length of rubber tubing to the one end, insert the new probe halfway into the stack, and attach the other end of the rubber tubing to the flue gas analyzer’s probe tip.  Allow the analyzer to draw a flue gas sample through the new probe and hose assembly and measure the percentage of excess air.  Finally, always watch the analyzer’s temperature indication to prevent exceeding its own upper temperature limit.  Make sure the tubing makes a leak-free seal on both probe tips.  If the rubber covering the high temperature probe tip becomes hardened, just snip off this small portion and re-attach it to the remaining soft portion of the probe.  Most analyzers can accommodate up to 10 feet of extension (probe and rubber tubing) without causing excessive drag on a sampling motor.

Do not forget the industrial furnace application.  Even though the temperatures are quite high, as compared to boilers, usually the temperature is already being measured and is known.  The application, for the 300, is to merely measure the oxygen level (excess air) and in the process use enough sampling line or probe extension to cool the flue gases to within the temperature specification of the analyzer.  The combustion efficiency indication or its calculation is not correct; however, the excess air measurement is correct.